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avformat/matroskaenc: Avoid seeking when writing level 1 elements

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Up until now, the writing process for level 1 elements (those elements
for which CRC-32 elements are written by default) was this in case the
output was seekable: Write the EBML ID, write an "unkown length" EBML
number of the desired length, then write the element into a dynamic
buffer, then write the dynamic buffer (after possible calculation and
writing of the CRC-element), then seek back to the size element and
overwrite the unknown-size element with the real size. The seeking and
overwriting part has been eliminated by not writing the size initially.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>
This commit is contained in:
Andreas Rheinhardt 2019-04-20 01:41:05 +02:00 committed by James Almer
parent 8f53fd2dfd
commit e04a24e701
1 changed files with 10 additions and 5 deletions
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15
libavformat/matroskaenc.c
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@ -58,8 +58,9 @@
#include "libavcodec/internal.h"
typedef struct ebml_master {
int64_t pos; ///< absolute offset in the file where the master's elements start
int sizebytes; ///< how many bytes were reserved for the size
int64_t pos; ///< absolute offset in the containing AVIOContext where the size field starts
///< for level 1 elements or else where the master's elements start
int sizebytes; ///< how many bytes were reserved/shall be used for the size
} ebml_master;
typedef struct mkv_seekhead_entry {
@ -316,6 +317,7 @@ static ebml_master start_ebml_master(AVIOContext *pb, uint32_t elementid,
uint64_t expectedsize)
{
int bytes = expectedsize ? ebml_num_size(expectedsize) : 8;
put_ebml_id(pb, elementid);
put_ebml_size_unknown(pb, bytes);
return (ebml_master) {avio_tell(pb), bytes };
@ -340,7 +342,10 @@ static int start_ebml_master_crc32(AVIOContext *pb, AVIOContext **dyn_cp, Matros
return ret;
if (pb->seekable & AVIO_SEEKABLE_NORMAL) {
*master = start_ebml_master(pb, elementid, expectedsize);
int bytes = expectedsize ? ebml_num_size(expectedsize) : 8;
put_ebml_id(pb, elementid);
*master = (ebml_master) { avio_tell(pb), bytes };
if (mkv->write_crc)
put_ebml_void(*dyn_cp, 6); /* Reserve space for CRC32 so position/size calculations using avio_tell() take it into account */
} else
@ -357,13 +362,13 @@ static void end_ebml_master_crc32(AVIOContext *pb, AVIOContext **dyn_cp, Matrosk
if (pb->seekable & AVIO_SEEKABLE_NORMAL) {
size = avio_close_dyn_buf(*dyn_cp, &buf);
put_ebml_num(pb, size, master.sizebytes);
if (mkv->write_crc) {
skip = 6; /* Skip reserved 6-byte long void element from the dynamic buffer. */
AV_WL32(crc, av_crc(av_crc_get_table(AV_CRC_32_IEEE_LE), UINT32_MAX, buf + skip, size - skip) ^ UINT32_MAX);
put_ebml_binary(pb, EBML_ID_CRC32, crc, sizeof(crc));
}
avio_write(pb, buf + skip, size - skip);
end_ebml_master(pb, master);
} else {
end_ebml_master(*dyn_cp, master);
size = avio_close_dyn_buf(*dyn_cp, &buf);
@ -383,8 +388,8 @@ static void end_ebml_master_crc32_preliminary(AVIOContext *pb, AVIOContext **dyn
uint8_t *buf;
int size = avio_get_dyn_buf(*dyn_cp, &buf);
put_ebml_num(pb, size, master.sizebytes);
avio_write(pb, buf, size);
end_ebml_master(pb, master);
}
static void put_xiph_size(AVIOContext *pb, int size)