dfpwmenc.c

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/*
 * DFPWM encoder
 * Copyright (c) 2022 Jack Bruienne
 * Copyright (c) 2012, 2016 Ben "GreaseMonkey" Russell
 *
 * 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
 * DFPWM1a encoder
 */
 
#include "avcodec.h"
#include "codec_id.h"
#include "codec_internal.h"
#include "encode.h"
#include "internal.h"
 
typedef struct {
    int q, s, lt;
} DFPWMState;
 
// DFPWM codec from https://github.com/ChenThread/dfpwm/blob/master/1a/
// Licensed in the public domain
 
// note, len denotes how many compressed bytes there are (uncompressed bytes / 8).
static void au_compress(DFPWMState *state, int len, uint8_t *outbuf, const uint8_t *inbuf)
{
    unsigned d = 0;
    for (int i = 0; i < len; i++) {
        for (int j = 0; j < 8; j++) {
            int nq, st, ns;
            // get sample
            int v = *(inbuf++) - 128;
            // set bit / target
            int t = (v > state->q || (v == state->q && v == 127) ? 127 : -128);
            d >>= 1;
            if(t > 0)
                d |= 0x80;
 
            // adjust charge
            nq = state->q + ((state->s * (t-state->q) + 512)>>10);
            if(nq == state->q && nq != t)
                nq += (t == 127 ? 1 : -1);
            state->q = nq;
 
            // adjust strength
            st = (t != state->lt ? 0 : 1023);
            ns = state->s;
            if(ns != st)
                ns += (st != 0 ? 1 : -1);
            if(ns < 8) ns = 8;
            state->s = ns;
 
            state->lt = t;
        }
 
        // output bits
        *(outbuf++) = d;
    }
}
 
static av_cold int dfpwm_enc_init(struct AVCodecContext *ctx)
{
    DFPWMState *state = ctx->priv_data;
 
    state->q = 0;
    state->s = 0;
    state->lt = -128;
 
    ctx->bits_per_coded_sample = 1;
    // Pad so that nb_samples * nb_channels is always a multiple of eight.
    ctx->internal->pad_samples = (const uint8_t[]){ 1, 8, 4, 8, 2, 8, 4, 8 }[ctx->ch_layout.nb_channels & 7];
    if (ctx->frame_size <= 0 || ctx->frame_size * ctx->ch_layout.nb_channels % 8U)
        ctx->frame_size = 4096;
 
    return 0;
}
 
static int dfpwm_enc_frame(struct AVCodecContext *ctx, struct AVPacket *packet,
    const struct AVFrame *frame, int *got_packet)
{
    DFPWMState *state = ctx->priv_data;
    int size = frame->nb_samples * frame->ch_layout.nb_channels / 8U;
    int ret = ff_get_encode_buffer(ctx, packet, size, 0);
 
    if (ret) {
        *got_packet = 0;
        return ret;
    }
 
    au_compress(state, size, packet->data, frame->data[0]);
 
    *got_packet = 1;
    return 0;
}
 
const FFCodec ff_dfpwm_encoder = {
    .p.name          = "dfpwm",
    CODEC_LONG_NAME("DFPWM1a audio"),
    .p.type          = AVMEDIA_TYPE_AUDIO,
    .p.id            = AV_CODEC_ID_DFPWM,
    .p.capabilities  = AV_CODEC_CAP_DR1 | AV_CODEC_CAP_ENCODER_REORDERED_OPAQUE,
    .priv_data_size  = sizeof(DFPWMState),
    .init            = dfpwm_enc_init,
    FF_CODEC_ENCODE_CB(dfpwm_enc_frame),
    CODEC_SAMPLEFMTS(AV_SAMPLE_FMT_U8),
};