vpx/vp8/decoder/dboolhuff.h

/*
 *  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.
 */


#ifndef DBOOLHUFF_H_
#define DBOOLHUFF_H_

#include <stddef.h>
#include <limits.h>

#include "vpx_config.h"
#include "vpx_ports/mem.h"
#include "vpx/vpx_integer.h"

typedef size_t VP8_BD_VALUE;

#define VP8_BD_VALUE_SIZE ((int)sizeof(VP8_BD_VALUE)*CHAR_BIT)

/*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 VP8_LOTS_OF_BITS (0x40000000)

static unsigned char decrypt_byte(const unsigned char *ch,
                                  const unsigned char *origin,
                                  const unsigned char *key)
{
#if CONFIG_DECRYPT
    const int offset = (int)(ch - origin);
    return *ch ^ key[offset % 32];  // VP8_DECRYPT_KEY_SIZE
#else
    return *ch;
#endif
}

typedef struct
{
    const unsigned char *user_buffer_end;
    const unsigned char *user_buffer;
    VP8_BD_VALUE         value;
    int                  count;
    unsigned int         range;
    const unsigned char *origin;
    const unsigned char *key;
} BOOL_DECODER;

DECLARE_ALIGNED(16, extern const unsigned char, vp8_norm[256]);

int vp8dx_start_decode(BOOL_DECODER *br,
                       const unsigned char *source,
                       unsigned int source_sz,
                       const unsigned char *origin,
                       const unsigned char *key);

void vp8dx_bool_decoder_fill(BOOL_DECODER *br);


static int vp8dx_decode_bool(BOOL_DECODER *br, int probability) {
    unsigned int bit = 0;
    VP8_BD_VALUE value;
    unsigned int split;
    VP8_BD_VALUE bigsplit;
    int count;
    unsigned int range;

    split = 1 + (((br->range - 1) * probability) >> 8);

    if(br->count < 0)
        vp8dx_bool_decoder_fill(br);

    value = br->value;
    count = br->count;

    bigsplit = (VP8_BD_VALUE)split << (VP8_BD_VALUE_SIZE - 8);

    range = split;

    if (value >= bigsplit)
    {
        range = br->range - split;
        value = value - bigsplit;
        bit = 1;
    }

    {
        register unsigned int shift = vp8_norm[range];
        range <<= shift;
        value <<= shift;
        count -= shift;
    }
    br->value = value;
    br->count = count;
    br->range = range;

    return bit;
}

static int vp8_decode_value(BOOL_DECODER *br, int bits)
{
    int z = 0;
    int bit;

    for (bit = bits - 1; bit >= 0; bit--)
    {
        z |= (vp8dx_decode_bool(br, 0x80) << bit);
    }

    return z;
}

static int vp8dx_bool_error(BOOL_DECODER *br)
{
    /* 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 VP8_LOTS_OF_BITS. So when
     * count == VP8_LOTS_OF_BITS - 1, the user's data has been exhausted.
     */
    if ((br->count > VP8_BD_VALUE_SIZE) && (br->count < VP8_LOTS_OF_BITS))
    {
       /* We have tried to decode bits after the end of
        * stream was encountered.
        */
        return 1;
    }

    /* No error. */
    return 0;
}

#endif  // DBOOLHUFF_H_