[Core] Add Chordal Hold, an "opposite hands rule" tap-hold option similar to Achordion, Bilateral Combinations.

docs/tap_hold.md

@@ -425,6 +425,167 @@ uint16_t get_quick_tap_term(uint16_t keycode, keyrecord_t *record) {
 If `QUICK_TAP_TERM` is set higher than `TAPPING_TERM`, it will default to `TAPPING_TERM`.
 :::
 
+## Chordal Hold
+
+Chordal Hold is intended to be used together with either Permissive Hold or Hold
+On Other Key Press. Chordal Hold is enabled by adding to your `config.h`:
+
+```c
+#define CHORDAL_HOLD
+```
+
+Chordal Hold implements, by default, an "opposite hands" rule. Suppose a
+tap-hold key is pressed and then, before the tapping term, another key is
+pressed. With Chordal Hold, the tap-hold key is settled as tapped if the two
+keys are on the same hand.
+
+Otherwise, if the keys are on opposite hands, Chordal Hold introduces no new
+behavior. Hold On Other Key Press or Permissive Hold may be used together with
+Chordal Hold to configure the behavior in the opposite hands case. With Hold On
+Other Key Press, an opposite hands chord is settled immediately as held. Or with
+Permissive Hold, an opposite hands chord is settled as held provided the other
+key is pressed and released (nested press) before releasing the tap-hold key.
+
+Chordal Hold may be useful to avoid accidental modifier activation with
+mod-taps, particularly in rolled keypresses when using home row mods.
+
+Notes:
+
+* Chordal Hold has no effect after the tapping term. 
+
+* Chordal Hold has no effect when the other key is also a tap-hold key. This is
+  so that multiple tap-hold keys may be held on the same hand, which is common
+  to do with home row mods.
+
+* Combos are exempt from the opposite hands rule, since "handedness" is
+  ill-defined in this case. Even so, Chordal Hold's behavior involving combos
+  may be customized through the `get_chordal_hold()` callback.
+
+An example of a sequence that is affected by “chordal hold”: 
+
+- `SFT_T(KC_A)` Down
+- `KC_C` Down
+- `KC_C` Up
+- `SFT_T(KC_A)` Up
+
+```
+                         TAPPING_TERM   
+  +---------------------------|--------+
+  | +----------------------+  |        |
+  | | SFT_T(KC_A)          |  |        |
+  | +----------------------+  |        |
+  |    +--------------+       |        |
+  |    | KC_C         |       |        |
+  |    +--------------+       |        |
+  +---------------------------|--------+
+```
+
+If the two keys are on the same hand, then this will produce `ac` with 
+`SFT_T(KC_A)` settled as tapped the moment that `KC_C` is pressed. 
+
+If the two keys are on opposite hands and the `HOLD_ON_OTHER_KEY_PRESS`
+option enabled, this will produce `C` with `SFT_T(KC_A)` settled as held the
+moment that `KC_C` is pressed.
+
+Or if the two keys are on opposite hands and the `PERMISSIVE_HOLD` option is
+enabled, this will produce `C` with `SFT_T(KC_A)` settled as held the
+moment that `KC_C` is released.
+
+### Chordal Hold Handedness
+
+Determining whether keys are on the same or opposite hands involves defining the
+"handedness" of each key position. By default, if nothing is specified,
+handedness is guessed based on keyboard matrix dimensions. If this is
+inaccurate, handedness may be specified in several ways.
+
+The easiest way to specify handedness is by `chordal_hold_layout`. Define in
+config.h:
+
+```c
+#define CHORDAL_HOLD_LAYOUT
+```
+
+Then in keymap.c, define `chordal_hold_layout` in the following form:
+
+```c
+const char chordal_hold_layout[MATRIX_ROWS][MATRIX_COLS] PROGMEM =
+    LAYOUT(
+        'L', 'L', 'L', 'L', 'L', 'L',  'R', 'R', 'R', 'R', 'R', 'R', 
+        'L', 'L', 'L', 'L', 'L', 'L',  'R', 'R', 'R', 'R', 'R', 'R', 
+        'L', 'L', 'L', 'L', 'L', 'L',  'R', 'R', 'R', 'R', 'R', 'R', 
+                       'L', 'L', 'L',  'R', 'R', 'R'
+    );
+```
+
+Use the same `LAYOUT` macro as used to define your keymap layers. Each entry is
+a character, either `'L'` for left, `'R'` for right, or `'*'` to exempt keys
+from the "opposite hands rule." When a key has `'*'` handedness, pressing it
+with either hand results in a hold. This could be used perhaps on thumb keys or
+other places where you want to allow same-hand chords.
+
+Alternatively, handedness may be defined functionally with
+`chordal_hold_handedness_user()`. For example, in keymap.c define:
+
+```c
+char chordal_hold_handedness_user(keypos_t key) {
+    if (key.col == 0 || key.col == MATRIX_COLS - 1) {
+        return '*';  // Exempt the outer columns.
+    }
+    // On split keyboards, typically, the first half of the rows are on the
+    // left, and the other half are on the right.
+    return key.row < MATRIX_ROWS / 2 ? 'L' : 'R';
+}
+```
+
+Given the matrix position of a key, the function should return `'L'`, `'R'`, or
+`'*'`. Adapt the logic in this function according to the keyboard's matrix.
+Similarly at the keyboard level, `chordal_hold_handedness_kb()` may be defined
+to specify handedness.
+
+::: warning
+Note the matrix may have irregularities around larger keys, around the edges of
+the board, and around thumb clusters. You may find it helpful to use [this
+debugging example](faq_debug#which-matrix-position-is-this-keypress) to
+correspond physical keys to matrix positions.
+:::
+
+
+### Per-chord customization
+
+Beyond the per-key configuration possible through handedness, Chordal Hold may
+be configured at a *per-chord* granularity for detailed tuning. In keymap.c,
+define `get_chordal_hold()`. Returning true settles the chord as held, while
+returning false settles as tapped.
+
+For example:
+
+```c
+bool get_chordal_hold(uint16_t tap_hold_keycode, keyrecord_t* tap_hold_record,
+                      uint16_t other_keycode, keyrecord_t* other_record) {
+    // Exceptionally allow some one-handed chords for hotkeys.
+    switch (tap_hold_keycode) {
+        case LCTL_T(KC_A):
+           if (other_keycode == KC_C || other_keycode == KC_V) {
+               return true;
+           }
+           break;
+
+        case RCTL_T(KC_SCLN):
+           if (other_keycode == KC_N) {
+               return true;
+           }
+           break;
+    }
+    // Otherwise defer to the opposite hands rule.
+    return get_chordal_hold_default(tap_hold_record, other_record);
+}
+```
+
+As shown above, the function may call `get_chordal_hold_default(tap_hold_record,
+other_record)` to get the default tap vs. hold decision according to the
+opposite hands rule.
+
+
 ## Retro Tapping
 
 To enable `retro tapping`, add the following to your `config.h`:

quantum/action_tapping.c

@@ -49,6 +49,45 @@ __attribute__((weak)) bool get_permissive_hold(uint16_t keycode, keyrecord_t *re
 }
 #    endif
 
+#    ifdef CHORDAL_HOLD
+__attribute__((weak)) bool get_chordal_hold(uint16_t tap_hold_keycode, keyrecord_t *tap_hold_record, uint16_t other_keycode, keyrecord_t *other_record) {
+    return get_chordal_hold_default(tap_hold_record, other_record);
+}
+
+bool get_chordal_hold_default(keyrecord_t *tap_hold_record, keyrecord_t *other_record) {
+    if (tap_hold_record->event.type != KEY_EVENT || other_record->event.type != KEY_EVENT) {
+        return true; // Return true on combos or other non-key events.
+    }
+
+    char tap_hold_hand = chordal_hold_handedness_user(tap_hold_record->event.key);
+    if (tap_hold_hand == '*') {
+        return true;
+    }
+    char other_hand = chordal_hold_handedness_user(other_record->event.key);
+    return other_hand == '*' || tap_hold_hand != other_hand;
+}
+
+__attribute__((weak)) char chordal_hold_handedness_kb(keypos_t key) {
+#        if defined(SPLIT_KEYBOARD) || ((MATRIX_ROWS) > (MATRIX_COLS))
+    // If the keyboard is split or if MATRIX_ROWS > MATRIX_COLS, assume that the
+    // first half of the rows are left and the latter half are right.
+    return (key.row < (MATRIX_ROWS) / 2) ? /*left*/ 'L' : /*right*/ 'R';
+#        else
+    // Otherwise, assume the first half of the cols are left, others are right.
+    return (key.col < (MATRIX_COLS) / 2) ? /*left*/ 'L' : /*right*/ 'R';
+#        endif
+}
+
+__attribute__((weak)) char chordal_hold_handedness_user(keypos_t key) {
+#        if defined(CHORDAL_HOLD_LAYOUT)
+    // If given, read handedness from `chordal_hold_layout` array.
+    return (char)pgm_read_byte(&chordal_hold_layout[key.row][key.col]);
+#        else
+    return chordal_hold_handedness_kb(key);
+#        endif
+}
+#    endif // CHORDAL_HOLD
+
 #    ifdef HOLD_ON_OTHER_KEY_PRESS_PER_KEY
 __attribute__((weak)) bool get_hold_on_other_key_press(uint16_t keycode, keyrecord_t *record) {
     return false;
@@ -188,7 +227,7 @@ bool process_tapping(keyrecord_t *keyp) {
         return true;
     }
 
-#    if (defined(AUTO_SHIFT_ENABLE) && defined(RETRO_SHIFT)) || defined(PERMISSIVE_HOLD_PER_KEY) || defined(HOLD_ON_OTHER_KEY_PRESS_PER_KEY)
+#    if (defined(AUTO_SHIFT_ENABLE) && defined(RETRO_SHIFT)) || defined(PERMISSIVE_HOLD_PER_KEY) || defined(CHORDAL_HOLD) || defined(HOLD_ON_OTHER_KEY_PRESS_PER_KEY)
     TAP_DEFINE_KEYCODE;
 #    endif
 
@@ -271,19 +310,34 @@ bool process_tapping(keyrecord_t *keyp) {
                     // set interrupted flag when other key pressed during tapping
                     if (event.pressed) {
                         tapping_key.tap.interrupted = true;
-                        if (TAP_GET_HOLD_ON_OTHER_KEY_PRESS
+
+#    if defined(CHORDAL_HOLD)
+                        if (!is_tap_record(keyp) && !get_chordal_hold(tapping_keycode, &tapping_key, get_record_keycode(keyp, true), keyp)) {
+                            // Settle the tapping key as *tapped*, since it
+                            // is not considered a held chord with keyp.
+                            ac_dprintf("Tapping: End. Chord considered a tap\n");
+                            tapping_key.tap.count = 1;
+                            // In process_action(), HOLD_ON_OTHER_KEY_PRESS
+                            // will revert interrupted events to holds, so
+                            // this needs to be set false.
+                            tapping_key.tap.interrupted = false;
+                            process_record(&tapping_key);
+                            debug_tapping_key();
+                        } else
+#    endif
+                            if (TAP_GET_HOLD_ON_OTHER_KEY_PRESS
 #    if defined(AUTO_SHIFT_ENABLE) && defined(RETRO_SHIFT)
-                            // Auto Shift cannot evaluate this early
-                            // Retro Shift uses the hold action for all nested taps even without HOLD_ON_OTHER_KEY_PRESS, so this is fine to skip
-                            && !(MAYBE_RETRO_SHIFTING(event, keyp) && get_auto_shifted_key(get_record_keycode(keyp, false), keyp))
+                                // Auto Shift cannot evaluate this early
+                                // Retro Shift uses the hold action for all nested taps even without HOLD_ON_OTHER_KEY_PRESS, so this is fine to skip
+                                && !(MAYBE_RETRO_SHIFTING(event, keyp) && get_auto_shifted_key(get_record_keycode(keyp, false), keyp))
 #    endif
-                        ) {
+                            ) {
+                            // Settle the tapping key as *held*, since
+                            // HOLD_ON_OTHER_KEY_PRESS is enabled for this key.
                             ac_dprintf("Tapping: End. No tap. Interfered by pressed key\n");
                             process_record(&tapping_key);
                             tapping_key = (keyrecord_t){0};
                             debug_tapping_key();
-                            // enqueue
-                            return false;
                         }
                     }
                     // enqueue

quantum/action_tapping.h

@@ -46,6 +46,81 @@ bool     get_permissive_hold(uint16_t keycode, keyrecord_t *record);
 bool     get_retro_tapping(uint16_t keycode, keyrecord_t *record);
 bool     get_hold_on_other_key_press(uint16_t keycode, keyrecord_t *record);
 
+#ifdef CHORDAL_HOLD
+/**
+ * Callback to say when a key chord before the tapping term may be held.
+ *
+ * In keymap.c, define the callback
+ *
+ *     bool get_chordal_hold(uint16_t tap_hold_keycode,
+ *                           keyrecord_t* tap_hold_record,
+ *                           uint16_t other_keycode,
+ *                           keyrecord_t* other_record) {
+ *        // Conditions...
+ *     }
+ *
+ * This callback is called when:
+ *
+ * 1. `tap_hold_keycode` is pressed.
+ * 2. `other_keycode` is pressed while `tap_hold_keycode` is still held,
+ *     provided `other_keycode` is *not* also a tap-hold key and it is pressed
+ *     before the tapping term.
+ *
+ * If false is returned, this has the effect of immediately settling the
+ * tap-hold key as tapped. If true is returned, the tap-hold key is still
+ * unsettled, and may be settled as held depending on configuration and
+ * subsequent events.
+ *
+ * @param tap_hold_keycode   Keycode of the tap-hold key.
+ * @param tap_hold_record    Record from the tap-hold press event.
+ * @param other_keycode      Keycode of the other key.
+ * @param other_record       Record from the other key's press event.
+ * @return True if the tap-hold key may be considered held; false if tapped.
+ */
+bool get_chordal_hold(uint16_t tap_hold_keycode, keyrecord_t *tap_hold_record, uint16_t other_keycode, keyrecord_t *other_record);
+
+/**
+ * Default "opposite hands rule" for whether a key chord may settle as held.
+ *
+ * This function returns true when the tap-hold key and other key are on
+ * "opposite hands." In detail, handedness of the two keys are compared. If
+ * handedness values differ, or if either handedness is '*', the function
+ * returns true, indicating that it may be held. Otherwise, it returns false,
+ * in which case the tap-hold key is immediately settled at tapped.
+ *
+ * "Handedness" is determined as follows, in order of decending precedence:
+ * 1. `chordal_hold_handedness_user()`, if defined.
+ * 2. `chordal_hold_layout`, if CHORDAL_HOLD_LAYOUT is defined.
+ * 3. `chordal_hold_handedness_kb()`, if defined.
+ * 4. fallback assumption based on keyboard matrix dimensions.
+ *
+ * @param tap_hold_record  Record of the active tap-hold key press.
+ * @param other_record     Record of the other, interrupting key press.
+ * @return True if the tap-hold key may be considered held; false if tapped.
+ */
+bool get_chordal_hold_default(keyrecord_t *tap_hold_record, keyrecord_t *other_record);
+
+/**
+ * Keyboard-level callback to determine handedness of a key.
+ *
+ * This function should return:
+ *   'L' for keys pressed by the left hand,
+ *   'R' for keys on the right hand,
+ *   '*' for keys exempt from the "opposite hands rule." This could be used
+ *       perhaps on thumb keys or keys that might be pressed by either hand.
+ *
+ * @param key   A key matrix position.
+ * @return Handedness value.
+ */
+char chordal_hold_handedness_kb(keypos_t key);
+/** User callback to determine handedness of a key. */
+char chordal_hold_handedness_user(keypos_t key);
+
+#    ifdef CHORDAL_HOLD_LAYOUT
+extern const char chordal_hold_layout[MATRIX_ROWS][MATRIX_COLS] PROGMEM;
+#    endif
+#endif
+
 #ifdef DYNAMIC_TAPPING_TERM_ENABLE
 extern uint16_t g_tapping_term;
 #endif

tests/tap_hold_configurations/chordal_hold_and_hold_on_other_key_press/config.h

@@ -0,0 +1,21 @@
+/* Copyright 2022 Vladislav Kucheriavykh
+ *
+ * This program 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.
+ *
+ * This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "test_common.h"
+#define CHORDAL_HOLD
+#define HOLD_ON_OTHER_KEY_PRESS

tests/tap_hold_configurations/chordal_hold_and_hold_on_other_key_press/test.mk

@@ -0,0 +1,18 @@
+# Copyright 2022 Vladislav Kucheriavykh
+#
+# This program 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.
+#
+# This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# --------------------------------------------------------------------------------
+# Keep this file, even if it is empty, as a marker that this folder contains tests
+# --------------------------------------------------------------------------------