diff --git a/examples/ex_4.c b/examples/ex_4.c
new file mode 100644
index 0000000..f8e97d9
--- /dev/null
+++ b/examples/ex_4.c
@@ -0,0 +1,192 @@
+#include <assert.h>
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+#include <unistd.h>
+
+#include "../include/sparsemap.h"
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wvariadic-macros"
+#define __diag(...)                                                \
+  do {                                                             \
+    fprintf(stderr, "%s:%d:%s(): ", __FILE__, __LINE__, __func__); \
+    fprintf(stderr, __VA_ARGS__);                                  \
+  } while (0)
+#pragma GCC diagnostic pop
+
+#define SEED
+
+/* https://burtleburtle.net/bob/rand/smallprng.html */
+typedef struct rnd_ctx {
+  uint32_t a;
+  uint32_t b;
+  uint32_t c;
+  uint32_t d;
+} rnd_ctx_t;
+#define __rot(x, k) (((x) << (k)) | ((x) >> (32 - (k))))
+uint32_t
+__random(rnd_ctx_t *x)
+{
+  uint32_t e = x->a - __rot(x->b, 27);
+  x->a = x->b ^ __rot(x->c, 17);
+  x->b = x->c + x->d;
+  x->c = x->d + e;
+  x->d = e + x->a;
+  return x->d;
+}
+
+void
+__random_seed(rnd_ctx_t *x, uint32_t seed)
+{
+  uint32_t i;
+  x->a = 0xf1ea5eed, x->b = x->c = x->d = seed;
+  for (i = 0; i < 20; ++i) {
+    (void)__random(x);
+  }
+}
+
+void
+shuffle(rnd_ctx_t *prng, int *array, size_t n)
+{
+  size_t i, j;
+
+  if (n > 1) {
+    for (i = n - 1; i > 0; i--) {
+      j = (unsigned int)(__random(prng) % (i + 1));
+      // XOR swap algorithm
+      if (i != j) { // avoid self-swap leading to zero-ing the element
+        array[i] = array[i] ^ array[j];
+        array[j] = array[i] ^ array[j];
+        array[i] = array[i] ^ array[j];
+      }
+    }
+  }
+}
+
+bool
+was_set(size_t bit, int array[])
+{
+  for (int i = 0; i < 1024; i++) {
+    if (array[i] == bit)
+      return true;
+  }
+  return false;
+}
+
+#define TEST_ARRAY_SIZE 1024
+
+int
+main(void)
+{
+  int i = 0;
+  size_t rank;
+  rnd_ctx_t prng;
+  int array[TEST_ARRAY_SIZE];
+
+  // disable buffering
+  setbuf(stderr, 0);
+
+  // start with a 1KiB buffer, TEST_ARRAY_SIZE bits
+  uint8_t *buf = calloc(TEST_ARRAY_SIZE, sizeof(uint8_t));
+
+  // create the sparse bitmap
+  sparsemap_t *map = sparsemap(buf, sizeof(uint8_t) * TEST_ARRAY_SIZE, 0);
+
+  for (i = 0; i < 8; i++)
+    sparsemap_set(map, i, true);
+  rank = sparsemap_rank(map, 0, 8);
+  __diag("rank was %lu at offset 0\n", rank);
+  assert(rank == 8);
+  for (i = 0; i < 8; i++) {
+    bool set = sparsemap_is_set(map, i);
+    if (set)
+      __diag("verified %d was set, %s\n", i,
+        was_set(i, array) ? "but we thought it was" : "because it wasn't");
+    else
+      __diag("darn, %d was not really set, %s\n", i,
+        was_set(i, array) ? "but we thought it was" : "because it wasn't");
+  }
+  __diag("and %d was %s", i, sparsemap_is_set(map, i + 1) ? "set" : "not set");
+  rank = sparsemap_span(map, 0, 8);
+  __diag("span was found at %lu\n", rank);
+  sparsemap_clear(map);
+
+  for (i = 1; i < 9; i++)
+    sparsemap_set(map, i, true);
+  rank = sparsemap_rank(map, 0, 10);
+  __diag("rank was %lu at offset 1\n", rank);
+  assert(rank == 8);
+  for (i = 100; i < 108; i++) {
+    bool set = sparsemap_is_set(map, i);
+    if (set)
+      __diag("verified %d was set, %s\n", i,
+        was_set(i, array) ? "but knew that" : "because it wasn't");
+    else
+      __diag("darn, %d was not set, %s\n", i,
+        was_set(i, array) ? "and yet we did set it" : "because it wasn't");
+  }
+  __diag("and %d was %s", i, sparsemap_is_set(map, i + 1) ? "set" : "not set");
+  rank = sparsemap_rank(map, 109, 8);
+  __diag("rank was %lu at offset 109\n", rank);
+  rank = sparsemap_span(map, 0, 8);
+  __diag("span was found at %lu\n", rank);
+  sparsemap_clear(map);
+
+  for (i = 2049; i < 2057; i++)
+    sparsemap_set(map, i, true);
+  rank = sparsemap_rank(map, 2048, 8);
+  __diag("rank was %lu at offset 108\n", rank);
+  assert(rank == 8);
+  for (i = 100; i < 108; i++) {
+    bool set = sparsemap_is_set(map, i);
+    if (set)
+      __diag("verified %d was set, %s\n", i,
+        was_set(i, array) ? "but we thought it was" : "because it wasn't");
+    else
+      __diag("darn, %d was not really set, %s\n", i,
+        was_set(i, array) ? "but we thought it was" : "because it wasn't");
+  }
+  __diag("and %d was %s", i, sparsemap_is_set(map, i + 1) ? "set" : "not set");
+  rank = sparsemap_rank(map, 2048, 2060);
+  __diag("rank was %lu at offset 109\n", rank);
+  rank = sparsemap_span(map, 2048, 8);
+  __diag("span was found at %lu\n", rank);
+  sparsemap_clear(map);
+
+  // seed the PRNG
+#ifdef SEED
+  __random_seed(&prng, 8675309);
+#else
+  __random_seed(&prng, (unsigned int)time(NULL) ^ getpid());
+#endif
+
+  for (i = 0; i < TEST_ARRAY_SIZE; i++) {
+    array[i] = (int)__random(&prng) % 7000 + 1;
+    if (array[i] < 0)
+      i--;
+  }
+  // randomize setting the bits on
+  shuffle(&prng, array, TEST_ARRAY_SIZE);
+
+  // set all the bits on in a random order
+  for (i = 0; i < TEST_ARRAY_SIZE; i++) {
+    //__diag("set %d\n", array[i]);
+    sparsemap_set(map, array[i], true);
+    assert(sparsemap_is_set(map, array[i]) == true);
+  }
+
+  size_t l = sparsemap_span(map, 0, 8);
+  __diag("found span of 8 at %lu starting from 0\n", l);
+  for (i = l; i < l + 8; i++) {
+    bool set = sparsemap_is_set(map, l + i);
+    if (set)
+      __diag("verified %lu was set\n", l + i);
+    else
+      __diag("darn, %lu was not really set, %s\n", l + i,
+        was_set(l + i, array) ? "but we thought it was" : "because it wasn't");
+  }
+
+  return 0;
+}
diff --git a/include/sparsemap.h b/include/sparsemap.h
index 23d32b9..786e642 100644
--- a/include/sparsemap.h
+++ b/include/sparsemap.h
@@ -118,4 +118,6 @@ size_t sparsemap_select(sparsemap_t *map, size_t n);
 /* Counts the set bits in the range [offset, idx]. */
 size_t sparsemap_rank(sparsemap_t *map, size_t offset, size_t idx);
 
+size_t sparsemap_span(sparsemap_t *map, size_t loc, size_t len);
+
 #endif