/src/libvpx/vp8/decoder/dboolhuff.h

Source
/*
 *  Copyright (c) 2010 The WebM project authors. All Rights Reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE 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.
 */

#ifndef VPX_VP8_DECODER_DBOOLHUFF_H_
#define VPX_VP8_DECODER_DBOOLHUFF_H_

#include <stddef.h>
#include <limits.h>

#include "./vpx_config.h"
#include "vpx_ports/compiler_attributes.h"
#include "vpx_ports/mem.h"
#include "vpx/vp8dx.h"
#include "vpx/vpx_integer.h"

#ifdef __cplusplus
extern "C" {
#endif

typedef size_t VP8_BD_VALUE;

#define VP8_BD_VALUE_SIZE ((int)sizeof(VP8_BD_VALUE) * CHAR_BIT)

/*This is meant to be a large, positive constant that can still be efficiently
   loaded as an immediate (on platforms like ARM, for example).
  Even relatively modest values like 100 would work fine.*/
#define VP8_LOTS_OF_BITS (0x40000000)

typedef struct {
  const unsigned char *user_buffer_end;
  const unsigned char *user_buffer;
  VP8_BD_VALUE value;
  int count;
  unsigned int range;
  vpx_decrypt_cb decrypt_cb;
  void *decrypt_state;
} BOOL_DECODER;

DECLARE_ALIGNED(16, extern const unsigned char, vp8_norm[256]);

int vp8dx_start_decode(BOOL_DECODER *br, const unsigned char *source,
                       unsigned int source_sz, vpx_decrypt_cb decrypt_cb,
                       void *decrypt_state);

void vp8dx_bool_decoder_fill(BOOL_DECODER *br);

static VPX_NO_UNSIGNED_SHIFT_CHECK int vp8dx_decode_bool(BOOL_DECODER *br,
                                                         int probability) {
  unsigned int bit = 0;
  VP8_BD_VALUE value;
  unsigned int split;
  VP8_BD_VALUE bigsplit;
  int count;
  unsigned int range;

  split = 1 + (((br->range - 1) * probability) >> 8);

  if (br->count < 0) vp8dx_bool_decoder_fill(br);

  value = br->value;
  count = br->count;

  bigsplit = (VP8_BD_VALUE)split << (VP8_BD_VALUE_SIZE - 8);

  range = split;

  if (value >= bigsplit) {
    range = br->range - split;
    value = value - bigsplit;
    bit = 1;
  }

  {
    const unsigned char shift = vp8_norm[(unsigned char)range];
    range <<= shift;
    value <<= shift;
    count -= shift;
  }
  br->value = value;
  br->count = count;
  br->range = range;

  return bit;
}

static INLINE int vp8_decode_value(BOOL_DECODER *br, int bits) {
  int z = 0;
  int bit;

  for (bit = bits - 1; bit >= 0; bit--) {
    z |= (vp8dx_decode_bool(br, 0x80) << bit);
  }

  return z;
}

static INLINE int vp8dx_bool_error(BOOL_DECODER *br) {
  /* Check if we have reached the end of the buffer.
   *
   * Variable 'count' stores the number of bits in the 'value' buffer, minus
   * 8. The top byte is part of the algorithm, and the remainder is buffered
   * to be shifted into it. So if count == 8, the top 16 bits of 'value' are
   * occupied, 8 for the algorithm and 8 in the buffer.
   *
   * When reading a byte from the user's buffer, count is filled with 8 and
   * one byte is filled into the value buffer. When we reach the end of the
   * data, count is additionally filled with VP8_LOTS_OF_BITS. So when
   * count == VP8_LOTS_OF_BITS - 1, the user's data has been exhausted.
   */
  if ((br->count > VP8_BD_VALUE_SIZE) && (br->count < VP8_LOTS_OF_BITS)) {
    /* We have tried to decode bits after the end of
     * stream was encountered.
     */
    return 1;
  }

  /* No error. */
  return 0;
}

#ifdef __cplusplus
}  // extern "C"
#endif

#endif  // VPX_VP8_DECODER_DBOOLHUFF_H_

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Created: 2026-01-16 07:48