options: support chapters for --start and --end

The --start and --end switch now accept a chapter number. The chapter
number is prefixed with '#', e.g. "--start=#2" jumps to chapter 2.

The chapter support might be able to replace --chapter completely, but
for now I am not sure how well this works out with e.g. DVDs and BDs,
and a separate --chapter option is useful interface-wise.

(This was supposed to be added in 51503a, but apparently the fixup
commit adding it was lost in a rebase. This might also be the reason
for the mess-up fixed in 394285.)

DOCS/man/en/options.rst

@@ -1703,7 +1703,9 @@
     The general format for absolute times is ``[[hh:]mm:]ss[.ms]``. If the time
     is negated with ``-``, the seek is relative from the end of the file.
 
-    It's also possible to seek to a percent position with ``pp%``.
+    ``pp%`` seeks to percent position pp (0-100).
+
+    ``#c`` seeks to chapter number c. (Chapters start from 1.)
 
     *EXAMPLE*:
 
@@ -1718,6 +1720,8 @@
     ``--start=-3:20 --length=10``
         Seeks to 3 minutes and 20 seconds before the end of the file, plays
         10 seconds, and exits.
+    ``--start=#2 --end=#4``
+        Plays chapters 2 and 3, and exits.
 
 --ssf=<mode>
     Specifies software scaler parameters.

core/m_option.c

@@ -1268,6 +1268,16 @@ static int parse_rel_time(const m_option_t *opt, struct bstr name,
         }
     }
 
+    // Chapter pos
+    if (bstr_startswith0(param, "#")) {
+        int chapter = bstrtoll(bstr_cut(param, 1), &param, 10);
+        if (param.len == 0 && chapter >= 1) {
+            t.type = REL_TIME_CHAPTER;
+            t.pos = chapter - 1;
+            goto out;
+        }
+    }
+
     bool sign = bstr_eatstart0(&param, "-");
     double time;
     if (parse_timestring(param, &time, 0)) {
@@ -1277,7 +1287,7 @@ static int parse_rel_time(const m_option_t *opt, struct bstr name,
     }
 
     mp_msg(MSGT_CFGPARSER, MSGL_ERR,
-           "Option %.*s: invalid time or size: '%.*s'\n",
+           "Option %.*s: invalid time or position: '%.*s'\n",
            BSTR_P(name), BSTR_P(param));
     return M_OPT_INVALID;
 

core/m_option.h

@@ -63,6 +63,7 @@ enum m_rel_time_type {
     REL_TIME_ABSOLUTE,
     REL_TIME_NEGATIVE,
     REL_TIME_PERCENT,
+    REL_TIME_CHAPTER,
 };
 
 struct m_rel_time {

core/mplayer.c

@@ -270,6 +270,10 @@ static double rel_time_to_abs(struct MPContext *mpctx, struct m_rel_time t,
         if (length != 0)
             return length * (t.pos / 100.0);
         break;
+    case REL_TIME_CHAPTER:
+        if (chapter_start_time(mpctx, t.pos) >= 0)
+            return chapter_start_time(mpctx, t.pos);
+        break;
     }
     return fallback_time;
 }