vpx_dsp/bitreader.c

   1 /*
   2  *  Copyright (c) 2010 The WebM project authors. All Rights Reserved.
   3  *
   4  *  Use of this source code is governed by a BSD-style license
   5  *  that can be found in the LICENSE file in the root of the source
   6  *  tree. An additional intellectual property rights grant can be found
   7  *  in the file PATENTS.  All contributing project authors may
   8  *  be found in the AUTHORS file in the root of the source tree.
   9  */
  10 #include "./bitreader.h"
  11 #include "./prob.h"
  12
  13 #include "vpx_ports/mem.h"
  14 #include "vpx_mem/vpx_mem.h"
  15
  16 int vpx_reader_init(vpx_reader *r,
  17                     const uint8_t *buffer,
  18                     size_t size,
  19                     vpx_decrypt_cb decrypt_cb,
  20                     void *decrypt_state) {
  21   if (size && !buffer) {
  22     return 1;
  23   } else {
  24     r->buffer_end = buffer + size;
  25     r->buffer = buffer;
  26     r->value = 0;
  27     r->count = -8;
  28     r->range = 255;
  29     r->decrypt_cb = decrypt_cb;
  30     r->decrypt_state = decrypt_state;
  31     vpx_reader_fill(r);
  32     return vpx_read_bit(r) != 0;  // marker bit
  33   }
  34 }
  35
  36 void vpx_reader_fill(vpx_reader *r) {
  37   const uint8_t *const buffer_end = r->buffer_end;
  38   const uint8_t *buffer = r->buffer;
  39   const uint8_t *buffer_start = buffer;
  40   BD_VALUE value = r->value;
  41   int count = r->count;
  42   int shift = BD_VALUE_SIZE - CHAR_BIT - (count + CHAR_BIT);
  43   int loop_end = 0;
  44   const size_t bytes_left = buffer_end - buffer;
  45   const size_t bits_left = bytes_left * CHAR_BIT;
  46   const int x = (int)(shift + CHAR_BIT - bits_left);
  47
  48   if (r->decrypt_cb) {
  49     size_t n = MIN(sizeof(r->clear_buffer), bytes_left);
  50     r->decrypt_cb(r->decrypt_state, buffer, r->clear_buffer, (int)n);
  51     buffer = r->clear_buffer;
  52     buffer_start = r->clear_buffer;
  53   }
  54
  55   if (x >= 0) {
  56     count += LOTS_OF_BITS;
  57     loop_end = x;
  58   }
  59
  60   if (x < 0 || bits_left) {
  61     while (shift >= loop_end) {
  62       count += CHAR_BIT;
  63       value |= (BD_VALUE)*buffer++ << shift;
  64       shift -= CHAR_BIT;
  65     }
  66   }
  67
  68   // NOTE: Variable 'buffer' may not relate to 'r->buffer' after decryption,
  69   // so we increase 'r->buffer' by the amount that 'buffer' moved, rather than
  70   // assign 'buffer' to 'r->buffer'.
  71   r->buffer += buffer - buffer_start;
  72   r->value = value;
  73   r->count = count;
  74 }
  75
  76 const uint8_t *vpx_reader_find_end(vpx_reader *r) {
  77   // Find the end of the coded buffer
  78   while (r->count > CHAR_BIT && r->count < BD_VALUE_SIZE) {
  79     r->count -= CHAR_BIT;
  80     r->buffer--;
  81   }
  82   return r->buffer;
  83 }