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✏️ Editing: random_utils.h

// 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/dsp/cpu.h"
#include "src/utils/bounds_safety.h"
#include "src/webp/types.h"

WEBP_ASSUME_UNSAFE_INDEXABLE_ABI

#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_UBSAN_IGNORE_UNSIGNED_OVERFLOW 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_

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