/*
 * Copyright (c) 2024 Gregory Burd <greg@burd.me>.  All rights reserved.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#include <sys/types.h>

#include <assert.h>
#include <errno.h>
#include <popcount.h>
#include <sparsemap.h>
#include <stdarg.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#ifdef SPARSEMAP_DIAGNOSTIC
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpedantic"
#pragma GCC diagnostic ignored "-Wvariadic-macros"
#define __sm_diag(format, ...) __sm_diag_(__FILE__, __LINE__, __func__, format, ##__VA_ARGS__)
#pragma GCC diagnostic pop
void __attribute__((format(printf, 4, 5))) __sm_diag_(const char *file, int line, const char *func, const char *format, ...)
{
  va_list args;
  fprintf(stderr, "%s:%d:%s(): ", file, line, func);
  va_start(args, format);
  vfprintf(stderr, format, args);
  va_end(args);
}

#define __sm_assert(expr) \
  if (!(expr))            \
  fprintf(stderr, "%s:%d:%s(): assertion failed! %s\n", __FILE__, __LINE__, __func__, #expr)

#define __sm_when_diag(expr) \
  if (1)                     \
  expr
#else
#define __sm_diag(file, line, func, format, ...) ((void)0)
#define __sm_assert(expr) ((void)0)
#define __sm_when_diag(expr) \
  if (0)                     \
  expr
#endif

#define IS_8_BYTE_ALIGNED(addr) (((uintptr_t)(addr)&0x7) == 0)

typedef uint64_t __sm_bitvec_t;
typedef uint32_t __sm_idx_t;

typedef struct {
  __sm_bitvec_t *m_data;
} __sm_chunk_t;

// TODO remove me...
static char *QCC_showChunk(void *value, int len);
static char *_qcc_format_chunk(__sm_idx_t start, __sm_chunk_t *chunk);

enum __SM_CHUNK_INFO {
  /* metadata overhead: 4 bytes for __sm_chunk_t count */
  SM_SIZEOF_OVERHEAD = sizeof(__sm_idx_t),

  /* number of bits that can be stored in a __sm_bitvec_t */
  SM_BITS_PER_VECTOR = (sizeof(__sm_bitvec_t) * 8),

  /* number of flags that can be stored in a single index byte */
  SM_FLAGS_PER_INDEX_BYTE = 4,

  /* number of flags that can be stored in the index */
  SM_FLAGS_PER_INDEX = (sizeof(__sm_bitvec_t) * SM_FLAGS_PER_INDEX_BYTE),

  /* maximum capacity of a __sm_chunk_t (in bits) */
  SM_CHUNK_MAX_CAPACITY = (SM_BITS_PER_VECTOR * SM_FLAGS_PER_INDEX),

  /* maximum capacity of a __sm_chunk_t (in bits) is */
  SM_CHUNK_RLE_MAX_CAPACITY = 0x7FFFFFFF,

  /* minimum capacity of a __sm_chunk_t (in bits) */
  SM_CHUNK_MIN_CAPACITY = (SM_BITS_PER_VECTOR - 2),

  /* __sm_bitvec_t payload is all zeros (2#00) */
  SM_PAYLOAD_ZEROS = 0,

  /* __sm_bitvec_t payload is all ones (2#11) */
  SM_PAYLOAD_ONES = 3,

  /* __sm_bitvec_t payload is mixed (2#10) */
  SM_PAYLOAD_MIXED = 2,

  /* __sm_bitvec_t is not used (2#01) */
  SM_PAYLOAD_NONE = 1,

  /* a mask for checking flags (2 bits, 2#11) */
  SM_FLAG_MASK = 3,

  /* return code for set(): ok, no further action required */
  SM_OK = 0,

  /* return code for set(): needs to grow this __sm_chunk_t */
  SM_NEEDS_TO_GROW = 1,

  /* return code for set(): needs to shrink this __sm_chunk_t */
  SM_NEEDS_TO_SHRINK = 2
};

#define SM_ENOUGH_SPACE(need)                          \
  do {                                                 \
    if (map->m_data_used + (need) > map->m_capacity) { \
      errno = ENOSPC;                                  \
      return SPARSEMAP_IDX_MAX;                        \
    }                                                  \
  } while (0)

#define SM_CHUNK_GET_FLAGS(data, at) ((((data)) & ((__sm_bitvec_t)SM_FLAG_MASK << ((at)*2))) >> ((at)*2))

#define SM_CHUNK_SET_FLAGS(data, at, to) (data) = ((data) & ~((__sm_bitvec_t)SM_FLAG_MASK << ((at)*2))) | ((__sm_bitvec_t)(to) << ((at)*2))

#define SM_IS_CHUNK_RLE(chunk) \
  (((*((__sm_bitvec_t *)(chunk)->m_data) & (((__sm_bitvec_t)0x3) << (SM_BITS_PER_VECTOR - 2))) >> (SM_BITS_PER_VECTOR - 2)) == SM_PAYLOAD_NONE)

#define SM_CHUNK_SET_RLE(chunk) (*(((__sm_bitvec_t *)(chunk)->m_data)) = (((__sm_bitvec_t)1) << (SM_BITS_PER_VECTOR - 2)))

#define SM_RLE_FLAGS_MASK 0xC000000000000000
#define SM_RLE_CAPACITY_MASK 0x3FFFFFFF80000000
#define SM_RLE_LENGTH_MASK 0x7FFFFFFF

static inline size_t
__sm_chunk_rle_get_capacity(__sm_chunk_t *chunk)
{
  __sm_bitvec_t w = chunk->m_data[0] & (__sm_bitvec_t)SM_RLE_CAPACITY_MASK;
  w >>= 31;
  return w;
}

static inline void
__sm_chunk_rle_set_capacity(__sm_chunk_t *chunk, size_t capacity)
{
  __sm_assert(capacity <= SM_CHUNK_RLE_MAX_CAPACITY);
  __sm_bitvec_t w = chunk->m_data[0];
  w &= ~SM_RLE_CAPACITY_MASK;
  w |= (capacity << 31) & SM_RLE_CAPACITY_MASK;
  chunk->m_data[0] = w;
}

static inline size_t
__sm_chunk_rle_get_length(__sm_chunk_t *chunk)
{
  __sm_bitvec_t w = chunk->m_data[0] & (__sm_bitvec_t)SM_RLE_LENGTH_MASK;
  return w;
}

static inline void
__sm_chunk_rle_set_length(__sm_chunk_t *chunk, size_t length)
{
  __sm_assert(length <= SM_CHUNK_RLE_MAX_CAPACITY);
  __sm_bitvec_t w = chunk->m_data[0];
  w &= ~SM_RLE_LENGTH_MASK;
  w |= length & SM_RLE_LENGTH_MASK;
  chunk->m_data[0] = w;
}

struct __attribute__((aligned(8))) sparsemap {
  size_t m_capacity;  /* The total size of m_data */
  size_t m_data_used; /* The used size of m_data */
  uint8_t *m_data;    /* The serialized bitmap data */
};

/** @brief Calculates the additional vectors required based on \b b.
 *
 * This function uses a precomputed lookup table to efficiently determine the
 * number of vectors required based on the value of the input byte \b b.
 *
 * Each entry in the lookup table represents a possible combination of 4 2-bit
 * values (00, 01, 10, 11).  The value at each index corresponds to the count of
 * "10" patterns in that 4-bit combination.  For example, lookup[10] is 2
 * because the binary representation of 10 (0000 1010) contains the "01" pattern
 * twice.
 *
 * @param[in] b The input byte used for the calculation.
 * @return The calculated number of vectors.
 * @see bin/gen_chunk_vector_size_table.py
 */
static size_t
__sm_chunk_calc_vector_size(uint8_t b)
{
  // clang-format off
  static int lookup[] = {
    0,  0,  1,  0,  0,  0,  1,  0,  1,  1,  2,  1,  0,  0,  1,  0,
    0,  0,  1,  0,  0,  0,  1,  0,  1,  1,  2,  1,  0,  0,  1,  0,
    1,  1,  2,  1,  1,  1,  2,  1,  2,  2,  3,  2,  1,  1,  2,  1,
    0,  0,  1,  0,  0,  0,  1,  0,  1,  1,  2,  1,  0,  0,  1,  0,
    0,  0,  1,  0,  0,  0,  1,  0,  1,  1,  2,  1,  0,  0,  1,  0,
    0,  0,  1,  0,  0,  0,  1,  0,  1,  1,  2,  1,  0,  0,  1,  0,
    1,  1,  2,  1,  1,  1,  2,  1,  2,  2,  3,  2,  1,  1,  2,  1,
    0,  0,  1,  0,  0,  0,  1,  0,  1,  1,  2,  1,  0,  0,  1,  0,
    1,  1,  2,  1,  1,  1,  2,  1,  2,  2,  3,  2,  1,  1,  2,  1,
    1,  1,  2,  1,  1,  1,  2,  1,  2,  2,  3,  2,  1,  1,  2,  1,
    2,  2,  3,  2,  2,  2,  3,  2,  3,  3,  4,  3,  2,  2,  3,  2,
    1,  1,  2,  1,  1,  1,  2,  1,  2,  2,  3,  2,  1,  1,  2,  1,
    0,  0,  1,  0,  0,  0,  1,  0,  1,  1,  2,  1,  0,  0,  1,  0,
    0,  0,  1,  0,  0,  0,  1,  0,  1,  1,  2,  1,  0,  0,  1,  0,
    1,  1,  2,  1,  1,  1,  2,  1,  2,  2,  3,  2,  1,  1,  2,  1,
    0,  0,  1,  0,  0,  0,  1,  0,  1,  1,  2,  1,  0,  0,  1,  0
  };
  // clang-format on
  return (size_t)lookup[b];
}

/** @brief Calculates the offset into set set of vectors within a chunk for a
 * given index.
 *
 * This function determines the array index into m_data of the requested
 * position.  The code examines the flags within the descriptor and calculates
 * the index within the chunk's data.
 *
 * @param[in] chunk Pointer to the chunk.
 * @param[in] bv Index within the descriptor (0-based).
 * @return Array index of the vector within the chunk's data iff the descriptor
 * was SM_PAYLOAD_MIXED, otherwise meaningless.
 */
static size_t
__sm_chunk_get_position(__sm_chunk_t *chunk, size_t bv)
{
  /* Handle 4 indices (1 byte) at a time. */
  size_t num_bytes;
  size_t position = 0;
  register uint8_t *p = (uint8_t *)chunk->m_data;

  /* Handle RLE by examining the first byte. */
  if (!SM_IS_CHUNK_RLE(chunk)) {
    num_bytes = bv / ((size_t)SM_FLAGS_PER_INDEX_BYTE * SM_BITS_PER_VECTOR);
    for (size_t i = 0; i < num_bytes; i++, p++) {
      position += __sm_chunk_calc_vector_size(*p);
    }

    bv -= num_bytes * SM_FLAGS_PER_INDEX_BYTE;
    for (size_t i = 0; i < bv; i++) {
      size_t flags = SM_CHUNK_GET_FLAGS(*chunk->m_data, i);
      if (flags == SM_PAYLOAD_MIXED) {
        position++;
      }
    }
  }

  return position;
}

/** @brief Initializes a chunk structure with raw data.
 *
 * This function casts the provided raw data pointer to a `__sm_bitvec_t`
 * pointer and stores it in the `m_data` member of the `__sm_chunk_t` structure.
 *
 * @param chunk Pointer to the chunk structure to initialize.
 * @param data Pointer to the raw data to be used by the chunk.
 */
static inline void
__sm_chunk_init(__sm_chunk_t *chunk, uint8_t *data)
{
  chunk->m_data = (__sm_bitvec_t *)data;
}

/** @brief Calculates the capacity of a chunk in bits.
 *
 * Determines the maximum number of bit available for storing data within the
 * chunk.  The capacity is typically `SM_CHUNK_MAX_CAPACITY` bits, but it can
 * be reduced if the chunk contains flags indicating an unused portion of the
 * chunk or larger than max capacity when this chunk represents RLE-encoded
 * data.
 *
 * @param[in] map The sparsemap that contains this chunk.
 * @param[in] chunk Pointer to the chunk to examine.
 * @param[in] start The starting offset of this chunk.
 * @return The maximum usable capacity of the chunk in bits.
 */
static size_t
__sm_chunk_get_capacity(__sm_chunk_t *chunk)
{
  /* Handle RLE which encodes the capacity in the vector. */
  if (SM_IS_CHUNK_RLE(chunk)) {
    return __sm_chunk_rle_get_capacity(chunk);
  }

  size_t capacity = SM_CHUNK_MAX_CAPACITY;
  register uint8_t *p = (uint8_t *)chunk->m_data;

  for (size_t i = 0; i < sizeof(__sm_bitvec_t); i++, p++) {
    if (!*p || *p == 0xff) {
      continue;
    }
    for (int j = 0; j < SM_FLAGS_PER_INDEX_BYTE; j++) {
      size_t flags = SM_CHUNK_GET_FLAGS(*p, j);
      if (flags == SM_PAYLOAD_NONE) {
        capacity -= SM_BITS_PER_VECTOR;
      }
    }
  }
  return capacity;
}

static void
__sm_chunk_increase_capacity(__sm_chunk_t *chunk, size_t capacity)
{
  __sm_assert(capacity % SM_BITS_PER_VECTOR == 0);
  __sm_assert(capacity <= SM_CHUNK_MAX_CAPACITY);
  __sm_assert(capacity > __sm_chunk_get_capacity(chunk));

  size_t initial_capacity = __sm_chunk_get_capacity(chunk);
  if (capacity <= initial_capacity || capacity > SM_CHUNK_MAX_CAPACITY) {
    return;
  }

  size_t increased = 0;
  register uint8_t *p = (uint8_t *)chunk->m_data;
  for (size_t i = 0; i < sizeof(__sm_bitvec_t); i++, p++) {
    if (!*p || *p == 0xff) {
      continue;
    }
    for (int j = 0; j < SM_FLAGS_PER_INDEX_BYTE; j++) {
      size_t flags = SM_CHUNK_GET_FLAGS(*p, j);
      if (flags == SM_PAYLOAD_NONE) {
        *p &= ~((__sm_bitvec_t)SM_PAYLOAD_ONES << (j * 2));
        *p |= ((__sm_bitvec_t)SM_PAYLOAD_ZEROS << (j * 2));
        increased += SM_BITS_PER_VECTOR;
        if (increased + initial_capacity == capacity) {
          __sm_assert(__sm_chunk_get_capacity(chunk) == capacity);
          return;
        }
      }
    }
  }
  __sm_assert(__sm_chunk_get_capacity(chunk) == capacity);
}

/** @brief Examines the chunk to determine if it is empty.
 *
 * @param[in] chunk The chunk in question.
 * @returns true if this __sm_chunk_t is empty
 */
static bool
__sm_chunk_is_empty(__sm_chunk_t *chunk)
{
  if (chunk->m_data[0] != 0) {
    /* A chunk is considered empty if all flags are SM_PAYLOAD_ZERO or _NONE. */
    register uint8_t *p = (uint8_t *)chunk->m_data;
    for (size_t i = 0; i < sizeof(__sm_bitvec_t); i++, p++) {
      if (*p) {
        for (int j = 0; j < SM_FLAGS_PER_INDEX_BYTE; j++) {
          size_t flags = SM_CHUNK_GET_FLAGS(*p, j);
          if (flags != SM_PAYLOAD_NONE && flags != SM_PAYLOAD_ZEROS) {
            return false;
          }
        }
      }
    }
  }
  /* The __sm_chunk_t is empty if all flags (in m_data[0]) are zero. */
  return true;
}

/** @brief Examines the chunk to determine its size.
 *
 * @param[in] chunk The chunk in question.
 * @returns the size of the data buffer, in bytes.
 */
static size_t
__sm_chunk_get_size(__sm_chunk_t *chunk)
{
  /* At least one __sm_bitvec_t is required for the flags (m_data[0]) */
  size_t size = sizeof(__sm_bitvec_t);
  if (!SM_IS_CHUNK_RLE(chunk)) {
    /* Use a lookup table for each byte of the flags */
    register uint8_t *p = (uint8_t *)chunk->m_data;
    for (size_t i = 0; i < sizeof(__sm_bitvec_t); i++, p++) {
      size += sizeof(__sm_bitvec_t) * __sm_chunk_calc_vector_size(*p);
    }
  }
  return size;
}

/** @brief Examines the chunk at \b idx to determine that bit's state (set,
 * or unset).
 *
 * @param[in] chunk The chunk in question.
 * @param[in] idx The 0-based index into this chunk to examine.
 * @returns the value of a bit at index \b idx
 */
static bool
__sm_chunk_is_set(__sm_chunk_t *chunk, size_t idx)
{
  /* in which __sm_bitvec_t is |idx| stored? */
  size_t bv = idx / SM_BITS_PER_VECTOR;
  __sm_assert(bv < SM_FLAGS_PER_INDEX);

  /* now retrieve the flags of that __sm_bitvec_t */
  size_t flags = SM_CHUNK_GET_FLAGS(*chunk->m_data, bv);
  switch (flags) {
  case SM_PAYLOAD_ZEROS:
  case SM_PAYLOAD_NONE:
    return false;
  case SM_PAYLOAD_ONES:
    return true;
  default:
    __sm_assert(flags == SM_PAYLOAD_MIXED);
    /* FALLTHROUGH */
  }

  /* get the __sm_bitvec_t at |bv| */
  __sm_bitvec_t w = chunk->m_data[1 + __sm_chunk_get_position(chunk, bv)];
  /* and finally check the bit in that __sm_bitvec_t */
  return (w & ((__sm_bitvec_t)1 << (idx % SM_BITS_PER_VECTOR))) > 0;
}

/*
 * TODO
 */
static int
__sm_chunk_clr_bit(__sm_chunk_t *chunk, sparsemap_idx_t idx, size_t *pos)
{
  /* Where in the descriptor does this idx fall, which flag should we examine? */
  size_t bv = idx / SM_BITS_PER_VECTOR;
  __sm_assert(bv < SM_FLAGS_PER_INDEX);

  switch (SM_CHUNK_GET_FLAGS(*chunk->m_data, bv)) {
    __sm_bitvec_t w;
  case SM_PAYLOAD_ZEROS:
    /* The bit is already clear, no-op. */
    return SM_OK;
    break;
  case SM_PAYLOAD_ONES:
    /* What was all ones transitions to mixed, which requires another vector. */
    if (*pos == 0) {
      *pos = (size_t)1 + __sm_chunk_get_position(chunk, bv);
      return SM_NEEDS_TO_GROW;
    }
    SM_CHUNK_SET_FLAGS(*chunk->m_data, bv, SM_PAYLOAD_MIXED);
    w = chunk->m_data[*pos];
    w &= ~((__sm_bitvec_t)1 << (idx % SM_BITS_PER_VECTOR));
    /* Update the mixed vector. */
    chunk->m_data[*pos] = w;
    return SM_OK;
    break;
  case SM_PAYLOAD_MIXED:
    *pos = 1 + __sm_chunk_get_position(chunk, bv);
    w = chunk->m_data[*pos];
    w &= ~((__sm_bitvec_t)1 << (idx % SM_BITS_PER_VECTOR));
    /* Did the vector transition from mixed to all zeros? Remove it if so. */
    if (w == 0) {
      SM_CHUNK_SET_FLAGS(*chunk->m_data, bv, SM_PAYLOAD_ZEROS);
      return SM_NEEDS_TO_SHRINK;
    }
    /* Update the mixed vector. */
    chunk->m_data[*pos] = w;
    break;
  case SM_PAYLOAD_NONE:
    /* FALLTHROUGH */
  default:
    __sm_assert(!"shouldn't be here");
#ifdef DEBUG
    abort();
#endif
    break;
  }
  return SM_OK;
}

/*
 * TODO
 */
static int
__sm_chunk_set_bit(__sm_chunk_t *chunk, sparsemap_idx_t idx, size_t *pos)
{
  /* Where in the descriptor does this idx fall, which flag should we examine? */
  size_t bv = idx / SM_BITS_PER_VECTOR;
  __sm_assert(bv < SM_FLAGS_PER_INDEX);

  switch (SM_CHUNK_GET_FLAGS(*chunk->m_data, bv)) {
  case SM_PAYLOAD_ONES:
    /* The bit is already set, no-op. */
    return SM_OK;
    break;
  case SM_PAYLOAD_ZEROS:
    /* What was all zeros transitions to mixed, which requires another vector. */
    if (*pos == 0) {
      *pos = (size_t)1 + __sm_chunk_get_position(chunk, bv);
      return SM_NEEDS_TO_GROW;
    }
    SM_CHUNK_SET_FLAGS(*chunk->m_data, bv, SM_PAYLOAD_MIXED);
    /* FALLTHROUGH */
  case SM_PAYLOAD_MIXED:
    *pos = 1 + __sm_chunk_get_position(chunk, bv);
    __sm_bitvec_t w = chunk->m_data[*pos];
    w |= (__sm_bitvec_t)1 << (idx % SM_BITS_PER_VECTOR);
    /* Did the vector transition from mixed to all ones? Remove it if so. */
    if (w == ~(__sm_bitvec_t)0) {
      SM_CHUNK_SET_FLAGS(*chunk->m_data, bv, SM_PAYLOAD_ONES);
      return SM_NEEDS_TO_SHRINK;
    }
    /* Update the mixed vector. */
    chunk->m_data[*pos] = w;
    break;
  case SM_PAYLOAD_NONE:
    /* FALLTHROUGH */
  default:
    __sm_assert(!"shouldn't be here");
#ifdef DEBUG
    abort();
#endif
    break;
  }
  return SM_OK;
}

/** @brief Assigns a state to a bit in the chunk (set or unset).
 *
 * Sets the value of a bit at index \b idx. Then updates position \b pos to the
 * position of the __sm_bitvec_t which is inserted/deleted and \b fill - the value
 * of the fill word (used when growing).
 *
 * @param[in] chunk The chunk in question.
 * @param[in] idx The 0-based index into this chunk to mutate.
 * @param[in] value The new state for the \b idx'th bit.
 * @param[in,out] pos The position of the __sm_bitvec_t inserted/deleted within the chunk.
 * @param[in,out] fill The value of the fill word (when growing).
 * @param[in] retired When not retried, grow the chunk by a bitvec.
 * @returns \b SM_NEEDS_TO_GROW, \b SM_NEEDS_TO_SHRINK, or \b SM_OK
 * @note, the caller MUST to perform the relevant actions and call set() again,
 * this time with \b retried = true.
 */
static int
__sm_chunk_set(__sm_chunk_t *chunk, size_t idx, bool value, size_t *pos, __sm_bitvec_t *fill, bool retried)
{
  /* Where in the descriptor does this idx fall, which flag should we examine? */
  size_t bv = idx / SM_BITS_PER_VECTOR;
  __sm_assert(bv < SM_FLAGS_PER_INDEX);

  size_t flags = SM_CHUNK_GET_FLAGS(*chunk->m_data, bv);
  assert(flags != SM_PAYLOAD_NONE);
  if (flags == SM_PAYLOAD_ZEROS) {
    /* Easy - set bit to 0 in a __sm_bitvec_t of zeroes. */
    if (value == false) {
      *pos = 0;
      *fill = 0;
      return SM_OK;
    }
    /* The sparsemap must grow this __sm_chunk_t by one additional __sm_bitvec_t,
     * then try again. */
    if (!retried) {
      *pos = 1 + __sm_chunk_get_position(chunk, bv);
      *fill = 0;
      return SM_NEEDS_TO_GROW;
    }
    /* New flags are 2#10 meaning SM_PAYLOAD_MIXED. Currently, flags are set
     * to 2#00, so 2#00 | 2#10 = 2#10. */
    *chunk->m_data |= ((__sm_bitvec_t)SM_PAYLOAD_MIXED << (bv * 2));
    /* FALLTHROUGH */
  } else if (flags == SM_PAYLOAD_ONES) {
    /* Easy - set bit to 1 in a __sm_bitvec_t of ones. */
    if (value == true) {
      *pos = 0;
      *fill = 0;
      return SM_OK;
    }
    /* The sparsemap must grow this __sm_chunk_t by one additional __sm_bitvec_t,
       then try again. */
    if (!retried) {
      *pos = 1 + __sm_chunk_get_position(chunk, bv);
      *fill = ~(__sm_bitvec_t)0;
      return SM_NEEDS_TO_GROW;
    }
    /* New flags are 2#10 meaning SM_PAYLOAD_MIXED. Currently, flags are
       set to 2#11, so 2#11 ^ 2#01 = 2#10. */
    chunk->m_data[0] ^= ((__sm_bitvec_t)SM_PAYLOAD_NONE << (bv * 2));
    /* FALLTHROUGH */
  }

  /* Now flip the bit. */
  size_t position = 1 + __sm_chunk_get_position(chunk, bv);
  __sm_bitvec_t w = chunk->m_data[position];
  if (value) {
    w |= (__sm_bitvec_t)1 << (idx % SM_BITS_PER_VECTOR);
  } else {
    w &= ~((__sm_bitvec_t)1 << (idx % SM_BITS_PER_VECTOR));
  }

  /* If this __sm_bitvec_t is now all zeroes or ones then we can remove it. */
  if (w == 0) {
    chunk->m_data[0] &= ~((__sm_bitvec_t)SM_PAYLOAD_ONES << (bv * 2));
    *pos = position;
    *fill = 0;
    return SM_NEEDS_TO_SHRINK;
  }
  if (w == ~(__sm_bitvec_t)0) {
    chunk->m_data[0] |= (__sm_bitvec_t)SM_PAYLOAD_ONES << (bv * 2);
    *pos = position;
    *fill = 0;
    return SM_NEEDS_TO_SHRINK;
  }

  chunk->m_data[position] = w;
  *pos = 0;
  *fill = 0;
  return SM_OK;
}

/** @brief Finds the index of the \b n'th bit after \b offset bits with \b
 * value.
 *
 * Scans the \b chunk until after \b offset bits (of any value) have
 * passed and then begins counting the bits that match \b value looking
 * for the \b n'th bit.  It may not be in this chunk, when it is offset is set.
 *
 * @param[in] chunk The chunk in question.
 * @param[in] value Informs what we're seeking, a set or unset bit's position.
 * @param offset[in,out] Sets \b offset to n if the n'th bit was found
 * in this __sm_chunk_t, or reduced value of \b n bits observed the search up
 * to a maximum of SM_BITS_PER_VECTOR.
 * @returns the 0-based index of the n'th set bit when found, otherwise
 * SM_BITS_PER_VECTOR
 */
static size_t
__sm_chunk_select(__sm_chunk_t *chunk, ssize_t n, ssize_t *offset, bool value)
{
  size_t ret = 0;
  register uint8_t *p;

  p = (uint8_t *)chunk->m_data;
  for (size_t i = 0; i < sizeof(__sm_bitvec_t); i++, p++) {
    if (*p == 0 && value) {
      ret += (size_t)SM_FLAGS_PER_INDEX_BYTE * SM_BITS_PER_VECTOR;
      continue;
    }

    for (int j = 0; j < SM_FLAGS_PER_INDEX_BYTE; j++) {
      size_t flags = SM_CHUNK_GET_FLAGS(*p, j);
      if (flags == SM_PAYLOAD_NONE) {
        continue;
      }
      if (flags == SM_PAYLOAD_ZEROS) {
        if (value == true) {
          ret += SM_BITS_PER_VECTOR;
          continue;
        } else {
          if (n > SM_BITS_PER_VECTOR) {
            n -= SM_BITS_PER_VECTOR;
            ret += SM_BITS_PER_VECTOR;
            continue;
          }
          *offset = -1;
          return ret + n;
        }
      }
      if (flags == SM_PAYLOAD_ONES) {
        if (value == true) {
          if (n > SM_BITS_PER_VECTOR) {
            n -= SM_BITS_PER_VECTOR;
            ret += SM_BITS_PER_VECTOR;
            continue;
          }
          *offset = -1;
          return ret + n;
        } else {
          ret += SM_BITS_PER_VECTOR;
          continue;
        }
      }
      if (flags == SM_PAYLOAD_MIXED) {
        __sm_bitvec_t w = chunk->m_data[1 + __sm_chunk_get_position(chunk, i * SM_FLAGS_PER_INDEX_BYTE + j)];
        for (int k = 0; k < SM_BITS_PER_VECTOR; k++) {
          if (value) {
            if (w & ((__sm_bitvec_t)1 << k)) {
              if (n == 0) {
                *offset = -1;
                return ret;
              }
              n--;
            }
            ret++;
          } else {
            if (!(w & ((__sm_bitvec_t)1 << k))) {
              if (n == 0) {
                *offset = -1;
                return ret;
              }
              n--;
            }
            ret++;
          }
        }
      }
    }
  }
  *offset = n;
  return ret;
}

/** @brief Counts the bits matching \b value in the range [0, \b idx]
 * inclusive after ignoring the first \b offset bits in the chunk.
 *
 * Scans the \b chunk until after \b offset bits (of any value) have
 * passed and then begins counting the bits that match \b value. The
 * result should never be greater than \b idx + 1 maxing out at
 * SM_BITS_PER_VECTOR.  A range of [0, 0] will count 1 bit at \b offset
 * + 1 in this chunk.  A range of [0, 9] will count 10 bits, starting
 * with the 0th and ending with the 9th and return at most a count of
 * 10.
 *
 * @param[in] chunk The chunk in question.
 * @param[in,out] begin Decreases \b offset by the number of bits ignored,
 * at most by SM_BITS_PER_VECTOR.
 * @param[in] end The ending value of the range (inclusive) to count.
 * @param[out] pos_in_chunk The position of the last bit examined in this chunk,
 * always
 * <= SM_BITS_PER_VECTOR, used when counting unset bits that fall within this
 * chunk's range but after the last set bit.
 * @param[out] last_bitvec The last __sm_bitvec_t, masked and shifted, so as to be able
 * to examine the bits used in the last portion of the ranking as a way to
 * skip forward during a #span() operation.
 * @param[in] value Informs what we're seeking, set or unset bits.
 * @returns the count of the bits matching \b value within the range.
 */
static size_t
__sm_chunk_rank(__sm_chunk_t *chunk, size_t *begin, size_t end, size_t *pos_in_chunk, __sm_bitvec_t *last_bitvec, bool value)
{
  size_t ret = 0;

  *pos_in_chunk = 0;

  /* A chunk can only hold at most SM_CHUNK_MAX_CAPACITY bits, so if
   * begin is larger than that, we're basically done. */
  if (*begin >= SM_CHUNK_MAX_CAPACITY) {
    *pos_in_chunk = SM_CHUNK_MAX_CAPACITY;
    *begin -= SM_CHUNK_MAX_CAPACITY;
    return 0;
  }

  register uint8_t *p = (uint8_t *)chunk->m_data;
  for (size_t i = 0; i < sizeof(__sm_bitvec_t); i++, p++) {
    for (int j = 0; j < SM_FLAGS_PER_INDEX_BYTE; j++) {
      size_t flags = SM_CHUNK_GET_FLAGS(*p, j);
      if (flags == SM_PAYLOAD_NONE) {
        continue;
      }
      if (flags == SM_PAYLOAD_ZEROS) {
        *last_bitvec = 0;
        if (end >= SM_BITS_PER_VECTOR) {
          *pos_in_chunk += SM_BITS_PER_VECTOR;
          end -= SM_BITS_PER_VECTOR;
          if (*begin >= SM_BITS_PER_VECTOR) {
            *begin = *begin - SM_BITS_PER_VECTOR;
          } else {
            if (value == false) {
              ret += SM_BITS_PER_VECTOR - *begin;
            }
            *begin = 0;
          }
        } else {
          *pos_in_chunk += end + 1;
          if (value == false) {
            if (*begin > end) {
              *begin = *begin - end;
            } else {
              ret += end + 1 - *begin;
              *begin = 0;
              return ret;
            }
          } else {
            return ret;
          }
        }
      } else if (flags == SM_PAYLOAD_ONES) {
        *last_bitvec = UINT64_MAX;
        if (end >= SM_BITS_PER_VECTOR) {
          *pos_in_chunk += SM_BITS_PER_VECTOR;
          end -= SM_BITS_PER_VECTOR;
          if (*begin >= SM_BITS_PER_VECTOR) {
            *begin = *begin - SM_BITS_PER_VECTOR;
          } else {
            if (value == true) {
              ret += SM_BITS_PER_VECTOR - *begin;
            }
            *begin = 0;
          }
        } else {
          *pos_in_chunk += end + 1;
          if (value == true) {
            if (*begin > end) {
              *begin = *begin - end;
            } else {
              ret += end + 1 - *begin;
              *begin = 0;
              return ret;
            }
          } else {
            return ret;
          }
        }
      } else if (flags == SM_PAYLOAD_MIXED) {
        __sm_bitvec_t w = chunk->m_data[1 + __sm_chunk_get_position(chunk, i * SM_FLAGS_PER_INDEX_BYTE + j)];
        if (end >= SM_BITS_PER_VECTOR) {
          *pos_in_chunk += SM_BITS_PER_VECTOR;
          end -= SM_BITS_PER_VECTOR;
          uint64_t mask = *begin == 0 ? UINT64_MAX : ~(UINT64_MAX >> (SM_BITS_PER_VECTOR - (*begin >= 64 ? 64 : *begin)));
          __sm_bitvec_t mw;
          if (value == true) {
            mw = w & mask;
          } else {
            mw = ~w & mask;
          }
          size_t pc = popcountll(mw);
          ret += pc;
          *begin = (*begin > SM_BITS_PER_VECTOR) ? *begin - SM_BITS_PER_VECTOR : 0;
        } else {
          *pos_in_chunk += end + 1;
          __sm_bitvec_t mw;
          uint64_t mask;
          uint64_t end_mask = (end == 63) ? UINT64_MAX : ((uint64_t)1 << (end + 1)) - 1;
          uint64_t begin_mask = *begin == 0 ? UINT64_MAX : ~(UINT64_MAX >> (SM_BITS_PER_VECTOR - (*begin >= 64 ? 64 : *begin)));
          /* To count the set bits we need to mask off the portion of the vector that we need
           * to count then call popcount().  So, let's create a mask for the range between
           * begin and end inclusive [*begin, end]. */
          mask = end_mask & begin_mask;
          if (value) {
            mw = w & mask;
          } else {
            mw = ~w & mask;
          }
          int pc = popcountll(mw);
          ret += pc;
          *last_bitvec = mw >> ((*begin > 63) ? 63 : *begin);
          *begin = *begin > end ? *begin - end + 1 : 0;
          return ret;
        }
      }
    }
  }
  return ret;
}

/** @brief Calls \b scanner with sm_bitmap_t for each vector in this chunk.
 *
 * Decompresses the whole chunk into separate bitmaps then calls visitor's
 * \b #operator() function for all bits that are set.
 *
 * @param[in] chunk The chunk in question.
 * @param[in] start Starting offset
 * @param[in] scanner Callback function which receives an array of indices (with
 * bits set to 1), the size of the array and an auxiliary pointer provided by
 * the caller.
 * @param[in] skip The number of bits to skip in the beginning.
 * @returns the number of (set) bits that were passed to the scanner
 */
static size_t
__sm_chunk_scan(__sm_chunk_t *chunk, __sm_idx_t start, void (*scanner)(uint32_t[], size_t, void *aux), size_t skip, void *aux)
{
  size_t ret = 0;
  register uint8_t *p = (uint8_t *)chunk->m_data;
  uint32_t buffer[SM_BITS_PER_VECTOR];
  for (size_t i = 0; i < sizeof(__sm_bitvec_t); i++, p++) {
    if (*p == 0) {
      /* Skip chunks that are all zeroes. */
      skip -= skip > SM_BITS_PER_VECTOR ? SM_BITS_PER_VECTOR : skip;
      continue;
    }

    for (int j = 0; j < SM_FLAGS_PER_INDEX_BYTE; j++) {
      size_t flags = SM_CHUNK_GET_FLAGS(*p, j);
      if (flags == SM_PAYLOAD_NONE || flags == SM_PAYLOAD_ZEROS) {
        /* Skip when all zeroes. */
        skip -= skip > SM_BITS_PER_VECTOR ? SM_BITS_PER_VECTOR : skip;
      } else if (flags == SM_PAYLOAD_ONES) {
        if (skip) {
          if (skip >= SM_BITS_PER_VECTOR) {
            skip -= SM_BITS_PER_VECTOR;
            ret += SM_BITS_PER_VECTOR;
            continue;
          }
          size_t n = 0;
          for (size_t b = 0; b < SM_BITS_PER_VECTOR; b++) {
            buffer[n++] = start + ret + b;
          }
          scanner(&buffer[0], n, aux);
          ret += n;
          skip = 0;
        } else {
          for (size_t b = 0; b < SM_BITS_PER_VECTOR; b++) {
            buffer[b] = start + ret + b;
          }
          scanner(&buffer[0], SM_BITS_PER_VECTOR, aux);
          ret += SM_BITS_PER_VECTOR;
        }
      } else if (flags == SM_PAYLOAD_MIXED) {
        __sm_bitvec_t w = chunk->m_data[1 + __sm_chunk_get_position(chunk, i * SM_FLAGS_PER_INDEX_BYTE + j)];
        size_t n = 0;
        if (skip) {
          if (skip >= SM_BITS_PER_VECTOR) {
            skip -= SM_BITS_PER_VECTOR;
            ret += SM_BITS_PER_VECTOR;
            continue;
          }
          for (int b = 0; b < SM_BITS_PER_VECTOR; b++) {
            if (skip > 0) {
              skip--;
              continue;
            }
            if (w & ((__sm_bitvec_t)1 << b)) {
              buffer[n++] = start + ret + b;
              ret++;
            }
          }
        } else {
          for (int b = 0; b < SM_BITS_PER_VECTOR; b++) {
            if (w & ((__sm_bitvec_t)1 << b)) {
              buffer[n++] = start + ret + b;
            }
          }
          ret += n;
        }
        __sm_assert(n > 0);
        scanner(&buffer[0], n, aux);
      }
    }
  }
  return ret;
}

/** @brief Provides the number of chunks currently in the map.
 *
 * @param[in] chunk  The sparsemap_t in question.
 * @returns the number of chunks in the sparsemap
 */
static size_t
__sm_get_chunk_count(sparsemap_t *map)
{
  return *(uint32_t *)&map->m_data[0];
}

/** @brief Encapsulates the method to find the starting address of a chunk's
 * data.
 *
 * @param[in] map The sparsemap_t in question.
 * @param[in] offset The offset in bytes for the desired chunk.
 * @returns the data for the specified \b offset
 */
static inline uint8_t *
__sm_get_chunk_data(sparsemap_t *map, size_t offset)
{
  return &map->m_data[SM_SIZEOF_OVERHEAD + offset];
}

/**
 * TODO only call this with an offset of an RLE chunk
 */
static size_t
__sm_chunk_rle_capacity_limit(sparsemap_t *map, __sm_idx_t start, size_t offset)
{
  size_t next_offset = offset + SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t);
  if (next_offset < map->m_data_used - (SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t))) {
    uint8_t *p = __sm_get_chunk_data(map, next_offset);
    __sm_idx_t next_start = *(__sm_idx_t *)p;
    return next_start - start;
  }
  return SM_CHUNK_RLE_MAX_CAPACITY;
}

/** @brief Encapsulates the method to find the address of the first unused byte
 * in \b m_data.
 *
 * @param[in] map The sparsemap_t in question.
 * @returns a pointer after the end of the used data
 */
static uint8_t *
__sm_get_chunk_end(sparsemap_t *map)
{
  uint8_t *p = __sm_get_chunk_data(map, 0);
  size_t count = __sm_get_chunk_count(map);
  for (size_t i = 0; i < count; i++) {
    p += SM_SIZEOF_OVERHEAD;
    __sm_chunk_t chunk;
    __sm_chunk_init(&chunk, p);
    p += __sm_chunk_get_size(&chunk);
  }
  return p;
}

/** @brief Provides the byte size amount of \b m_data consumed.
 *
 * @param[in] map The sparsemap_t in question.
 * @returns the used size in the data buffer
 */
static size_t
__sm_get_size_impl(sparsemap_t *map)
{
  uint8_t *start = __sm_get_chunk_data(map, 0);
  uint8_t *p = start;

  size_t count = __sm_get_chunk_count(map);
  for (size_t i = 0; i < count; i++) {
    p += SM_SIZEOF_OVERHEAD;
    __sm_chunk_t chunk;
    __sm_chunk_init(&chunk, p);
    p += __sm_chunk_get_size(&chunk);
  }
  return SM_SIZEOF_OVERHEAD + p - start;
}

/** @brief Aligns to SM_CHUNK_CAPACITY a given index \b idx.
 *
 * Due to integer division discarding the remainder, the final return value is
 * always rounded down to the nearest multiple of SM_CHUNK_MAX_CAPACITY.
 *
 * @param[in] idx The index to align.
 * @returns the aligned offset (aligned to __sm_chunk_t capacity)
 */
static __sm_idx_t
__sm_get_chunk_aligned_offset(size_t idx)
{
  const size_t capacity = SM_CHUNK_MAX_CAPACITY;
  return (idx / capacity) * capacity;
}

/** @brief Provides the byte offset of the chunk containing the bit at \b idx.
 *
 * TODO...
 *
 * @param[in] map A sparsemap_t.
 * @param[in] idx Seeking the offset of a chunk for this index.
 * @returns the offset of the __sm_chunk_t in m_data, or -1 if there
 * are no chunks.
 */
static ssize_t
__sm_get_chunk_offset(sparsemap_t *map, sparsemap_idx_t idx)
{
  size_t count = __sm_get_chunk_count(map);

  if (count == 0) {
    return -1;
  }

  uint8_t *start = __sm_get_chunk_data(map, 0);
  uint8_t *p = start;

  for (size_t i = 0; i < count - 1; i++) {
    __sm_idx_t s = *(__sm_idx_t *)p;
    __sm_chunk_t chunk;
    __sm_chunk_init(&chunk, p + SM_SIZEOF_OVERHEAD);
    __sm_assert(s == __sm_get_chunk_aligned_offset(s));
    if (s >= idx || idx < s + __sm_chunk_get_capacity(&chunk)) {
      break;
    }
    p += SM_SIZEOF_OVERHEAD + __sm_chunk_get_size(&chunk);
  }

  return (ssize_t)(p - start);
}

/** @brief Sets the number of __sm_chunk_t's.
 *
 * @param[in] map The sparsemap_t in question.
 * @param[in] new_count The new number of chunks in the map.
 */
static void
__sm_set_chunk_count(sparsemap_t *map, size_t new_count)
{
  *(uint32_t *)&map->m_data[0] = (uint32_t)new_count;
}

/** @brief Appends raw data at the end of used portion of \b m_data.
 *
 * @param[in] map The sparsemap_t in question.
 * @param[in] buffer The bytes to copy into \b m_data.
 * @param[in] buffer_size The size of the byte array \b buffer to copy.
 */
static void
__sm_append_data(sparsemap_t *map, uint8_t *buffer, size_t buffer_size)
{
  __sm_assert(map->m_data_used + buffer_size <= map->m_capacity);

  memcpy(&map->m_data[map->m_data_used], buffer, buffer_size);
  map->m_data_used += buffer_size;
}

/** @brief Inserts data at \b offset in the middle of \b m_data.
 *
 * @param[in] map The sparsemap_t in question.
 * @param[in] offset The offset in bytes into \b m_data to place the buffer.
 * @param[in] buffer The bytes to copy into \b m_data.
 * @param[in] buffer_size The size of the byte array \b buffer to copy.
 */
void
__sm_insert_data(sparsemap_t *map, size_t offset, uint8_t *buffer, size_t buffer_size)
{
  __sm_assert(map->m_data_used + buffer_size <= map->m_capacity);

  uint8_t *p = __sm_get_chunk_data(map, offset);
  memmove(p + buffer_size, p, map->m_data_used - offset);
  memcpy(p, buffer, buffer_size);
  map->m_data_used += buffer_size;
}

/** @brief Removes data from \b m_data.
 *
 * @param[in] map The sparsemap_t in question.
 * @param[in] offset The offset in bytes into \b m_data at which to excise data.
 * @param[in] gap_size The size of the excision.
 */
static void
__sm_remove_data(sparsemap_t *map, size_t offset, size_t gap_size)
{
  __sm_assert(map->m_data_used >= gap_size);
  uint8_t *p = __sm_get_chunk_data(map, offset);
  memmove(p, p + gap_size, map->m_data_used - offset - gap_size);
  map->m_data_used -= gap_size;
}

/** @brief Merges into the chunk at \b offset all set bits from \b src.
 *
 * @param[in] map The map the chunk belongs too.
 * @param[in] offset The offset of the first bit in the chunk to be merged.
 * @todo merge at the vector level not offset
 */
void
__sm_merge_chunk(sparsemap_t *map, sparsemap_idx_t src_start, sparsemap_idx_t dst_start, sparsemap_idx_t capacity, __sm_chunk_t *dst_chunk,
  __sm_chunk_t *src_chunk)
{
  ssize_t delta = src_start - dst_start;
  for (sparsemap_idx_t j = 0; j < capacity; j++) {
    ssize_t offset = __sm_get_chunk_offset(map, src_start + j);
    if (__sm_chunk_is_set(src_chunk, j) && !__sm_chunk_is_set(dst_chunk, j + delta)) {
      size_t position;
      __sm_bitvec_t fill;
      switch (__sm_chunk_set(dst_chunk, j + delta, true, &position, &fill, false)) {
      case SM_NEEDS_TO_GROW:
        offset += SM_SIZEOF_OVERHEAD + position * sizeof(__sm_bitvec_t);
        __sm_insert_data(map, offset, (uint8_t *)&fill, sizeof(__sm_bitvec_t));
        __sm_chunk_set(dst_chunk, j + delta, true, &position, &fill, true);
        break;
      case SM_NEEDS_TO_SHRINK:
        if (__sm_chunk_is_empty(src_chunk)) {
          __sm_assert(position == 1);
          __sm_remove_data(map, offset, SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2);
          __sm_set_chunk_count(map, __sm_get_chunk_count(map) - 1);
        } else {
          offset += SM_SIZEOF_OVERHEAD + position * sizeof(__sm_bitvec_t);
          __sm_remove_data(map, offset, sizeof(__sm_bitvec_t));
        }
        break;
      case SM_OK:
      default:
        break;
      }
    }
  }
}

/*
 * The following is the "Sparsemap" implementation, it uses chunks (code above)
 * and is the public API for this compressed bitmap representation.
 */

void
sparsemap_clear(sparsemap_t *map)
{
  if (map == NULL) {
    return;
  }
  memset(map->m_data, 0, map->m_capacity);
  map->m_data_used = SM_SIZEOF_OVERHEAD;
  __sm_set_chunk_count(map, 0);
}

sparsemap_t *
sparsemap(size_t size)
{
  if (size == 0) {
    size = 1024;
  }

  size_t data_size = (size * sizeof(uint8_t));

  /* Ensure that m_data is 8-byte aligned. */
  size_t total_size = sizeof(sparsemap_t) + data_size;
  size_t padding = total_size % 8 == 0 ? 0 : 8 - (total_size % 8);
  total_size += padding;

  sparsemap_t *map = (sparsemap_t *)calloc(1, total_size);
  if (map) {
    uint8_t *data = (uint8_t *)(((uintptr_t)map + sizeof(sparsemap_t)) & ~(uintptr_t)7);
    sparsemap_init(map, data, size);
    __sm_when_diag({ __sm_assert(IS_8_BYTE_ALIGNED(map->m_data)); });
  }
  return map;
}

sparsemap_t *
sparsemap_copy(sparsemap_t *other)
{
  size_t cap = sparsemap_get_capacity(other);
  sparsemap_t *map = sparsemap(cap);
  if (map) {
    map->m_capacity = other->m_capacity;
    map->m_data_used = other->m_data_used;
    memcpy(map->m_data, other->m_data, cap);
  }
  return map;
}

sparsemap_t *
sparsemap_wrap(uint8_t *data, size_t size)
{
  sparsemap_t *map = (sparsemap_t *)calloc(1, sizeof(sparsemap_t));
  if (map) {
    map->m_data = data;
    map->m_data_used = 0;
    map->m_capacity = size;
  }
  return map;
}

void
sparsemap_init(sparsemap_t *map, uint8_t *data, size_t size)
{
  map->m_data = data;
  map->m_data_used = 0;
  map->m_capacity = size;
  sparsemap_clear(map);
}

void
sparsemap_open(sparsemap_t *map, uint8_t *data, size_t size)
{
  map->m_data = data;
  map->m_data_used = __sm_get_size_impl(map);
  map->m_capacity = size;
}

sparsemap_t *
sparsemap_set_data_size(sparsemap_t *map, uint8_t *data, size_t size)
{
  size_t data_size = (size * sizeof(uint8_t));

  /* If this sparsemap was allocated by the sparsemap() API and we're not handed
   * a new data, it's up to us to resize it. */
  if (data == NULL && (uintptr_t)map->m_data == (uintptr_t)map + sizeof(sparsemap_t) && size > map->m_capacity) {

    /* Ensure that m_data is 8-byte aligned. */
    size_t total_size = sizeof(sparsemap_t) + data_size;
    size_t padding = total_size % 8 == 0 ? 0 : 8 - (total_size % 8);
    total_size += padding;

    sparsemap_t *m = (sparsemap_t *)realloc(map, total_size);
    if (!m) {
      return NULL;
    }
    memset(((uint8_t *)m) + sizeof(sparsemap_t) + (m->m_capacity * sizeof(uint8_t)), 0, size - m->m_capacity + padding);
    m->m_capacity = data_size;
    m->m_data = (uint8_t *)(((uintptr_t)m + sizeof(sparsemap_t)) & ~(uintptr_t)7);
    __sm_when_diag({ __sm_assert(IS_8_BYTE_ALIGNED(m->m_data)); }) return m;
  } else {
    /* NOTE: It is up to the caller to realloc their buffer and provide it here
     * for reassignment. */
    if (data != NULL && data != map->m_data) {
      map->m_data = data;
    }
    map->m_capacity = size;
    return map;
  }
}

double
sparsemap_capacity_remaining(sparsemap_t *map)
{
  if (map->m_data_used >= map->m_capacity) {
    return 0;
  }
  if (map->m_capacity == 0) {
    return 100.0;
  }
  return 100 - (((double)map->m_data_used / (double)map->m_capacity) * 100);
}

size_t
sparsemap_get_capacity(sparsemap_t *map)
{
  return map->m_capacity;
}

bool
sparsemap_is_set(sparsemap_t *map, sparsemap_idx_t idx)
{
  __sm_assert(sparsemap_get_size(map) >= SM_SIZEOF_OVERHEAD);

  /* Get the __sm_chunk_t which manages this index */
  ssize_t offset = __sm_get_chunk_offset(map, idx);

  /* No __sm_chunk_t's available -> the bit is not set */
  if (offset == -1) {
    return false;
  }

  /* Otherwise load the __sm_chunk_t */
  uint8_t *p = __sm_get_chunk_data(map, offset);
  __sm_idx_t start = *(__sm_idx_t *)p;
  __sm_chunk_t chunk;
  __sm_chunk_init(&chunk, p + SM_SIZEOF_OVERHEAD);

  /* Determine if the bit is out of bounds of the __sm_chunk_t; if yes then
   * the bit is not set. */
  if (idx < start || (unsigned long)idx - start >= __sm_chunk_get_capacity(&chunk)) {
    return false;
  }

  /* Otherwise ask the __sm_chunk_t whether the bit is set. */
  return __sm_chunk_is_set(&chunk, idx - start);
}

sparsemap_idx_t
bidx_clear(sparsemap_t *map, sparsemap_idx_t idx)
{
  __sm_assert(sparsemap_get_size(map) >= SM_SIZEOF_OVERHEAD);

  /* Clearing a bit could require an additional vector, let's ensure we have that
   * space available in the buffer first, or ENOMEM now. */
  SM_ENOUGH_SPACE(SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t));

  /* Determine if there is a chunk that could contain this index. */
  size_t offset = (size_t)__sm_get_chunk_offset(map, idx);

  if ((ssize_t)offset == -1) {
    /* There are no chunks in the map, there is nothing to clear, this is a
     * no-op. */
    return idx;
  }

  /* Try to locate a chunk for this idx.  We could find that:
   * - the first chunk's offset is greater than the index, or
   * - the index is beyond the end of the last chunk, or
   * - we found a chunk that can contain this index. */
  uint8_t *p = __sm_get_chunk_data(map, offset);
  __sm_idx_t start = *(__sm_idx_t *)p;
  __sm_assert(start == __sm_get_chunk_aligned_offset(start));

  if (idx < start) {
    /* Our search resulted in the first chunk that starts after the index but
     * that means there is no chunk that contains this index, so again this is
     * a no-op. */
    return idx;
  }

  __sm_chunk_t chunk;
  __sm_chunk_init(&chunk, p + SM_SIZEOF_OVERHEAD);
  size_t capacity = __sm_chunk_get_capacity(&chunk);

  if (idx - start >= capacity) {
    /* Our search resulted in a chunk however it's capacity doesn't encompass
     * this index, so again a no-op. */
    return idx;
  }

  if (SM_IS_CHUNK_RLE(&chunk)) {
    /* Our search resulted in a chunk that is run-length encoded (RLE).  There
     * are three possibilities at this point: 1) the index is at the end of the
     * run, so we just shorten then length; 2) the index is between start and
     * end [start, end) so we have to split this chunk up; 3) the index is
     * beyond the length but within the capacity, then clearing it is a no-op.
     * If the chunk length shrinks to the max capacity of sparse encoding we
     * have to transition its encoding. */

    /* Is the 0-based index beyond the run length? */
    size_t length = __sm_chunk_rle_get_length(&chunk);
    if (idx > start + length) {
      return idx;
    }

    /* Is the 0-based index referencing the last bit in the run? */
    if (idx - start + 1 == length) {
      /* Should the run-length chunk transition into a sparse chunk? */
      if (length - 1 == SM_CHUNK_MAX_CAPACITY) {
        chunk.m_data[0] = ~(__sm_bitvec_t)0;
      } else {
        __sm_chunk_rle_set_length(&chunk, length - 1);
      }
      return idx;
    }

    /* Now that we've addressed (1) and (3) we have to work on (2) where the
     * index is within the body of this RLE chunk.  This will lead to:
     *  - a) TODO...
     *  - b) TODO...
     *  - c) ...
     *
     * Chunks must have an aligned starting offset, so let's first find what
     * we'll call the "pivot" chunk wherein we'll find the index we need to
     * clear. That chunk will be sparse.
     */
    size_t pos = 0;
    uint8_t buf[(SM_SIZEOF_OVERHEAD * 3) + (sizeof(__sm_bitvec_t) * 6)] = { 0 };
    uint8_t *pivot_p;
    __sm_chunk_t pivot_chunk;
    size_t pivot_offset;

    /* Find the starting offset for our pivot chunk. */
    size_t aligned_idx = __sm_get_chunk_aligned_offset(idx);
    __sm_assert(idx >= aligned_idx && idx < (aligned_idx + SM_CHUNK_MAX_CAPACITY));
    /* Let's avoid changing the actual map and for now work in our static buf. */
    pivot_p = buf;
    *(__sm_idx_t *)pivot_p = aligned_idx;
    __sm_chunk_init(&pivot_chunk, pivot_p + SM_SIZEOF_OVERHEAD);
    /* Set the chunk flags to all ones, ... */
    pivot_chunk.m_data[0] = ~(__sm_bitvec_t)0;
    /* ... set the flag for the position containing the index to mixed ... */
    SM_CHUNK_SET_FLAGS(pivot_chunk.m_data[0], aligned_idx / SM_BITS_PER_VECTOR, SM_PAYLOAD_MIXED);
    /* ... and clear only the bit at that index in this chunk. */
    pivot_chunk.m_data[1] = ~(__sm_bitvec_t)0 & ~((__sm_bitvec_t)1 << (idx % SM_BITS_PER_VECTOR));
    __sm_when_diag({
      /* Sanity check the chunk */
      // __sm_when_diag({ fprintf(stdout, "\n%s\n", QCC_showChunk(pivot_p, 0)); } );
      for (size_t i = aligned_idx; i < aligned_idx + SM_CHUNK_MAX_CAPACITY; i++) {
        __sm_assert(__sm_chunk_is_set(&pivot_chunk, i) == (i >= idx ? false : true));
      }
    });
    /* Where did the pivot chunk fall within the original chunk? */
    __sm_idx_t lr_start[2], lr_end[2];
    uint8_t *lr[2] = { 0 };
    size_t expand_by;

    do {
      if (aligned_idx == start) {
        /* The pivot is left aligned, there will be two chunks in total. */
        lr_start[1] = aligned_idx + SM_CHUNK_MAX_CAPACITY;
        lr_end[1] = length;
        /* Used later for constructing the remaining right chunk */
        lr[1] = (uint8_t *)((uintptr_t)buf + (SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2));
        /* Calculate space needed in the buffer, reuse the left chunk bytes. */
        expand_by = (SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 3);
        break;
      }

      if (aligned_idx + SM_CHUNK_MAX_CAPACITY >= start + length) {
        /* The pivot is right aligned, there will be two chunks in total. */
        lr_start[0] = start;
        lr_end[0] = aligned_idx - 1;
        /* Move the pivot chunk over to make room for the new left chunk. */
        size_t amt = SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2;
        memmove((uint8_t *)((uintptr_t)buf + amt), buf, amt);
        memset(buf, 0, amt);
        /* Used later for constructing the remaining left chunk */
        lr[0] = buf;
        /* Calculate space needed in the buffer, reuse the left chunk bytes. */
        expand_by = (SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 3);
        break;
      }

      /* The pivot's range is central, there will be three chunks in total. */
      lr_start[0] = start;
      lr_end[0] = aligned_idx;
      lr_start[1] = aligned_idx + SM_CHUNK_MAX_CAPACITY;
      lr_end[1] = length;
      /* Move the pivot chunk over to make room for the new left chunk. */
      size_t amt = SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2;
      memmove((uint8_t *)((uintptr_t)buf + amt), buf, amt);
      memset(buf, 0, amt);
      /* Used later for constructing the remaining left and right chunks */
      lr[0] = buf;
      lr[1] = (uint8_t *)((uintptr_t)buf + amt * 2);
      /* Calculate space needed in the buffer, reuse the left chunk bytes. */
      expand_by = (amt * 2) + sizeof(__sm_bitvec_t);
    } while (0);

    for (int i = 0; i < 2; i++) {
      __sm_chunk_t lrc;
      if (lr[i]) {
        /* First assign the starting offset ... */
        *(__sm_idx_t *)lr[i] = lr_start[i];
        /* ... then, construct a chunk ... */
        __sm_chunk_init(&lrc, lr[i] + SM_SIZEOF_OVERHEAD);
        /* ... determine the type of chunk required ... */
        if (lr_end[i] - lr_start[i] - 1 >= SM_CHUNK_MAX_CAPACITY) {
          /* ... we need a run-length encoding (RLE), chunk ... */
          SM_CHUNK_SET_RLE(&lrc);
          /* ... now assign the length ... */
          __sm_chunk_rle_set_length(&lrc, lr_end[i] - lr_start[i]);
          /* ... and capacity, which differs left to right ... */
          if (i == 0) {
            /* ... left: extend to the start of the pivot chunk or, */
            __sm_chunk_rle_set_capacity(&lrc, aligned_idx - lr_start[i]);
          } else {
            /* ... right: extend to either max or the start of the next chunk */
            size_t right_offset = offset + SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t);
            __sm_chunk_rle_set_capacity(&lrc, __sm_chunk_rle_capacity_limit(map, aligned_idx, right_offset));
          }
        } else {
          /* ... we need a new sparse chunk ... */
          size_t lrl = lr_end[i] - lr_start[i];
          /* ... how many flags can we mark as all ones? ... */
          if (lrl > SM_BITS_PER_VECTOR) {
            lrc.m_data[0] = ~(__sm_bitvec_t)0 >> (SM_FLAGS_PER_INDEX - (lrl / SM_BITS_PER_VECTOR)) * 2;
          }
          /* ... do we have a mixed flag to create and vector to assign? ... */
          if (lrl % SM_BITS_PER_VECTOR) {
            SM_CHUNK_SET_FLAGS(lrc.m_data[0], (aligned_idx + lrl) / SM_BITS_PER_VECTOR, SM_PAYLOAD_MIXED);
            lrc.m_data[1] |= ~(__sm_bitvec_t)0 >> (SM_CHUNK_MAX_CAPACITY - (lrl % SM_BITS_PER_VECTOR));
          } else {
            /* ... earlier size estimates were all pessimistic, adjust them ... */
            if (i == 0) {
              /* ... slide the pivot chunk over a tad ... */
              size_t amt = SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t);
              uint8_t *loc = (uint8_t *)((uintptr_t)buf + amt);
              // fprintf(stdout, "\n%s\n", QCC_showChunk((uint8_t *)((uintptr_t)loc + sizeof(__sm_bitvec_t)), 0));
              memmove(loc, (uint8_t *)((uintptr_t)loc + sizeof(__sm_bitvec_t)), amt + sizeof(__sm_bitvec_t));
              // fprintf(stdout, "\n%s\n", QCC_showChunk(loc, 0));
              memset(((uint8_t *)(uintptr_t)buf + (2 * amt) + sizeof(__sm_bitvec_t)), 0, sizeof(__sm_bitvec_t));
              // fprintf(stdout, "\n%s\n", QCC_showChunk(loc, 0));
              lr[1] = (uint8_t *)((uintptr_t)lr[1] - sizeof(__sm_bitvec_t));
            }
            /* ... if not, our size estimate shrinks ... */
            expand_by -= sizeof(__sm_bitvec_t);
          }
        }
        __sm_when_diag({
          /* Sanity check the chunk */
          // fprintf(stdout, "\n%s\n", QCC_showChunk(lr[i], 0));
          for (size_t j = lr_start[i]; j < lr_end[i]; j++) {
            __sm_assert(__sm_chunk_is_set(&pivot_chunk, j) == true);
          }
          if (!SM_IS_CHUNK_RLE(&lrc)) {
            for (size_t j = lr_end[i]; j < SM_CHUNK_MAX_CAPACITY; j++) {
              __sm_assert(__sm_chunk_is_set(&pivot_chunk, j) == false);
            }
          }
        });
      }
    }
    /* Determine if we have room for this construct. */
    SM_ENOUGH_SPACE(expand_by);

    /* We do, so let's knit this into place within the map. */
    __sm_when_diag({ fprintf(stdout, "\n%s\n", QCC_showChunk(p, 0)); });
    size_t amt = SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t);
    __sm_insert_data(map, offset + amt, buf + amt, expand_by);
    memcpy(p, buf, expand_by + amt);
    //__sm_when_diag({
    //  fprintf(stdout, "\n%s\n", QCC_showChunk(p, 0));
    //  fprintf(stdout, "\n%s\n", QCC_showChunk(p + SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2, 0));
    //});

    /* And update the chunk count in the map. */
    __sm_set_chunk_count(map, __sm_get_chunk_count(map) + (lr[0] ? 1 : 0) + (lr[1] ? 1 : 0));

    __sm_when_diag({
      /* Sanity check all indexes in the region. */
      __sm_chunk_t c;
      for (size_t j = start; j < length; j++) {
        __sm_chunk_init(&c, p + __sm_get_chunk_offset(map, start));
        __sm_assert(__sm_chunk_is_set(&c, j) == (j == idx ? false : true));
        __sm_assert(sparsemap_is_set(map, j) == (j == idx ? false : true));
      }
    });
    return idx;
  }

  size_t pos = 0;
  __sm_bitvec_t vec = ~(__sm_bitvec_t)0;
  switch (__sm_chunk_clr_bit(&chunk, idx - start, &pos)) {
  case SM_OK:
    break;
  case SM_NEEDS_TO_GROW:
    offset += (SM_SIZEOF_OVERHEAD + pos * sizeof(__sm_bitvec_t));
    __sm_insert_data(map, offset, (uint8_t *)&vec, sizeof(__sm_bitvec_t));
    __sm_chunk_clr_bit(&chunk, idx - start, &pos);
    break;
  case SM_NEEDS_TO_SHRINK:
    /* The vector is empty, perhaps the entire chunk is empty? */
    if (__sm_chunk_is_empty(&chunk)) {
      __sm_remove_data(map, offset, SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2);
      __sm_set_chunk_count(map, __sm_get_chunk_count(map) - 1);
    } else {
      offset += (SM_SIZEOF_OVERHEAD + pos * sizeof(__sm_bitvec_t));
      __sm_remove_data(map, offset, sizeof(__sm_bitvec_t));
    }
    break;
  default:
    __sm_assert(!"shouldn't be here");
#ifdef DEBUG
    abort();
#endif
    break;
  }

  return idx;
}

/*
 * When v is non-NULL we've just added a new chunk and we knew in advance that a
 * new chunk will result in a SM_PAYLOAD_MIXED which in turn requires space to
 * store the bit pattern, so given that we allocated the space ahead of time and
 * don't need to allocate it now.
 */
static sparsemap_idx_t
__bidx_set(sparsemap_t *map, sparsemap_idx_t idx, uint8_t *p, size_t offset, __sm_bitvec_t *v)
{
  size_t pos = v ? -1 : 0;
  __sm_chunk_t chunk;
  __sm_idx_t start = *(__sm_idx_t *)p;

  __sm_chunk_init(&chunk, p + SM_SIZEOF_OVERHEAD);

  switch (__sm_chunk_set_bit(&chunk, idx - start, &pos)) {
  case SM_OK:
    break;
  case SM_NEEDS_TO_GROW:
    if (!v) {
      __sm_bitvec_t vec = 0;
      offset += (SM_SIZEOF_OVERHEAD + pos * sizeof(__sm_bitvec_t));
      __sm_insert_data(map, offset, (uint8_t *)&vec, sizeof(__sm_bitvec_t));
      pos = -1;
    }
    __sm_chunk_set_bit(&chunk, idx - start, &pos);
    break;
  case SM_NEEDS_TO_SHRINK:
    /* The vector is empty, perhaps the entire chunk is empty? */
    if (__sm_chunk_is_empty(&chunk)) {
      __sm_remove_data(map, offset, SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2);
      __sm_set_chunk_count(map, __sm_get_chunk_count(map) - 1);
    } else {
      offset += (SM_SIZEOF_OVERHEAD + pos * sizeof(__sm_bitvec_t));
      __sm_remove_data(map, offset, sizeof(__sm_bitvec_t));
    }
    break;
  default:
    __sm_assert(!"shouldn't be here");
#ifdef DEBUG
    abort();
#endif
    break;
  }

  return idx;
}

sparsemap_idx_t
bidx_set(sparsemap_t *map, sparsemap_idx_t idx)
{
  __sm_assert(sparsemap_get_size(map) >= SM_SIZEOF_OVERHEAD);

  /* Setting a bit could require an additional vector, let's ensure we have that
   * space available in the buffer first, or ENOMEM now. */
  SM_ENOUGH_SPACE(SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t));

  /* Determine if there is a chunk that could contain this index. */
  size_t offset = (size_t)__sm_get_chunk_offset(map, idx);

  if ((ssize_t)offset == -1) {
    /* No chunks exist, the map is empty, so we must append a new chunk to the
     * end of the buffer and initialize it so that it can contain this index. */
    uint8_t buf[SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2] = { 0 };
    __sm_append_data(map, &buf[0], sizeof(buf));
    uint8_t *p = __sm_get_chunk_data(map, 0);
    *(__sm_idx_t *)p = __sm_get_chunk_aligned_offset(idx);
    __sm_set_chunk_count(map, 1);

    __sm_bitvec_t *v = (__sm_bitvec_t *)(uintptr_t)p + SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t);
    return __bidx_set(map, idx, p, 0, v);
  }

  /* Try to locate a chunk for this idx.  We could find that:
   *  - the first chunk's offset is greater than the index, or
   *  - the index is beyond the end of the last chunk, or
   *  - we found a chunk that can contain this index. */
  uint8_t *p = __sm_get_chunk_data(map, offset);
  __sm_idx_t start = *(__sm_idx_t *)p;
  __sm_assert(start == __sm_get_chunk_aligned_offset(start));

  if (idx < start) {
    /* Our search resulted in the first chunk that starts after the index but
     * that means there is no chunk that can contain this index, so we need to
     * insert a new chunk before this one and initialize it so that it can
     * contain this index. */
    uint8_t buf[SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2] = { 0 };
    __sm_insert_data(map, offset, &buf[0], sizeof(buf));
    /* NOTE: insert moves the memory over meaning `p` is now the new chunk */
    *(__sm_idx_t *)p = __sm_get_chunk_aligned_offset(idx);
    __sm_set_chunk_count(map, __sm_get_chunk_count(map) + 1);

    __sm_bitvec_t *v = (__sm_bitvec_t *)(uintptr_t)p + SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t);
    return __bidx_set(map, idx, p, offset, v);
  }

  __sm_chunk_t chunk;
  __sm_chunk_init(&chunk, p + SM_SIZEOF_OVERHEAD);
  size_t capacity = __sm_chunk_get_capacity(&chunk);

  if (capacity < SM_CHUNK_MAX_CAPACITY && idx - start < SM_CHUNK_MAX_CAPACITY) {
    /* Special case, we have a chunk with one or more flags set to
     * SM_PAYLOAD_NONE which reduces the carrying capacity of the chunk. In
     * this case we should remove those flags and try again. */

    // GSB TODO
    capacity = __sm_chunk_get_capacity(&chunk);
  }

  if (chunk.m_data[0] == ~(__sm_bitvec_t)0 && idx - start == SM_CHUNK_MAX_CAPACITY) {
    /* Our search resulted in a chunk that is full of ones and this index is the
     * next one after the capacity, we have a run of ones longer than the
     * capacity of the sparse encoding, let's transition this chunk to
     * run-length encoding (RLE).
     *
     * NOTE: Keep in mind that idx is 0-based, so idx=2048 is the 2049th bit.
     * When a chunk is at maximum capacity it is storing indexes [0, 2048).
     *
     * ALSO: Keep in mind the RLE "length" is the current length of 1s in the
     * run, so in this case we transition from 2048 to a length of 2049.
     * in this run. */

    SM_CHUNK_SET_RLE(&chunk);
    __sm_chunk_rle_set_length(&chunk, SM_CHUNK_MAX_CAPACITY + 1);
    __sm_chunk_rle_set_capacity(&chunk, __sm_chunk_rle_capacity_limit(map, start, offset));
    return idx;
  }

  /* Is this an RLE chunk and the index within its range? */
  if (SM_IS_CHUNK_RLE(&chunk) && idx >= start && idx - start < capacity) {
    /* This RLE contains the bits in [start, start + length] so the index of
     * the last bit in this RLE chunk is `start + length - 1` which is why
     * we test index (0-based) against current length (1-based) below. */
    size_t l = __sm_chunk_rle_get_length(&chunk);
    if ((idx - start) == l) {
      __sm_chunk_rle_set_length(&chunk, l + 1);
      __sm_assert(__sm_chunk_rle_get_length(&chunk) == l + 1);
      return idx;
    }
  }

  if (idx - start >= capacity) {
    /* Our search resulted in a chunk however it's capacity doesn't encompass
     * this index, so we need to insert a new chunk after this one and
     * initialize it so that it can contain this index.  */
    uint8_t buf[SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2] = { 0 };
    size_t size = __sm_chunk_get_size(&chunk);
    offset += (SM_SIZEOF_OVERHEAD + size);
    p += SM_SIZEOF_OVERHEAD + size;
    __sm_insert_data(map, offset, &buf[0], sizeof(buf));

    start += __sm_chunk_get_capacity(&chunk);
    if (start + SM_CHUNK_MAX_CAPACITY <= idx) {
      start = __sm_get_chunk_aligned_offset(idx);
    }
    *(__sm_idx_t *)p = start;
    __sm_assert(start == __sm_get_chunk_aligned_offset(start));
    __sm_set_chunk_count(map, __sm_get_chunk_count(map) + 1);

    __sm_bitvec_t *v = (__sm_bitvec_t *)(uintptr_t)p + SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t);
    return __bidx_set(map, idx, p, offset, v);
  }

  __sm_idx_t ret_idx = __bidx_set(map, idx, p, offset, NULL);

  // Did this chunk become all ones?  Can we compact with adjacent chunks?
  if (chunk.m_data[0] == ~(__sm_bitvec_t)0) {

  }
  return ret_idx;
}

sparsemap_idx_t
bidx_set_to(sparsemap_t *map, sparsemap_idx_t idx, bool value)
{
  if (value) {
    return bidx_set(map, idx);
  } else {
    return bidx_clear(map, idx);
  }
}

sparsemap_idx_t
sparsemap_set(sparsemap_t *map, sparsemap_idx_t idx, bool value)
{
  return bidx_set_to(map, idx, value);
  __sm_assert(sparsemap_get_size(map) >= SM_SIZEOF_OVERHEAD);

  /* Locate the __sm_chunk_t for this index */
  ssize_t offset = __sm_get_chunk_offset(map, idx);
  bool dont_grow = false;

  /* If we're going to set a new bit there is the potential that we'll need
   * additional space in the buffer, ensure we have enough. */
  if (value && map->m_data_used + SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2 > map->m_capacity) {
    errno = ENOSPC;
    return SPARSEMAP_IDX_MAX;
  }

  /* No chunks exists, the map is empty, create one now... */
  if (offset == -1) {
    /* ...unless we're trying to turn a bit off (when `value` is false) in which
     * case we're done (because the bit is implictly "unset" at that `idx`). */
    if (value == false) {
      return idx;
    }

    /* Append a newly initialized vector to the end of the buffer. */
    uint8_t buf[SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2] = { 0 };
    __sm_append_data(map, &buf[0], sizeof(buf));

    /* Fetch that new chunk at index 0 and set its starting offset relative to
     * `idx`. */
    offset = 0;
    uint8_t *p = __sm_get_chunk_data(map, offset);
    *(__sm_idx_t *)p = __sm_get_chunk_aligned_offset(idx);

    __sm_set_chunk_count(map, 1);

    /* Now that we have inserted a chunk should avoid doing so again; there is
     * no need to grow the vector. */
    dont_grow = true;
  }

  /* Now we either find the pre-existing chunk for this `idx` or the one we just
   * created above. */
  uint8_t *p = __sm_get_chunk_data(map, offset);
  __sm_idx_t start = *(__sm_idx_t *)p;
  __sm_assert(start == __sm_get_chunk_aligned_offset(start));

  /* The new index is smaller than the first __sm_chunk_t: create a new
   * __sm_chunk_t and insert it at the front. */
  if (idx < start) {
    if (value == false) {
      /* nothing to do */
      return idx;
    }

    uint8_t buf[SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2] = { 0 };
    __sm_insert_data(map, offset, &buf[0], sizeof(buf));

    size_t aligned_idx = __sm_get_chunk_aligned_offset(idx);
#if 0
    if (start - aligned_idx < SM_CHUNK_MAX_CAPACITY) {
      __sm_chunk_t chunk;
      __sm_chunk_init(&chunk, p + SM_SIZEOF_OVERHEAD);
      if (__sm_chunk_reduce_capacity(&chunk, start - aligned_idx)) {
        /* The __sm_chunk_t is empty then remove it. */
        __sm_remove_data(map, offset, SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2);
        __sm_set_chunk_count(map, __sm_get_chunk_count(map) - 1);
      }
    }
#endif
    *(__sm_idx_t *)p = start = aligned_idx;

    /* We just added another chunk! */
    __sm_set_chunk_count(map, __sm_get_chunk_count(map) + 1);

    /* We already inserted an additional __sm_bitvec_t; later on there
     * is no need to grow the vector even further. */
    dont_grow = true;
  }

  /* A __sm_chunk_t exists, but the new index exceeds its capacities: create
   * a new __sm_chunk_t and insert it after the current one. */
  else {
    __sm_chunk_t chunk;
    __sm_chunk_init(&chunk, p + SM_SIZEOF_OVERHEAD);
    if (idx - start >= (sparsemap_idx_t)__sm_chunk_get_capacity(&chunk)) {
      if (value == false) {
        /* nothing to do */
        return idx;
      }

      size_t size = __sm_chunk_get_size(&chunk);
      offset += (SM_SIZEOF_OVERHEAD + size);
      p += SM_SIZEOF_OVERHEAD + size;

      uint8_t buf[SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2] = { 0 };
      __sm_insert_data(map, offset, &buf[0], sizeof(buf));

      start += __sm_chunk_get_capacity(&chunk);
      if ((sparsemap_idx_t)start + SM_CHUNK_MAX_CAPACITY <= idx) {
        start = __sm_get_chunk_aligned_offset(idx);
      }
      *(__sm_idx_t *)p = start;
      __sm_assert(start == __sm_get_chunk_aligned_offset(start));

      /* We just added another chunk! */
      __sm_set_chunk_count(map, __sm_get_chunk_count(map) + 1);

      /* We already inserted an additional __sm_bitvec_t; later on there
       * is no need to grow the vector even further. */
      dont_grow = true;
    }
  }

  __sm_chunk_t chunk;
  __sm_chunk_init(&chunk, p + SM_SIZEOF_OVERHEAD);

  /* Now update the __sm_chunk_t. */
  size_t position;
  __sm_bitvec_t fill;
  int code = __sm_chunk_set(&chunk, idx - start, value, &position, &fill, false);
  switch (code) {
  case SM_OK:
    break;
  case SM_NEEDS_TO_GROW:
    if (!dont_grow) {
      offset += (SM_SIZEOF_OVERHEAD + position * sizeof(__sm_bitvec_t));
      __sm_insert_data(map, offset, (uint8_t *)&fill, sizeof(__sm_bitvec_t));
    }
    __sm_chunk_set(&chunk, idx - start, value, &position, &fill, true);
    break;
  case SM_NEEDS_TO_SHRINK:
    /* If the __sm_chunk_t is empty then remove it. */
    if (__sm_chunk_is_empty(&chunk)) {
      __sm_assert(position == 1);
      __sm_remove_data(map, offset, SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2);
      __sm_set_chunk_count(map, __sm_get_chunk_count(map) - 1);
    } else {
      offset += (SM_SIZEOF_OVERHEAD + position * sizeof(__sm_bitvec_t));
      __sm_remove_data(map, offset, sizeof(__sm_bitvec_t));
    }
    break;
  default:
    __sm_assert(!"shouldn't be here");
#ifdef DEBUG
    abort();
#endif
    break;
  }
  __sm_assert(sparsemap_get_size(map) >= SM_SIZEOF_OVERHEAD);
  return idx;
}

sparsemap_idx_t
sparsemap_get_starting_offset(sparsemap_t *map)
{
  sparsemap_idx_t offset = 0;
  size_t count = __sm_get_chunk_count(map);
  if (count == 0) {
    return 0;
  }
  uint8_t *p = __sm_get_chunk_data(map, 0);
  sparsemap_idx_t relative_position = *(__sm_idx_t *)p;
  p += SM_SIZEOF_OVERHEAD;
  __sm_chunk_t chunk;
  __sm_chunk_init(&chunk, p);
  for (size_t m = 0; m < sizeof(__sm_bitvec_t); m++, p++) {
    for (int n = 0; n < SM_FLAGS_PER_INDEX_BYTE; n++) {
      size_t flags = SM_CHUNK_GET_FLAGS(*p, n);
      if (flags == SM_PAYLOAD_NONE) {
        continue;
      } else if (flags == SM_PAYLOAD_ZEROS) {
        relative_position += SM_BITS_PER_VECTOR;
      } else if (flags == SM_PAYLOAD_ONES) {
        offset = relative_position;
        goto done;
      } else if (flags == SM_PAYLOAD_MIXED) {
        __sm_bitvec_t w = chunk.m_data[1 + __sm_chunk_get_position(&chunk, m * SM_FLAGS_PER_INDEX_BYTE + n)];
        for (int k = 0; k < SM_BITS_PER_VECTOR; k++) {
          if (w & ((__sm_bitvec_t)1 << k)) {
            offset = relative_position + k;
            goto done;
          }
        }
        relative_position += SM_BITS_PER_VECTOR;
      }
    }
  }
done:;
  return offset;
}

sparsemap_idx_t
sparsemap_get_ending_offset(sparsemap_t *map)
{
  sparsemap_idx_t offset = 0;
  size_t count = __sm_get_chunk_count(map);
  if (count == 0) {
    return 0;
  }
  uint8_t *p = __sm_get_chunk_data(map, 0);
  for (size_t i = 0; i < count - 1; i++) {
    p += SM_SIZEOF_OVERHEAD;
    __sm_chunk_t chunk;
    __sm_chunk_init(&chunk, p);
    p += __sm_chunk_get_size(&chunk);
  }
  __sm_idx_t start = *(__sm_idx_t *)p;
  p += SM_SIZEOF_OVERHEAD;
  __sm_chunk_t chunk;
  __sm_chunk_init(&chunk, p);
  sparsemap_idx_t relative_position = start;
  for (size_t m = 0; m < sizeof(__sm_bitvec_t); m++, p++) {
    for (int n = 0; n < SM_FLAGS_PER_INDEX_BYTE; n++) {
      size_t flags = SM_CHUNK_GET_FLAGS(*p, n);
      if (flags == SM_PAYLOAD_NONE) {
        continue;
      } else if (flags == SM_PAYLOAD_ZEROS) {
        relative_position += SM_BITS_PER_VECTOR;
      } else if (flags == SM_PAYLOAD_ONES) {
        relative_position += SM_BITS_PER_VECTOR;
        offset = relative_position;
      } else if (flags == SM_PAYLOAD_MIXED) {
        __sm_bitvec_t w = chunk.m_data[1 + __sm_chunk_get_position(&chunk, m * SM_FLAGS_PER_INDEX_BYTE + n)];
        int idx = 0;
        for (int k = 0; k < SM_BITS_PER_VECTOR; k++) {
          if (w & ((__sm_bitvec_t)1 << k)) {
            idx = k;
          }
        }
        offset = relative_position + idx;
        relative_position += SM_BITS_PER_VECTOR;
      }
    }
  }
  return offset;
}

double
sparsemap_fill_factor(sparsemap_t *map)
{
  size_t rank = sparsemap_rank(map, 0, SPARSEMAP_IDX_MAX, true);
  sparsemap_idx_t end = sparsemap_get_ending_offset(map);
  return (double)rank / (double)end * 100.0;
}

void *
sparsemap_get_data(sparsemap_t *map)
{
  return map->m_data;
}

size_t
sparsemap_get_size(sparsemap_t *map)
{
  if (map->m_data_used) {
    size_t size = __sm_get_size_impl(map);
    if (size != map->m_data_used) {
      map->m_data_used = size;
    }
    __sm_when_diag({ __sm_assert(map->m_data_used == __sm_get_size_impl(map)); });
    return map->m_data_used;
  }
  return map->m_data_used = __sm_get_size_impl(map);
}

size_t
sparsemap_count(sparsemap_t *map)
{
  return sparsemap_rank(map, 0, SPARSEMAP_IDX_MAX, true);
}

void
sparsemap_scan(sparsemap_t *map, void (*scanner)(__sm_idx_t[], size_t, void *aux), size_t skip, void *aux)
{
  uint8_t *p = __sm_get_chunk_data(map, 0);
  size_t count = __sm_get_chunk_count(map);

  for (size_t i = 0; i < count; i++) {
    __sm_idx_t start = *(__sm_idx_t *)p;
    p += SM_SIZEOF_OVERHEAD;
    __sm_chunk_t chunk;
    __sm_chunk_init(&chunk, p);
    size_t skipped = __sm_chunk_scan(&chunk, start, scanner, skip, aux);
    if (skip) {
      assert(skip >= skipped);
      skip -= skipped;
    }
    p += __sm_chunk_get_size(&chunk);
  }
}

int
sparsemap_merge(sparsemap_t *destination, sparsemap_t *source)
{
  uint8_t *src, *dst;
  size_t src_count = __sm_get_chunk_count(source);
  sparsemap_idx_t dst_ending_offset = sparsemap_get_ending_offset(destination);

  if (src_count == 0) {
    return 0;
  }

  ssize_t remaining_capacity = destination->m_capacity - destination->m_data_used -
    (source->m_data_used + src_count * (SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2));

  /* Estimate worst-case overhead required for merge. */
  if (remaining_capacity <= 0) {
    errno = ENOSPC;
    return -remaining_capacity;
  }

  src = __sm_get_chunk_data(source, 0);
  while (src_count) {
    __sm_idx_t src_start = *(__sm_idx_t *)src;
    __sm_chunk_t src_chunk;
    __sm_chunk_init(&src_chunk, src + SM_SIZEOF_OVERHEAD);
    size_t src_capacity = __sm_chunk_get_capacity(&src_chunk);
    ssize_t dst_offset = __sm_get_chunk_offset(destination, src_start);
    if (dst_offset >= 0) {
      dst = __sm_get_chunk_data(destination, dst_offset);
      __sm_idx_t dst_start = *(__sm_idx_t *)dst;
      __sm_chunk_t dst_chunk;
      __sm_chunk_init(&dst_chunk, dst + SM_SIZEOF_OVERHEAD);
      size_t dst_capacity = __sm_chunk_get_capacity(&dst_chunk);

      /* Try to expand the capacity if there's room before the start of the next chunk. */
      if (src_start == dst_start && dst_capacity < src_capacity) {
        ssize_t nxt_offset = __sm_get_chunk_offset(destination, dst_start + dst_capacity + 1);
        uint8_t *nxt_dst = __sm_get_chunk_data(destination, nxt_offset);
        __sm_idx_t nxt_dst_start = *(__sm_idx_t *)nxt_dst;
        if (nxt_dst_start > dst_start + src_capacity) {
          __sm_chunk_increase_capacity(&dst_chunk, src_capacity);
          dst_capacity = __sm_chunk_get_capacity(&dst_chunk);
        }
      }

      /* Source chunk precedes next destination chunk. */
      if ((src_start + src_capacity) <= dst_start) {
        size_t src_size = __sm_chunk_get_size(&src_chunk);
        ssize_t offset = __sm_get_chunk_offset(destination, dst_start);
        __sm_insert_data(destination, offset, src, SM_SIZEOF_OVERHEAD + src_size);
        /* Update the chunk count and data_used. */
        __sm_set_chunk_count(destination, __sm_get_chunk_count(destination) + 1);
        src_count--;
        src += SM_SIZEOF_OVERHEAD + __sm_chunk_get_size(&src_chunk);
        continue;
      }

      /* Source chunk follows next destination chunk. */
      if (src_start >= (dst_start + dst_capacity)) {
        size_t src_size = __sm_chunk_get_size(&src_chunk);
        if (dst_offset == __sm_get_chunk_offset(destination, SPARSEMAP_IDX_MAX)) {
          __sm_append_data(destination, src, SM_SIZEOF_OVERHEAD + src_size);
        } else {
          ssize_t offset = __sm_get_chunk_offset(destination, src_start);
          __sm_insert_data(destination, offset, src, SM_SIZEOF_OVERHEAD + src_size);
        }
        /* Update the chunk count and data_used. */
        __sm_set_chunk_count(destination, __sm_get_chunk_count(destination) + 1);
        src_count--;
        src += SM_SIZEOF_OVERHEAD + __sm_chunk_get_size(&src_chunk);
        continue;
      }

      /* Source and destination and a perfect overlapping pair. */
      if (src_start == dst_start && src_capacity == dst_capacity) {
        __sm_merge_chunk(destination, src_start, dst_start, dst_capacity, &dst_chunk, &src_chunk);
        src_count--;
        src += SM_SIZEOF_OVERHEAD + __sm_chunk_get_size(&src_chunk);
        continue;
      }

      /* Non-uniform overlapping chunks. */
      if (dst_start < src_start || (dst_start == src_start && dst_capacity != src_capacity)) {
        size_t src_end = src_start + src_capacity;
        size_t dst_end = dst_start + dst_capacity;
        size_t overlap = src_end > dst_end ? src_capacity - (src_end - dst_end) : src_capacity;
        __sm_merge_chunk(destination, src_start, dst_start, overlap, &dst_chunk, &src_chunk);
        for (size_t n = src_start + overlap; n <= src_end; n++) {
          if (sparsemap_is_set(source, n)) {
            sparsemap_set(destination, n, true);
          }
        }
        src_count--;
        src += SM_SIZEOF_OVERHEAD + __sm_chunk_get_size(&src_chunk);
        continue;
      }
    } else {
      if (src_start >= dst_ending_offset) {
        /* Starting offset is after destination chunks, so append data. */
        size_t src_size = __sm_chunk_get_size(&src_chunk);
        __sm_append_data(destination, src, SM_SIZEOF_OVERHEAD + src_size);

        /* Update the chunk count and data_used. */
        __sm_set_chunk_count(destination, __sm_get_chunk_count(destination) + 1);

        src_count--;
        src += SM_SIZEOF_OVERHEAD + __sm_chunk_get_size(&src_chunk);
        continue;
      } else {
        /* Source chunk precedes next destination chunk. */
        size_t src_size = __sm_chunk_get_size(&src_chunk);
        ssize_t offset = __sm_get_chunk_offset(destination, src_start);
        __sm_insert_data(destination, offset, src, SM_SIZEOF_OVERHEAD + src_size);

        /* Update the chunk count and data_used. */
        __sm_set_chunk_count(destination, __sm_get_chunk_count(destination) + 1);

        src_count--;
        src += SM_SIZEOF_OVERHEAD + __sm_chunk_get_size(&src_chunk);
        continue;
      }
    }
  }
  return 0;
}

sparsemap_idx_t
sparsemap_split(sparsemap_t *map, sparsemap_idx_t offset, sparsemap_t *other)
{
  if (sparsemap_count(other) > 0) {
    return 0;
  }

  if (offset == SPARSEMAP_IDX_MAX) {
    sparsemap_idx_t begin = sparsemap_get_starting_offset(map);
    sparsemap_idx_t end = sparsemap_get_ending_offset(map);
    if (begin != end) {
      size_t count = sparsemap_rank(map, begin, end, true);
      offset = sparsemap_select(map, count / 2, true);
    } else {
      return SPARSEMAP_IDX_MAX;
    }
  }

  if (offset >= sparsemap_get_ending_offset(map)) {
    return 0;
  }

  /* |dst| points to the destination buffer */
  uint8_t *dst = __sm_get_chunk_end(other);

  /* |src| points to the source-chunk */
  uint8_t *src = __sm_get_chunk_data(map, 0);

  bool in_middle = false;
  uint8_t *prev = src;
  size_t i, count = __sm_get_chunk_count(map);
  for (i = 0; i < count; i++) {
    __sm_idx_t start = *(__sm_idx_t *)src;
    __sm_chunk_t chunk;
    __sm_chunk_init(&chunk, src + SM_SIZEOF_OVERHEAD);
    if (start == offset) {
      break;
    }
    if (start + __sm_chunk_get_capacity(&chunk) > (unsigned long)offset) {
      in_middle = true;
      break;
    }
    if (start > offset) {
      src = prev;
      i--;
      break;
    }

    prev = src;
    src += SM_SIZEOF_OVERHEAD + __sm_chunk_get_size(&chunk);
  }
  if (i == count) {
    __sm_assert(sparsemap_get_size(map) > SM_SIZEOF_OVERHEAD);
    __sm_assert(sparsemap_get_size(other) > SM_SIZEOF_OVERHEAD);
    return offset;
  }

  /* Now copy all the remaining chunks. */
  int moved = 0;

  /* If |offset| is in the middle of a chunk then this chunk has to be split */
  if (in_middle) {
    uint8_t buf[SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t) * 2] = { 0 };
    memcpy(dst, &buf[0], sizeof(buf));

    __sm_idx_t start = *(__sm_idx_t *)src;
    *(__sm_idx_t *)dst = start;
    dst += SM_SIZEOF_OVERHEAD;

    /* The |other| sparsemap_t now has one additional chunk */
    __sm_set_chunk_count(other, __sm_get_chunk_count(other) + 1);
    if (other->m_data_used != 0) {
      other->m_data_used += SM_SIZEOF_OVERHEAD + sizeof(__sm_bitvec_t);
    }

    src += SM_SIZEOF_OVERHEAD;
    __sm_chunk_t s_chunk;
    __sm_chunk_init(&s_chunk, src);
    size_t capacity = __sm_chunk_get_capacity(&s_chunk);

    __sm_chunk_t d_chunk;
    __sm_chunk_init(&d_chunk, dst);

    /* Now copy the bits. */
    for (size_t j = start; j < capacity + start; j++) {
      if (j >= offset) {
        if (__sm_chunk_is_set(&s_chunk, j - start)) {
          __sm_bitvec_t fill;
          size_t pos;
          __sm_chunk_set(&d_chunk, j - start, true, &pos, &fill, true);
          sparsemap_set(map, j, false);
        }
      }
    }
    src += __sm_chunk_get_size(&s_chunk);
    size_t dsize = __sm_chunk_get_size(&d_chunk);
    dst += dsize;
    i++;
  }

  /* Now continue with all remaining chunks. */
  for (; i < count; i++) {
    __sm_idx_t start = *(__sm_idx_t *)src;
    src += SM_SIZEOF_OVERHEAD;
    __sm_chunk_t chunk;
    __sm_chunk_init(&chunk, src);
    size_t s = __sm_chunk_get_size(&chunk);

    *(__sm_idx_t *)dst = start;
    dst += SM_SIZEOF_OVERHEAD;
    memcpy(dst, src, s);
    src += s;
    dst += s;

    moved++;
  }

  /* Force new calculation. */
  other->m_data_used = 0;
  map->m_data_used = 0;

  /* Update the Chunk counters. */
  __sm_set_chunk_count(map, __sm_get_chunk_count(map) - moved);
  __sm_set_chunk_count(other, __sm_get_chunk_count(other) + moved);

  __sm_assert(sparsemap_get_size(map) >= SM_SIZEOF_OVERHEAD);
  __sm_assert(sparsemap_get_size(other) > SM_SIZEOF_OVERHEAD);

  return offset;
}

sparsemap_idx_t
sparsemap_select(sparsemap_t *map, sparsemap_idx_t n, bool value)
{
  __sm_assert(sparsemap_get_size(map) >= SM_SIZEOF_OVERHEAD);
  __sm_idx_t start;
  size_t count = __sm_get_chunk_count(map);

  if (count == 0 && value == false) {
    return n;
  }

  uint8_t *p = __sm_get_chunk_data(map, 0);

  for (size_t i = 0; i < count; i++) {
    start = *(__sm_idx_t *)p;
    /* Start of this chunk is greater than n meaning there are a set of 0s
     * before the first 1 sufficient to consume n. */
    if (value == false && i == 0 && start > n) {
      return n;
    }
    p += SM_SIZEOF_OVERHEAD;
    __sm_chunk_t chunk;
    __sm_chunk_init(&chunk, p);

    ssize_t new_n = n;
    size_t index = __sm_chunk_select(&chunk, n, &new_n, value);
    if (new_n == -1) {
      return start + index;
    }
    n = new_n;

    p += __sm_chunk_get_size(&chunk);
  }
  return SPARSEMAP_IDX_MAX;
}

static size_t
__sm_rank_vec(sparsemap_t *map, size_t begin, size_t end, bool value, __sm_bitvec_t *vec)
{
  assert(sparsemap_get_size(map) >= SM_SIZEOF_OVERHEAD);
  size_t amt, gap, pos = 0, result = 0, prev = 0, count, len = end - begin + 1;
  uint8_t *p;

  if (begin > end) {
    return 0;
  }

  count = __sm_get_chunk_count(map);

  if (count == 0) {
    if (value == false) {
      /* The count/rank of unset bits in an empty map is inf, so what you requested is the answer. */
      return len;
    }
  }

  p = __sm_get_chunk_data(map, 0);

  for (size_t i = 0; i < count; i++) {
    __sm_idx_t start = *(__sm_idx_t *)p;
    /* [prev, start + pos), prev is the last bit examined 0-based. */
    if (i == 0) {
      gap = start;
    } else {
      if (prev + SM_CHUNK_MAX_CAPACITY == start) {
        gap = 0;
      } else {
        gap = start - (prev + pos);
      }
    }
    /* Start of this chunk is greater than the end of the desired range. */
    if (start > end) {
      if (value == true) {
        /* We're counting set bits and this chunk starts after the range
         * [begin, end], we're done. */
        return result;
      } else {
        if (i == 0) {
          /* We're counting unset bits and the first chunk starts after the
           * range meaning everything proceeding this chunk was zero and should
           * be counted, also we're done. */
          result += (end - begin) + 1;
          return result;
        } else {
          /* We're counting unset bits and some chunk starts after the range, so
           * we've counted enough, we're done. */
          if (pos > end) {
            return result;
          } else {
            if (end - pos < gap) {
              result += end - pos;
              return result;
            } else {
              result += gap;
              return result;
            }
          }
        }
      }
    } else {
      /* The range and this chunk overlap. */
      if (value == false) {
        if (begin > gap) {
          begin -= gap;
        } else {
          result += gap - begin;
          begin = 0;
        }
      } else {
        if (begin >= gap) {
          begin -= gap;
        }
      }
    }
    prev = start;
    p += SM_SIZEOF_OVERHEAD;
    __sm_chunk_t chunk;
    __sm_chunk_init(&chunk, p);

    /* Count all the set/unset inside this chunk. */
    amt = __sm_chunk_rank(&chunk, &begin, end - start, &pos, vec, value);
    result += amt;
    p += __sm_chunk_get_size(&chunk);
  }
  /* Count any additional unset bits that fall outside the last chunk but
   * within the range. */
  if (value == false) {
    size_t last = prev - 1 + pos;
    if (end > last) {
      result += end - last - begin;
    }
  }
  return result;
}

size_t
sparsemap_rank(sparsemap_t *map, size_t begin, size_t end, bool value)
{
  __sm_bitvec_t vec;
  return __sm_rank_vec(map, begin, end, value, &vec);
}

size_t
sparsemap_span(sparsemap_t *map, sparsemap_idx_t idx, size_t len, bool value)
{
  size_t rank, nth;
  __sm_bitvec_t vec = 0;
  sparsemap_idx_t offset;

  /* When skipping forward to `idx` offset in the map we can determine how
   * many selects we can avoid by taking the rank of the range and starting
   * at that bit. */
  nth = (idx == 0) ? 0 : sparsemap_rank(map, 0, idx - 1, value);
  if (SPARSEMAP_NOT_FOUND(nth)) {
    return nth;
  }
  /* Find the first bit that matches value, then... */
  offset = sparsemap_select(map, nth, value);
  do {
    /* See if the rank of the bits in the range starting at offset is equal
     * to the desired amount. */
    rank = (len == 1) ? 1 : __sm_rank_vec(map, offset, offset + len - 1, value, &vec);
    if (rank >= len) {
      /* We've found what we're looking for, return the index of the first
       * bit in the range. */
      break;
    }
    /* Now we try to jump forward as much as possible before we look for a
     * new match. We do this by counting the remaining bits in the returned
     * vec from the call to rank_vec(). */
    int amt = 1;
    if (vec > 0) {
      /* The returned vec had som set bits, let's move forward in the map as much
       * as possible (max: 64 bit positions). */
      int max = len > SM_BITS_PER_VECTOR ? SM_BITS_PER_VECTOR : len;
      while (amt < max && (vec & 1 << amt)) {
        amt++;
      }
    }
    nth += amt;
    offset = sparsemap_select(map, nth, value);
  } while (SPARSEMAP_FOUND(offset));

  return offset;
}

#ifdef SPARSEMAP_TESTING

#include <qc.h>

static double
_tst_pow(double base, int exponent)
{
  if (exponent == 0) {
    return 1.0; // 0^0 is 1
  } else if (base == 0.0) {
    return 0.0; // 0 raised to any positive exponent is 0 (except 0^0)
  } else if (base < 0.0 && (exponent & 1) != 0) {
    // negative base with odd exponent, results in a negative
    return -_tst_pow(-base, exponent);
  }

  double result = base;
  for (unsigned int i = 1; i < exponent; i++) {
    result *= base;
  }
  return result;
}

static char *
_qcc_format_chunk(__sm_idx_t start, __sm_chunk_t *chunk)
{
  char *buf = NULL;
  __sm_bitvec_t desc = chunk->m_data[0];

  buf = malloc(sizeof(char) * ((SM_FLAGS_PER_INDEX * 16) + (SM_BITS_PER_VECTOR * 64) + 16) * 2);

  if (!SM_IS_CHUNK_RLE(chunk)) {
    char desc_str[(2 * SM_FLAGS_PER_INDEX) + 1] = { 0 };
    char *str = desc_str;
    int mixed = 0;
    for (int i = SM_FLAGS_PER_INDEX - 1; i >= 0; i--) {
      uint8_t flag = SM_CHUNK_GET_FLAGS(desc, i);
      switch (flag) {
      case SM_PAYLOAD_NONE:
        str += sprintf(str, "∘");
        break;
      case SM_PAYLOAD_ONES:
        str += sprintf(str, "1");
        break;
      case SM_PAYLOAD_ZEROS:
        str += sprintf(str, "0");
        break;
      case SM_PAYLOAD_MIXED:
        str += sprintf(str, "①");
        mixed++;
        break;
      default:
      }
    }
    str = buf + sprintf(buf, "%.10u\t%s%s", start, desc_str, mixed ? " :: " : "");
    for (int i = 0; i < mixed; i++) {
      str += sprintf(str, "0x%lx%s", chunk->m_data[1 + i], i + 1 < mixed ? " " : "");
    }
  } else {
    sprintf(buf, "%.10u\t1»%zu of %zu", start, __sm_chunk_rle_get_length(chunk), __sm_chunk_rle_get_capacity(chunk));
  }
  return buf;
}

static char *
QCC_showChunk(void *value, int len)
{
  __sm_idx_t start = *(__sm_idx_t *)value;
  __sm_chunk_t chunk;
  // TODO: __sm_chunk_t *chunk = (__sm_chunk_t *)((uintptr_t)value + SM_SIZEOF_OVERHEAD);
  __sm_chunk_init(&chunk, value + SM_SIZEOF_OVERHEAD);

  return _qcc_format_chunk(start, &chunk);
}

static char *
QCC_showSparsemap(void *value, int len)
{
  sparsemap_t *map = (sparsemap_t *)value;
  size_t count = __sm_get_chunk_count(map);
  size_t clen = 0;
  char *str, *buf = NULL;

  if (count > 0) {
    uint8_t *p = __sm_get_chunk_data(map, 0);
    for (size_t i = 0; i < count; i++) {
      __sm_chunk_t chunk;
      __sm_idx_t start = *(__sm_idx_t *)p;
      __sm_chunk_init(&chunk, p + SM_SIZEOF_OVERHEAD);
      char *c = _qcc_format_chunk(start, &chunk);
      if (buf) {
        char *new = realloc(buf, strlen(buf) + strlen(c) + 2);
        if (new) {
          buf = new;
          sprintf(str, "\n%s", c);
          str += strlen(c);
        }
      } else {
        buf = c;
        str = buf + strlen(c);
      }
      p += SM_SIZEOF_OVERHEAD;
      p += __sm_chunk_get_size(&chunk);
    }
  }

  return buf;
}

static void
QCC_freeChunkValue(void *value)
{
  free(value);
}

static void
QCC_freeSparsemapValue(void *value)
{
  free(value);
}

QCC_GenValue *
QCC_genChunk()
{
  if (((double)random() / (double)RAND_MAX) > 0.5) {
    // Generate a run-length encoded (RLE) chunk:
    sparsemap_idx_t from = 1, to = SM_CHUNK_RLE_MAX_CAPACITY;
    unsigned int len = ((unsigned int)random() % (to - from)) + from;
    // First allocate enough room for the chunk data ...
    uint8_t *p = malloc(SM_SIZEOF_OVERHEAD + sizeof(__sm_chunk_t) + (sizeof(__sm_bitvec_t) * 2));
    __sm_chunk_t *chunk;
    // ... then set the offset to the length so we can test for that later ...
    *(__sm_idx_t *)p = len;
    // ... next is the chunk begins after the offset ...
    chunk = (__sm_chunk_t *)((uintptr_t)p + SM_SIZEOF_OVERHEAD);
    // ... this contains a single vector ...
    chunk->m_data = (__sm_bitvec_t *)((uintptr_t)chunk + sizeof(__sm_chunk_t));
    chunk->m_data[0] = 0;
    // ... set the flags on this vector to indicate that is it RLE ...
    SM_CHUNK_SET_RLE(chunk);
    // ... set the RLE chunk's initial capacity ...
    __sm_chunk_rle_set_capacity(chunk, SM_CHUNK_RLE_MAX_CAPACITY);
    // ... and set the RLE chunk's length of 1s to len.
    __sm_chunk_rle_set_length(chunk, len);
    // Now, test what we've generated to ensure it's correct.
    assert(*(__sm_idx_t *)p == len);
    assert(SM_IS_CHUNK_RLE(chunk));
    assert(__sm_chunk_rle_get_capacity(chunk) == SM_CHUNK_RLE_MAX_CAPACITY);
    assert(__sm_chunk_rle_get_length(chunk) == len);
    return QCC_initGenValue(p, 1, QCC_showChunk, QCC_freeChunkValue);
  } else {
    // Generate a chunk with the offset equal to the number of additional
    // vectors (len) and a descriptor that matches that with random data.
    unsigned int from = 0, to = SM_FLAGS_PER_INDEX;
    unsigned int len = ((unsigned int)random() % (to - from)) + from;
    unsigned int cut = ((unsigned int)random() % ((SM_FLAGS_PER_INDEX - len) - from)) + from;
    // First allocate enough room for the chunk data ...
    uint8_t *p = malloc(SM_SIZEOF_OVERHEAD + sizeof(__sm_chunk_t) + (sizeof(__sm_bitvec_t) * (len + 1)));
    __sm_chunk_t *chunk;
    // ... then set the offset to the capacity ...
    *(__sm_idx_t *)p = SM_CHUNK_MAX_CAPACITY - (cut * SM_BITS_PER_VECTOR);
    // ... next is the chunk begins after the offset ...
    chunk = (__sm_chunk_t *)((uintptr_t)p + SM_SIZEOF_OVERHEAD);
    // ... this contains a len + 1 vectors ...
    chunk->m_data = (__sm_bitvec_t *)((uintptr_t)chunk + sizeof(__sm_chunk_t));
    // ... the first is the descriptor with the flags ...
    __sm_bitvec_t *desc = chunk->m_data;
    *desc = 0;
    // ... ensure that exactly `len` flags are set to SM_PAYLOAD_MIXED ...
    for (size_t i = 0; i < len; i++) {
      SM_CHUNK_SET_FLAGS(*desc, i, SM_PAYLOAD_MIXED);
      chunk->m_data[1 + i] = (uintptr_t)chunk + i;
    }
    // ... and, on average, 50% of the rest are SM_PAYLOAD_ONES ...
    for (size_t i = len; i < SM_FLAGS_PER_INDEX - cut; i++) {
      double coin = (double)random() / (double)RAND_MAX;
      if (SM_CHUNK_GET_FLAGS(*desc, i) != SM_PAYLOAD_MIXED && coin >= 0.5) {
        SM_CHUNK_SET_FLAGS(*desc, i, SM_PAYLOAD_ONES);
      }
    }
    // ... shuffle those around ...
    for (size_t i = 0; i < SM_FLAGS_PER_INDEX - cut - 1; i++) {
      size_t j = ((size_t)random() % ((SM_FLAGS_PER_INDEX - cut) - i)) + i;
      int flags = SM_CHUNK_GET_FLAGS(*desc, j);
      SM_CHUNK_SET_FLAGS(*desc, j, SM_CHUNK_GET_FLAGS(*desc, i));
      SM_CHUNK_SET_FLAGS(*desc, i, flags);
    }
    // ... reduce the capacity by setting trailing flags to SM_PAYLOAD_NONE ...
    *desc <<= (cut * 2);
    for (int i = 0; i < cut; i++) {
      SM_CHUNK_SET_FLAGS(*desc, i, SM_PAYLOAD_NONE);
    }
#if 0
    char *s = QCC_showChunk(p, 0);
    fprintf(stdout, "\n%s\n", s);
    fflush(stdout);
    free(s);
#endif
    // ... and check that our franken-chunk appears to be correct.
    assert(SM_IS_CHUNK_RLE(chunk) == false);
    return QCC_initGenValue(p, 1, QCC_showChunk, QCC_freeChunkValue);
  }
}

extern void populate_map(sparsemap_t *map, int size, int max_value);

QCC_GenValue *
QCC_genSparsemap()
{
  sparsemap_t *map = sparsemap(1024);
  return QCC_initGenValue(map, 1, QCC_showSparsemap, QCC_freeSparsemapValue);
}

static size_t
_tst_sm_chunk_calc_vector_size(uint8_t b)
{
  int count = 0;

  for (int i = 0; i < 4; i++) {
    if (((b >> (i * 2)) & 0x03) == 0x02) {
      count++;
    }
  }

  return count;
}

QCC_TestStatus
_tst_chunk_calc_vector_size_equality(QCC_GenValue **vals, int len, QCC_Stamp **stamp)
{
  unsigned int a = *QCC_getValue(vals, 0, unsigned int *) % 256;
  if (_tst_sm_chunk_calc_vector_size(a) != __sm_chunk_calc_vector_size(a)) {
    return QCC_FAIL;
  }
  return QCC_OK;
}

QCC_TestStatus
_tst_chunk_get_position(QCC_GenValue **vals, int len, QCC_Stamp **stamp)
{
  uint8_t *p = (uint8_t *)QCC_getValue(vals, 0, void *);
  __sm_idx_t start = *(__sm_idx_t *)p;
  __sm_chunk_t *chunk = (__sm_chunk_t *)((uintptr_t)p + SM_SIZEOF_OVERHEAD);
  size_t pos;

  if (SM_IS_CHUNK_RLE(chunk)) {
    for (size_t i = 0; i < SM_FLAGS_PER_INDEX; i++) {
      pos = __sm_chunk_get_position(chunk, i);
      if (pos != 0) {
        return QCC_FAIL;
      }
    }
  } else {
    size_t mixed = 0;
    for (size_t i = 0; i < SM_FLAGS_PER_INDEX; i++) {
      uint8_t flag = SM_CHUNK_GET_FLAGS(*chunk->m_data, i);
      switch (flag) {
      case SM_PAYLOAD_MIXED:
        pos = __sm_chunk_get_position(chunk, i);
        if (chunk->m_data[1 + pos] != (uintptr_t)chunk + pos) {
          return QCC_FAIL;
        }
        mixed++;
        break;
      case SM_PAYLOAD_ONES:
      case SM_PAYLOAD_ZEROS:
        pos = __sm_chunk_get_position(chunk, i);
        if (pos != mixed) {
          return QCC_FAIL;
        }
        break;
      case SM_PAYLOAD_NONE:
      default:
        break;
      }
    }
  }
  return QCC_OK;
}

QCC_TestStatus
_tst_chunk_get_capacity(QCC_GenValue **vals, int len, QCC_Stamp **stamp)
{
  uint8_t *p = (uint8_t *)QCC_getValue(vals, 0, void *);
  __sm_idx_t start = *(__sm_idx_t *)p;
  __sm_chunk_t *chunk = (__sm_chunk_t *)((uintptr_t)p + SM_SIZEOF_OVERHEAD);

  if (SM_IS_CHUNK_RLE(chunk)) {
    if (__sm_chunk_rle_get_length(chunk) != start) {
      return QCC_FAIL;
    }
  } else {
    if (__sm_chunk_get_capacity(chunk) != start) {
      return QCC_FAIL;
    }
  }
  return QCC_OK;
}

QCC_TestStatus
_tst_get_chunk_offset(QCC_GenValue **vals, int len, QCC_Stamp **stamp)
{
  unsigned int idx = *QCC_getValue(vals, 0, unsigned int *);
  sparsemap_t *map = QCC_getValue(vals, 1, sparsemap_t *);
  unsigned int max_offset = ((SM_FLAGS_PER_INDEX - 1) * sizeof(__sm_bitvec_t));
  unsigned int rnd_offset = (idx % max_offset) - ((idx % max_offset) % sizeof(__sm_bitvec_t));
  unsigned int rnd_nvec = rnd_offset / sizeof(__sm_bitvec_t);
  __sm_idx_t offset = __sm_get_chunk_aligned_offset(idx);

  // an empty map should return -1 (no chunks present, so offset of -1)
  for (unsigned int i = offset; i < SM_CHUNK_MAX_CAPACITY + offset; i++) {
    if (__sm_get_chunk_offset(map, idx) != -1) {
      return QCC_FAIL;
    }
  }

  // by setting the first bit in each of rnd_nvec chunks we create one chunk
  // per and with exactly one additional bitvec per so we should observe...
  for (int i = 0; i < rnd_nvec; i++) {
    sparsemap_idx_t l = offset + (i * SM_CHUNK_MAX_CAPACITY);
    sparsemap_set(map, l, true);
  }
  for (int i = 0; i < rnd_nvec; i++) {
    size_t expected_offset = __sm_get_chunk_offset(map, offset + (i * SM_CHUNK_MAX_CAPACITY));
    size_t calculated_offset = i * (SM_SIZEOF_OVERHEAD + (sizeof(__sm_bitvec_t) * 2));
    if (calculated_offset != expected_offset) {
      return QCC_FAIL;
    }
  }

  // now for RLE, first let's clear and check a full chunk
  sparsemap_clear(map);
  for (int i = 0; i < SM_CHUNK_MAX_CAPACITY; i++) {
    sparsemap_set(map, i, true);
  }
  for (int i = 0; i < SM_CHUNK_MAX_CAPACITY; i++) {
    if (__sm_get_chunk_offset(map, i) != 0) {
      return QCC_FAIL;
    }
  }
  if (__sm_get_chunk_offset(map, SM_CHUNK_MAX_CAPACITY) != 0) {
    return QCC_FAIL;
  }

  // this should trigger the transformation of the 0th chunk into RLE
  sparsemap_set(map, SM_CHUNK_MAX_CAPACITY, true);
  if (__sm_get_chunk_offset(map, SM_CHUNK_MAX_CAPACITY) != 0) {
    return QCC_FAIL;
  }
  // this should trigger the transformation of the 0th chunk back to sparse
  sparsemap_set(map, SM_CHUNK_MAX_CAPACITY, false);
  if (__sm_get_chunk_offset(map, SM_CHUNK_MAX_CAPACITY) != 0) {
    return QCC_FAIL;
  }

  // this should trigger the transformation of the 0th chunk into RLE again
  for (int i = 0; i < 3000; i++) {
    sparsemap_set(map, SM_CHUNK_MAX_CAPACITY + i, true);
  }
  // this should trigger the transformation of the 0th chunk back to sparse,
  // but also create a second and third sparse chunks
  sparsemap_set(map, 0, false);
  if (__sm_get_chunk_offset(map, 0) != 0) {
    return QCC_FAIL;
  }
  sparsemap_set(map, 0, true);

  sparsemap_set(map, 129, false);
  if (__sm_get_chunk_offset(map, 129) != 0) {
    return QCC_FAIL;
  }
  sparsemap_set(map, 129, true);

  sparsemap_set(map, 2050, false);
  if (__sm_get_chunk_offset(map, 2050) != 1) {
    return QCC_FAIL;
  }
  sparsemap_set(map, 2050, true);

  sparsemap_set(map, 5048, false);
  if (__sm_get_chunk_offset(map, 5048) != 2) {
    return QCC_FAIL;
  }

  return QCC_OK;
}

#endif