ffmpeg: Implement support for seeking relative to EOF

Fixes Ticket227
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/ffmpeg.texi

@@ -307,6 +307,11 @@ input until the timestamps reach @var{position}.
 
 @var{position} may be either in seconds or in @code{hh:mm:ss[.xxx]} form.
 
+@item -sseof @var{position} (@emph{input/output})
+
+Like the -ss option but relative to the "end of file". That is negative
+values are earlier in the file, 0 is at EOF.
+
 @item -itsoffset @var{offset} (@emph{input})
 Set the input time offset.
 

ffmpeg.h

@@ -92,6 +92,7 @@ typedef struct OptionsContext {
 
     /* input/output options */
     int64_t start_time;
+    int64_t start_time_eof;
     int seek_timestamp;
     const char *format;
 

ffmpeg_opt.c

@@ -156,6 +156,7 @@ static void init_options(OptionsContext *o)
     o->stop_time = INT64_MAX;
     o->mux_max_delay  = 0.7;
     o->start_time     = AV_NOPTS_VALUE;
+    o->start_time_eof = AV_NOPTS_VALUE;
     o->recording_time = INT64_MAX;
     o->limit_filesize = UINT64_MAX;
     o->chapters_input_file = INT_MAX;
@@ -933,6 +934,8 @@ static int open_input_file(OptionsContext *o, const char *filename)
         }
     }
 
+    if (o->start_time_eof != AV_NOPTS_VALUE && ic->duration>0)
+        o->start_time = o->start_time_eof + ic->duration;
     timestamp = (o->start_time == AV_NOPTS_VALUE) ? 0 : o->start_time;
     /* add the stream start time */
     if (!o->seek_timestamp && ic->start_time != AV_NOPTS_VALUE)
@@ -3019,6 +3022,9 @@ const OptionDef options[] = {
     { "ss",             HAS_ARG | OPT_TIME | OPT_OFFSET |
                         OPT_INPUT | OPT_OUTPUT,                      { .off = OFFSET(start_time) },
         "set the start time offset", "time_off" },
+    { "sseof",          HAS_ARG | OPT_TIME | OPT_OFFSET |
+                        OPT_INPUT | OPT_OUTPUT,                      { .off = OFFSET(start_time_eof) },
+        "set the start time offset relative to EOF", "time_off" },
     { "seek_timestamp", HAS_ARG | OPT_INT | OPT_OFFSET |
                         OPT_INPUT,                                   { .off = OFFSET(seek_timestamp) },
         "enable/disable seeking by timestamp with -ss" },