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