107 lines
3.4 KiB
C
107 lines
3.4 KiB
C
/*
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* Copyright (c) 2010 The WebM project authors. All Rights Reserved.
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*
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* Use of this source code is governed by a BSD-style license
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* that can be found in the LICENSE file in the root of the source
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* tree. An additional intellectual property rights grant can be found
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* in the file PATENTS. All contributing project authors may
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* be found in the AUTHORS file in the root of the source tree.
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*/
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#include "vpx_ports/mem.h"
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#include "vpx_mem/vpx_mem.h"
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#include "vp9/decoder/vp9_reader.h"
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// This is meant to be a large, positive constant that can still be efficiently
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// loaded as an immediate (on platforms like ARM, for example).
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// Even relatively modest values like 100 would work fine.
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#define LOTS_OF_BITS 0x40000000
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int vp9_reader_init(vp9_reader *r,
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const uint8_t *buffer,
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size_t size,
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vpx_decrypt_cb decrypt_cb,
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void *decrypt_state) {
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if (size && !buffer) {
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return 1;
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} else {
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r->buffer_end = buffer + size;
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r->buffer = buffer;
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r->value = 0;
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r->count = -8;
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r->range = 255;
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r->decrypt_cb = decrypt_cb;
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r->decrypt_state = decrypt_state;
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vp9_reader_fill(r);
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return vp9_read_bit(r) != 0; // marker bit
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}
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}
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void vp9_reader_fill(vp9_reader *r) {
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const uint8_t *const buffer_end = r->buffer_end;
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const uint8_t *buffer = r->buffer;
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const uint8_t *buffer_start = buffer;
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BD_VALUE value = r->value;
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int count = r->count;
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int shift = BD_VALUE_SIZE - CHAR_BIT - (count + CHAR_BIT);
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int loop_end = 0;
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const size_t bytes_left = buffer_end - buffer;
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const size_t bits_left = bytes_left * CHAR_BIT;
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const int x = (int)(shift + CHAR_BIT - bits_left);
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if (r->decrypt_cb) {
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size_t n = MIN(sizeof(r->clear_buffer), bytes_left);
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r->decrypt_cb(r->decrypt_state, buffer, r->clear_buffer, (int)n);
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buffer = r->clear_buffer;
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buffer_start = r->clear_buffer;
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}
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if (x >= 0) {
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count += LOTS_OF_BITS;
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loop_end = x;
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}
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if (x < 0 || bits_left) {
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while (shift >= loop_end) {
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count += CHAR_BIT;
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value |= (BD_VALUE)*buffer++ << shift;
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shift -= CHAR_BIT;
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}
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}
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// NOTE: Variable 'buffer' may not relate to 'r->buffer' after decryption,
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// so we increase 'r->buffer' by the amount that 'buffer' moved, rather than
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// assign 'buffer' to 'r->buffer'.
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r->buffer += buffer - buffer_start;
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r->value = value;
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r->count = count;
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}
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const uint8_t *vp9_reader_find_end(vp9_reader *r) {
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// Find the end of the coded buffer
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while (r->count > CHAR_BIT && r->count < BD_VALUE_SIZE) {
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r->count -= CHAR_BIT;
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r->buffer--;
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}
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return r->buffer;
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}
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int vp9_reader_has_error(vp9_reader *r) {
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// Check if we have reached the end of the buffer.
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//
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// Variable 'count' stores the number of bits in the 'value' buffer, minus
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// 8. The top byte is part of the algorithm, and the remainder is buffered
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// to be shifted into it. So if count == 8, the top 16 bits of 'value' are
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// occupied, 8 for the algorithm and 8 in the buffer.
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//
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// When reading a byte from the user's buffer, count is filled with 8 and
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// one byte is filled into the value buffer. When we reach the end of the
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// data, count is additionally filled with LOTS_OF_BITS. So when
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// count == LOTS_OF_BITS - 1, the user's data has been exhausted.
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//
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// 1 if we have tried to decode bits after the end of stream was encountered.
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// 0 No error.
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return r->count > BD_VALUE_SIZE && r->count < LOTS_OF_BITS;
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}
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