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/* Copyright 2013 Google Inc. All Rights Reserved.
Distributed under MIT license.
See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
*/
/* Utilities for fast computation of logarithms. */
#ifndef BROTLI_ENC_FAST_LOG_H_
#define BROTLI_ENC_FAST_LOG_H_
#include <math.h>
#include "../common/platform.h"
#include <brotli/types.h>
#if defined(__cplusplus) || defined(c_plusplus)
extern "C" {
#endif
static BROTLI_INLINE uint32_t Log2FloorNonZero(size_t n) {
#if defined(BROTLI_BSR32)
return BROTLI_BSR32((uint32_t)n);
#else
uint32_t result = 0;
while (n >>= 1) result++;
return result;
#endif
}
#define BROTLI_LOG2_TABLE_SIZE 256
/* A lookup table for small values of log2(int) to be used in entropy
computation. */
BROTLI_INTERNAL extern const double kBrotliLog2Table[BROTLI_LOG2_TABLE_SIZE];
/* Visual Studio 2012 and Android API levels < 18 do not have the log2()
* function defined, so we use log() and a multiplication instead. */
#if !defined(BROTLI_HAVE_LOG2)
#if ((defined(_MSC_VER) && _MSC_VER <= 1700) || \
(defined(__ANDROID_API__) && __ANDROID_API__ < 18))
#define BROTLI_HAVE_LOG2 0
#else
#define BROTLI_HAVE_LOG2 1
#endif
#endif
#define LOG_2_INV 1.4426950408889634
/* Faster logarithm for small integers, with the property of log2(0) == 0. */
static BROTLI_INLINE double FastLog2(size_t v) {
if (v < BROTLI_LOG2_TABLE_SIZE) {
return kBrotliLog2Table[v];
}
#if !(BROTLI_HAVE_LOG2)
return log((double)v) * LOG_2_INV;
#else
return log2((double)v);
#endif
}
#if defined(__cplusplus) || defined(c_plusplus)
} /* extern "C" */
#endif
#endif /* BROTLI_ENC_FAST_LOG_H_ */
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