1 files changed, 63 insertions, 0 deletions
diff --git a/media/libwebp/utils/random_utils.h b/media/libwebp/utils/random_utils.h
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+++ b/media/libwebp/utils/random_utils.h
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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_H_
+#define WEBP_UTILS_RANDOM_H_
+
+#include <assert.h>
+#include "../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_H_ */