command: if only ab-loop-b is set, loop from start of file

Commit 382bafcb changed the behavior for ab-loop-a. This commit changes
ab-loop-b so that the behavior is symmetric.

Adjust the OSD rendering accordingly to the two changes.

Also fix mentions of the "ab_loop" command to the now preferred
"ab-loop".

DOCS/interface-changes.rst

@@ -20,8 +20,9 @@ Interface changes
 ::
 
  --- mpv 0.17.1 ---
-    - now ab-loops are active even if the "ab-loop-b" property is unset ("no"),
-      in which case the end of the file is used as B loop point
+    - now ab-loops are active even if one of the "ab-loop-a"/"-b" properties is
+      unset ("no"), in which case the start of the file is used if the A loop
+      point is unsert, and the end of the file for an unset B loop point
  --- mpv 0.17.0 ---
     - deprecate "track-list/N/audio-channels" property (use
       "track-list/N/demux-channel-count" instead)

DOCS/man/input.rst

@@ -1081,9 +1081,8 @@ Property list
                 "default"           MPV_FORMAT_FLAG
 
 ``ab-loop-a``, ``ab-loop-b`` (RW)
-    Set/get A-B loop points. See corresponding options and ``ab-loop`` command.
-    The special value ``no`` on ``ab-loop-a`` disables looping, while setting
-    ``ab-loop-b`` to ``no`` loops when the end of the file is reached.
+    Set/get A-B loop points. See corresponding options and ``ab-loop`` command
+    for details.
 
 ``angle`` (RW)
     Current DVD angle.

DOCS/man/options.rst

@@ -261,8 +261,13 @@ Playback Control
 ``--ab-loop-a=<time>``, ``--ab-loop-b=<time>``
     Set loop points. If playback passes the ``b`` timestamp, it will seek to
     the ``a`` timestamp. Seeking past the ``b`` point doesn't loop (this is
-    intentional). The loop-points can be adjusted at runtime with the
-    corresponding properties. See also ``ab_loop`` command.
+    intentional).
+
+    If both options are set to ``no``, looping is disabled. Otherwise, the
+    start/end of the file is used if one of the options is set to ``no``.
+
+    The loop-points can be adjusted at runtime with the corresponding
+    properties. See also ``ab-loop`` command.
 
 ``--ordered-chapters``, ``--no-ordered-chapters``
     Enabled by default.

etc/input.conf

@@ -142,7 +142,7 @@
 #CLOSE_WIN quit
 #CLOSE_WIN {encode} quit 4
 #E cycle edition                        # next edition
-#l ab_loop                              # Set/clear A-B loop points
+#l ab-loop                              # Set/clear A-B loop points
 #L cycle-values loop "inf" "no"         # toggle infinite looping
 #ctrl+c quit 4
 

player/command.c

@@ -5245,16 +5245,20 @@ void handle_ab_loop(struct MPContext *mpctx)
     struct MPOpts *opts = mpctx->opts;
 
     double now = mpctx->restart_complete ? mpctx->playback_pts : MP_NOPTS_VALUE;
-    if (now != MP_NOPTS_VALUE && opts->ab_loop[0] != MP_NOPTS_VALUE) {
+    if (now != MP_NOPTS_VALUE && (opts->ab_loop[0] != MP_NOPTS_VALUE ||
+                                  opts->ab_loop[1] != MP_NOPTS_VALUE))
+    {
+        double start = opts->ab_loop[0];
+        if (start == MP_NOPTS_VALUE)
+            start = 0;
         double end = opts->ab_loop[1];
         if (end == MP_NOPTS_VALUE)
             end = INFINITY;
-        if (ctx->prev_pts >= opts->ab_loop[0] &&
-            ctx->prev_pts < end &&
+        if (ctx->prev_pts >= start && ctx->prev_pts < end &&
             (now >= end || mpctx->stop_play == AT_END_OF_FILE))
         {
             mark_seek(mpctx);
-            queue_seek(mpctx, MPSEEK_ABSOLUTE, opts->ab_loop[0],
+            queue_seek(mpctx, MPSEEK_ABSOLUTE, start,
                        MPSEEK_EXACT, false);
         }
     }

player/osd.c

@@ -362,14 +362,15 @@ void set_osd_bar_chapters(struct MPContext *mpctx, int type)
     mpctx->osd_progbar.num_stops = 0;
     double len = get_time_length(mpctx);
     if (len > 0) {
-        if (opts->ab_loop[0] != MP_NOPTS_VALUE &&
-            opts->ab_loop[1] != MP_NOPTS_VALUE)
-        {
+        if (opts->ab_loop[0] != MP_NOPTS_VALUE) {
             MP_TARRAY_APPEND(mpctx, mpctx->osd_progbar.stops,
                         mpctx->osd_progbar.num_stops, opts->ab_loop[0] / len);
+        }
+        if (opts->ab_loop[1] != MP_NOPTS_VALUE) {
             MP_TARRAY_APPEND(mpctx, mpctx->osd_progbar.stops,
                         mpctx->osd_progbar.num_stops, opts->ab_loop[1] / len);
-        } else {
+        }
+        if (mpctx->osd_progbar.stops == 0) {
             int num = get_chapter_count(mpctx);
             for (int n = 0; n < num; n++) {
                 double time = chapter_start_time(mpctx, n);