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 11#include "vpx_ports/mem.h" 12#include "vpx_mem/vpx_mem.h" 13 14#include "vp9/decoder/vp9_reader.h" 15 16// This is meant to be a large, positive constant that can still be efficiently 17// loaded as an immediate (on platforms like ARM, for example). 18// Even relatively modest values like 100 would work fine. 19#define LOTS_OF_BITS 0x40000000 20 21int vp9_reader_init(vp9_reader *r, const uint8_t *buffer, size_t size) { 22 if (size && !buffer) { 23 return 1; 24 } else { 25 r->buffer_end = buffer + size; 26 r->buffer = buffer; 27 r->value = 0; 28 r->count = -8; 29 r->range = 255; 30 vp9_reader_fill(r); 31 return vp9_read_bit(r) != 0; // marker bit 32 } 33} 34 35void vp9_reader_fill(vp9_reader *r) { 36 const uint8_t *const buffer_end = r->buffer_end; 37 const uint8_t *buffer = r->buffer; 38 BD_VALUE value = r->value; 39 int count = r->count; 40 int shift = BD_VALUE_SIZE - CHAR_BIT - (count + CHAR_BIT); 41 int loop_end = 0; 42 const int bits_left = (int)((buffer_end - buffer) * CHAR_BIT); 43 const int x = shift + CHAR_BIT - bits_left; 44 45 if (x >= 0) { 46 count += LOTS_OF_BITS; 47 loop_end = x; 48 } 49 50 if (x < 0 || bits_left) { 51 while (shift >= loop_end) { 52 count += CHAR_BIT; 53 value |= (BD_VALUE)*buffer++ << shift; 54 shift -= CHAR_BIT; 55 } 56 } 57 58 r->buffer = buffer; 59 r->value = value; 60 r->count = count; 61} 62 63const uint8_t *vp9_reader_find_end(vp9_reader *r) { 64 // Find the end of the coded buffer 65 while (r->count > CHAR_BIT && r->count < BD_VALUE_SIZE) { 66 r->count -= CHAR_BIT; 67 r->buffer--; 68 } 69 return r->buffer; 70} 71 72int vp9_reader_has_error(vp9_reader *r) { 73 // Check if we have reached the end of the buffer. 74 // 75 // Variable 'count' stores the number of bits in the 'value' buffer, minus 76 // 8. The top byte is part of the algorithm, and the remainder is buffered 77 // to be shifted into it. So if count == 8, the top 16 bits of 'value' are 78 // occupied, 8 for the algorithm and 8 in the buffer. 79 // 80 // When reading a byte from the user's buffer, count is filled with 8 and 81 // one byte is filled into the value buffer. When we reach the end of the 82 // data, count is additionally filled with LOTS_OF_BITS. So when 83 // count == LOTS_OF_BITS - 1, the user's data has been exhausted. 84 // 85 // 1 if we have tried to decode bits after the end of stream was encountered. 86 // 0 No error. 87 return r->count > BD_VALUE_SIZE && r->count < LOTS_OF_BITS; 88} 89