Add combo hook to allow per layer combo reference layers. (#16699)

Co-authored-by: Drashna Jaelre <drashna@live.com>
Co-authored-by: Sergey Vlasov <sigprof@gmail.com>

docs/feature_combo.md

@@ -328,6 +328,51 @@ If you, for example, use multiple base layers for different key layouts, one for
 
 With `#define COMBO_ONLY_FROM_LAYER 0` in config.h, the combos' keys are always checked from layer `0`, even if other layers are active.
 
+### Combo reference layers by layer.
+
+If not using `COMBO_ONLY_FROM_LAYER` it is possible to specify a
+combo reference layer for any layer using the `combo_ref_from_layer` hook. 
+The combo macros automatically create this function from the `COMBO_REF_LAYER()`
+entries given.
+
+This function returns the assigned reference layer for the current layer.
+if there is no match, it returns the  default reference layer if set, 
+or the current layer otherwise. A default layer can be set with
+`DEFAULT_REF_LAYER(_MY_COMBO_REF_LAYER)`
+
+If not set, the default reference layer selection from the automatically generated 
+`combo-ref-from-layer()` will be the current layer.
+
+The following `combo_ref_from_layer` function 
+will give a reference layer of _QWERTY for the _DVORAK layer and
+will give the _NAV layer as a reference to it's self. All other layers
+will have the default for their combo reference layer. If the default
+is not set, all other layers will reference themselves.
+
+    ```c
+    #define COMBO_REF_DEFAULT _MY_COMBO_LAYER
+    ...
+
+    uint8_t combo_ref_from_layer(uint8_t layer){
+        switch (get_highest_layer(layer_state)){
+            case _DVORAK: return _QWERTY;
+            case _NAV: return _NAV;
+            default: return _MY_COMBO_LAYER;
+        }
+        return layer;  // important if default is not in case.
+    }
+
+    ```
+    
+    The equivalent definition using the combo macros is this: 
+
+    ```c
+    COMBO_REF_LAYER(_DVORAK, _QWERTY)
+    COMBO_REF_LAYER(_NAV, _NAV)
+    DEFAULT_REF_LAYER(_MY_COMBO_LAYER).
+    ```
+    
+
 ## User callbacks
 
 In addition to the keycodes, there are a few functions that you can use to set the status, or check it:
@@ -350,6 +395,11 @@ First, you need to add `VPATH += keyboards/gboards` to your `rules.mk`. Next, in
 Then, write your combos in `combos.def` file in the following manner:
 
 ```c
+// Alternate reference layers by layer
+//               Layer     Reference layer
+COMBO_REF_LAYER(_DVORAK, _QWERTY)  // reference the qwerty layer for dvorak.
+COMBO_REF_LAYER(_NAV, _NAV) // explicit reference to self instead of the default.
+
 //   name     result    chord keys
 COMB(AB_ESC,   KC_ESC,   KC_A, KC_B)
 COMB(JK_TAB,   KC_TAB,   KC_J, KC_K)

keyboards/gboards/g/keymap_combo.h

@@ -1,4 +1,10 @@
 // Keymap helpers
+// define reference layers per layer.
+#define REF_LAYER_FOR_LAYER(LAYER, REF_LAYER)   \
+  case LAYER: return REF_LAYER;
+
+#define DEF_REF_LAYER(LAYER)                    \
+  default: return LAYER;
 
 #define K_ENUM(name, key, ...) name,
 #define K_DATA(name, key, ...) const uint16_t PROGMEM cmb_##name[] = {__VA_ARGS__, COMBO_END};
@@ -18,6 +24,11 @@
         break;
 
 #define BLANK(...)
+#undef COMBO_REF_LAYER
+#undef DEFAULT_REF_LAYER
+#define COMBO_REF_LAYER BLANK
+#define DEFAULT_REF_LAYER BLANK
+
 // Generate data needed for combos/actions
 // Create Enum
 #undef COMB
@@ -73,3 +84,26 @@ void process_combo_event(uint16_t combo_index, bool pressed) {
 #undef COMB
 #undef SUBS
 #undef TOGG
+
+// Allow reference layers per layer.
+#define COMB BLANK
+#define SUBS BLANK
+#define TOGG BLANK
+
+#undef DEFAULT_REF_LAYER
+#undef COMBO_REF_LAYER
+#define COMBO_REF_LAYER REF_LAYER_FOR_LAYER
+#define DEFAULT_REF_LAYER DEF_REF_LAYER
+
+uint8_t combo_ref_from_layer(uint8_t current_layer){
+  switch (current_layer){
+#include "combos.def"
+  }
+  return current_layer;
+}
+
+#undef COMB
+#undef SUBS
+#undef TOGG
+#undef COMBO_REF_LAYER
+#undef DEFAULT_REF_LAYER

quantum/process_keycode/process_combo.c

@@ -29,6 +29,12 @@ extern uint16_t COMBO_LEN;
 
 __attribute__((weak)) void process_combo_event(uint16_t combo_index, bool pressed) {}
 
+#ifndef COMBO_ONLY_FROM_LAYER
+__attribute__((weak)) uint8_t combo_ref_from_layer(uint8_t layer) {
+    return layer;
+}
+#endif
+
 #ifdef COMBO_MUST_HOLD_PER_COMBO
 __attribute__((weak)) bool get_combo_must_hold(uint16_t index, combo_t *combo) {
     return false;
@@ -549,6 +555,12 @@ bool process_combo(uint16_t keycode, keyrecord_t *record) {
 #ifdef COMBO_ONLY_FROM_LAYER
     /* Only check keycodes from one layer. */
     keycode = keymap_key_to_keycode(COMBO_ONLY_FROM_LAYER, record->event.key);
+#else
+    uint8_t  highest_layer = get_highest_layer(layer_state);
+    uint8_t  ref_layer     = combo_ref_from_layer(highest_layer);
+    if (ref_layer != highest_layer) {
+        keycode = keymap_key_to_keycode(ref_layer, record->event.key);
+    }
 #endif
 
     for (uint16_t idx = 0; idx < COMBO_LEN; ++idx) {