Support for generic multi-stream key frame finding for new seek API.

Originally committed as revision 19680 to svn://svn.ffmpeg.org/ffmpeg/trunk

4 files changed, 677 insertions, 2 deletions

diff --git a/libavformat/Makefile b/libavformat/Makefile
index e4aa53abd0..89b6ce2a45 100644
--- a/libavformat/Makefile
+++ b/libavformat/Makefile
@@ -5,7 +5,7 @@ FFLIBS = avcodec avutil
 
 HEADERS = avformat.h avio.h
 
-OBJS = allformats.o cutils.o metadata.o metadata_compat.o options.o os_support.o sdp.o utils.o
+OBJS = allformats.o cutils.o metadata.o metadata_compat.o options.o os_support.o sdp.o seek.o utils.o
 
 # muxers/demuxers
 OBJS-$(CONFIG_AAC_DEMUXER)               += raw.o id3v1.o id3v2.o
diff --git a/libavformat/seek.c b/libavformat/seek.c
new file mode 100644
index 0000000000..2f4595304d
--- /dev/null
+++ b/libavformat/seek.c
@@ -0,0 +1,578 @@
+/*
+ * Utility functions for seeking for use within FFmpeg format handlers.
+ *
+ * Copyright (c) 2009 Ivan Schreter
+ *
+ * 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
+ */
+
+#include "seek.h"
+#include "libavutil/mem.h"
+
+// NOTE: implementation should be moved here in another patch, to keep patches
+// separated.
+extern void av_read_frame_flush(AVFormatContext *s);
+
+/**
+ * Helper structure to store parser state of AVStream.
+ */
+typedef struct AVStreamState {
+    // Saved members of AVStream
+    AVCodecParserContext   *parser;
+    AVPacket                cur_pkt;
+    int64_t                 last_IP_pts;
+    int64_t                 cur_dts;
+    int64_t                 reference_dts;
+    const uint8_t          *cur_ptr;
+    int                     cur_len;
+    int                     probe_packets;
+} AVStreamState;
+
+/**
+ * Helper structure to store parser state of AVFormat.
+ */
+struct AVParserState {
+    int64_t         fpos;                   ///< File position at the time of call.
+
+    // Saved members of AVFormatContext
+    AVStream       *cur_st;                 ///< Current stream.
+    AVPacketList   *packet_buffer;          ///< Packet buffer of original state.
+    AVPacketList   *raw_packet_buffer;      ///< Raw packet buffer of original state.
+    int raw_packet_buffer_remaining_size;   ///< Remaining size available for raw_packet_buffer.
+
+    // Saved info for streams.
+    int             nb_streams;             ///< Number of streams with stored state.
+    AVStreamState  *stream_states;          ///< States of individual streams (array).
+};
+
+/**
+ * Helper structure describing keyframe search state of one stream.
+ */
+typedef struct {
+    int64_t pos_lo;     ///< Position of the frame with low timestamp in file or INT64_MAX if not found (yet).
+    int64_t ts_lo;      ///< Frame presentation timestamp or same as pos_lo for byte seeking.
+
+    int64_t pos_hi;     ///< Position of the frame with high timestamp in file or INT64_MAX if not found (yet).
+    int64_t ts_hi;      ///< Frame presentation timestamp or same as pos_hi for byte seeking.
+
+    int64_t last_pos;   ///< Last known position of a frame, for multi-frame packets.
+
+    int64_t     term_ts;    ///< Termination timestamp (which TS we already read).
+    AVRational  term_ts_tb; ///< Timebase for term_ts.
+    int64_t     first_ts;   ///< First packet timestamp in this iteration (to fill term_ts later).
+    AVRational  first_ts_tb;///< Timebase for first_ts.
+
+    int         terminated; ///< Termination flag for current iteration.
+} AVSyncPoint;
+
+/**
+ * Compare two timestamps exactly, taking into account their respective time bases.
+ *
+ * @param ts_a timestamp A.
+ * @param tb_a time base for timestamp A.
+ * @param ts_b timestamp B.
+ * @param tb_b time base for timestamp A.
+ * @return -1. 0 or 1 if timestamp A is less than, equal or greater than timestamp B.
+ */
+static int compare_ts(int64_t ts_a, AVRational tb_a, int64_t ts_b, AVRational tb_b)
+{
+    int64_t a, b, res;
+
+    if (ts_a == INT64_MIN)
+        return ts_a < ts_b ? -1 : 0;
+    if (ts_a == INT64_MAX)
+        return ts_a > ts_b ? 1 : 0;
+    if (ts_b == INT64_MIN)
+        return ts_a > ts_b ? 1 : 0;
+    if (ts_b == INT64_MAX)
+        return ts_a < ts_b ? -1 : 0;
+
+    a = ts_a * tb_a.num * tb_b.den;
+    b = ts_b * tb_b.num * tb_a.den;
+
+    res = a - b;
+    if (res == 0)
+        return 0;
+    else
+        return (res >> 63) | 1;
+}
+
+/**
+ * Compute a distance between timestamps.
+ *
+ * Distances are only comparable, if same time bases are used for computing
+ * distances.
+ *
+ * @param ts_hi high timestamp.
+ * @param tb_hi high timestamp time base.
+ * @param ts_lo low timestamp.
+ * @param tb_lo low timestamp time base.
+ * @return representation of distance between high and low timestamps.
+ */
+static int64_t ts_distance(int64_t ts_hi, AVRational tb_hi, int64_t ts_lo, AVRational tb_lo)
+{
+    int64_t hi, lo;
+
+    hi = ts_hi * tb_hi.num * tb_lo.den;
+    lo = ts_lo * tb_lo.num * tb_hi.den;
+
+    return hi - lo;
+}
+
+/**
+ * Partial search for keyframes in multiple streams.
+ *
+ * This routine searches for the next lower and next higher timestamp to
+ * given target timestamp in each stream, starting at current file position
+ * and ending at position, where all streams have already been examined
+ * (or when all higher key frames found in first iteration).
+ *
+ * This routine is called iteratively with exponential backoff to find lower
+ * timestamp.
+ *
+ * @param s                 format context.
+ * @param timestamp         target timestamp (or position, if AVSEEK_FLAG_BYTE).
+ * @param timebase          time base for timestamps.
+ * @param flags             seeking flags.
+ * @param sync              array with information per stream.
+ * @param keyframes_to_find count of keyframes to find in total.
+ * @param found_lo          pointer to count of already found low timestamp keyframes.
+ * @param found_hi          pointer to count of already found high timestamp keyframes.
+ * @param first_iter        flag for first iteration.
+ */
+static void search_hi_lo_keyframes(AVFormatContext *s,
+                                   int64_t timestamp,
+                                   AVRational timebase,
+                                   int flags,
+                                   AVSyncPoint *sync,
+                                   int keyframes_to_find,
+                                   int *found_lo,
+                                   int *found_hi,
+                                   int first_iter)
+{
+    AVPacket pkt;
+    AVSyncPoint *sp;
+    AVStream *st;
+    int idx;
+    int flg;
+    int terminated_count = 0;
+    int64_t pos;
+    int64_t pts, dts;   // PTS/DTS from stream
+    int64_t ts;         // PTS in stream-local time base or position for byte seeking
+    AVRational ts_tb;   // Time base of the stream or 1:1 for byte seeking
+
+    for (;;) {
+        if (av_read_frame(s, &pkt) < 0) {
+            // EOF or error, make sure high flags are set
+            for (idx = 0; idx < s->nb_streams; ++idx) {
+                if (s->streams[idx]->discard < AVDISCARD_ALL) {
+                    sp = &sync[idx];
+                    if (sp->pos_hi == INT64_MAX) {
+                        // No high frame exists for this stream
+                        (*found_hi)++;
+                        sp->ts_hi  = INT64_MAX;
+                        sp->pos_hi = INT64_MAX - 1;
+                    }
+                }
+            }
+            break;
+        }
+
+        idx = pkt.stream_index;
+        st = s->streams[idx];
+        if (st->discard >= AVDISCARD_ALL) {
+            // This stream is not active, skip packet.
+            continue;
+        }
+        sp = &sync[idx];
+
+        flg = pkt.flags;
+        pos = pkt.pos;
+        pts = pkt.pts;
+        dts = pkt.dts;
+        if (pts == AV_NOPTS_VALUE) {
+            // Some formats don't provide PTS, only DTS.
+            pts = dts;
+        }
+        av_free_packet(&pkt);
+
+        // Multi-frame packets only return position for the very first frame.
+        // Other frames are read with position == -1. Therefore, we note down
+        // last known position of a frame and use it if a frame without
+        // position arrives. In this way, it's possible to seek to proper
+        // position. Additionally, for parsers not providing position at all,
+        // an approximation will be used (starting position of this iteration).
+        if (pos < 0) {
+            pos = sp->last_pos;
+        } else {
+            sp->last_pos = pos;
+        }
+
+        // Evaluate key frames with known TS (or any frames, if AVSEEK_FLAG_ANY set).
+        if (pts != AV_NOPTS_VALUE && ((flg & PKT_FLAG_KEY) || (flags & AVSEEK_FLAG_ANY))) {
+            if (flags & AVSEEK_FLAG_BYTE) {
+                // For byte seeking, use position as timestamp.
+                ts        = pos;
+                ts_tb.num = 1;
+                ts_tb.den = 1;
+            } else {
+                // Get stream time_base.
+                ts    = pts;
+                ts_tb = st->time_base;
+            }
+
+            if (sp->first_ts == AV_NOPTS_VALUE) {
+                // Note down termination timestamp for the next iteration - when
+                // we encounter a packet with the same timestamp, we will ignore
+                // any further packets for this stream in next iteration (as they
+                // are already evaluated).
+                sp->first_ts    = ts;
+                sp->first_ts_tb = ts_tb;
+            }
+
+            if (sp->term_ts != AV_NOPTS_VALUE && compare_ts(ts, ts_tb, sp->term_ts, sp->term_ts_tb) > 0) {
+                // We are past the end position from last iteration, ignore packet.
+                if (!sp->terminated) {
+                    sp->terminated = 1;
+                    ++terminated_count;
+                    if (sp->pos_hi == INT64_MAX) {
+                        // No high frame exists for this stream
+                        (*found_hi)++;
+                        sp->ts_hi  = INT64_MAX;
+                        sp->pos_hi = INT64_MAX - 1;
+                    }
+                    if (terminated_count == keyframes_to_find)
+                        break;  // all terminated, iteration done
+                }
+                continue;
+            }
+
+            if (compare_ts(ts, ts_tb, timestamp, timebase) <= 0) {
+                // Keyframe found before target timestamp.
+                if (sp->pos_lo == INT64_MAX) {
+                    // Found first keyframe lower than target timestamp.
+                    (*found_lo)++;
+                    sp->ts_lo  = ts;
+                    sp->pos_lo = pos;
+                } else if (sp->ts_lo < ts) {
+                    // Found a better match (closer to target timestamp).
+                    sp->ts_lo  = ts;
+                    sp->pos_lo = pos;
+                }
+            }
+            if (compare_ts(ts, ts_tb, timestamp, timebase) >= 0) {
+                // Keyframe found after target timestamp.
+                if (sp->pos_hi == INT64_MAX) {
+                    // Found first keyframe higher than target timestamp.
+                    (*found_hi)++;
+                    sp->ts_hi  = ts;
+                    sp->pos_hi = pos;
+                    if (*found_hi >= keyframes_to_find && first_iter) {
+                        // We found high frame for all. They may get updated
+                        // to TS closer to target TS in later iterations (which
+                        // will stop at start position of previous iteration).
+                        break;
+                    }
+                } else if (sp->ts_hi > ts) {
+                    // Found a better match (actually, shouldn't happen).
+                    sp->ts_hi  = ts;
+                    sp->pos_hi = pos;
+                }
+            }
+        }
+    }
+
+    // Clean up the parser.
+    av_read_frame_flush(s);
+}
+
+int64_t ff_gen_syncpoint_search(AVFormatContext *s,
+                                int stream_index,
+                                int64_t pos,
+                                int64_t ts_min,
+                                int64_t ts,
+                                int64_t ts_max,
+                                int flags)
+{
+    AVSyncPoint *sync, *sp;
+    AVStream *st;
+    int i;
+    int keyframes_to_find = 0;
+    int64_t curpos;
+    int64_t step;
+    int found_lo = 0, found_hi = 0;
+    int64_t min_distance, distance;
+    int64_t min_pos = 0;
+    int first_iter = 1;
+    AVRational time_base;
+
+    if (flags & AVSEEK_FLAG_BYTE) {
+        /* For byte seeking, we have exact 1:1 "timestamps" - positions */
+        time_base.num = 1;
+        time_base.den = 1;
+    } else {
+        if (stream_index >= 0) {
+            /* We have a reference stream, which time base we use */
+            st = s->streams[stream_index];
+            time_base = st->time_base;
+        } else {
+            /* No reference stream, use AV_TIME_BASE as reference time base */
+            time_base.num = 1;
+            time_base.den = AV_TIME_BASE;
+        }
+    }
+
+    // Initialize syncpoint structures for each stream.
+    sync = (AVSyncPoint*) av_malloc(s->nb_streams * sizeof(AVSyncPoint));
+    if (!sync) {
+        // cannot allocate helper structure
+        return -1;
+    }
+    for (i = 0; i < s->nb_streams; ++i) {
+        st = s->streams[i];
+        sp = &sync[i];
+
+        sp->pos_lo     = INT64_MAX;
+        sp->ts_lo      = INT64_MAX;
+        sp->pos_hi     = INT64_MAX;
+        sp->ts_hi      = INT64_MAX;
+        sp->terminated = 0;
+        sp->first_ts   = AV_NOPTS_VALUE;
+        sp->term_ts    = ts_max;
+        sp->term_ts_tb = time_base;
+        sp->last_pos   = pos;
+
+        st->cur_dts    = AV_NOPTS_VALUE;
+
+        if (st->discard < AVDISCARD_ALL)
+            ++keyframes_to_find;
+    }
+
+    if (keyframes_to_find == 0) {
+        // No stream active, error.
+        av_free(sync);
+        return -1;
+    }
+
+    // Find keyframes in all active streams with timestamp/position just before
+    // and just after requested timestamp/position.
+    step = 1024;
+    curpos = pos;
+    for (;;) {
+        url_fseek(s->pb, curpos, SEEK_SET);
+        search_hi_lo_keyframes(s,
+                               ts, time_base,
+                               flags,
+                               sync,
+                               keyframes_to_find,
+                               &found_lo, &found_hi,
+                               first_iter);
+        if (found_lo == keyframes_to_find && found_hi == keyframes_to_find)
+            break;  // have all keyframes we wanted
+        if (curpos == 0)
+            break;  // cannot go back anymore
+
+        curpos = pos - step;
+        if (curpos < 0)
+            curpos = 0;
+        step *= 2;
+
+        // switch termination positions
+        for (i = 0; i < s->nb_streams; ++i) {
+            st = s->streams[i];
+            st->cur_dts = AV_NOPTS_VALUE;
+
+            sp = &sync[i];
+            if (sp->first_ts != AV_NOPTS_VALUE) {
+                sp->term_ts    = sp->first_ts;
+                sp->term_ts_tb = sp->first_ts_tb;
+                sp->first_ts   = AV_NOPTS_VALUE;
+            }
+            sp->terminated = 0;
+            sp->last_pos = curpos;
+        }
+        first_iter = 0;
+    }
+
+    // Find actual position to start decoding so that decoder synchronizes
+    // closest to ts and between ts_min and ts_max.
+    pos = INT64_MAX;
+
+    for (i = 0; i < s->nb_streams; ++i) {
+        st = s->streams[i];
+        if (st->discard < AVDISCARD_ALL) {
+            sp = &sync[i];
+            min_distance = INT64_MAX;
+            // Find timestamp closest to requested timestamp within min/max limits.
+            if (sp->pos_lo != INT64_MAX
+                && compare_ts(ts_min, time_base, sp->ts_lo, st->time_base) <= 0
+                && compare_ts(sp->ts_lo, st->time_base, ts_max, time_base) <= 0) {
+                // low timestamp is in range
+                min_distance = ts_distance(ts, time_base, sp->ts_lo, st->time_base);
+                min_pos = sp->pos_lo;
+            }
+            if (sp->pos_hi != INT64_MAX
+                && compare_ts(ts_min, time_base, sp->ts_hi, st->time_base) <= 0
+                && compare_ts(sp->ts_hi, st->time_base, ts_max, time_base) <= 0) {
+                // high timestamp is in range, check distance
+                distance = ts_distance(sp->ts_hi, st->time_base, ts, time_base);
+                if (distance < min_distance) {
+                    min_distance = distance;
+                    min_pos = sp->pos_hi;
+                }
+            }
+            if (min_distance == INT64_MAX) {
+                // no timestamp is in range, cannot seek
+                av_free(sync);
+                return -1;
+            }
+            if (min_pos < pos)
+                pos = min_pos;
+        }
+    }
+
+    url_fseek(s->pb, pos, SEEK_SET);
+    av_free(sync);
+    return pos;
+}
+
+AVParserState *ff_store_parser_state(AVFormatContext *s)
+{
+    int i;
+    AVStream *st;
+    AVStreamState *ss;
+    AVParserState *state = (AVParserState*) av_malloc(sizeof(AVParserState));
+    if (!state)
+        return NULL;
+
+    state->stream_states = (AVStreamState*) av_malloc(sizeof(AVStreamState) * s->nb_streams);
+    if (!state->stream_states) {
+        av_free(state);
+        return NULL;
+    }
+
+    state->fpos = url_ftell(s->pb);
+
+    // copy context structures
+    state->cur_st                           = s->cur_st;
+    state->packet_buffer                    = s->packet_buffer;
+    state->raw_packet_buffer                = s->raw_packet_buffer;
+    state->raw_packet_buffer_remaining_size = s->raw_packet_buffer_remaining_size;
+
+    s->cur_st                               = NULL;
+    s->packet_buffer                        = NULL;
+    s->raw_packet_buffer                    = NULL;
+    s->raw_packet_buffer_remaining_size     = RAW_PACKET_BUFFER_SIZE;
+
+    // copy stream structures
+    state->nb_streams = s->nb_streams;
+    for (i = 0; i < s->nb_streams; i++) {
+        st = s->streams[i];
+        ss = &state->stream_states[i];
+
+        ss->parser        = st->parser;
+        ss->last_IP_pts   = st->last_IP_pts;
+        ss->cur_dts       = st->cur_dts;
+        ss->reference_dts = st->reference_dts;
+        ss->cur_ptr       = st->cur_ptr;
+        ss->cur_len       = st->cur_len;
+        ss->probe_packets = st->probe_packets;
+        ss->cur_pkt       = st->cur_pkt;
+
+        st->parser        = NULL;
+        st->last_IP_pts   = AV_NOPTS_VALUE;
+        st->cur_dts       = AV_NOPTS_VALUE;
+        st->reference_dts = AV_NOPTS_VALUE;
+        st->cur_ptr       = NULL;
+        st->cur_len       = 0;
+        st->probe_packets = MAX_PROBE_PACKETS;
+        av_init_packet(&st->cur_pkt);
+    }
+
+    return state;
+}
+
+void ff_restore_parser_state(AVFormatContext *s, AVParserState *state)
+{
+    int i;
+    AVStream *st;
+    AVStreamState *ss;
+    av_read_frame_flush(s);
+
+    if (!state)
+        return;
+
+    url_fseek(s->pb, state->fpos, SEEK_SET);
+
+    // copy context structures
+    s->cur_st                           = state->cur_st;
+    s->packet_buffer                    = state->packet_buffer;
+    s->raw_packet_buffer                = state->raw_packet_buffer;
+    s->raw_packet_buffer_remaining_size = state->raw_packet_buffer_remaining_size;
+
+    // copy stream structures
+    for (i = 0; i < state->nb_streams; i++) {
+        st = s->streams[i];
+        ss = &state->stream_states[i];
+
+        st->parser        = ss->parser;
+        st->last_IP_pts   = ss->last_IP_pts;
+        st->cur_dts       = ss->cur_dts;
+        st->reference_dts = ss->reference_dts;
+        st->cur_ptr       = ss->cur_ptr;
+        st->cur_len       = ss->cur_len;
+        st->probe_packets = ss->probe_packets;
+        st->cur_pkt       = ss->cur_pkt;
+    }
+
+    av_free(state->stream_states);
+    av_free(state);
+}
+
+static void free_packet_list(AVPacketList *pktl)
+{
+    AVPacketList *cur;
+    while (pktl) {
+        cur = pktl;
+        pktl = cur->next;
+        av_free_packet(&cur->pkt);
+        av_free(cur);
+    }
+}
+
+void ff_free_parser_state(AVFormatContext *s, AVParserState *state)
+{
+    int i;
+    AVStreamState *ss;
+
+    if (!state)
+        return;
+
+    for (i = 0; i < state->nb_streams; i++) {
+        ss = &state->stream_states[i];
+        if (ss->parser)
+            av_parser_close(ss->parser);
+        av_free_packet(&ss->cur_pkt);
+    }
+
+    free_packet_list(state->packet_buffer);
+    free_packet_list(state->raw_packet_buffer);
+
+    av_free(state->stream_states);
+    av_free(state);
+}
+
diff --git a/libavformat/seek.h b/libavformat/seek.h
new file mode 100644
index 0000000000..570c2bcedf
--- /dev/null
+++ b/libavformat/seek.h
@@ -0,0 +1,97 @@
+/*
+ * Utility functions for seeking for use within FFmpeg format handlers.
+ *
+ * Copyright (c) 2009 Ivan Schreter
+ *
+ * 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
+ */
+
+#ifndef AVFORMAT_SEEK_H
+#define AVFORMAT_SEEK_H
+
+#include "avformat.h"
+
+/// Opaque structure for parser state.
+typedef struct AVParserState AVParserState;
+
+/**
+ * Search for sync point of all active streams.
+ *
+ * This is not supposed to be called directly by a user application,
+ * but by demuxers.
+ *
+ * A sync point is a point in stream, so that decoding from this point,
+ * output of decoders of all streams synchronizes closest to given timestamp
+ * ts (but taking timestamp limits into account, i.e., no sooner than ts_min
+ * and no later than ts_max).
+ *
+ * @param stream_index stream index for time base reference of timestamps.
+ * @param pos approximate position where to start searching for key frames.
+ * @param min_ts minimum allowed timestamp (position, if AVSEEK_FLAG_BYTE set).
+ * @param ts target timestamp (or position, if AVSEEK_FLAG_BYTE set in flags).
+ * @param max_ts maximum allowed timestamp (position, if AVSEEK_FLAG_BYTE set).
+ * @param flags if AVSEEK_FLAG_ANY is set, seek to any frame, otherwise only
+ *              to a keyframe. If AVSEEK_FLAG_BYTE is set, search by
+ *              position, not by timestamp.
+ * @return < 0 if no such sync point could be found, otherwise stream position
+ *              (stream is repositioned to this position).
+ */
+int64_t ff_gen_syncpoint_search(AVFormatContext *s,
+                                int stream_index,
+                                int64_t pos,
+                                int64_t min_ts,
+                                int64_t ts,
+                                int64_t max_ts,
+                                int flags);
+
+/**
+ * Store current parser state and file position.
+ *
+ * This function can be used by demuxers before destructive seeking algorithm
+ * to store parser state. After the seek, depending on outcome, original state
+ * can be restored or new state kept and original state freed.
+ *
+ * @note As a side effect, original parser state is reset, since structures
+ *       are relinked to stored state instead of being deeply-copied (for
+ *       performance reasons and to keep code simple).
+ *
+ * @param s context from which to save state.
+ * @return parser state object or NULL if memory could not be allocated.
+ */
+AVParserState *ff_store_parser_state(AVFormatContext *s);
+
+/**
+ * Restore previously saved parser state and file position.
+ *
+ * Saved state will be invalidated and freed by this call, since internal
+ * structures will be relinked back to stored state instead of being
+ * deeply-copied.
+ *
+ * @param s context to which to restore state (same as used for storing state).
+ * @param state state to restore.
+ */
+void ff_restore_parser_state(AVFormatContext *s, AVParserState *state);
+
+/**
+ * Free previously saved parser state.
+ *
+ * @param s context to which the state belongs (same as used for storing state).
+ * @param state state to free.
+ */
+void ff_free_parser_state(AVFormatContext *s, AVParserState *state);
+
+#endif /* AVFORMAT_SEEK_H */
diff --git a/libavformat/utils.c b/libavformat/utils.c
index b9f6c004c9..4cec2863f3 100644
--- a/libavformat/utils.c
+++ b/libavformat/utils.c
@@ -1140,7 +1140,7 @@ int av_find_default_stream_index(AVFormatContext *s)
 /**
  * Flush the frame reader.
  */
-static void av_read_frame_flush(AVFormatContext *s)
+void av_read_frame_flush(AVFormatContext *s)
 {
     AVStream *st;
     int i;