command: more intuitive chapter seek behavior

If close to chapter start, skipping back goes to previous chapter (no change).
If more than <threshold> seconds in, skipping back will now go to the beginning
of the current chapter instead.

The threshold is set by the new option --chapter-seek-threshold and defaults to
5 seconds.  A negative value disables the new functionality.

5 files changed, 36 insertions, 2 deletions

diff --git a/DOCS/man/en/mpv.rst b/DOCS/man/en/mpv.rst
index af905539f0..8931a129f2 100644
--- a/DOCS/man/en/mpv.rst
+++ b/DOCS/man/en/mpv.rst
@@ -176,7 +176,9 @@ P
     Show progression bar, elapsed time and total duration on the OSD.
 
 ! and @
-    Seek to the beginning of the previous/next chapter.
+    Seek to the beginning of the previous/next chapter. In most cases,
+    "previous" will actually go to the beginning of the current chapter; see
+    ``--chapter-seek-threshold``.
 
 D (``--vo=vdpau``, ``--vf=yadif`` only)
     Activate/deactivate deinterlacer.
diff --git a/DOCS/man/en/options.rst b/DOCS/man/en/options.rst
index 5a9aa5fc12..21e4a7991a 100644
--- a/DOCS/man/en/options.rst
+++ b/DOCS/man/en/options.rst
@@ -459,6 +459,13 @@
     the start of the next one then keep playing video normally over the
     chapter change instead of doing a seek.
 
+``--chapter-seek-threshold=<seconds>``
+    Distance in seconds from the beginning of a chapter within which a backward
+    chapter seek will go to the previous chapter (default: 5.0). Past this
+    threshold, a backward chapter seek will go to the beginning of the current
+    chapter instead. A negative value means always go back to the previous
+    chapter.
+
 ``--colormatrix=<colorspace>``
     Controls the YUV to RGB color space conversion when playing video. There
     are various standards. Normally, BT.601 should be used for SD video, and
diff --git a/mpvcore/command.c b/mpvcore/command.c
index b04c05c9e6..b7aa9dfe6a 100644
--- a/mpvcore/command.c
+++ b/mpvcore/command.c
@@ -390,9 +390,30 @@ static int mp_property_chapter(m_option_t *prop, int action, void *arg,
         *(char **) arg = chapter_name;
         return M_PROPERTY_OK;
     }
+    case M_PROPERTY_SWITCH:
     case M_PROPERTY_SET: ;
-        int step_all = *(int *)arg - chapter;
+        int step_all;
+        if (action == M_PROPERTY_SWITCH) {
+            struct m_property_switch_arg *sarg = arg;
+            step_all = ROUND(sarg->inc);
+            // Check threshold for relative backward seeks
+            if (mpctx->opts->chapter_seek_threshold >= 0 && step_all < 0) {
+                if (chapter < -1)
+                    return M_PROPERTY_UNAVAILABLE;
+                double current_chapter_start =
+                    chapter_start_time(mpctx, chapter);
+                // If we are far enough into a chapter, seek back to the
+                // beginning of current chapter instead of previous one
+                if (current_chapter_start >= 0 &&
+                    get_current_time(mpctx) - current_chapter_start >
+                    mpctx->opts->chapter_seek_threshold)
+                    step_all++;
+            }
+        } else // Absolute set
+            step_all = *(int *)arg - chapter;
         chapter += step_all;
+        if (chapter < -1)
+            chapter = -1;
         if (chapter >= get_chapter_count(mpctx) && step_all > 0) {
             mpctx->stop_play = PT_NEXT_ENTRY;
         } else {
diff --git a/mpvcore/options.c b/mpvcore/options.c
index e0d138a545..4a3c067404 100644
--- a/mpvcore/options.c
+++ b/mpvcore/options.c
@@ -661,6 +661,8 @@ const m_option_t mp_opts[] = {
     OPT_FLAG("ordered-chapters", ordered_chapters, 0),
     OPT_INTRANGE("chapter-merge-threshold", chapter_merge_threshold, 0, 0, 10000),
 
+    OPT_DOUBLE("chapter-seek-threshold", chapter_seek_threshold, 0),
+
     // a-v sync stuff:
     OPT_FLAG("correct-pts", correct_pts, 0),
     OPT_CHOICE("pts-association-mode", user_pts_assoc_mode, 0,
@@ -772,6 +774,7 @@ const struct MPOpts mp_default_opts = {
     .loop_times = -1,
     .ordered_chapters = 1,
     .chapter_merge_threshold = 100,
+    .chapter_seek_threshold = 5.0,
     .load_config = 1,
     .position_resume = 1,
     .stream_cache_min_percent = 20.0,
diff --git a/mpvcore/options.h b/mpvcore/options.h
index ff2b5954ff..9309cc1a85 100644
--- a/mpvcore/options.h
+++ b/mpvcore/options.h
@@ -83,6 +83,7 @@ typedef struct MPOpts {
     int loop_times;
     int ordered_chapters;
     int chapter_merge_threshold;
+    double chapter_seek_threshold;
     int quiet;
     int load_config;
     int use_filedir_conf;