ffmpeg/libavcodec/4xm.c

/*
 * 4XM codec
 * Copyright (c) 2003 Michael Niedermayer
 *
 * This file is part of FFmpeg.
 *
 * FFmpeg is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * FFmpeg is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with FFmpeg; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

/**
 * @file 4xm.c
 * 4XM codec.
 */

#include "avcodec.h"
#include "dsputil.h"
#include "mpegvideo.h"

//#undef NDEBUG
//#include <assert.h>

#define BLOCK_TYPE_VLC_BITS 5
#define ACDC_VLC_BITS 9

#define CFRAME_BUFFER_COUNT 100

static const uint8_t block_type_tab[4][8][2]={
  {   //{8,4,2}x{8,4,2}
    { 0,1}, { 2,2}, { 6,3}, {14,4}, {30,5}, {31,5}, { 0,0}
  },{ //{8,4}x1
    { 0,1}, { 0,0}, { 2,2}, { 6,3}, {14,4}, {15,4}, { 0,0}
  },{ //1x{8,4}
    { 0,1}, { 2,2}, { 0,0}, { 6,3}, {14,4}, {15,4}, { 0,0}
  },{ //1x2, 2x1
    { 0,1}, { 0,0}, { 0,0}, { 2,2}, { 6,3}, {14,4}, {15,4}
  }
};

static const uint8_t size2index[4][4]={
  {-1, 3, 1, 1},
  { 3, 0, 0, 0},
  { 2, 0, 0, 0},
  { 2, 0, 0, 0},
};

static const int8_t mv[256][2]={
{  0,  0},{  0, -1},{ -1,  0},{  1,  0},{  0,  1},{ -1, -1},{  1, -1},{ -1,  1},
{  1,  1},{  0, -2},{ -2,  0},{  2,  0},{  0,  2},{ -1, -2},{  1, -2},{ -2, -1},
{  2, -1},{ -2,  1},{  2,  1},{ -1,  2},{  1,  2},{ -2, -2},{  2, -2},{ -2,  2},
{  2,  2},{  0, -3},{ -3,  0},{  3,  0},{  0,  3},{ -1, -3},{  1, -3},{ -3, -1},
{  3, -1},{ -3,  1},{  3,  1},{ -1,  3},{  1,  3},{ -2, -3},{  2, -3},{ -3, -2},
{  3, -2},{ -3,  2},{  3,  2},{ -2,  3},{  2,  3},{  0, -4},{ -4,  0},{  4,  0},
{  0,  4},{ -1, -4},{  1, -4},{ -4, -1},{  4, -1},{  4,  1},{ -1,  4},{  1,  4},
{ -3, -3},{ -3,  3},{  3,  3},{ -2, -4},{ -4, -2},{  4, -2},{ -4,  2},{ -2,  4},
{  2,  4},{ -3, -4},{  3, -4},{  4, -3},{ -5,  0},{ -4,  3},{ -3,  4},{  3,  4},
{ -1, -5},{ -5, -1},{ -5,  1},{ -1,  5},{ -2, -5},{  2, -5},{  5, -2},{  5,  2},
{ -4, -4},{ -4,  4},{ -3, -5},{ -5, -3},{ -5,  3},{  3,  5},{ -6,  0},{  0,  6},
{ -6, -1},{ -6,  1},{  1,  6},{  2, -6},{ -6,  2},{  2,  6},{ -5, -4},{  5,  4},
{  4,  5},{ -6, -3},{  6,  3},{ -7,  0},{ -1, -7},{  5, -5},{ -7,  1},{ -1,  7},
{  4, -6},{  6,  4},{ -2, -7},{ -7,  2},{ -3, -7},{  7, -3},{  3,  7},{  6, -5},
{  0, -8},{ -1, -8},{ -7, -4},{ -8,  1},{  4,  7},{  2, -8},{ -2,  8},{  6,  6},
{ -8,  3},{  5, -7},{ -5,  7},{  8, -4},{  0, -9},{ -9, -1},{  1,  9},{  7, -6},
{ -7,  6},{ -5, -8},{ -5,  8},{ -9,  3},{  9, -4},{  7, -7},{  8, -6},{  6,  8},
{ 10,  1},{-10,  2},{  9, -5},{ 10, -3},{ -8, -7},{-10, -4},{  6, -9},{-11,  0},
{ 11,  1},{-11, -2},{ -2, 11},{  7, -9},{ -7,  9},{ 10,  6},{ -4, 11},{  8, -9},
{  8,  9},{  5, 11},{  7,-10},{ 12, -3},{ 11,  6},{ -9, -9},{  8, 10},{  5, 12},
{-11,  7},{ 13,  2},{  6,-12},{ 10,  9},{-11,  8},{ -7, 12},{  0, 14},{ 14, -2},
{ -9, 11},{ -6, 13},{-14, -4},{ -5,-14},{  5, 14},{-15, -1},{-14, -6},{  3,-15},
{ 11,-11},{ -7, 14},{ -5, 15},{  8,-14},{ 15,  6},{  3, 16},{  7,-15},{-16,  5},
{  0, 17},{-16, -6},{-10, 14},{-16,  7},{ 12, 13},{-16,  8},{-17,  6},{-18,  3},
{ -7, 17},{ 15, 11},{ 16, 10},{  2,-19},{  3,-19},{-11,-16},{-18,  8},{-19, -6},
{  2,-20},{-17,-11},{-10,-18},{  8, 19},{-21, -1},{-20,  7},{ -4, 21},{ 21,  5},
{ 15, 16},{  2,-22},{-10,-20},{-22,  5},{ 20,-11},{ -7,-22},{-12, 20},{ 23, -5},
{ 13,-20},{ 24, -2},{-15, 19},{-11, 22},{ 16, 19},{ 23,-10},{-18,-18},{ -9,-24},
{ 24,-10},{ -3, 26},{-23, 13},{-18,-20},{ 17, 21},{ -4, 27},{ 27,  6},{  1,-28},
{-11, 26},{-17,-23},{  7, 28},{ 11,-27},{ 29,  5},{-23,-19},{-28,-11},{-21, 22},
{-30,  7},{-17, 26},{-27, 16},{ 13, 29},{ 19,-26},{ 10,-31},{-14,-30},{ 20,-27},
{-29, 18},{-16,-31},{-28,-22},{ 21,-30},{-25, 28},{ 26,-29},{ 25,-32},{-32,-32}
};

// this is simply the scaled down elementwise product of the standard jpeg quantizer table and the AAN premul table
static const uint8_t dequant_table[64]={
 16, 15, 13, 19, 24, 31, 28, 17,
 17, 23, 25, 31, 36, 63, 45, 21,
 18, 24, 27, 37, 52, 59, 49, 20,
 16, 28, 34, 40, 60, 80, 51, 20,
 18, 31, 48, 66, 68, 86, 56, 21,
 19, 38, 56, 59, 64, 64, 48, 20,
 27, 48, 55, 55, 56, 51, 35, 15,
 20, 35, 34, 32, 31, 22, 15,  8,
};

static VLC block_type_vlc[4];


typedef struct CFrameBuffer{
    unsigned int allocated_size;
    unsigned int size;
    int id;
    uint8_t *data;
}CFrameBuffer;

typedef struct FourXContext{
    AVCodecContext *avctx;
    DSPContext dsp;
    AVFrame current_picture, last_picture;
    GetBitContext pre_gb;          ///< ac/dc prefix
    GetBitContext gb;
    uint8_t *bytestream;
    uint16_t *wordstream;
    int mv[256];
    VLC pre_vlc;
    int last_dc;
    DECLARE_ALIGNED_8(DCTELEM, block[6][64]);
    uint8_t *bitstream_buffer;
    unsigned int bitstream_buffer_size;
    CFrameBuffer cfrm[CFRAME_BUFFER_COUNT];
} FourXContext;


#define FIX_1_082392200  70936
#define FIX_1_414213562  92682
#define FIX_1_847759065 121095
#define FIX_2_613125930 171254

#define MULTIPLY(var,const)  (((var)*(const)) >> 16)

static void idct(DCTELEM block[64]){
    int tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7;
    int tmp10, tmp11, tmp12, tmp13;
    int z5, z10, z11, z12, z13;
    int i;
    int temp[64];

    for(i=0; i<8; i++){
        tmp10 = block[8*0 + i] + block[8*4 + i];
        tmp11 = block[8*0 + i] - block[8*4 + i];

        tmp13 =          block[8*2 + i] + block[8*6 + i];
        tmp12 = MULTIPLY(block[8*2 + i] - block[8*6 + i], FIX_1_414213562) - tmp13;

        tmp0 = tmp10 + tmp13;
        tmp3 = tmp10 - tmp13;
        tmp1 = tmp11 + tmp12;
        tmp2 = tmp11 - tmp12;

        z13 = block[8*5 + i] + block[8*3 + i];
        z10 = block[8*5 + i] - block[8*3 + i];
        z11 = block[8*1 + i] + block[8*7 + i];
        z12 = block[8*1 + i] - block[8*7 + i];

        tmp7  =          z11 + z13;
        tmp11 = MULTIPLY(z11 - z13, FIX_1_414213562);

        z5    = MULTIPLY(z10 + z12, FIX_1_847759065);
        tmp10 = MULTIPLY(z12, FIX_1_082392200) - z5;
        tmp12 = MULTIPLY(z10, - FIX_2_613125930) + z5;

        tmp6 = tmp12 - tmp7;
        tmp5 = tmp11 - tmp6;
        tmp4 = tmp10 + tmp5;

        temp[8*0 + i] = tmp0 + tmp7;
        temp[8*7 + i] = tmp0 - tmp7;
        temp[8*1 + i] = tmp1 + tmp6;
        temp[8*6 + i] = tmp1 - tmp6;
        temp[8*2 + i] = tmp2 + tmp5;
        temp[8*5 + i] = tmp2 - tmp5;
        temp[8*4 + i] = tmp3 + tmp4;
        temp[8*3 + i] = tmp3 - tmp4;
    }

    for(i=0; i<8*8; i+=8){
        tmp10 = temp[0 + i] + temp[4 + i];
        tmp11 = temp[0 + i] - temp[4 + i];

        tmp13 = temp[2 + i] + temp[6 + i];
        tmp12 = MULTIPLY(temp[2 + i] - temp[6 + i], FIX_1_414213562) - tmp13;

        tmp0 = tmp10 + tmp13;
        tmp3 = tmp10 - tmp13;
        tmp1 = tmp11 + tmp12;
        tmp2 = tmp11 - tmp12;

        z13 = temp[5 + i] + temp[3 + i];
        z10 = temp[5 + i] - temp[3 + i];
        z11 = temp[1 + i] + temp[7 + i];
        z12 = temp[1 + i] - temp[7 + i];

        tmp7 = z11 + z13;
        tmp11 = MULTIPLY(z11 - z13, FIX_1_414213562);

        z5 = MULTIPLY(z10 + z12, FIX_1_847759065);
        tmp10 = MULTIPLY(z12, FIX_1_082392200) - z5;
        tmp12 = MULTIPLY(z10, - FIX_2_613125930) + z5;

        tmp6 = tmp12 - tmp7;
        tmp5 = tmp11 - tmp6;
        tmp4 = tmp10 + tmp5;

        block[0 + i] = (tmp0 + tmp7)>>6;
        block[7 + i] = (tmp0 - tmp7)>>6;
        block[1 + i] = (tmp1 + tmp6)>>6;
        block[6 + i] = (tmp1 - tmp6)>>6;
        block[2 + i] = (tmp2 + tmp5)>>6;
        block[5 + i] = (tmp2 - tmp5)>>6;
        block[4 + i] = (tmp3 + tmp4)>>6;
        block[3 + i] = (tmp3 - tmp4)>>6;
    }
}

static void init_vlcs(FourXContext *f){
    int i;

    for(i=0; i<4; i++){
        init_vlc(&block_type_vlc[i], BLOCK_TYPE_VLC_BITS, 7,
                 &block_type_tab[i][0][1], 2, 1,
                 &block_type_tab[i][0][0], 2, 1, 1);
    }
}

static void init_mv(FourXContext *f){
    int i;

    for(i=0; i<256; i++){
        f->mv[i] = mv[i][0] + mv[i][1]*f->current_picture.linesize[0]/2;
    }
}

static inline void mcdc(uint16_t *dst, uint16_t *src, int log2w, int h, int stride, int scale, int dc){
   int i;
   dc*= 0x10001;

   switch(log2w){
   case 0:
        for(i=0; i<h; i++){
            dst[0] = scale*src[0] + dc;
            if(scale) src += stride;
            dst += stride;
        }
        break;
    case 1:
        for(i=0; i<h; i++){
            ((uint32_t*)dst)[0] = scale*((uint32_t*)src)[0] + dc;
            if(scale) src += stride;
            dst += stride;
        }
        break;
    case 2:
        for(i=0; i<h; i++){
            ((uint32_t*)dst)[0] = scale*((uint32_t*)src)[0] + dc;
            ((uint32_t*)dst)[1] = scale*((uint32_t*)src)[1] + dc;
            if(scale) src += stride;
            dst += stride;
        }
        break;
    case 3:
        for(i=0; i<h; i++){
            ((uint32_t*)dst)[0] = scale*((uint32_t*)src)[0] + dc;
            ((uint32_t*)dst)[1] = scale*((uint32_t*)src)[1] + dc;
            ((uint32_t*)dst)[2] = scale*((uint32_t*)src)[2] + dc;
            ((uint32_t*)dst)[3] = scale*((uint32_t*)src)[3] + dc;
            if(scale) src += stride;
            dst += stride;
        }
        break;
    default: assert(0);
    }
}

static void decode_p_block(FourXContext *f, uint16_t *dst, uint16_t *src, int log2w, int log2h, int stride){
    const int index= size2index[log2h][log2w];
    const int h= 1<<log2h;
    int code= get_vlc2(&f->gb, block_type_vlc[index].table, BLOCK_TYPE_VLC_BITS, 1);

    assert(code>=0 && code<=6);

    if(code == 0){
        src += f->mv[ *f->bytestream++ ];
        mcdc(dst, src, log2w, h, stride, 1, 0);
    }else if(code == 1){
        log2h--;
        decode_p_block(f, dst                  , src                  , log2w, log2h, stride);
        decode_p_block(f, dst + (stride<<log2h), src + (stride<<log2h), log2w, log2h, stride);
    }else if(code == 2){
        log2w--;
        decode_p_block(f, dst             , src             , log2w, log2h, stride);
        decode_p_block(f, dst + (1<<log2w), src + (1<<log2w), log2w, log2h, stride);
    }else if(code == 4){
        src += f->mv[ *f->bytestream++ ];
        mcdc(dst, src, log2w, h, stride, 1, le2me_16(*f->wordstream++));
    }else if(code == 5){
        mcdc(dst, src, log2w, h, stride, 0, le2me_16(*f->wordstream++));
    }else if(code == 6){
        if(log2w){
            dst[0] = le2me_16(*f->wordstream++);
            dst[1] = le2me_16(*f->wordstream++);
        }else{
            dst[0     ] = le2me_16(*f->wordstream++);
            dst[stride] = le2me_16(*f->wordstream++);
        }
    }
}

static int get32(void *p){
    return le2me_32(*(uint32_t*)p);
}

static int decode_p_frame(FourXContext *f, uint8_t *buf, int length){
    int x, y;
    const int width= f->avctx->width;
    const int height= f->avctx->height;
    uint16_t *src= (uint16_t*)f->last_picture.data[0];
    uint16_t *dst= (uint16_t*)f->current_picture.data[0];
    const int stride= f->current_picture.linesize[0]>>1;
    const unsigned int bitstream_size= get32(buf+8);
    const unsigned int bytestream_size= get32(buf+16);
    const unsigned int wordstream_size= get32(buf+12);

    if(bitstream_size+ bytestream_size+ wordstream_size + 20 != length
       || bitstream_size  > (1<<26)
       || bytestream_size > (1<<26)
       || wordstream_size > (1<<26)
       ){
        av_log(f->avctx, AV_LOG_ERROR, "lengths %d %d %d %d\n", bitstream_size, bytestream_size, wordstream_size,
        bitstream_size+ bytestream_size+ wordstream_size - length);
        return -1;
    }

    f->bitstream_buffer= av_fast_realloc(f->bitstream_buffer, &f->bitstream_buffer_size, bitstream_size + FF_INPUT_BUFFER_PADDING_SIZE);
    f->dsp.bswap_buf((uint32_t*)f->bitstream_buffer, (uint32_t*)(buf + 20), bitstream_size/4);
    init_get_bits(&f->gb, f->bitstream_buffer, 8*bitstream_size);

    f->wordstream= (uint16_t*)(buf + 20 + bitstream_size);
    f->bytestream= buf + 20 + bitstream_size + wordstream_size;

    init_mv(f);

    for(y=0; y<height; y+=8){
        for(x=0; x<width; x+=8){
            decode_p_block(f, dst + x, src + x, 3, 3, stride);
        }
        src += 8*stride;
        dst += 8*stride;
    }

    if(bitstream_size != (get_bits_count(&f->gb)+31)/32*4)
        av_log(f->avctx, AV_LOG_ERROR, " %d %td %td bytes left\n",
            bitstream_size - (get_bits_count(&f->gb)+31)/32*4,
            bytestream_size - (f->bytestream - (buf + 20 + bitstream_size + wordstream_size)),
            wordstream_size - (((uint8_t*)f->wordstream) - (buf + 20 + bitstream_size))
        );

    return 0;
}

/**
 * decode block and dequantize.
 * Note this is allmost identical to mjpeg
 */
static int decode_i_block(FourXContext *f, DCTELEM *block){
    int code, i, j, level, val;

    /* DC coef */
    val = get_vlc2(&f->pre_gb, f->pre_vlc.table, ACDC_VLC_BITS, 3);
    if (val>>4){
        av_log(f->avctx, AV_LOG_ERROR, "error dc run != 0\n");
    }

    if(val)
        val = get_xbits(&f->gb, val);

    val = val * dequant_table[0] + f->last_dc;
    f->last_dc =
    block[0] = val;
    /* AC coefs */
    i = 1;
    for(;;) {
        code = get_vlc2(&f->pre_gb, f->pre_vlc.table, ACDC_VLC_BITS, 3);

        /* EOB */
        if (code == 0)
            break;
        if (code == 0xf0) {
            i += 16;
        } else {
            level = get_xbits(&f->gb, code & 0xf);
            i += code >> 4;
            if (i >= 64) {
                av_log(f->avctx, AV_LOG_ERROR, "run %d oveflow\n", i);
                return 0;
            }

            j= ff_zigzag_direct[i];
            block[j] = level * dequant_table[j];
            i++;
            if (i >= 64)
                break;
        }
    }

    return 0;
}

static inline void idct_put(FourXContext *f, int x, int y){
    DCTELEM (*block)[64]= f->block;
    int stride= f->current_picture.linesize[0]>>1;
    int i;
    uint16_t *dst = ((uint16_t*)f->current_picture.data[0]) + y * stride + x;

    for(i=0; i<4; i++){
        block[i][0] += 0x80*8*8;
        idct(block[i]);
    }

    if(!(f->avctx->flags&CODEC_FLAG_GRAY)){
        for(i=4; i<6; i++) idct(block[i]);
    }

/* Note transform is:
y= ( 1b + 4g + 2r)/14
cb=( 3b - 2g - 1r)/14
cr=(-1b - 4g + 5r)/14
*/
    for(y=0; y<8; y++){
        for(x=0; x<8; x++){
            DCTELEM *temp= block[(x>>2) + 2*(y>>2)] + 2*(x&3) + 2*8*(y&3); //FIXME optimize
            int cb= block[4][x + 8*y];
            int cr= block[5][x + 8*y];
            int cg= (cb + cr)>>1;
            int y;

            cb+=cb;

            y = temp[0];
            dst[0       ]= ((y+cb)>>3) + (((y-cg)&0xFC)<<3) + (((y+cr)&0xF8)<<8);
            y = temp[1];
            dst[1       ]= ((y+cb)>>3) + (((y-cg)&0xFC)<<3) + (((y+cr)&0xF8)<<8);
            y = temp[8];
            dst[  stride]= ((y+cb)>>3) + (((y-cg)&0xFC)<<3) + (((y+cr)&0xF8)<<8);
            y = temp[9];
            dst[1+stride]= ((y+cb)>>3) + (((y-cg)&0xFC)<<3) + (((y+cr)&0xF8)<<8);
            dst += 2;
        }
        dst += 2*stride - 2*8;
    }
}

static int decode_i_mb(FourXContext *f){
    int i;

    f->dsp.clear_blocks(f->block[0]);

    for(i=0; i<6; i++){
        if(decode_i_block(f, f->block[i]) < 0)
            return -1;
    }

    return 0;
}

static uint8_t *read_huffman_tables(FourXContext *f, uint8_t * const buf){
    int frequency[512];
    uint8_t flag[512];
    int up[512];
    uint8_t len_tab[257];
    int bits_tab[257];
    int start, end;
    uint8_t *ptr= buf;
    int j;

    memset(frequency, 0, sizeof(frequency));
    memset(up, -1, sizeof(up));

    start= *ptr++;
    end= *ptr++;
    for(;;){
        int i;

        for(i=start; i<=end; i++){
            frequency[i]= *ptr++;
        }
        start= *ptr++;
        if(start==0) break;

        end= *ptr++;
    }
    frequency[256]=1;

    while((ptr - buf)&3) ptr++; // 4byte align

    for(j=257; j<512; j++){
        int min_freq[2]= {256*256, 256*256};
        int smallest[2]= {0, 0};
        int i;
        for(i=0; i<j; i++){
            if(frequency[i] == 0) continue;
            if(frequency[i] < min_freq[1]){
                if(frequency[i] < min_freq[0]){
                    min_freq[1]= min_freq[0]; smallest[1]= smallest[0];
                    min_freq[0]= frequency[i];smallest[0]= i;
                }else{
                    min_freq[1]= frequency[i];smallest[1]= i;
                }
            }
        }
        if(min_freq[1] == 256*256) break;

        frequency[j]= min_freq[0] + min_freq[1];
        flag[ smallest[0] ]= 0;
        flag[ smallest[1] ]= 1;
        up[ smallest[0] ]=
        up[ smallest[1] ]= j;
        frequency[ smallest[0] ]= frequency[ smallest[1] ]= 0;
    }

    for(j=0; j<257; j++){
        int node;
        int len=0;
        int bits=0;

        for(node= j; up[node] != -1; node= up[node]){
            bits += flag[node]<<len;
            len++;
            if(len > 31) av_log(f->avctx, AV_LOG_ERROR, "vlc length overflow\n"); //can this happen at all ?
        }

        bits_tab[j]= bits;
        len_tab[j]= len;
    }

    init_vlc(&f->pre_vlc, ACDC_VLC_BITS, 257,
             len_tab , 1, 1,
             bits_tab, 4, 4, 0);

    return ptr;
}

static int decode_i_frame(FourXContext *f, uint8_t *buf, int length){
    int x, y;
    const int width= f->avctx->width;
    const int height= f->avctx->height;
    uint16_t *dst= (uint16_t*)f->current_picture.data[0];
    const int stride= f->current_picture.linesize[0]>>1;
    const unsigned int bitstream_size= get32(buf);
    const int token_count __attribute__((unused)) = get32(buf + bitstream_size + 8);
    unsigned int prestream_size= 4*get32(buf + bitstream_size + 4);
    uint8_t *prestream= buf + bitstream_size + 12;

    if(prestream_size + bitstream_size + 12 != length
       || bitstream_size > (1<<26)
       || prestream_size > (1<<26)){
        av_log(f->avctx, AV_LOG_ERROR, "size mismatch %d %d %d\n", prestream_size, bitstream_size, length);
        return -1;
    }

    prestream= read_huffman_tables(f, prestream);

    init_get_bits(&f->gb, buf + 4, 8*bitstream_size);

    prestream_size= length + buf - prestream;

    f->bitstream_buffer= av_fast_realloc(f->bitstream_buffer, &f->bitstream_buffer_size, prestream_size + FF_INPUT_BUFFER_PADDING_SIZE);
    f->dsp.bswap_buf((uint32_t*)f->bitstream_buffer, (uint32_t*)prestream, prestream_size/4);
    init_get_bits(&f->pre_gb, f->bitstream_buffer, 8*prestream_size);

    f->last_dc= 0*128*8*8;

    for(y=0; y<height; y+=16){
        for(x=0; x<width; x+=16){
            if(decode_i_mb(f) < 0)
                return -1;

            idct_put(f, x, y);
        }
        dst += 16*stride;
    }

    if(get_vlc2(&f->pre_gb, f->pre_vlc.table, ACDC_VLC_BITS, 3) != 256)
        av_log(f->avctx, AV_LOG_ERROR, "end mismatch\n");

    return 0;
}

static int decode_frame(AVCodecContext *avctx,
                        void *data, int *data_size,
                        uint8_t *buf, int buf_size)
{
    FourXContext * const f = avctx->priv_data;
    AVFrame *picture = data;
    AVFrame *p, temp;
    int i, frame_4cc, frame_size;

    frame_4cc= get32(buf);
    if(buf_size != get32(buf+4)+8 || buf_size < 20){
        av_log(f->avctx, AV_LOG_ERROR, "size mismatch %d %d\n", buf_size, get32(buf+4));
    }

    if(frame_4cc == ff_get_fourcc("cfrm")){
        int free_index=-1;
        const int data_size= buf_size - 20;
        const int id= get32(buf+12);
        const int whole_size= get32(buf+16);
        CFrameBuffer *cfrm;

        for(i=0; i<CFRAME_BUFFER_COUNT; i++){
            if(f->cfrm[i].id && f->cfrm[i].id < avctx->frame_number)
                av_log(f->avctx, AV_LOG_ERROR, "lost c frame %d\n", f->cfrm[i].id);
        }

        for(i=0; i<CFRAME_BUFFER_COUNT; i++){
            if(f->cfrm[i].id   == id) break;
            if(f->cfrm[i].size == 0 ) free_index= i;
        }

        if(i>=CFRAME_BUFFER_COUNT){
            i= free_index;
            f->cfrm[i].id= id;
        }
        cfrm= &f->cfrm[i];

        cfrm->data= av_fast_realloc(cfrm->data, &cfrm->allocated_size, cfrm->size + data_size + FF_INPUT_BUFFER_PADDING_SIZE);
        if(!cfrm->data){ //explicit check needed as memcpy below might not catch a NULL
            av_log(f->avctx, AV_LOG_ERROR, "realloc falure");
            return -1;
        }

        memcpy(cfrm->data + cfrm->size, buf+20, data_size);
        cfrm->size += data_size;

        if(cfrm->size >= whole_size){
            buf= cfrm->data;
            frame_size= cfrm->size;

            if(id != avctx->frame_number){
                av_log(f->avctx, AV_LOG_ERROR, "cframe id mismatch %d %d\n", id, avctx->frame_number);
            }

            cfrm->size= cfrm->id= 0;
            frame_4cc= ff_get_fourcc("pfrm");
        }else
            return buf_size;
    }else{
        buf= buf + 12;
        frame_size= buf_size - 12;
    }

    temp= f->current_picture;
    f->current_picture= f->last_picture;
    f->last_picture= temp;

    p= &f->current_picture;
    avctx->coded_frame= p;

    avctx->flags |= CODEC_FLAG_EMU_EDGE; // alternatively we would have to use our own buffer management

    if(p->data[0])
        avctx->release_buffer(avctx, p);

    p->reference= 1;
    if(avctx->get_buffer(avctx, p) < 0){
        av_log(avctx, AV_LOG_ERROR, "get_buffer() failed\n");
        return -1;
    }

    if(frame_4cc == ff_get_fourcc("ifrm")){
        p->pict_type= I_TYPE;
        if(decode_i_frame(f, buf, frame_size) < 0)
            return -1;
    }else if(frame_4cc == ff_get_fourcc("pfrm")){
        p->pict_type= P_TYPE;
        if(decode_p_frame(f, buf, frame_size) < 0)
            return -1;
    }else if(frame_4cc == ff_get_fourcc("snd_")){
        av_log(avctx, AV_LOG_ERROR, "ignoring snd_ chunk length:%d\n", buf_size);
    }else{
        av_log(avctx, AV_LOG_ERROR, "ignoring unknown chunk length:%d\n", buf_size);
    }

    p->key_frame= p->pict_type == I_TYPE;

    *picture= *p;
    *data_size = sizeof(AVPicture);

    emms_c();

    return buf_size;
}


static void common_init(AVCodecContext *avctx){
    FourXContext * const f = avctx->priv_data;

    dsputil_init(&f->dsp, avctx);

    f->avctx= avctx;
}

static int decode_init(AVCodecContext *avctx){
    FourXContext * const f = avctx->priv_data;

    common_init(avctx);
    init_vlcs(f);

    avctx->pix_fmt= PIX_FMT_RGB565;

    return 0;
}


static int decode_end(AVCodecContext *avctx){
    FourXContext * const f = avctx->priv_data;
    int i;

    av_freep(&f->bitstream_buffer);
    f->bitstream_buffer_size=0;
    for(i=0; i<CFRAME_BUFFER_COUNT; i++){
        av_freep(&f->cfrm[i].data);
        f->cfrm[i].allocated_size= 0;
    }
    free_vlc(&f->pre_vlc);

    return 0;
}

AVCodec fourxm_decoder = {
    "4xm",
    CODEC_TYPE_VIDEO,
    CODEC_ID_4XM,
    sizeof(FourXContext),
    decode_init,
    NULL,
    decode_end,
    decode_frame,
    /*CODEC_CAP_DR1,*/
};