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/*
* 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;
}
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