16-bit integer logarithm

int_log2_16bit.h

#include <inttypes.h>
#include <stdio.h>


static inline uint8_t bitwise_ceil8(uint8_t x) {
	x |= x >> 1;
	x |= x >> 2;
	x |= x >> 4;
	return x;
}

static inline uint16_t bitwise_ceil16(uint16_t x) {
	x |= x >> 1;
	x |= x >> 2;
	x |= x >> 4;
	x |= x >> 8;
	return x;
}

static inline uint8_t int_log2_8bit(uint8_t const x) {
	size_t const de_bruijn_tab[/** 8 */] = {
		 0, 5, 1, 6, 4, 3, 2, 7,
	};
	enum { DeBruijnLog2Magic = 0x1Du };
	uint8_t const seq = bitwise_ceil8(x) * (uint8_t)DeBruijnLog2Magic;
	return de_bruijn_tab[seq >> 5];
}

static inline uint16_t int_log2_16bit(uint16_t const x) {
	size_t const de_bruijn_tab[/** 16 */] = {
		 0,  1, 4, 10, 2,  8,  5, 14,
		11,  3, 7,  9, 6, 12, 11, 15,
	};
	enum { DeBruijnLog2Magic = 0x09AFu };
	uint16_t const seq = bitwise_ceil16(x) * (uint16_t)DeBruijnLog2Magic;
	return de_bruijn_tab[seq >> 12];
}