[FFmpeg-devel] [PATCH 4/8] avformat/flvenc: Use array instead of linked list for index

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Using a linked list had very much overhead (the pointer to the next
entry increased the size of the index entry struct from 16 to 24 bytes,
not to mention the overhead of having separate allocations), so it is
better to (re)allocate a continuous array for the index.
av_fast_realloc_array() is used for this purpose, in order not to
reallocate the array for each entry. It's feature of allowing to
restrict the number of elements of the array is put to good use here:
Each entry will lead to 18 bytes being written and the array is
contained in an element whose size field has a length of three bytes,
so that more than (2^24 - 1) / 18 entries make no sense.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
---
 libavformat/flvenc.c | 60 +++++++++++++++++---------------------------
 1 file changed, 23 insertions(+), 37 deletions(-)

diff --git a/libavformat/flvenc.c b/libavformat/flvenc.c
index b6135b25bf..426135aa15 100644
--- a/libavformat/flvenc.c
+++ b/libavformat/flvenc.c
@@ -34,6 +34,8 @@
 #include "libavutil/opt.h"
 #include "libavcodec/put_bits.h"
 
+/* Each FLVFileposition entry is written as two AMF double values. */
+#define FILEPOSITION_ENTRY_SIZE (2 * 9)
 
 static const AVCodecTag flv_video_codec_ids[] = {
     { AV_CODEC_ID_FLV1,     FLV_CODECID_H263 },
@@ -73,7 +75,6 @@ typedef enum {
 typedef struct FLVFileposition {
     int64_t keyframe_position;
     double keyframe_timestamp;
-    struct FLVFileposition *next;
 } FLVFileposition;
 
 typedef struct FLVContext {
@@ -107,9 +108,9 @@ typedef struct FLVContext {
     int acurframeindex;
     int64_t keyframes_info_offset;
 
-    int64_t filepositions_count;
     FLVFileposition *filepositions;
-    FLVFileposition *head_filepositions;
+    unsigned filepositions_count;
+    size_t filepositions_allocated;
 
     AVCodecParameters *audio_par;
     AVCodecParameters *video_par;
@@ -548,27 +549,20 @@ static void flv_write_codec_header(AVFormatContext* s, AVCodecParameters* par, i
 
 static int flv_append_keyframe_info(AVFormatContext *s, FLVContext *flv, double ts, int64_t pos)
 {
-    FLVFileposition *position = av_malloc(sizeof(FLVFileposition));
-
-    if (!position) {
-        av_log(s, AV_LOG_WARNING, "no mem for add keyframe index!\n");
-        return AVERROR(ENOMEM);
-    }
-
-    position->keyframe_timestamp = ts;
-    position->keyframe_position = pos;
-
-    if (!flv->filepositions_count) {
-        flv->filepositions = position;
-        flv->head_filepositions = flv->filepositions;
-        position->next = NULL;
-    } else {
-        flv->filepositions->next = position;
-        position->next = NULL;
-        flv->filepositions = flv->filepositions->next;
+    /* The filepositions array is part of a metadata element whose size field
+     * is three bytes long; so bound the number of filepositions in order not
+     * to allocate more than could ever be written. */
+    int ret = av_fast_realloc_array(&flv->filepositions,
+                                    &flv->filepositions_allocated,
+                                    flv->filepositions_count + 1,
+                                    (((1 << 24) - 1) - 10) / FILEPOSITION_ENTRY_SIZE,
+                                    sizeof(*flv->filepositions));
+    if (ret < 0) {
+        av_log(s, AV_LOG_WARNING, "Adding entry to keyframe index failed.\n");
+        return ret;
     }
 
-    flv->filepositions_count++;
+    flv->filepositions[flv->filepositions_count++] = (FLVFileposition){ pos, ts };
 
     return 0;
 }
@@ -733,7 +727,7 @@ static int flv_write_trailer(AVFormatContext *s)
     int64_t cur_pos = avio_tell(s->pb);
 
     if (build_keyframes_idx) {
-        FLVFileposition *newflv_posinfo;
+        const FLVFileposition *flv_posinfo = flv->filepositions;
 
         avio_seek(pb, flv->videosize_offset, SEEK_SET);
         put_amf_double(pb, flv->videosize);
@@ -758,15 +752,13 @@ static int flv_write_trailer(AVFormatContext *s)
         avio_seek(pb, flv->keyframes_info_offset, SEEK_SET);
         put_amf_string(pb, "filepositions");
         put_amf_dword_array(pb, flv->filepositions_count);
-        for (newflv_posinfo = flv->head_filepositions; newflv_posinfo; newflv_posinfo = newflv_posinfo->next) {
-            put_amf_double(pb, newflv_posinfo->keyframe_position + flv->keyframe_index_size);
-        }
+        for (unsigned i = 0; i < flv->filepositions_count; i++)
+            put_amf_double(pb, flv_posinfo[i].keyframe_position + flv->keyframe_index_size);
 
         put_amf_string(pb, "times");
         put_amf_dword_array(pb, flv->filepositions_count);
-        for (newflv_posinfo = flv->head_filepositions; newflv_posinfo; newflv_posinfo = newflv_posinfo->next) {
-            put_amf_double(pb, newflv_posinfo->keyframe_timestamp);
-        }
+        for (unsigned i = 0; i < flv->filepositions_count; i++)
+            put_amf_double(pb, flv_posinfo[i].keyframe_timestamp);
 
         put_amf_string(pb, "");
         avio_w8(pb, AMF_END_OF_OBJECT);
@@ -1037,15 +1029,9 @@ static int flv_check_bitstream(AVFormatContext *s, AVStream *st,
 static void flv_deinit(AVFormatContext *s)
 {
     FLVContext *flv = s->priv_data;
-    FLVFileposition *filepos = flv->head_filepositions;
 
-    while (filepos) {
-        FLVFileposition *next = filepos->next;
-        av_free(filepos);
-        filepos = next;
-    }
-    flv->filepositions = flv->head_filepositions = NULL;
-    flv->filepositions_count = 0;
+    flv->filepositions_allocated = flv->filepositions_count = 0;
+    av_freep(&flv->filepositions);
 }
 
 static const AVOption options[] = {
-- 
2.34.1

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