155 строки
4.3 KiB
C
155 строки
4.3 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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#ifndef DBOOLHUFF_H
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#define DBOOLHUFF_H
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#include <stddef.h>
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#include <limits.h>
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#include "vpx_config.h"
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#include "vpx_ports/mem.h"
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#include "vpx/vpx_integer.h"
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typedef size_t VP8_BD_VALUE;
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# define VP8_BD_VALUE_SIZE ((int)sizeof(VP8_BD_VALUE)*CHAR_BIT)
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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 VP8_LOTS_OF_BITS (0x40000000)
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typedef struct
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{
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const unsigned char *user_buffer_end;
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const unsigned char *user_buffer;
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VP8_BD_VALUE value;
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int count;
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unsigned int range;
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} BOOL_DECODER;
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DECLARE_ALIGNED(16, extern const unsigned char, vp8_norm[256]);
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int vp8dx_start_decode(BOOL_DECODER *br,
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const unsigned char *source,
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unsigned int source_sz);
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void vp8dx_bool_decoder_fill(BOOL_DECODER *br);
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/*The refill loop is used in several places, so define it in a macro to make
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sure they're all consistent.
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An inline function would be cleaner, but has a significant penalty, because
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multiple BOOL_DECODER fields must be modified, and the compiler is not smart
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enough to eliminate the stores to those fields and the subsequent reloads
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from them when inlining the function.*/
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#define VP8DX_BOOL_DECODER_FILL(_count,_value,_bufptr,_bufend) \
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do \
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{ \
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int shift = VP8_BD_VALUE_SIZE - 8 - ((_count) + 8); \
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int loop_end, x; \
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size_t bits_left = ((_bufend)-(_bufptr))*CHAR_BIT; \
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\
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x = (int)(shift + CHAR_BIT - bits_left); \
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loop_end = 0; \
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if(x >= 0) \
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{ \
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(_count) += VP8_LOTS_OF_BITS; \
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loop_end = x; \
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if(!bits_left) break; \
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} \
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while(shift >= loop_end) \
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{ \
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(_count) += CHAR_BIT; \
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(_value) |= (VP8_BD_VALUE)*(_bufptr)++ << shift; \
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shift -= CHAR_BIT; \
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} \
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} \
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while(0) \
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static int vp8dx_decode_bool(BOOL_DECODER *br, int probability) {
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unsigned int bit = 0;
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VP8_BD_VALUE value;
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unsigned int split;
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VP8_BD_VALUE bigsplit;
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int count;
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unsigned int range;
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split = 1 + (((br->range - 1) * probability) >> 8);
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if(br->count < 0)
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vp8dx_bool_decoder_fill(br);
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value = br->value;
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count = br->count;
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bigsplit = (VP8_BD_VALUE)split << (VP8_BD_VALUE_SIZE - 8);
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range = split;
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if (value >= bigsplit)
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{
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range = br->range - split;
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value = value - bigsplit;
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bit = 1;
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}
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{
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register unsigned int shift = vp8_norm[range];
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range <<= shift;
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value <<= shift;
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count -= shift;
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}
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br->value = value;
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br->count = count;
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br->range = range;
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return bit;
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}
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static int vp8_decode_value(BOOL_DECODER *br, int bits)
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{
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int z = 0;
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int bit;
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for (bit = bits - 1; bit >= 0; bit--)
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{
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z |= (vp8dx_decode_bool(br, 0x80) << bit);
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}
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return z;
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}
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static int vp8dx_bool_error(BOOL_DECODER *br)
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{
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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 VP8_LOTS_OF_BITS. So when
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* count == VP8_LOTS_OF_BITS - 1, the user's data has been exhausted.
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*/
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if ((br->count > VP8_BD_VALUE_SIZE) && (br->count < VP8_LOTS_OF_BITS))
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{
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/* We have tried to decode bits after the end of
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* stream was encountered.
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*/
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return 1;
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}
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/* No error. */
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return 0;
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}
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#endif
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