ref: 49fef904054b668881dfb21e37bcb60d571cb2c5
dir: /vp9/decoder/vp9_reader.c/
/* * 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; }