avformat/segafilmenc: Avoid seek when writing header

Up until now, the Sega FILM muxer would first write all the packet data,
then shift the data (in the muxer's write_trailer function) by the amount
necessary to write the header at the front (which entails a seek to the
front), then seek back to the beginning and actually write the header.

This commit changes this: The dynamic buffer that is used to write the
sample table (containing information about each sample in the file) is
now used to write the complete header. This is possible because the size
of everything in the header except the sample table is known in advance.
Said buffer can then be used as one of the two temporary buffers used
for shifting which also reduces the amount one has to allocate for this.
Thereby the header will be written when shifting, so that the second
seek to the beginning is unnecessary.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/segafilmenc.c

@@ -29,7 +29,9 @@
  *   http://wiki.multimedia.cx/index.php?title=Sega_FILM
  */
 
+#include "libavutil/avassert.h"
 #include "libavutil/intreadwrite.h"
+#include "libavcodec/bytestream.h"
 #include "avformat.h"
 #include "internal.h"
 #include "avio_internal.h"
@@ -160,11 +162,13 @@ static int film_init(AVFormatContext *format_context)
     }
     if ((ret = avio_open_dyn_buf(&film->header)) < 0)
         return ret;
+    ffio_fill(film->header, 0, 16 + 32 + 16);
 
     return 0;
 }
 
-static int shift_data(AVFormatContext *format_context, int64_t shift_size)
+static int write_header(AVFormatContext *format_context, uint8_t *header,
+                        unsigned header_size)
 {
     int ret = 0;
     int64_t pos, pos_end;
@@ -173,11 +177,12 @@ static int shift_data(AVFormatContext *format_context, int64_t shift_size)
     int read_size[2];
     AVIOContext *read_pb;
 
-    buf = av_malloc(shift_size * 2);
+    buf = av_malloc(header_size);
     if (!buf)
         return AVERROR(ENOMEM);
     read_buf[0] = buf;
-    read_buf[1] = buf + shift_size;
+    read_buf[1]  = header;
+    read_size[1] = header_size;
 
     /* Write the header at the beginning of the file, shifting all content as necessary;
      * based on the approach used by MOV faststart. */
@@ -190,25 +195,20 @@ static int shift_data(AVFormatContext *format_context, int64_t shift_size)
         return ret;
     }
 
-    /* mark the end of the shift to up to the last data we wrote, and get ready
-     * for writing */
-    pos_end = avio_tell(format_context->pb);
-    avio_seek(format_context->pb, shift_size, SEEK_SET);
+    /* Mark the end of the shift to up to the last data we are going to write,
+     * and get ready for writing */
+    pos_end = avio_tell(format_context->pb) + header_size;
+    pos = avio_seek(format_context->pb, 0, SEEK_SET);
 
     /* start reading at where the new header will be placed */
     avio_seek(read_pb, 0, SEEK_SET);
-    pos = avio_tell(read_pb);
 
-#define READ_BLOCK do {                                                             \
-    read_size[read_buf_id] = avio_read(read_pb, read_buf[read_buf_id], shift_size);  \
-    read_buf_id ^= 1;                                                               \
-} while (0)
-
-    /* shift data by chunk of at most shift_size */
-    READ_BLOCK;
+    /* shift data by chunk of at most header_size */
     do {
         int n;
-        READ_BLOCK;
+        read_size[read_buf_id] = avio_read(read_pb, read_buf[read_buf_id],
+                                           header_size);
+        read_buf_id ^= 1;
         n = read_size[read_buf_id];
         if (n <= 0)
             break;
@@ -224,81 +224,75 @@ static int shift_data(AVFormatContext *format_context, int64_t shift_size)
 static int film_write_header(AVFormatContext *format_context)
 {
     int ret = 0;
-    unsigned sample_table_size, stabsize, headersize, packet_count;
-    AVIOContext *pb = format_context->pb;
+    unsigned stabsize, headersize, packet_count;
     FILMOutputContext *film = format_context->priv_data;
     AVStream *video = NULL;
-    uint8_t *sample_table;
+    uint8_t *header, *ptr;
 
     /* Calculate how much we need to reserve for the header;
      * this is the amount the rest of the data will be shifted up by. */
-    sample_table_size = avio_get_dyn_buf(film->header, &sample_table);
-    packet_count      = sample_table_size / 16;
-    sample_table_size = packet_count * 16;
-    stabsize = 16 + sample_table_size;
+    headersize = avio_get_dyn_buf(film->header, &header);
+    if (headersize < 64) {
+        av_assert1(film->header->error < 0);
+        return film->header->error;
+    }
+    packet_count = (headersize - 64) / 16;
+    stabsize = 16 + 16 * packet_count;
     headersize = 16 + /* FILM header base */
                  32 + /* FDSC chunk */
                  stabsize;
 
-    ret = shift_data(format_context, headersize);
-    if (ret < 0)
-        return ret;
-    /* Seek back to the beginning to start writing the header now */
-    avio_seek(pb, 0, SEEK_SET);
-
-    /* First, write the FILM header; this is very simple */
-
-    ffio_wfourcc(pb, "FILM");
-    avio_wb32(pb, 48 + stabsize);
+    /* Write the header at the position in the buffer reserved for it.
+     * First, write the FILM header; this is very simple */
+    ptr = header;
+    bytestream_put_be32(&ptr, MKBETAG('F', 'I', 'L', 'M'));
+    bytestream_put_be32(&ptr, headersize);
     /* This seems to be okay to hardcode, since this muxer targets 1.09 features;
      * videos produced by this muxer are readable by 1.08 and lower players. */
-    ffio_wfourcc(pb, "1.09");
-    /* I have no idea what this field does, might be reserved */
-    avio_wb32(pb, 0);
+    bytestream_put_be32(&ptr, MKBETAG('1', '.', '0', '9'));
+    /* I have no idea what the next four bytes do, might be reserved */
+    ptr += 4;
 
     /* Next write the FDSC (file description) chunk */
-    ffio_wfourcc(pb, "FDSC");
-    avio_wb32(pb, 0x20); /* Size of FDSC chunk */
+    bytestream_put_be32(&ptr, MKBETAG('F', 'D', 'S', 'C'));
+    bytestream_put_be32(&ptr, 0x20); /* Size of FDSC chunk */
 
     video = format_context->streams[film->video_index];
 
     /* The only two supported codecs; raw video is rare */
     switch (video->codecpar->codec_id) {
     case AV_CODEC_ID_CINEPAK:
-        ffio_wfourcc(pb, "cvid");
+        bytestream_put_be32(&ptr, MKBETAG('c', 'v', 'i', 'd'));
         break;
     case AV_CODEC_ID_RAWVIDEO:
-        ffio_wfourcc(pb, "raw ");
+        bytestream_put_be32(&ptr, MKBETAG('r', 'a', 'w', ' '));
         break;
     }
 
-    avio_wb32(pb, video->codecpar->height);
-    avio_wb32(pb, video->codecpar->width);
-    avio_w8(pb, 24); /* Bits per pixel - observed to always be 24 */
+    bytestream_put_be32(&ptr, video->codecpar->height);
+    bytestream_put_be32(&ptr, video->codecpar->width);
+    bytestream_put_byte(&ptr, 24); /* Bits per pixel - observed to always be 24 */
 
     if (film->audio_index > -1) {
         AVStream *audio = format_context->streams[film->audio_index];
         int audio_codec = get_audio_codec_id(audio->codecpar->codec_id);
 
-        avio_w8(pb, audio->codecpar->channels); /* Audio channels */
-        avio_w8(pb, audio->codecpar->bits_per_coded_sample); /* Audio bit depth */
-        avio_w8(pb, audio_codec); /* Compression - 0 is PCM, 2 is ADX */
-        avio_wb16(pb, audio->codecpar->sample_rate); /* Audio sampling rate */
+        bytestream_put_byte(&ptr, audio->codecpar->channels); /* Audio channels */
+        bytestream_put_byte(&ptr, audio->codecpar->bits_per_coded_sample); /* Audio bit depth */
+        bytestream_put_byte(&ptr, audio_codec); /* Compression - 0 is PCM, 2 is ADX */
+        bytestream_put_be16(&ptr, audio->codecpar->sample_rate); /* Audio sampling rate */
     } else {
-        /* Set all these fields to 0 if there's no audio */
-        avio_w8(pb, 0);
-        avio_w8(pb, 0);
-        avio_w8(pb, 0);
-        avio_wb16(pb, 0);
+        /* If there is no audio, all the audio fields should be set to zero.
+         * ffio_fill() already did this for us. */
+        ptr += 1 + 1 + 1 + 2;
     }
 
     /* I have no idea what this pair of fields does either, might be reserved */
-    avio_wb32(pb, 0);
-    avio_wb16(pb, 0);
+    ptr += 4 + 2;
 
     /* Finally, write the STAB (sample table) chunk */
-    ffio_wfourcc(pb, "STAB");
-    avio_wb32(pb, stabsize);
+    bytestream_put_be32(&ptr, MKBETAG('S', 'T', 'A', 'B'));
+    bytestream_put_be32(&ptr, stabsize);
     /* Framerate base frequency. Here we're assuming that the frame rate is even.
      * In real world Sega FILM files, there are usually a couple of approaches:
      * a) framerate base frequency is the same as the framerate, and ticks
@@ -308,12 +302,14 @@ static int film_write_header(AVFormatContext *format_context)
      * The latter occurs even in cases where the frame rate is even; for example, in
      * Lunar: Silver Star Story, the base frequency is 600 and each frame, the ticks
      * are incremented by 25 for an evenly spaced framerate of 24fps. */
-    avio_wb32(pb, av_q2d(av_inv_q(video->time_base)));
+    bytestream_put_be32(&ptr, av_q2d(av_inv_q(video->time_base)));
 
-    avio_wb32(pb, packet_count);
+    bytestream_put_be32(&ptr, packet_count);
 
-    /* Finally, write out each packet's data to the header */
-    avio_write(pb, sample_table, sample_table_size);
+    /* Finally, shift the data and write out the header. */
+    ret = write_header(format_context, header, headersize);
+    if (ret < 0)
+        return ret;
 
     return 0;
 }