audio: add channel map selection function

The point is selecting a minimal fallback. The AOs will call this
through the AO API, so it will be possible to add options affecting
the general channel layout selection.

It provides the following mechanism to AOs:
- forcing the correct channel order
- downmixing to stereo if no layout is available
- allow 5.1 <-> 5.1(side) fallback
- handling "unknown" channel layouts

This is quite weak and lots of code/complexity for little gain. All AOs
already made sure the channel order was correct, and the fallback is of
little value, and could perhaps be done in the frontend instead, like
stereo downmixing with --channels=2 is handled. But I'm not really sure
how this stuff should _really_ work, and the new code will hopefully
provides enough flexibility to make radical changes to channel layout
negotiation easier.

7 files changed, 297 insertions, 20 deletions

diff --git a/audio/audio.c b/audio/audio.c
index 8af6a20a1f..c9d5c9231c 100644
--- a/audio/audio.c
+++ b/audio/audio.c
@@ -37,8 +37,7 @@ void mp_audio_set_num_channels(struct mp_audio *mpa, int num_channels)
 void mp_audio_set_channels_old(struct mp_audio *mpa, int num_channels)
 {
     struct mp_chmap map;
-    mp_chmap_from_channels(&map, num_channels);
-    mp_chmap_reorder_to_alsa(&map);
+    mp_chmap_from_channels_alsa(&map, num_channels);
     mp_audio_set_channels(mpa, &map);
 }
 
diff --git a/audio/chmap.c b/audio/chmap.c
index 61df408e02..fcdb95edb3 100644
--- a/audio/chmap.c
+++ b/audio/chmap.c
@@ -100,6 +100,17 @@ static const struct mp_chmap default_layouts[MP_NUM_CHANNELS + 1] = {
     MP_CHMAP8(FL, FR, FC, LFE, BL, BR, SL, SR), // 7.1
 };
 
+// The channel order was lavc/waveex, but differs from lavc for 5, 6 and 8
+// channels. 3 and 7 channels were likely undefined (no ALSA support).
+static const char *mplayer_layouts[MP_NUM_CHANNELS + 1] = {
+    [1] = "mono",
+    [2] = "stereo",
+    [4] = "4.0",
+    [5] = "5.0(alsa)",
+    [6] = "5.1(alsa)",
+    [8] = "7.1(alsa)",
+};
+
 // Returns true if speakers are mapped uniquely, and there's at least 1 channel.
 bool mp_chmap_is_valid(const struct mp_chmap *src)
 {
@@ -192,6 +203,19 @@ void mp_chmap_from_channels(struct mp_chmap *dst, int num_channels)
     }
 }
 
+// Try to do what mplayer/mplayer2/mpv did before channel layouts were
+// introduced, i.e. get the old default channel order.
+void mp_chmap_from_channels_alsa(struct mp_chmap *dst, int num_channels)
+{
+    if (num_channels < 0 || num_channels > MP_NUM_CHANNELS) {
+        *dst = (struct mp_chmap) {0};
+    } else {
+        mp_chmap_from_str(dst, bstr0(mplayer_layouts[num_channels]));
+        if (!dst->num)
+            mp_chmap_from_channels(dst, num_channels);
+    }
+}
+
 // Set *dst to an unknown layout for the given numbers of channels.
 // If the number of channels is invalid, an invalid map is set, and
 // mp_chmap_is_valid(dst) will return false.
@@ -314,22 +338,6 @@ void mp_chmap_reorder_to_lavc(struct mp_chmap *map)
     mp_chmap_from_lavc(map, mask);
 }
 
-// Try to do what mplayer/mplayer2/mpv did before channel layouts were
-// introduced, i.e. get the old default channel order.
-void mp_chmap_reorder_to_alsa(struct mp_chmap *map)
-{
-    // The channel order was lavc/waveex, but differs from lavc for 5, 6 and 8
-    // channels. 3 and 7 channels were likely undefined (no ALSA support).
-    mp_chmap_from_channels(map, map->num);
-    if (map->num == 5) {
-        mp_chmap_from_str(map, bstr0("5.0(alsa)"));
-    } else if (map->num == 6) {
-        mp_chmap_from_str(map, bstr0("5.1(alsa)"));
-    } else if (map->num == 8) {
-        mp_chmap_from_str(map, bstr0("7.1(alsa)"));
-    }
-}
-
 // Get reordering array for from->to reordering. from->to must have the same set
 // of speakers (i.e. same number and speaker IDs, just different order). Then,
 // for each speaker n, dst[n] will be set such that:
diff --git a/audio/chmap.h b/audio/chmap.h
index 9ab97ac8ba..929283dfd5 100644
--- a/audio/chmap.h
+++ b/audio/chmap.h
@@ -104,6 +104,7 @@ void mp_chmap_reorder_norm(struct mp_chmap *map);
 
 void mp_chmap_from_channels(struct mp_chmap *dst, int num_channels);
 void mp_chmap_set_unknown(struct mp_chmap *dst, int num_channels);
+void mp_chmap_from_channels_alsa(struct mp_chmap *dst, int num_channels);
 
 void mp_chmap_remove_useless_channels(struct mp_chmap *map,
                                       const struct mp_chmap *requested);
@@ -115,8 +116,6 @@ void mp_chmap_from_lavc(struct mp_chmap *dst, uint64_t src);
 bool mp_chmap_is_lavc(const struct mp_chmap *src);
 void mp_chmap_reorder_to_lavc(struct mp_chmap *map);
 
-void mp_chmap_reorder_to_alsa(struct mp_chmap *map);
-
 void mp_chmap_get_reorder(int dst[MP_NUM_CHANNELS], const struct mp_chmap *from,
                           const struct mp_chmap *to);
 
@@ -130,4 +129,6 @@ void mp_chmap_print_help(int msgt, int msgl);
 #define mp_chmap_is_waveext mp_chmap_is_lavc
 #define mp_chmap_reorder_to_waveext mp_chmap_reorder_to_lavc
 
+#define mp_chmap_reorder_to_alsa(x) mp_chmap_from_channels_alsa((x), (x)->num)
+
 #endif
diff --git a/audio/chmap_sel.c b/audio/chmap_sel.c
new file mode 100644
index 0000000000..8e5be5c86e
--- /dev/null
+++ b/audio/chmap_sel.c
@@ -0,0 +1,210 @@
+/*
+ * This file is part of mpv.
+ *
+ * mpv is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * mpv is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with mpv.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <stdlib.h>
+#include <assert.h>
+
+#include "chmap_sel.h"
+
+// 5.1 and 5.1(side) are practically the same. It doesn't make much sense to
+// reject either of them.
+static const int replaceable_speakers[][2] = {
+    {MP_SPEAKER_ID_SL, MP_SPEAKER_ID_BL},
+    {MP_SPEAKER_ID_SR, MP_SPEAKER_ID_BR},
+    {-1},
+};
+
+// list[] contains a list of speaker pairs, with each pair indicating how
+// a speaker can be swapped for another speaker. Try to replace speakers from
+// the left of the list with the ones on the right, or the other way around.
+static bool replace_speakers(struct mp_chmap *map, const int list[][2])
+{
+    if (!mp_chmap_is_valid(map))
+        return false;
+    for (int dir = 0; dir < 2; dir++) {
+        int from = dir ? 0 : 1;
+        int to   = dir ? 1 : 0;
+        bool replaced = false;
+        struct mp_chmap t = *map;
+        for (int n = 0; n < t.num; n++) {
+            for (int i = 0; list[i][0] != -1; i++) {
+                if (t.speaker[n] == list[i][from]) {
+                    t.speaker[n] = list[i][to];
+                    replaced = true;
+                    break;
+                }
+            }
+        }
+        if (replaced && mp_chmap_is_valid(&t)) {
+            *map = t;
+            return true;
+        }
+    }
+    return false;
+}
+
+// Allow all channel layouts that can be expressed with mp_chmap.
+// (By default, all layouts are rejected.)
+void mp_chmap_sel_add_any(struct mp_chmap_sel *s)
+{
+    s->allow_any = true;
+}
+
+// Allow all waveext formats, and force waveext channel order.
+void mp_chmap_sel_add_waveext(struct mp_chmap_sel *s)
+{
+    s->allow_waveext = true;
+}
+
+void mp_chmap_sel_add_alsa_def(struct mp_chmap_sel *s)
+{
+    for (int n = 0; n < MP_NUM_CHANNELS; n++) {
+        struct mp_chmap t;
+        mp_chmap_from_channels_alsa(&t, n);
+        if (t.num)
+            mp_chmap_sel_add_map(s, &t);
+    }
+}
+
+#define ARRAY_LEN(x) (sizeof(x) / sizeof((x)[0]))
+
+// Add a channel map that should be allowed.
+void mp_chmap_sel_add_map(struct mp_chmap_sel *s, const struct mp_chmap *map)
+{
+    assert(s->num_chmaps < ARRAY_LEN(s->chmaps));
+    if (mp_chmap_is_valid(map))
+        s->chmaps[s->num_chmaps++] = *map;
+}
+
+// Allow all waveext formats in default order.
+void mp_chmap_sel_add_waveext_def(struct mp_chmap_sel *s)
+{
+    for (int n = 1; n < MP_NUM_CHANNELS; n++) {
+        struct mp_chmap map;
+        mp_chmap_from_channels(&map, n);
+        mp_chmap_sel_add_map(s, &map);
+    }
+}
+
+// Whitelist a speaker (MP_SPEAKER_ID_...). All layouts that contain whitelisted
+// speakers are allowed.
+void mp_chmap_sel_add_speaker(struct mp_chmap_sel *s, int id)
+{
+    assert(id >= 0 && id < MP_SPEAKER_ID_COUNT);
+    s->speakers[id] = true;
+}
+
+static bool test_speakers(const struct mp_chmap_sel *s, struct mp_chmap *map)
+{
+    for (int n = 0; n < map->num; n++) {
+        if (!s->speakers[map->speaker[n]])
+            return false;
+    }
+    return true;
+}
+
+static bool test_maps(const struct mp_chmap_sel *s, struct mp_chmap *map)
+{
+    for (int n = 0; n < s->num_chmaps; n++) {
+        if (mp_chmap_equals_reordered(&s->chmaps[n], map)) {
+            *map = s->chmaps[n];
+            return true;
+        }
+    }
+    return false;
+}
+
+static bool test_waveext(const struct mp_chmap_sel *s, struct mp_chmap *map)
+{
+    if (s->allow_waveext) {
+        struct mp_chmap t = *map;
+        mp_chmap_reorder_to_waveext(&t);
+        if (mp_chmap_is_waveext(&t)) {
+            *map = t;
+            return true;
+        }
+    }
+    return false;
+}
+
+static bool test_layout(const struct mp_chmap_sel *s, struct mp_chmap *map)
+{
+    if (!mp_chmap_is_valid(map))
+        return false;
+
+    return s->allow_any || test_waveext(s, map) || test_speakers(s, map) ||
+           test_maps(s, map);
+}
+
+// Determine which channel map to use given a source channel map, and various
+// parameters restricting possible choices. If the map doesn't match, select
+// a fallback and set it.
+// If no matching layout is found, a reordered layout may be returned.
+// If that is not possible, a fallback for up/downmixing may be returned.
+// If no choice is possible, set *map to empty.
+bool mp_chmap_sel_adjust(const struct mp_chmap_sel *s, struct mp_chmap *map)
+{
+    if (test_layout(s, map))
+        return true;
+    if (mp_chmap_is_unknown(map)) {
+        struct mp_chmap t = {0};
+        if (mp_chmap_sel_get_def(s, &t, map->num) && test_layout(s, &t)) {
+            *map = t;
+            return true;
+        }
+    }
+    // 5.1 <-> 5.1(side)
+    if (replace_speakers(map, replaceable_speakers) && test_layout(s, map))
+        return true;
+    // Fallback to mono/stereo as last resort
+    if (map->num == 1) {
+        *map = (struct mp_chmap) MP_CHMAP_INIT_MONO;
+    } else if (map->num >= 2) {
+        *map = (struct mp_chmap) MP_CHMAP_INIT_STEREO;
+    }
+    if (test_layout(s, map))
+        return true;
+    *map = (struct mp_chmap) {0};
+    return false;
+}
+
+// Set map to a default layout with num channels. Used for audio APIs that
+// return a channel count as part of format negotiation, but give no
+// information about the channel layout.
+// If the channel count is correct, do nothing and leave *map untouched.
+bool mp_chmap_sel_get_def(const struct mp_chmap_sel *s, struct mp_chmap *map,
+                          int num)
+{
+    if (map->num != num) {
+        *map = (struct mp_chmap) {0};
+        // Set of speakers or waveext might allow it.
+        struct mp_chmap t;
+        mp_chmap_from_channels(&t, num);
+        mp_chmap_reorder_to_waveext(&t);
+        if (test_layout(s, &t)) {
+            *map = t;
+        } else {
+            for (int n = 0; n < s->num_chmaps; n++) {
+                if (s->chmaps[n].num == num) {
+                    *map = s->chmaps[n];
+                    break;
+                }
+            }
+        }
+    }
+    return map->num > 0;
+}
diff --git a/audio/chmap_sel.h b/audio/chmap_sel.h
new file mode 100644
index 0000000000..c9d75196a5
--- /dev/null
+++ b/audio/chmap_sel.h
@@ -0,0 +1,43 @@
+/*
+ * This file is part of mpv.
+ *
+ * mpv is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * mpv is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with mpv.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef MP_CHMAP_SEL_H
+#define MP_CHMAP_SEL_H
+
+#include <stdbool.h>
+
+#include "chmap.h"
+
+struct mp_chmap_sel {
+    // should be considered opaque
+    bool allow_any, allow_waveext;
+    bool speakers[MP_SPEAKER_ID_COUNT];
+    struct mp_chmap chmaps[20];
+    int num_chmaps;
+};
+
+void mp_chmap_sel_add_any(struct mp_chmap_sel *s);
+void mp_chmap_sel_add_waveext(struct mp_chmap_sel *s);
+void mp_chmap_sel_add_waveext_def(struct mp_chmap_sel *s);
+void mp_chmap_sel_add_alsa_def(struct mp_chmap_sel *s);
+void mp_chmap_sel_add_map(struct mp_chmap_sel *s, const struct mp_chmap *map);
+void mp_chmap_sel_add_speaker(struct mp_chmap_sel *s, int id);
+bool mp_chmap_sel_adjust(const struct mp_chmap_sel *s, struct mp_chmap *map);
+bool mp_chmap_sel_get_def(const struct mp_chmap_sel *s, struct mp_chmap *map,
+                          int num);
+
+#endif
diff --git a/audio/out/ao.c b/audio/out/ao.c
index bf9b47a14e..10badcfa07 100644
--- a/audio/out/ao.c
+++ b/audio/out/ao.c
@@ -250,7 +250,17 @@ void ao_resume(struct ao *ao)
         ao->driver->resume(ao);
 }
 
+bool ao_chmap_sel_adjust(struct ao *ao, const struct mp_chmap_sel *s,
+                         struct mp_chmap *map)
+{
+    return mp_chmap_sel_adjust(s, map);
+}
 
+bool ao_chmap_sel_get_def(struct ao *ao, const struct mp_chmap_sel *s,
+                          struct mp_chmap *map, int num)
+{
+    return mp_chmap_sel_get_def(s, map, num);
+}
 
 int old_ao_init(struct ao *ao, char *params)
 {
diff --git a/audio/out/ao.h b/audio/out/ao.h
index 50f121bec2..d908841457 100644
--- a/audio/out/ao.h
+++ b/audio/out/ao.h
@@ -24,6 +24,7 @@
 #include "core/bstr.h"
 #include "core/mp_common.h"
 #include "audio/chmap.h"
+#include "audio/chmap_sel.h"
 
 enum aocontrol {
     // _VOLUME commands take struct ao_control_vol pointer for input/output.
@@ -121,6 +122,11 @@ void ao_reset(struct ao *ao);
 void ao_pause(struct ao *ao);
 void ao_resume(struct ao *ao);
 
+bool ao_chmap_sel_adjust(struct ao *ao, const struct mp_chmap_sel *s,
+                         struct mp_chmap *map);
+bool ao_chmap_sel_get_def(struct ao *ao, const struct mp_chmap_sel *s,
+                          struct mp_chmap *map, int num);
+
 int old_ao_control(struct ao *ao, enum aocontrol cmd, void *arg);
 int old_ao_init(struct ao *ao, char *params);
 void old_ao_uninit(struct ao *ao, bool cut_audio);