random_utils.h - mozsearch

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// Copyright 2013 Google Inc. All Rights Reserved.

//

// Use of this source code is governed by a BSD-style license

// that can be found in the COPYING file in the root of the source

// tree. An additional intellectual property rights grant can be found

// in the file PATENTS. All contributing project authors may

// be found in the AUTHORS file in the root of the source tree.

// -----------------------------------------------------------------------------

//

// Pseudo-random utilities

//

// Author: Skal (pascal.massimino@gmail.com)

#ifndef WEBP_UTILS_RANDOM_UTILS_H_

#define WEBP_UTILS_RANDOM_UTILS_H_

#include <assert.h>

#include "src/webp/types.h"

#ifdef __cplusplus

extern "C" {

#endif

#define VP8_RANDOM_DITHER_FIX 8   // fixed-point precision for dithering

#define VP8_RANDOM_TABLE_SIZE 55

typedef struct {

  int index1_, index2_;

  uint32_t tab_[VP8_RANDOM_TABLE_SIZE];

  int amp_;

} VP8Random;

// Initializes random generator with an amplitude 'dithering' in range [0..1].

void VP8InitRandom(VP8Random* const rg, float dithering);

// Returns a centered pseudo-random number with 'num_bits' amplitude.

// (uses D.Knuth's Difference-based random generator).

// 'amp' is in VP8_RANDOM_DITHER_FIX fixed-point precision.

static WEBP_INLINE int VP8RandomBits2(VP8Random* const rg, int num_bits,

                                      int amp) {

  int diff;

  assert(num_bits + VP8_RANDOM_DITHER_FIX <= 31);

  diff = rg->tab_[rg->index1_] - rg->tab_[rg->index2_];

  if (diff < 0) diff += (1u << 31);

  rg->tab_[rg->index1_] = diff;

  if (++rg->index1_ == VP8_RANDOM_TABLE_SIZE) rg->index1_ = 0;

  if (++rg->index2_ == VP8_RANDOM_TABLE_SIZE) rg->index2_ = 0;

  // sign-extend, 0-center

  diff = (int)((uint32_t)diff << 1) >> (32 - num_bits);

  diff = (diff * amp) >> VP8_RANDOM_DITHER_FIX;  // restrict range

  diff += 1 << (num_bits - 1);                   // shift back to 0.5-center

  return diff;

static WEBP_INLINE int VP8RandomBits(VP8Random* const rg, int num_bits) {

  return VP8RandomBits2(rg, num_bits, rg->amp_);

#ifdef __cplusplus

}    // extern "C"

#endif

#endif  // WEBP_UTILS_RANDOM_UTILS_H_