// 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_ */