72 lines
1.5 KiB
C
72 lines
1.5 KiB
C
/*
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* log2.h
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*
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* Created on: Sep 28, 2010
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* Author: sears
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*
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*
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* Methods that use table lookups to compute log base two of integers.
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*
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* These methods are based on the public domain methods at:
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*
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* http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogLookup
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*
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*/
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#ifndef LOG2_H_
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#define LOG2_H_
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#include <stasis/common.h>
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extern const uint8_t LogTable256[256];
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/*Taken from
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http://graphics.stanford.edu/~seander/bithacks.html
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@todo extend to handle unsigned long (this will mean 64bit on 64bit
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platforms; need compiler macro to test for sizeof(long), test
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harness to compare logBase2Slow's output with logBase2's output,
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etc...)
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*/
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/**
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* @param v 32-bit word to find the log of
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* @return lg_2(v)
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*/
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static inline uint8_t stasis_log_2_32(uint32_t v) {
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uint8_t r; // r will be lg(v)
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register uint32_t t, tt; // temporaries
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if ((tt = v >> 16))
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{
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r = ((t = tt >> 8)) ? 24 + LogTable256[t] : 16 + LogTable256[tt];
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}
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else
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{
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r = ((t = v >> 8)) ? 8 + LogTable256[t] : LogTable256[v];
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}
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return r;
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}
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static inline uint8_t stasis_log_2_64(uint64_t v) {
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uint8_t r; // r will be lg(v)
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register uint64_t tt; // temporaries
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if((tt = v >> 32))
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{
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r = stasis_log_2_32(tt) + 32;
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} else {
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r = stasis_log_2_32(v);
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}
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return r;
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}
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static inline uint64_t stasis_round_up_to_power_of_two(uint64_t v) {
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if(v) {
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return 1 << (stasis_log_2_64(v)-1);
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} else {
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return 0;
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}
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}
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#endif /* LOG2_H_ */
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