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_dboolhuff.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 21 22 int vp9_reader_init(vp9_reader *r, const uint8_t *buffer, size_t size) { 23 int marker_bit; 24 25 r->buffer_end = buffer + size; 26 r->buffer = buffer; 27 r->value = 0; 28 r->count = -8; 29 r->range = 255; 30 31 if (size && !buffer) 32 return 1; 33 34 vp9_reader_fill(r); 35 marker_bit = vp9_read_bit(r); 36 return marker_bit != 0; 37 } 38 39 void vp9_reader_fill(vp9_reader *r) { 40 const uint8_t *const buffer_end = r->buffer_end; 41 const uint8_t *buffer = r->buffer; 42 VP9_BD_VALUE value = r->value; 43 int count = r->count; 44 int shift = BD_VALUE_SIZE - 8 - (count + 8); 45 int loop_end = 0; 46 const int bits_left = (int)((buffer_end - buffer)*CHAR_BIT); 47 const int x = shift + CHAR_BIT - bits_left; 48 49 if (x >= 0) { 50 count += LOTS_OF_BITS; 51 loop_end = x; 52 } 53 54 if (x < 0 || bits_left) { 55 while (shift >= loop_end) { 56 count += CHAR_BIT; 57 value |= (VP9_BD_VALUE)*buffer++ << shift; 58 shift -= CHAR_BIT; 59 } 60 } 61 62 r->buffer = buffer; 63 r->value = value; 64 r->count = count; 65 } 66 67 const uint8_t *vp9_reader_find_end(vp9_reader *r) { 68 // Find the end of the coded buffer 69 while (r->count > CHAR_BIT && r->count < BD_VALUE_SIZE) { 70 r->count -= CHAR_BIT; 71 r->buffer--; 72 } 73 return r->buffer; 74 } 75 76 int vp9_reader_has_error(vp9_reader *r) { 77 // Check if we have reached the end of the buffer. 78 // 79 // Variable 'count' stores the number of bits in the 'value' buffer, minus 80 // 8. The top byte is part of the algorithm, and the remainder is buffered 81 // to be shifted into it. So if count == 8, the top 16 bits of 'value' are 82 // occupied, 8 for the algorithm and 8 in the buffer. 83 // 84 // When reading a byte from the user's buffer, count is filled with 8 and 85 // one byte is filled into the value buffer. When we reach the end of the 86 // data, count is additionally filled with LOTS_OF_BITS. So when 87 // count == LOTS_OF_BITS - 1, the user's data has been exhausted. 88 // 89 // 1 if we have tried to decode bits after the end of stream was encountered. 90 // 0 No error. 91 return r->count > BD_VALUE_SIZE && r->count < LOTS_OF_BITS; 92 } 93
