FFmpeg/libavcodec/vulkan/ffv1_vlc.comp

/*
 * FFv1 codec
 *
 * Copyright (c) 2024 Lynne <dev@lynne.ee>
 *
 * 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
 */

#define VLC_STATE_SIZE 8
layout(buffer_reference, buffer_reference_align = VLC_STATE_SIZE) buffer VlcState {
    uint32_t error_sum;
    int16_t  drift;
    int8_t   bias;
    uint8_t  count;
};

void update_vlc_state(inout VlcState state, const int v)
{
    int drift = state.drift;
    int count = state.count;
    int bias = state.bias;
    state.error_sum += uint16_t(abs(v));
    drift           += v;

    if (count == 128) { // FIXME: variable
        count           >>= 1;
        drift           >>= 1;
        state.error_sum >>= 1;
    }
    count++;

    if (drift <= -count) {
        bias = max(bias - 1, -128);
        drift = max(drift + count, -count + 1);
    } else if (drift > 0) {
        bias = min(bias + 1, 127);
        drift = min(drift - count, 0);
    }

    state.bias = int8_t(bias);
    state.drift = int16_t(drift);
    state.count = uint8_t(count);
}

struct Symbol {
    uint32_t bits;
    uint32_t val;
};

Symbol set_ur_golomb(int i, int k, int limit, int esc_len)
{
    int e;
    Symbol sym;

#ifdef DEBUG
    if (i < 0)
        debugPrintfEXT("Error: i is zero!");
#endif

    e = i >> k;
    if (e < limit) {
        sym.bits = e + k + 1;
        sym.val = (1 << k) + zero_extend(i, k);
    } else {
        sym.bits = limit + esc_len;
        sym.val = i - limit + 1;
    }

    return sym;
}

/**
 * write signed golomb rice code (ffv1).
 */
Symbol set_sr_golomb(int i, int k, int limit, int esc_len)
{
    int v;

    v  = -2 * i - 1;
    v ^= (v >> 31);

    return set_ur_golomb(v, k, limit, esc_len);
}

Symbol get_vlc_symbol(inout VlcState state, int v, int bits)
{
    int i, k, code;
    Symbol sym;
    v = fold(v - int(state.bias), bits);

    i = state.count;
    k = 0;
    while (i < state.error_sum) { // FIXME: optimize
        k++;
        i += i;
    }

#ifdef DEBUG
    if (k > 16)
        debugPrintfEXT("Error: k > 16!");
#endif

    code = v ^ ((2 * state.drift + state.count) >> 31);

    update_vlc_state(state, v);

    return set_sr_golomb(code, k, 12, bits);
}