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      1 // Copyright 2013 Google Inc. All Rights Reserved.
      2 //
      3 // Use of this source code is governed by a BSD-style license
      4 // that can be found in the COPYING file in the root of the source
      5 // tree. An additional intellectual property rights grant can be found
      6 // in the file PATENTS. All contributing project authors may
      7 // be found in the AUTHORS file in the root of the source tree.
      8 // -----------------------------------------------------------------------------
      9 //
     10 // Pseudo-random utilities
     11 //
     12 // Author: Skal (pascal.massimino (at) gmail.com)
     13 
     14 #ifndef WEBP_UTILS_RANDOM_UTILS_H_
     15 #define WEBP_UTILS_RANDOM_UTILS_H_
     16 
     17 #include <assert.h>
     18 #include "src/webp/types.h"
     19 
     20 #ifdef __cplusplus
     21 extern "C" {
     22 #endif
     23 
     24 #define VP8_RANDOM_DITHER_FIX 8   // fixed-point precision for dithering
     25 #define VP8_RANDOM_TABLE_SIZE 55
     26 
     27 typedef struct {
     28   int index1_, index2_;
     29   uint32_t tab_[VP8_RANDOM_TABLE_SIZE];
     30   int amp_;
     31 } VP8Random;
     32 
     33 // Initializes random generator with an amplitude 'dithering' in range [0..1].
     34 void VP8InitRandom(VP8Random* const rg, float dithering);
     35 
     36 // Returns a centered pseudo-random number with 'num_bits' amplitude.
     37 // (uses D.Knuth's Difference-based random generator).
     38 // 'amp' is in VP8_RANDOM_DITHER_FIX fixed-point precision.
     39 static WEBP_INLINE int VP8RandomBits2(VP8Random* const rg, int num_bits,
     40                                       int amp) {
     41   int diff;
     42   assert(num_bits + VP8_RANDOM_DITHER_FIX <= 31);
     43   diff = rg->tab_[rg->index1_] - rg->tab_[rg->index2_];
     44   if (diff < 0) diff += (1u << 31);
     45   rg->tab_[rg->index1_] = diff;
     46   if (++rg->index1_ == VP8_RANDOM_TABLE_SIZE) rg->index1_ = 0;
     47   if (++rg->index2_ == VP8_RANDOM_TABLE_SIZE) rg->index2_ = 0;
     48   // sign-extend, 0-center
     49   diff = (int)((uint32_t)diff << 1) >> (32 - num_bits);
     50   diff = (diff * amp) >> VP8_RANDOM_DITHER_FIX;  // restrict range
     51   diff += 1 << (num_bits - 1);                   // shift back to 0.5-center
     52   return diff;
     53 }
     54 
     55 static WEBP_INLINE int VP8RandomBits(VP8Random* const rg, int num_bits) {
     56   return VP8RandomBits2(rg, num_bits, rg->amp_);
     57 }
     58 
     59 #ifdef __cplusplus
     60 }    // extern "C"
     61 #endif
     62 
     63 #endif  /* WEBP_UTILS_RANDOM_UTILS_H_ */
     64