/* *  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. */ #include "vpx_ports/mem.h"#include "vpx_mem/vpx_mem.h" #include "vp9/decoder/vp9_reader.h" // 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 LOTS_OF_BITS 0x40000000 int vp9_reader_init(vp9_reader *r,                    const uint8_t *buffer,                    size_t size,                    vpx_decrypt_cb decrypt_cb,                    void *decrypt_state) {  if (size && !buffer) {    return 1;  } else {    r->buffer_end = buffer + size;    r->buffer = buffer;    r->value = 0;    r->count = -8;    r->range = 255;    r->decrypt_cb = decrypt_cb;    r->decrypt_state = decrypt_state;    vp9_reader_fill(r);    return vp9_read_bit(r) != 0;  // marker bit  }} void vp9_reader_fill(vp9_reader *r) {  const uint8_t *const buffer_end = r->buffer_end;  const uint8_t *buffer = r->buffer;  const uint8_t *buffer_start = buffer;  BD_VALUE value = r->value;  int count = r->count;  int shift = BD_VALUE_SIZE - CHAR_BIT - (count + CHAR_BIT);  int loop_end = 0;  const size_t bytes_left = buffer_end - buffer;  const size_t bits_left = bytes_left * CHAR_BIT;  const int x = (int)(shift + CHAR_BIT - bits_left);   if (r->decrypt_cb) {    size_t n = MIN(sizeof(r->clear_buffer), bytes_left);    r->decrypt_cb(r->decrypt_state, buffer, r->clear_buffer, (int)n);    buffer = r->clear_buffer;    buffer_start = r->clear_buffer;  }   if (x >= 0) {    count += LOTS_OF_BITS;    loop_end = x;  }   if (x < 0 || bits_left) {    while (shift >= loop_end) {      count += CHAR_BIT;      value |= (BD_VALUE)*buffer++ << shift;      shift -= CHAR_BIT;    }  }   // NOTE: Variable 'buffer' may not relate to 'r->buffer' after decryption,  // so we increase 'r->buffer' by the amount that 'buffer' moved, rather than  // assign 'buffer' to 'r->buffer'.  r->buffer += buffer - buffer_start;  r->value = value;  r->count = count;} const uint8_t *vp9_reader_find_end(vp9_reader *r) {  // Find the end of the coded buffer  while (r->count > CHAR_BIT && r->count < BD_VALUE_SIZE) {    r->count -= CHAR_BIT;    r->buffer--;  }  return r->buffer;} int vp9_reader_has_error(vp9_reader *r) {  // 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 LOTS_OF_BITS. So when  // count == LOTS_OF_BITS - 1, the user's data has been exhausted.  //  // 1 if we have tried to decode bits after the end of stream was encountered.  // 0 No error.  return r->count > BD_VALUE_SIZE && r->count < LOTS_OF_BITS;}