alac.c

ffmpeg的完整源代码和作者自己写的文档。不但有在Linux的工程哦

/*
 * ALAC (Apple Lossless Audio Codec) decoder
 * Copyright (c) 2005 David Hammerton
 *
 * 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 alac.c
 * ALAC (Apple Lossless Audio Codec) decoder
 * @author 2005 David Hammerton
 *
 * For more information on the ALAC format, visit:
 *  http://crazney.net/programs/itunes/alac.html
 *
 * Note: This decoder expects a 36- (0x24-)byte QuickTime atom to be
 * passed through the extradata[_size] fields. This atom is tacked onto
 * the end of an 'alac' stsd atom and has the following format:
 *  bytes 0-3   atom size (0x24), big-endian
 *  bytes 4-7   atom type ('alac', not the 'alac' tag from start of stsd)
 *  bytes 8-35  data bytes needed by decoder
 *
 * Extradata:
 * 32bit  size
 * 32bit  tag (=alac)
 * 32bit  zero?
 * 32bit  max sample per frame
 *  8bit  ?? (zero?)
 *  8bit  sample size
 *  8bit  history mult
 *  8bit  initial history
 *  8bit  kmodifier
 *  8bit  channels?
 * 16bit  ??
 * 32bit  max coded frame size
 * 32bit  bitrate?
 * 32bit  samplerate
 */


#include "avcodec.h"
#include "bitstream.h"
#include "bytestream.h"
#include "unary.h"

#define ALAC_EXTRADATA_SIZE 36
#define MAX_CHANNELS 2

typedef struct {

    AVCodecContext *avctx;
    GetBitContext gb;
    /* init to 0; first frame decode should initialize from extradata and
     * set this to 1 */
    int context_initialized;

    int samplesize;
    int numchannels;
    int bytespersample;

    /* buffers */
    int32_t *predicterror_buffer[MAX_CHANNELS];

    int32_t *outputsamples_buffer[MAX_CHANNELS];

    /* stuff from setinfo */
    uint32_t setinfo_max_samples_per_frame; /* 0x1000 = 4096 */    /* max samples per frame? */
    uint8_t setinfo_7a; /* 0x00 */
    uint8_t setinfo_sample_size; /* 0x10 */
    uint8_t setinfo_rice_historymult; /* 0x28 */
    uint8_t setinfo_rice_initialhistory; /* 0x0a */
    uint8_t setinfo_rice_kmodifier; /* 0x0e */
    uint8_t setinfo_7f; /* 0x02 */
    uint16_t setinfo_80; /* 0x00ff */
    uint32_t setinfo_82; /* 0x000020e7 */ /* max sample size?? */
    uint32_t setinfo_86; /* 0x00069fe4 */ /* bit rate (average)?? */
    uint32_t setinfo_8a_rate; /* 0x0000ac44 */
    /* end setinfo stuff */

} ALACContext;

static void allocate_buffers(ALACContext *alac)
{
    int chan;
    for (chan = 0; chan < MAX_CHANNELS; chan++) {
        alac->predicterror_buffer[chan] =
            av_malloc(alac->setinfo_max_samples_per_frame * 4);

        alac->outputsamples_buffer[chan] =
            av_malloc(alac->setinfo_max_samples_per_frame * 4);
    }
}

static int alac_set_info(ALACContext *alac)
{
    unsigned char *ptr = alac->avctx->extradata;

    ptr += 4; /* size */
    ptr += 4; /* alac */
    ptr += 4; /* 0 ? */

    if(AV_RB32(ptr) >= UINT_MAX/4){
        av_log(alac->avctx, AV_LOG_ERROR, "setinfo_max_samples_per_frame too large\n");
        return -1;
    }

    /* buffer size / 2 ? */
    alac->setinfo_max_samples_per_frame = bytestream_get_be32(&ptr);
    alac->setinfo_7a                    = *ptr++;
    alac->setinfo_sample_size           = *ptr++;
    alac->setinfo_rice_historymult      = *ptr++;
    alac->setinfo_rice_initialhistory   = *ptr++;
    alac->setinfo_rice_kmodifier        = *ptr++;
    /* channels? */
    alac->setinfo_7f                    = *ptr++;
    alac->setinfo_80                    = bytestream_get_be16(&ptr);
    /* max coded frame size */
    alac->setinfo_82                    = bytestream_get_be32(&ptr);
    /* bitrate ? */
    alac->setinfo_86                    = bytestream_get_be32(&ptr);
    /* samplerate */
    alac->setinfo_8a_rate               = bytestream_get_be32(&ptr);

    allocate_buffers(alac);

    return 0;
}

static inline int count_leading_zeros(int32_t input)
{
    return 31-av_log2(input);
}

static void bastardized_rice_decompress(ALACContext *alac,
                                 int32_t *output_buffer,
                                 int output_size,
                                 int readsamplesize, /* arg_10 */
                                 int rice_initialhistory, /* arg424->b */
                                 int rice_kmodifier, /* arg424->d */
                                 int rice_historymult, /* arg424->c */
                                 int rice_kmodifier_mask /* arg424->e */
        )
{
    int output_count;
    unsigned int history = rice_initialhistory;
    int sign_modifier = 0;

    for (output_count = 0; output_count < output_size; output_count++) {
        int32_t x;
        int32_t x_modified;
        int32_t final_val;

        /* read x - number of 1s before 0 represent the rice */
        x = get_unary_0_9(&alac->gb);

        if (x > 8) { /* RICE THRESHOLD */
            /* use alternative encoding */
            int32_t value;

            value = get_bits(&alac->gb, readsamplesize);

            /* mask value to readsamplesize size */
            if (readsamplesize != 32)
                value &= (0xffffffff >> (32 - readsamplesize));

            x = value;
        } else {
            /* standard rice encoding */
            int extrabits;
            int k; /* size of extra bits */

            /* read k, that is bits as is */
            k = 31 - rice_kmodifier - count_leading_zeros((history >> 9) + 3);

            if (k < 0)
                k += rice_kmodifier;
            else
                k = rice_kmodifier;

            if (k != 1) {
                extrabits = show_bits(&alac->gb, k);

                /* multiply x by 2^k - 1, as part of their strange algorithm */
                x = (x << k) - x;

                if (extrabits > 1) {
                    x += extrabits - 1;
                    skip_bits(&alac->gb, k);
                } else
                    skip_bits(&alac->gb, k - 1);
            }
        }

        x_modified = sign_modifier + x;
        final_val = (x_modified + 1) / 2;
        if (x_modified & 1) final_val *= -1;

        output_buffer[output_count] = final_val;

        sign_modifier = 0;

        /* now update the history */
        history += x_modified * rice_historymult
                   - ((history * rice_historymult) >> 9);

        if (x_modified > 0xffff)
            history = 0xffff;

        /* special case: there may be compressed blocks of 0 */
        if ((history < 128) && (output_count+1 < output_size)) {
            int block_size;

            sign_modifier = 1;

            x = get_unary_0_9(&alac->gb);

            if (x > 8) {
                block_size = get_bits(&alac->gb, 16);
                block_size &= 0xffff;
            } else {
                int k;
                int extrabits;

                k = count_leading_zeros(history) + ((history + 16) >> 6 /* / 64 */) - 24;

                extrabits = show_bits(&alac->gb, k);

                block_size = (((1 << k) - 1) & rice_kmodifier_mask) * x
                           + extrabits - 1;

                if (extrabits < 2) {
                    x = 1 - extrabits;
                    block_size += x;
                    skip_bits(&alac->gb, k - 1);
                } else {
                    skip_bits(&alac->gb, k);
                }
            }

            if (block_size > 0) {
                memset(&output_buffer[output_count+1], 0, block_size * 4);
                output_count += block_size;
            }

            if (block_size > 0xffff)
                sign_modifier = 0;

            history = 0;
        }
    }
}

static inline int32_t extend_sign32(int32_t val, int bits)
{
    return (val << (32 - bits)) >> (32 - bits);
}

static inline int sign_only(int v)
{
    return v ? FFSIGN(v) : 0;
}

static void predictor_decompress_fir_adapt(int32_t *error_buffer,
                                           int32_t *buffer_out,
                                           int output_size,
                                           int readsamplesize,
                                           int16_t *predictor_coef_table,
                                           int predictor_coef_num,
                                           int predictor_quantitization)
{
    int i;

    /* first sample always copies */
    *buffer_out = *error_buffer;

    if (!predictor_coef_num) {
        if (output_size <= 1)
            return;

        memcpy(buffer_out+1, error_buffer+1, (output_size-1) * 4);
        return;
    }

    if (predictor_coef_num == 0x1f) { /* 11111 - max value of predictor_coef_num */
      /* second-best case scenario for fir decompression,
       * error describes a small difference from the previous sample only
       */
        if (output_size <= 1)
            return;
        for (i = 0; i < output_size - 1; i++) {
            int32_t prev_value;
            int32_t error_value;

            prev_value = buffer_out[i];
            error_value = error_buffer[i+1];
            buffer_out[i+1] =
                extend_sign32((prev_value + error_value), readsamplesize);
        }
        return;
    }

    /* read warm-up samples */
    if (predictor_coef_num > 0)
        for (i = 0; i < predictor_coef_num; i++) {
            int32_t val;

            val = buffer_out[i] + error_buffer[i+1];
            val = extend_sign32(val, readsamplesize);
            buffer_out[i+1] = val;
        }

#if 0
    /* 4 and 8 are very common cases (the only ones i've seen). these
     * should be unrolled and optimised
     */
    if (predictor_coef_num == 4) {
        /* FIXME: optimised general case */
        return;