FFmpeg/libavcodec/flac_parser.c

/*
 * FLAC parser
 * Copyright (c) 2010 Michael Chinen
 *
 * 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
 * FLAC parser
 *
 * The FLAC parser buffers input until FLAC_MIN_HEADERS has been found.
 * Each time it finds and verifies a CRC-8 header it sees which of the
 * FLAC_MAX_SEQUENTIAL_HEADERS that came before it have a valid CRC-16 footer
 * that ends at the newly found header.
 * Headers are scored by FLAC_HEADER_BASE_SCORE plus the max of its crc-verified
 * children, penalized by changes in sample rate, frame number, etc.
 * The parser returns the frame with the highest score.
 **/

#include "libavutil/attributes.h"
#include "libavutil/crc.h"
#include "libavutil/fifo.h"
#include "bytestream.h"
#include "parser.h"
#include "flac.h"

/** maximum number of adjacent headers that compare CRCs against each other   */
#define FLAC_MAX_SEQUENTIAL_HEADERS 3
/** minimum number of headers buffered and checked before returning frames    */
#define FLAC_MIN_HEADERS 10
/** estimate for average size of a FLAC frame                                 */
#define FLAC_AVG_FRAME_SIZE 8192

/** scoring settings for score_header */
#define FLAC_HEADER_BASE_SCORE        10
#define FLAC_HEADER_CHANGED_PENALTY   7
#define FLAC_HEADER_CRC_FAIL_PENALTY  50
#define FLAC_HEADER_NOT_PENALIZED_YET 100000
#define FLAC_HEADER_NOT_SCORED_YET    -100000

/** largest possible size of flac header */
#define MAX_FRAME_HEADER_SIZE 16

typedef struct FLACHeaderMarker {
    int offset;       /**< byte offset from start of FLACParseContext->buffer */
    int *link_penalty;  /**< pointer to array of local scores between this header
                           and the one at a distance equal array position     */
    int max_score;    /**< maximum score found after checking each child that
                           has a valid CRC                                    */
    FLACFrameInfo fi; /**< decoded frame header info                          */
    struct FLACHeaderMarker *next;       /**< next CRC-8 verified header that
                                              immediately follows this one in
                                              the bytestream                  */
    struct FLACHeaderMarker *best_child; /**< following frame header with
                                              which this frame has the best
                                              score with                      */
} FLACHeaderMarker;

typedef struct FLACParseContext {
    AVCodecParserContext *pc;      /**< parent context                        */
    AVCodecContext *avctx;         /**< codec context pointer for logging     */
    FLACHeaderMarker *headers;     /**< linked-list that starts at the first
                                        CRC-8 verified header within buffer   */
    FLACHeaderMarker *best_header; /**< highest scoring header within buffer  */
    int nb_headers_found;          /**< number of headers found in the last
                                        flac_parse() call                     */
    int nb_headers_buffered;       /**< number of headers that are buffered   */
    int best_header_valid;         /**< flag set when the parser returns junk;
                                        if set return best_header next time   */
    AVFifoBuffer *fifo_buf;        /**< buffer to store all data until headers
                                        can be verified                       */
    int end_padded;                /**< specifies if fifo_buf's end is padded */
    uint8_t *wrap_buf;             /**< general fifo read buffer when wrapped */
    int wrap_buf_allocated_size;   /**< actual allocated size of the buffer   */
    FLACFrameInfo last_fi;         /**< last decoded frame header info        */
    int last_fi_valid;             /**< set if last_fi is valid               */
} FLACParseContext;

static int frame_header_is_valid(AVCodecContext *avctx, const uint8_t *buf,
                                 FLACFrameInfo *fi)
{
    GetBitContext gb;
    init_get_bits(&gb, buf, MAX_FRAME_HEADER_SIZE * 8);
    return !ff_flac_decode_frame_header(avctx, &gb, fi, 127);
}

/**
 * Non-destructive fast fifo pointer fetching
 * Returns a pointer from the specified offset.
 * If possible the pointer points within the fifo buffer.
 * Otherwise (if it would cause a wrap around,) a pointer to a user-specified
 * buffer is used.
 * The pointer can be NULL.  In any case it will be reallocated to hold the size.
 * If the returned pointer will be used after subsequent calls to flac_fifo_read_wrap
 * then the subsequent calls should pass in a different wrap_buf so as to not
 * overwrite the contents of the previous wrap_buf.
 * This function is based on av_fifo_generic_read, which is why there is a comment
 * about a memory barrier for SMP.
 */
static uint8_t* flac_fifo_read_wrap(FLACParseContext *fpc, int offset, int len,
                               uint8_t** wrap_buf, int* allocated_size)
{
    AVFifoBuffer *f   = fpc->fifo_buf;
    uint8_t *start    = f->rptr + offset;
    uint8_t *tmp_buf;

    if (start >= f->end)
        start -= f->end - f->buffer;
    if (f->end - start >= len)
        return start;

    tmp_buf = av_fast_realloc(*wrap_buf, allocated_size, len);

    if (!tmp_buf) {
        av_log(fpc->avctx, AV_LOG_ERROR,
               "couldn't reallocate wrap buffer of size %d", len);
        return NULL;
    }
    *wrap_buf = tmp_buf;
    do {
        int seg_len = FFMIN(f->end - start, len);
        memcpy(tmp_buf, start, seg_len);
        tmp_buf = (uint8_t*)tmp_buf + seg_len;
// memory barrier needed for SMP here in theory

        start += seg_len - (f->end - f->buffer);
        len -= seg_len;
    } while (len > 0);

    return *wrap_buf;
}

/**
 * Return a pointer in the fifo buffer where the offset starts at until
 * the wrap point or end of request.
 * len will contain the valid length of the returned buffer.
 * A second call to flac_fifo_read (with new offset and len) should be called
 * to get the post-wrap buf if the returned len is less than the requested.
 **/
static uint8_t* flac_fifo_read(FLACParseContext *fpc, int offset, int *len)
{
    AVFifoBuffer *f   = fpc->fifo_buf;
    uint8_t *start    = f->rptr + offset;

    if (start >= f->end)
        start -= f->end - f->buffer;
    *len = FFMIN(*len, f->end - start);
    return start;
}

static int find_headers_search_validate(FLACParseContext *fpc, int offset)
{
    FLACFrameInfo fi;
    uint8_t *header_buf;
    int size = 0;
    header_buf = flac_fifo_read_wrap(fpc, offset,
                                     MAX_FRAME_HEADER_SIZE,
                                     &fpc->wrap_buf,
                                     &fpc->wrap_buf_allocated_size);
    if (frame_header_is_valid(fpc->avctx, header_buf, &fi)) {
        FLACHeaderMarker **end_handle = &fpc->headers;
        int i;

        size = 0;
        while (*end_handle) {
            end_handle = &(*end_handle)->next;
            size++;
        }

        *end_handle = av_mallocz(sizeof(FLACHeaderMarker));
        if (!*end_handle) {
            av_log(fpc->avctx, AV_LOG_ERROR,
                   "couldn't allocate FLACHeaderMarker\n");
            return AVERROR(ENOMEM);
        }
        (*end_handle)->fi           = fi;
        (*end_handle)->offset       = offset;
        (*end_handle)->link_penalty = av_malloc(sizeof(int) *
                                            FLAC_MAX_SEQUENTIAL_HEADERS);
        for (i = 0; i < FLAC_MAX_SEQUENTIAL_HEADERS; i++)
            (*end_handle)->link_penalty[i] = FLAC_HEADER_NOT_PENALIZED_YET;

        fpc->nb_headers_found++;
        size++;
    }
    return size;
}

static int find_headers_search(FLACParseContext *fpc, uint8_t *buf, int buf_size,
                               int search_start)

{
    int size = 0, mod_offset = (buf_size - 1) % 4, i, j;
    uint32_t x;

    for (i = 0; i < mod_offset; i++) {
        if ((AV_RB16(buf + i) & 0xFFFE) == 0xFFF8)
            size = find_headers_search_validate(fpc, search_start + i);
    }

    for (; i < buf_size - 1; i += 4) {
        x = AV_RB32(buf + i);
        if (((x & ~(x + 0x01010101)) & 0x80808080)) {
            for (j = 0; j < 4; j++) {
                if ((AV_RB16(buf + i + j) & 0xFFFE) == 0xFFF8)
                    size = find_headers_search_validate(fpc, search_start + i + j);
            }
        }
    }
    return size;
}

static int find_new_headers(FLACParseContext *fpc, int search_start)
{
    FLACHeaderMarker *end;
    int search_end, size = 0, read_len, temp;
    uint8_t *buf;
    fpc->nb_headers_found = 0;

    /* Search for a new header of at most 16 bytes. */
    search_end = av_fifo_size(fpc->fifo_buf) - (MAX_FRAME_HEADER_SIZE - 1);
    read_len   = search_end - search_start + 1;
    buf        = flac_fifo_read(fpc, search_start, &read_len);
    size       = find_headers_search(fpc, buf, read_len, search_start);
    search_start += read_len - 1;

    /* If fifo end was hit do the wrap around. */
    if (search_start != search_end) {
        uint8_t wrap[2];

        wrap[0]  = buf[read_len - 1];
        read_len = search_end - search_start + 1;

        /* search_start + 1 is the post-wrap offset in the fifo. */
        buf      = flac_fifo_read(fpc, search_start + 1, &read_len);
        wrap[1]  = buf[0];

        if ((AV_RB16(wrap) & 0xFFFE) == 0xFFF8) {
            temp = find_headers_search_validate(fpc, search_start);
            size = FFMAX(size, temp);
        }
        search_start++;

        /* Continue to do the last half of the wrap. */
        temp     = find_headers_search(fpc, buf, read_len, search_start);
        size     = FFMAX(size, temp);
        search_start += read_len - 1;
    }

    /* Return the size even if no new headers were found. */
    if (!size && fpc->headers)
        for (end = fpc->headers; end; end = end->next)
            size++;
    return size;
}

static int check_header_fi_mismatch(FLACParseContext  *fpc,
                                    FLACFrameInfo     *header_fi,
                                    FLACFrameInfo     *child_fi,
                                    int                log_level_offset)
{
    int deduction = 0;
    if (child_fi->samplerate != header_fi->samplerate) {
        deduction += FLAC_HEADER_CHANGED_PENALTY;
        av_log(fpc->avctx, AV_LOG_WARNING + log_level_offset,
               "sample rate change detected in adjacent frames\n");
    }
    if (child_fi->bps != header_fi->bps) {
        deduction += FLAC_HEADER_CHANGED_PENALTY;
        av_log(fpc->avctx, AV_LOG_WARNING + log_level_offset,
               "bits per sample change detected in adjacent frames\n");
    }
    if (child_fi->is_var_size != header_fi->is_var_size) {
        /* Changing blocking strategy not allowed per the spec */
        deduction += FLAC_HEADER_BASE_SCORE;
        av_log(fpc->avctx, AV_LOG_WARNING + log_level_offset,
               "blocking strategy change detected in adjacent frames\n");
    }
    if (child_fi->channels != header_fi->channels) {
        deduction += FLAC_HEADER_CHANGED_PENALTY;
        av_log(fpc->avctx, AV_LOG_WARNING + log_level_offset,
               "number of channels change detected in adjacent frames\n");
    }
    return deduction;
}

static int check_header_mismatch(FLACParseContext  *fpc,
                                 FLACHeaderMarker  *header,
                                 FLACHeaderMarker  *child,
                                 int                log_level_offset)
{
    FLACFrameInfo  *header_fi = &header->fi, *child_fi = &child->fi;
    int deduction, deduction_expected = 0, i;
    deduction = check_header_fi_mismatch(fpc, header_fi, child_fi,
                                         log_level_offset);
    /* Check sample and frame numbers. */
    if ((child_fi->frame_or_sample_num - header_fi->frame_or_sample_num
         != header_fi->blocksize) &&
        (child_fi->frame_or_sample_num
         != header_fi->frame_or_sample_num + 1)) {
        FLACHeaderMarker *curr;
        int expected_frame_num, expected_sample_num;
        /* If there are frames in the middle we expect this deduction,
           as they are probably valid and this one follows it */

        expected_frame_num = expected_sample_num = header_fi->frame_or_sample_num;
        curr = header;
        while (curr != child) {
            /* Ignore frames that failed all crc checks */
            for (i = 0; i < FLAC_MAX_SEQUENTIAL_HEADERS; i++) {
                if (curr->link_penalty[i] < FLAC_HEADER_CRC_FAIL_PENALTY) {
                    expected_frame_num++;
                    expected_sample_num += curr->fi.blocksize;
                    break;
                }
            }
            curr = curr->next;
        }

        if (expected_frame_num  == child_fi->frame_or_sample_num ||
            expected_sample_num == child_fi->frame_or_sample_num)
            deduction_expected = deduction ? 0 : 1;

        deduction += FLAC_HEADER_CHANGED_PENALTY;
        av_log(fpc->avctx, AV_LOG_WARNING + log_level_offset,
                   "sample/frame number mismatch in adjacent frames\n");
    }

    /* If we have suspicious headers, check the CRC between them */
    if (deduction && !deduction_expected) {
        FLACHeaderMarker *curr;
        int read_len;
        uint8_t *buf;
        uint32_t crc = 1;
        int inverted_test = 0;

        /* Since CRC is expensive only do it if we haven't yet.
           This assumes a CRC penalty is greater than all other check penalties */
        curr = header->next;
        for (i = 0; i < FLAC_MAX_SEQUENTIAL_HEADERS && curr != child; i++)
            curr = curr->next;

        if (header->link_penalty[i] < FLAC_HEADER_CRC_FAIL_PENALTY ||
            header->link_penalty[i] == FLAC_HEADER_NOT_PENALIZED_YET) {
            FLACHeaderMarker *start, *end;

            /* Although overlapping chains are scored, the crc should never
               have to be computed twice for a single byte. */
            start = header;
            end   = child;
            if (i > 0 &&
                header->link_penalty[i - 1] >= FLAC_HEADER_CRC_FAIL_PENALTY) {
                while (start->next != child)
                    start = start->next;
                inverted_test = 1;
            } else if (i > 0 &&
                       header->next->link_penalty[i-1] >=
                       FLAC_HEADER_CRC_FAIL_PENALTY ) {
                end = header->next;
                inverted_test = 1;
            }

            read_len = end->offset - start->offset;
            buf      = flac_fifo_read(fpc, start->offset, &read_len);
            crc      = av_crc(av_crc_get_table(AV_CRC_16_ANSI), 0, buf, read_len);
            read_len = (end->offset - start->offset) - read_len;

            if (read_len) {
                buf = flac_fifo_read(fpc, end->offset - read_len, &read_len);
                crc = av_crc(av_crc_get_table(AV_CRC_16_ANSI), crc, buf, read_len);
            }
        }

        if (!crc ^ !inverted_test) {
            deduction += FLAC_HEADER_CRC_FAIL_PENALTY;
            av_log(fpc->avctx, AV_LOG_WARNING + log_level_offset,
                   "crc check failed from offset %i (frame %"PRId64") to %i (frame %"PRId64")\n",
                   header->offset, header_fi->frame_or_sample_num,
                   child->offset, child_fi->frame_or_sample_num);
        }
    }
    return deduction;
}

/**
 * Score a header.
 *
 * Give FLAC_HEADER_BASE_SCORE points to a frame for existing.
 * If it has children, (subsequent frames of which the preceding CRC footer
 * validates against this one,) then take the maximum score of the children,
 * with a penalty of FLAC_HEADER_CHANGED_PENALTY applied for each change to
 * bps, sample rate, channels, but not decorrelation mode, or blocksize,
 * because it can change often.
 **/
static int score_header(FLACParseContext *fpc, FLACHeaderMarker *header)
{
    FLACHeaderMarker *child;
    int dist = 0;
    int child_score;
    int base_score = FLAC_HEADER_BASE_SCORE;
    if (header->max_score != FLAC_HEADER_NOT_SCORED_YET)
        return header->max_score;

    /* Modify the base score with changes from the last output header */
    if (fpc->last_fi_valid) {
        /* Silence the log since this will be repeated if selected */
        base_score -= check_header_fi_mismatch(fpc, &fpc->last_fi, &header->fi,
                                               AV_LOG_DEBUG);
    }

    header->max_score = base_score;

    /* Check and compute the children's scores. */
    child = header->next;
    for (dist = 0; dist < FLAC_MAX_SEQUENTIAL_HEADERS && child; dist++) {
        /* Look at the child's frame header info and penalize suspicious
           changes between the headers. */
        if (header->link_penalty[dist] == FLAC_HEADER_NOT_PENALIZED_YET) {
            header->link_penalty[dist] = check_header_mismatch(fpc, header,
                                                               child, AV_LOG_DEBUG);
        }
        child_score = score_header(fpc, child) - header->link_penalty[dist];

        if (FLAC_HEADER_BASE_SCORE + child_score > header->max_score) {
            /* Keep the child because the frame scoring is dynamic. */
            header->best_child = child;
            header->max_score  = base_score + child_score;
        }
        child = child->next;
    }

    return header->max_score;
}

static void score_sequences(FLACParseContext *fpc)
{
    FLACHeaderMarker *curr;
    int best_score = 0;//FLAC_HEADER_NOT_SCORED_YET;
    /* First pass to clear all old scores. */
    for (curr = fpc->headers; curr; curr = curr->next)
        curr->max_score = FLAC_HEADER_NOT_SCORED_YET;

    /* Do a second pass to score them all. */
    for (curr = fpc->headers; curr; curr = curr->next) {
        if (score_header(fpc, curr) > best_score) {
            fpc->best_header = curr;
            best_score       = curr->max_score;
        }
    }
}

static int get_best_header(FLACParseContext* fpc, const uint8_t **poutbuf,
                           int *poutbuf_size)
{
    FLACHeaderMarker *header = fpc->best_header;
    FLACHeaderMarker *child  = header->best_child;
    if (!child) {
        *poutbuf_size = av_fifo_size(fpc->fifo_buf) - header->offset;
    } else {
        *poutbuf_size = child->offset - header->offset;

        /* If the child has suspicious changes, log them */
        check_header_mismatch(fpc, header, child, 0);
    }

    if (header->fi.channels != fpc->avctx->channels ||
        !fpc->avctx->channel_layout) {
        fpc->avctx->channels = header->fi.channels;
        ff_flac_set_channel_layout(fpc->avctx);
    }
    fpc->avctx->sample_rate = header->fi.samplerate;
    fpc->pc->duration       = header->fi.blocksize;
    *poutbuf = flac_fifo_read_wrap(fpc, header->offset, *poutbuf_size,
                                        &fpc->wrap_buf,
                                        &fpc->wrap_buf_allocated_size);


    if (fpc->pc->flags & PARSER_FLAG_USE_CODEC_TS){
        if (header->fi.is_var_size)
          fpc->pc->pts = header->fi.frame_or_sample_num;
        else if (header->best_child)
          fpc->pc->pts = header->fi.frame_or_sample_num * header->fi.blocksize;
    }

    fpc->best_header_valid = 0;
    fpc->last_fi_valid = 1;
    fpc->last_fi = header->fi;

    /* Return the negative overread index so the client can compute pos.
       This should be the amount overread to the beginning of the child */
    if (child)
        return child->offset - av_fifo_size(fpc->fifo_buf);
    return 0;
}

static int flac_parse(AVCodecParserContext *s, AVCodecContext *avctx,
                      const uint8_t **poutbuf, int *poutbuf_size,
                      const uint8_t *buf, int buf_size)
{
    FLACParseContext *fpc = s->priv_data;
    FLACHeaderMarker *curr;
    int nb_headers;
    const uint8_t *read_end   = buf;
    const uint8_t *read_start = buf;

    if (s->flags & PARSER_FLAG_COMPLETE_FRAMES) {
        FLACFrameInfo fi;
        if (frame_header_is_valid(avctx, buf, &fi)) {
            s->duration = fi.blocksize;
            if (!avctx->sample_rate)
                avctx->sample_rate = fi.samplerate;
            if (fpc->pc->flags & PARSER_FLAG_USE_CODEC_TS){
                fpc->pc->pts = fi.frame_or_sample_num;
                if (!fi.is_var_size)
                  fpc->pc->pts *= fi.blocksize;
            }
        }
        *poutbuf      = buf;
        *poutbuf_size = buf_size;
        return buf_size;
    }

    fpc->avctx = avctx;
    if (fpc->best_header_valid)
        return get_best_header(fpc, poutbuf, poutbuf_size);

    /* If a best_header was found last call remove it with the buffer data. */
    if (fpc->best_header && fpc->best_header->best_child) {
        FLACHeaderMarker *temp;
        FLACHeaderMarker *best_child = fpc->best_header->best_child;

        /* Remove headers in list until the end of the best_header. */
        for (curr = fpc->headers; curr != best_child; curr = temp) {
            if (curr != fpc->best_header) {
                av_log(avctx, AV_LOG_DEBUG,
                       "dropping low score %i frame header from offset %i to %i\n",
                       curr->max_score, curr->offset, curr->next->offset);
            }
            temp = curr->next;
            av_freep(&curr->link_penalty);
            av_free(curr);
            fpc->nb_headers_buffered--;
        }
        /* Release returned data from ring buffer. */
        av_fifo_drain(fpc->fifo_buf, best_child->offset);

        /* Fix the offset for the headers remaining to match the new buffer. */
        for (curr = best_child->next; curr; curr = curr->next)
            curr->offset -= best_child->offset;

        fpc->nb_headers_buffered--;
        best_child->offset = 0;
        fpc->headers       = best_child;
        if (fpc->nb_headers_buffered >= FLAC_MIN_HEADERS) {
            fpc->best_header = best_child;
            return get_best_header(fpc, poutbuf, poutbuf_size);
        }
        fpc->best_header   = NULL;
    } else if (fpc->best_header) {
        /* No end frame no need to delete the buffer; probably eof */
        FLACHeaderMarker *temp;

        for (curr = fpc->headers; curr != fpc->best_header; curr = temp) {
            temp = curr->next;
            av_freep(&curr->link_penalty);
            av_free(curr);
        }
        fpc->headers = fpc->best_header->next;
        av_freep(&fpc->best_header->link_penalty);
        av_freep(&fpc->best_header);
    }

    /* Find and score new headers.                                     */
    /* buf_size is to zero when padding, so check for this since we do */
    /* not want to try to read more input once we have found the end.  */
    /* Note that as (non-modified) parameters, buf can be non-NULL,    */
    /* while buf_size is 0.                                            */
    while ((buf && buf_size && read_end < buf + buf_size &&
            fpc->nb_headers_buffered < FLAC_MIN_HEADERS)
           || ((!buf || !buf_size) && !fpc->end_padded)) {
        int start_offset;

        /* Pad the end once if EOF, to check the final region for headers. */
        if (!buf || !buf_size) {
            fpc->end_padded      = 1;
            buf_size = MAX_FRAME_HEADER_SIZE;
            read_end = read_start + MAX_FRAME_HEADER_SIZE;
        } else {
            /* The maximum read size is the upper-bound of what the parser
               needs to have the required number of frames buffered */
            int nb_desired = FLAC_MIN_HEADERS - fpc->nb_headers_buffered + 1;
            read_end       = read_end + FFMIN(buf + buf_size - read_end,
                                              nb_desired * FLAC_AVG_FRAME_SIZE);
        }

        /* Fill the buffer. */
        if (   av_fifo_space(fpc->fifo_buf) < read_end - read_start
            && av_fifo_realloc2(fpc->fifo_buf, (read_end - read_start) + 2*av_fifo_size(fpc->fifo_buf)) < 0) {
            av_log(avctx, AV_LOG_ERROR,
                   "couldn't reallocate buffer of size %"PTRDIFF_SPECIFIER"\n",
                   (read_end - read_start) + av_fifo_size(fpc->fifo_buf));
            goto handle_error;
        }

        if (buf && buf_size) {
            av_fifo_generic_write(fpc->fifo_buf, (void*) read_start,
                                  read_end - read_start, NULL);
        } else {
            int8_t pad[MAX_FRAME_HEADER_SIZE] = { 0 };
            av_fifo_generic_write(fpc->fifo_buf, (void*) pad, sizeof(pad), NULL);
        }

        /* Tag headers and update sequences. */
        start_offset = av_fifo_size(fpc->fifo_buf) -
                       ((read_end - read_start) + (MAX_FRAME_HEADER_SIZE - 1));
        start_offset = FFMAX(0, start_offset);
        nb_headers   = find_new_headers(fpc, start_offset);

        if (nb_headers < 0) {
            av_log(avctx, AV_LOG_ERROR,
                   "find_new_headers couldn't allocate FLAC header\n");
            goto handle_error;
        }

        fpc->nb_headers_buffered = nb_headers;
        /* Wait till FLAC_MIN_HEADERS to output a valid frame. */
        if (!fpc->end_padded && fpc->nb_headers_buffered < FLAC_MIN_HEADERS) {
            if (buf && read_end < buf + buf_size) {
                read_start = read_end;
                continue;
            } else {
                goto handle_error;
            }
        }

        /* If headers found, update the scores since we have longer chains. */
        if (fpc->end_padded || fpc->nb_headers_found)
            score_sequences(fpc);

        /* restore the state pre-padding */
        if (fpc->end_padded) {
            int warp = fpc->fifo_buf->wptr - fpc->fifo_buf->buffer < MAX_FRAME_HEADER_SIZE;
            /* HACK: drain the tail of the fifo */
            fpc->fifo_buf->wptr -= MAX_FRAME_HEADER_SIZE;
            fpc->fifo_buf->wndx -= MAX_FRAME_HEADER_SIZE;
            if (warp) {
                fpc->fifo_buf->wptr += fpc->fifo_buf->end -
                    fpc->fifo_buf->buffer;
            }
            buf_size = 0;
            read_start = read_end = NULL;
        }
    }

    curr = fpc->headers;
    for (curr = fpc->headers; curr; curr = curr->next) {
        if (curr->max_score > 0 &&
            (!fpc->best_header || curr->max_score > fpc->best_header->max_score)) {
            fpc->best_header = curr;
        }
    }

    if (fpc->best_header) {
        fpc->best_header_valid = 1;
        if (fpc->best_header->offset > 0) {
            /* Output a junk frame. */
            av_log(avctx, AV_LOG_DEBUG, "Junk frame till offset %i\n",
                   fpc->best_header->offset);

            /* Set duration to 0. It is unknown or invalid in a junk frame. */
            s->duration = 0;
            *poutbuf_size     = fpc->best_header->offset;
            *poutbuf          = flac_fifo_read_wrap(fpc, 0, *poutbuf_size,
                                                    &fpc->wrap_buf,
                                                    &fpc->wrap_buf_allocated_size);
            return buf_size ? (read_end - buf) : (fpc->best_header->offset -
                                           av_fifo_size(fpc->fifo_buf));
        }
        if (!buf_size)
            return get_best_header(fpc, poutbuf, poutbuf_size);
    }

handle_error:
    *poutbuf      = NULL;
    *poutbuf_size = 0;
    return read_end - buf;
}

static av_cold int flac_parse_init(AVCodecParserContext *c)
{
    FLACParseContext *fpc = c->priv_data;
    fpc->pc = c;
    /* There will generally be FLAC_MIN_HEADERS buffered in the fifo before
       it drains.  This is allocated early to avoid slow reallocation. */
    fpc->fifo_buf = av_fifo_alloc(FLAC_AVG_FRAME_SIZE * (FLAC_MIN_HEADERS + 3));
    if (!fpc->fifo_buf)
        return AVERROR(ENOMEM);
    return 0;
}

static void flac_parse_close(AVCodecParserContext *c)
{
    FLACParseContext *fpc = c->priv_data;
    FLACHeaderMarker *curr = fpc->headers, *temp;

    while (curr) {
        temp = curr->next;
        av_freep(&curr->link_penalty);
        av_free(curr);
        curr = temp;
    }
    av_fifo_free(fpc->fifo_buf);
    av_free(fpc->wrap_buf);
}

AVCodecParser ff_flac_parser = {
    .codec_ids      = { AV_CODEC_ID_FLAC },
    .priv_data_size = sizeof(FLACParseContext),
    .parser_init    = flac_parse_init,
    .parser_parse   = flac_parse,
    .parser_close   = flac_parse_close,
};