[Core] Add Repeat Key ("repeat last key") as a core feature. (#19700)

Co-authored-by: casuanoob <96005765+casuanoob@users.noreply.github.com>
Co-authored-by: Sergey Vlasov <sigprof@gmail.com>

builddefs/generic_features.mk

@@ -32,6 +32,7 @@ GENERIC_FEATURES = \
     KEY_OVERRIDE \
     LEADER \
     PROGRAMMABLE_BUTTON \
+    REPEAT_KEY \
     SECURE \
     SPACE_CADET \
     SWAP_HANDS \

builddefs/show_options.mk

@@ -85,7 +85,8 @@ OTHER_OPTION_NAMES = \
   SECURE_ENABLE \
   CAPS_WORD_ENABLE \
   AUTOCORRECT_ENABLE \
-  TRI_LAYER_ENABLE
+  TRI_LAYER_ENABLE \
+  REPEAT_KEY_ENABLE
 
 define NAME_ECHO
        @printf "  %-30s = %-16s # %s\\n" "$1" "$($1)" "$(origin $1)"

data/constants/keycodes/keycodes_0.0.3_quantum.hjson

@@ -0,0 +1,18 @@
+{
+    "keycodes": {
+       "0x7C79": {
+            "group": "quantum",
+            "key": "QK_REPEAT_KEY",
+            "aliases": [
+                "QK_REP"
+            ]
+        },
+        "0x7C7A": {
+            "group": "quantum",
+            "key": "QK_ALT_REPEAT_KEY",
+            "aliases": [
+                "QK_AREP"
+            ]
+        }
+    }
+}

docs/_summary.md

@@ -70,6 +70,7 @@
     * [Macros](feature_macros.md)
     * [Mouse Keys](feature_mouse_keys.md)
     * [Programmable Button](feature_programmable_button.md)
+    * [Repeat Key](feature_repeat_key.md)
     * [Space Cadet Shift](feature_space_cadet.md)
     * [US ANSI Shifted Keys](keycodes_us_ansi_shifted.md)
 

docs/ja/_summary.md

@@ -68,6 +68,7 @@
     * [モッドタップ](ja/mod_tap.md)
     * [マクロ](ja/feature_macros.md)
     * [マウスキー](ja/feature_mouse_keys.md)
+    * [Repeat Key](ja/feature_repeat_key.md)
     * [Space Cadet Shift](ja/feature_space_cadet.md)
     * [US ANSI シフトキー](ja/keycodes_us_ansi_shifted.md)
 

docs/keycodes.md

@@ -803,6 +803,15 @@ See also: [Programmable Button](feature_programmable_button.md)
 |`QK_PROGRAMMABLE_BUTTON_31`|`PB_31`|Programmable button 31|
 |`QK_PROGRAMMABLE_BUTTON_32`|`PB_32`|Programmable button 32|
 
+## Repeat Key :id=repeat-key
+
+See also: [Repeat Key](feature_repeat_key.md)
+
+|Keycode                |Aliases  |Description                          |
+|-----------------------|---------|-------------------------------------|
+|`QK_REPEAT_KEY`        |`QK_REP` |Repeat the last pressed key          |
+|`QK_ALT_REPEAT_KEY`    |`QK_AREP`|Perform alternate of the last key    |
+
 ## Space Cadet :id=space-cadet
 
 See also: [Space Cadet](feature_space_cadet.md)

docs/zh-cn/_summary.md

@@ -73,6 +73,7 @@
     * [Mod-Tap](zh-cn/mod_tap.md)
     * [宏](zh-cn/feature_macros.md)
     * [鼠标键](zh-cn/feature_mouse_keys.md)
+    * [Repeat Key](zh-cn/feature_repeat_key.md)
     * [Space Cadet Shift](zh-cn/feature_space_cadet.md)
     * [US ANSI上档键值](zh-cn/keycodes_us_ansi_shifted.md)
 

quantum/action.c

@@ -285,7 +285,7 @@ void process_record(keyrecord_t *record) {
 }
 
 void process_record_handler(keyrecord_t *record) {
-#ifdef COMBO_ENABLE
+#if defined(COMBO_ENABLE) || defined(REPEAT_KEY_ENABLE)
     action_t action;
     if (record->keycode) {
         action = action_for_keycode(record->keycode);
@@ -1109,7 +1109,7 @@ bool is_tap_record(keyrecord_t *record) {
         return false;
     }
 
-#ifdef COMBO_ENABLE
+#if defined(COMBO_ENABLE) || defined(REPEAT_KEY_ENABLE)
     action_t action;
     if (record->keycode) {
         action = action_for_keycode(record->keycode);

quantum/action.h

@@ -50,7 +50,7 @@ typedef struct {
 #ifndef NO_ACTION_TAPPING
     tap_t tap;
 #endif
-#ifdef COMBO_ENABLE
+#if defined(COMBO_ENABLE) || defined(REPEAT_KEY_ENABLE)
     uint16_t keycode;
 #endif
 } keyrecord_t;

quantum/keycodes.h

@@ -721,6 +721,8 @@ enum qk_keycode_defines {
     QK_AUTOCORRECT_TOGGLE = 0x7C76,
     QK_TRI_LAYER_LOWER = 0x7C77,
     QK_TRI_LAYER_UPPER = 0x7C78,
+    QK_REPEAT_KEY = 0x7C79,
+    QK_ALT_REPEAT_KEY = 0x7C7A,
     QK_KB_0 = 0x7E00,
     QK_KB_1 = 0x7E01,
     QK_KB_2 = 0x7E02,
@@ -1362,6 +1364,8 @@ enum qk_keycode_defines {
     AC_TOGG    = QK_AUTOCORRECT_TOGGLE,
     TL_LOWR    = QK_TRI_LAYER_LOWER,
     TL_UPPR    = QK_TRI_LAYER_UPPER,
+    QK_REP     = QK_REPEAT_KEY,
+    QK_AREP    = QK_ALT_REPEAT_KEY,
 };
 
 // Range Helpers
@@ -1413,6 +1417,6 @@ enum qk_keycode_defines {
 #define IS_MACRO_KEYCODE(code) ((code) >= QK_MACRO_0 && (code) <= QK_MACRO_31)
 #define IS_BACKLIGHT_KEYCODE(code) ((code) >= QK_BACKLIGHT_ON && (code) <= QK_BACKLIGHT_TOGGLE_BREATHING)
 #define IS_RGB_KEYCODE(code) ((code) >= RGB_TOG && (code) <= RGB_MODE_TWINKLE)
-#define IS_QUANTUM_KEYCODE(code) ((code) >= QK_BOOTLOADER && (code) <= QK_TRI_LAYER_UPPER)
+#define IS_QUANTUM_KEYCODE(code) ((code) >= QK_BOOTLOADER && (code) <= QK_ALT_REPEAT_KEY)
 #define IS_KB_KEYCODE(code) ((code) >= QK_KB_0 && (code) <= QK_KB_31)
 #define IS_USER_KEYCODE(code) ((code) >= QK_USER_0 && (code) <= QK_USER_31)

quantum/process_keycode/process_repeat_key.c

@@ -0,0 +1,109 @@
+// Copyright 2022-2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "process_repeat_key.h"
+
+// Default implementation of remember_last_key_user().
+__attribute__((weak)) bool remember_last_key_user(uint16_t keycode, keyrecord_t* record, uint8_t* remembered_mods) {
+    return true;
+}
+
+static bool remember_last_key(uint16_t keycode, keyrecord_t* record, uint8_t* remembered_mods) {
+    switch (keycode) {
+        // Ignore MO, TO, TG, TT, and TL layer switch keys.
+        case QK_MOMENTARY ... QK_MOMENTARY_MAX:
+        case QK_TO ... QK_TO_MAX:
+        case QK_TOGGLE_LAYER ... QK_TOGGLE_LAYER_MAX:
+        case QK_LAYER_TAP_TOGGLE ... QK_LAYER_TAP_TOGGLE_MAX:
+        // Ignore mod keys.
+        case KC_LCTL ... KC_RGUI:
+        case KC_HYPR:
+        case KC_MEH:
+#ifndef NO_ACTION_ONESHOT // Ignore one-shot keys.
+        case QK_ONE_SHOT_LAYER ... QK_ONE_SHOT_LAYER_MAX:
+        case QK_ONE_SHOT_MOD ... QK_ONE_SHOT_MOD_MAX:
+#endif                  // NO_ACTION_ONESHOT
+#ifdef TRI_LAYER_ENABLE // Ignore Tri Layer keys.
+        case QK_TRI_LAYER_LOWER:
+        case QK_TRI_LAYER_UPPER:
+#endif // TRI_LAYER_ENABLE
+            return false;
+
+            // Ignore hold events on tap-hold keys.
+#ifndef NO_ACTION_TAPPING
+        case QK_MOD_TAP ... QK_MOD_TAP_MAX:
+#    ifndef NO_ACTION_LAYER
+        case QK_LAYER_TAP ... QK_LAYER_TAP_MAX:
+#    endif // NO_ACTION_LAYER
+            if (record->tap.count == 0) {
+                return false;
+            }
+            break;
+#endif // NO_ACTION_TAPPING
+
+#ifdef SWAP_HANDS_ENABLE
+        case QK_SWAP_HANDS ... QK_SWAP_HANDS_MAX:
+            if (IS_SWAP_HANDS_KEYCODE(keycode) || record->tap.count == 0) {
+                return false;
+            }
+            break;
+#endif // SWAP_HANDS_ENABLE
+
+        case QK_REPEAT_KEY:
+#ifndef NO_ALT_REPEAT_KEY
+        case QK_ALT_REPEAT_KEY:
+#endif // NO_ALT_REPEAT_KEY
+            return false;
+    }
+
+    return remember_last_key_user(keycode, record, remembered_mods);
+}
+
+bool process_last_key(uint16_t keycode, keyrecord_t* record) {
+    if (get_repeat_key_count()) {
+        return true;
+    }
+
+    if (record->event.pressed) {
+        uint8_t remembered_mods = get_mods() | get_weak_mods();
+#ifndef NO_ACTION_ONESHOT
+        remembered_mods |= get_oneshot_mods();
+#endif // NO_ACTION_ONESHOT
+
+        if (remember_last_key(keycode, record, &remembered_mods)) {
+            set_last_record(keycode, record);
+            set_last_mods(remembered_mods);
+        }
+    }
+
+    return true;
+}
+
+bool process_repeat_key(uint16_t keycode, keyrecord_t* record) {
+    if (get_repeat_key_count()) {
+        return true;
+    }
+
+    if (keycode == QK_REPEAT_KEY) {
+        repeat_key_invoke(&record->event);
+        return false;
+#ifndef NO_ALT_REPEAT_KEY
+    } else if (keycode == QK_ALT_REPEAT_KEY) {
+        alt_repeat_key_invoke(&record->event);
+        return false;
+#endif // NO_ALT_REPEAT_KEY
+    }
+
+    return true;
+}

quantum/process_keycode/process_repeat_key.h

@@ -0,0 +1,62 @@
+// Copyright 2022-2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#pragma once
+
+#include "quantum.h"
+
+/**
+ * @brief Process handler for remembering the last key.
+ *
+ * @param keycode  Keycode registered by matrix press, per keymap
+ * @param record   keyrecord_t structure
+ * @return true    Continue processing keycodes, and send to host
+ * @return false   Stop processing keycodes, and don't send to host
+ */
+bool process_last_key(uint16_t keycode, keyrecord_t* record);
+
+/**
+ * @brief Optional callback defining which keys are remembered.
+ *
+ * @param keycode          Keycode that was just pressed
+ * @param record           keyrecord_t structure
+ * @param remembered_mods  Mods that will be remembered with this key
+ * @return true            Key is remembered
+ * @return false           Key is ignored
+ *
+ * Modifier and layer switch keys are always ignored. For all other keys, this
+ * callback is called on every key press. Returning true means that the key is
+ * remembered, false means it is ignored. By default, all non-modifier,
+ * non-layer switch keys are remembered.
+ *
+ * The `remembered_mods` arg represents the mods that will be remembered with
+ * this key. It can be modified to forget certain mods, for instance to forget
+ * capitalization when repeating shifted letters:
+ *
+ *     // Forget Shift on letter keys.
+ *     if (KC_A <= keycode && keycode <= KC_Z && (*remembered_mods & ~MOD_MASK_SHIFT) == 0) {
+ *         *remembered_mods = 0;
+ *     }
+ */
+bool remember_last_key_user(uint16_t keycode, keyrecord_t* record, uint8_t* remembered_mods);
+
+/**
+ * @brief Process handler for Repeat Key feature.
+ *
+ * @param keycode  Keycode registered by matrix press, per keymap
+ * @param record   keyrecord_t structure
+ * @return true    Continue processing keycodes, and send to host
+ * @return false   Stop processing keycodes, and don't send to host
+ */
+bool process_repeat_key(uint16_t keycode, keyrecord_t* record);

quantum/quantum.c

@@ -176,7 +176,7 @@ void soft_reset_keyboard(void) {
 
 /* Convert record into usable keycode via the contained event. */
 uint16_t get_record_keycode(keyrecord_t *record, bool update_layer_cache) {
-#ifdef COMBO_ENABLE
+#if defined(COMBO_ENABLE) || defined(REPEAT_KEY_ENABLE)
     if (record->keycode) {
         return record->keycode;
     }
@@ -273,6 +273,9 @@ bool process_record_quantum(keyrecord_t *record) {
             // Must run asap to ensure all keypresses are recorded.
             process_dynamic_macro(keycode, record) &&
 #endif
+#ifdef REPEAT_KEY_ENABLE
+            process_last_key(keycode, record) && process_repeat_key(keycode, record) &&
+#endif
 #if defined(AUDIO_ENABLE) && defined(AUDIO_CLICKY)
             process_clicky(keycode, record) &&
 #endif

quantum/quantum.h

@@ -251,6 +251,11 @@ extern layer_state_t layer_state;
 #    include "process_tri_layer.h"
 #endif
 
+#ifdef REPEAT_KEY_ENABLE
+#    include "repeat_key.h"
+#    include "process_repeat_key.h"
+#endif
+
 void set_single_persistent_default_layer(uint8_t default_layer);
 
 #define IS_LAYER_ON(layer) layer_state_is(layer)

quantum/repeat_key.c

@@ -0,0 +1,282 @@
+// Copyright 2022-2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "repeat_key.h"
+
+// Variables saving the state of the last key press.
+static keyrecord_t last_record = {0};
+static uint8_t     last_mods   = 0;
+// Signed count of the number of times the last key has been repeated or
+// alternate repeated: it is 0 when a key is pressed normally, positive when
+// repeated, and negative when alternate repeated.
+static int8_t last_repeat_count = 0;
+// The repeat_count, but set to 0 outside of repeat_key_invoke() so that it is
+// nonzero only while a repeated key is being processed.
+static int8_t processing_repeat_count = 0;
+
+uint16_t get_last_keycode(void) {
+    return last_record.keycode;
+}
+
+uint8_t get_last_mods(void) {
+    return last_mods;
+}
+
+void set_last_keycode(uint16_t keycode) {
+    set_last_record(keycode, &(keyrecord_t){
+#ifndef NO_ACTION_TAPPING
+                                 .tap.interrupted = false,
+                                 .tap.count       = 1,
+#endif
+                             });
+}
+
+void set_last_mods(uint8_t mods) {
+    last_mods = mods;
+}
+
+void set_last_record(uint16_t keycode, keyrecord_t* record) {
+    last_record         = *record;
+    last_record.keycode = keycode;
+    last_repeat_count   = 0;
+}
+
+/** @brief Updates `last_repeat_count` in direction `dir`. */
+static void update_last_repeat_count(int8_t dir) {
+    if (dir * last_repeat_count < 0) {
+        last_repeat_count = dir;
+    } else if (dir * last_repeat_count < 127) {
+        last_repeat_count += dir;
+    }
+}
+
+int8_t get_repeat_key_count(void) {
+    return processing_repeat_count;
+}
+
+void repeat_key_invoke(const keyevent_t* event) {
+    // It is possible (e.g. in rolled presses) that the last key changes while
+    // the Repeat Key is pressed. To prevent stuck keys, it is important to
+    // remember separately what key record was processed on press so that the
+    // the corresponding record is generated on release.
+    static keyrecord_t registered_record       = {0};
+    static int8_t      registered_repeat_count = 0;
+    // Since this function calls process_record(), it may recursively call
+    // itself. We return early if `processing_repeat_count` is nonzero to
+    // prevent infinite recursion.
+    if (processing_repeat_count || !last_record.keycode) {
+        return;
+    }
+
+    if (event->pressed) {
+        update_last_repeat_count(1);
+        // On press, apply the last mods state, stacking on top of current mods.
+        register_weak_mods(last_mods);
+        registered_record       = last_record;
+        registered_repeat_count = last_repeat_count;
+    }
+
+    // Generate a keyrecord and plumb it into the event pipeline.
+    registered_record.event = *event;
+    processing_repeat_count = registered_repeat_count;
+    process_record(&registered_record);
+    processing_repeat_count = 0;
+
+    // On release, restore the mods state.
+    if (!event->pressed) {
+        unregister_weak_mods(last_mods);
+    }
+}
+
+#ifndef NO_ALT_REPEAT_KEY
+/**
+ * @brief Find alternate keycode from a table of opposing keycode pairs.
+ * @param table Array of pairs of basic keycodes, declared as PROGMEM.
+ * @param table_size_bytes The size of the table in bytes.
+ * @param target The basic keycode to find.
+ * @return The alternate basic keycode, or KC_NO if none was found.
+ *
+ * @note The table keycodes and target must be basic keycodes.
+ *
+ * This helper is used several times below to define alternate keys. Given a
+ * table of pairs of basic keycodes, the function finds the pair containing
+ * `target` and returns the other keycode in the pair.
+ */
+static uint8_t find_alt_keycode(const uint8_t (*table)[2], uint8_t table_size_bytes, uint8_t target) {
+    const uint8_t* keycodes = (const uint8_t*)table;
+    for (uint8_t i = 0; i < table_size_bytes; ++i) {
+        if (target == pgm_read_byte(keycodes + i)) {
+            // Xor (i ^ 1) the index to get the other element in the pair.
+            return pgm_read_byte(keycodes + (i ^ 1));
+        }
+    }
+    return KC_NO;
+}
+
+uint16_t get_alt_repeat_key_keycode(void) {
+    uint16_t keycode = last_record.keycode;
+    uint8_t  mods    = last_mods;
+
+    // Call the user callback first to give it a chance to override the default
+    // alternate key definitions that follow.
+    uint16_t alt_keycode = get_alt_repeat_key_keycode_user(keycode, mods);
+
+    if (alt_keycode != KC_TRANSPARENT) {
+        return alt_keycode;
+    }
+
+    // Convert 8-bit mods to the 5-bit format used in keycodes. This is lossy:
+    // if left and right handed mods were mixed, they all become right handed.
+    mods = ((mods & 0xf0) ? /* set right hand bit */ 0x10 : 0)
+           // Combine right and left hand mods.
+           | (((mods >> 4) | mods) & 0xf);
+
+    switch (keycode) {
+        case QK_MODS ... QK_MODS_MAX: // Unpack modifier + basic key.
+            mods |= QK_MODS_GET_MODS(keycode);
+            keycode = QK_MODS_GET_BASIC_KEYCODE(keycode);
+            break;
+
+#    ifndef NO_ACTION_TAPPING
+        case QK_MOD_TAP ... QK_MOD_TAP_MAX:
+            keycode = QK_MOD_TAP_GET_TAP_KEYCODE(keycode);
+            break;
+#        ifndef NO_ACTION_LAYER
+        case QK_LAYER_TAP ... QK_LAYER_TAP_MAX:
+            keycode = QK_LAYER_TAP_GET_TAP_KEYCODE(keycode);
+            break;
+#        endif // NO_ACTION_LAYER
+#    endif     // NO_ACTION_TAPPING
+
+#    ifdef SWAP_HANDS_ENABLE
+        case QK_SWAP_HANDS ... QK_SWAP_HANDS_MAX:
+            if (IS_SWAP_HANDS_KEYCODE(keycode)) {
+                return KC_NO;
+            }
+            keycode = QK_SWAP_HANDS_GET_TAP_KEYCODE(keycode);
+            break;
+#    endif // SWAP_HANDS_ENABLE
+    }
+
+    if (IS_QK_BASIC(keycode)) {
+        if ((mods & (MOD_LCTL | MOD_LALT | MOD_LGUI))) {
+            // The last key was pressed with a modifier other than Shift.
+            // The following maps
+            //   mod + F <-> mod + B
+            // and a few others, supporting several core hotkeys used in
+            // Emacs, Vim, less, and other programs.
+            // clang-format off
+            static const uint8_t pairs[][2] PROGMEM = {
+                {KC_F   , KC_B   },  // Forward / Backward.
+                {KC_D   , KC_U   },  // Down / Up.
+                {KC_N   , KC_P   },  // Next / Previous.
+                {KC_A   , KC_E   },  // Home / End.
+                {KC_O   , KC_I   },  // Older / Newer in Vim jump list.
+            };
+            // clang-format on
+            alt_keycode = find_alt_keycode(pairs, sizeof(pairs), keycode);
+        } else {
+            // The last key was pressed with no mods or only Shift. The
+            // following map a few more Vim hotkeys.
+            // clang-format off
+            static const uint8_t pairs[][2] PROGMEM = {
+                {KC_J   , KC_K   },  // Down / Up.
+                {KC_H   , KC_L   },  // Left / Right.
+                // These two lines map W and E to B, and B to W.
+                {KC_W   , KC_B   },  // Forward / Backward by word.
+                {KC_E   , KC_B   },  // Forward / Backward by word.
+            };
+            // clang-format on
+            alt_keycode = find_alt_keycode(pairs, sizeof(pairs), keycode);
+        }
+
+        if (!alt_keycode) {
+            // The following key pairs are considered with any mods.
+            // clang-format off
+            static const uint8_t pairs[][2] PROGMEM = {
+                {KC_LEFT, KC_RGHT},  // Left / Right Arrow.
+                {KC_UP  , KC_DOWN},  // Up / Down Arrow.
+                {KC_HOME, KC_END },  // Home / End.
+                {KC_PGUP, KC_PGDN},  // Page Up / Page Down.
+                {KC_BSPC, KC_DEL },  // Backspace / Delete.
+                {KC_LBRC, KC_RBRC},  // Brackets [ ] and { }.
+#ifdef EXTRAKEY_ENABLE
+                {KC_WBAK, KC_WFWD},  // Browser Back / Forward.
+                {KC_MNXT, KC_MPRV},  // Next / Previous Media Track.
+                {KC_MFFD, KC_MRWD},  // Fast Forward / Rewind Media.
+                {KC_VOLU, KC_VOLD},  // Volume Up / Down.
+                {KC_BRIU, KC_BRID},  // Brightness Up / Down.
+#endif  // EXTRAKEY_ENABLE
+#ifdef MOUSEKEY_ENABLE
+                {KC_MS_L, KC_MS_R},  // Mouse Cursor Left / Right.
+                {KC_MS_U, KC_MS_D},  // Mouse Cursor Up / Down.
+                {KC_WH_L, KC_WH_R},  // Mouse Wheel Left / Right.
+                {KC_WH_U, KC_WH_D},  // Mouse Wheel Up / Down.
+#endif  // MOUSEKEY_ENABLE
+            };
+            // clang-format on
+            alt_keycode = find_alt_keycode(pairs, sizeof(pairs), keycode);
+        }
+
+        if (alt_keycode) {
+            // Combine basic keycode with mods.
+            return (mods << 8) | alt_keycode;
+        }
+    }
+
+    return KC_NO; // No alternate key found.
+}
+
+void alt_repeat_key_invoke(const keyevent_t* event) {
+    static keyrecord_t registered_record       = {0};
+    static int8_t      registered_repeat_count = 0;
+    // Since this function calls process_record(), it may recursively call
+    // itself. We return early if `processing_repeat_count` is nonzero to
+    // prevent infinite recursion.
+    if (processing_repeat_count) {
+        return;
+    }
+
+    if (event->pressed) {
+        registered_record = (keyrecord_t){
+#    ifndef NO_ACTION_TAPPING
+            .tap.interrupted = false,
+            .tap.count       = 0,
+#    endif
+            .keycode = get_alt_repeat_key_keycode(),
+        };
+    }
+
+    // Early return if there is no alternate key defined.
+    if (!registered_record.keycode) {
+        return;
+    }
+
+    if (event->pressed) {
+        update_last_repeat_count(-1);
+        registered_repeat_count = last_repeat_count;
+    }
+
+    // Generate a keyrecord and plumb it into the event pipeline.
+    registered_record.event = *event;
+    processing_repeat_count = registered_repeat_count;
+    process_record(&registered_record);
+    processing_repeat_count = 0;
+}
+
+// Default implementation of get_alt_repeat_key_keycode_user().
+__attribute__((weak)) uint16_t get_alt_repeat_key_keycode_user(uint16_t keycode, uint8_t mods) {
+    return KC_TRANSPARENT;
+}
+#endif // NO_ALT_REPEAT_KEY

quantum/repeat_key.h

@@ -0,0 +1,80 @@
+// Copyright 2022-2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#pragma once
+
+#include "quantum.h"
+
+uint16_t get_last_keycode(void);             /**< Keycode of the last key. */
+uint8_t  get_last_mods(void);                /**< Mods active with the last key. */
+void     set_last_keycode(uint16_t keycode); /**< Sets the last key. */
+void     set_last_mods(uint8_t mods);        /**< Sets the last mods. */
+
+/** @brief Gets the record for the last key. */
+keyrecord_t* get_last_record(void);
+
+/** @brief Sets keycode and record info for the last key. */
+void set_last_record(uint16_t keycode, keyrecord_t* record);
+
+/**
+ * @brief Signed count of times the key has been repeated or alternate repeated.
+ *
+ * @note The count is nonzero only while a repeated or alternate-repeated key is
+ *       being processed.
+ *
+ * When a key is pressed normally, the count is 0. When the Repeat Key is used
+ * to repeat a key, the count is 1 on the first repeat, 2 on the second repeat,
+ * and continuing up to 127.
+ *
+ * Negative counts are used similarly for alternate repeating. When the
+ * Alternate Repeat Key is used, the count is -1 on the first alternate repeat,
+ * -2 on the second, continuing down to -127.
+ */
+int8_t get_repeat_key_count(void);
+
+/**
+ * @brief Calls `process_record()` on a generated record repeating the last key.
+ * @param event Event information in the generated record.
+ */
+void repeat_key_invoke(const keyevent_t* event);
+
+#ifndef NO_ALT_REPEAT_KEY
+
+/**
+ * @brief Keycode to be used for alternate repeating.
+ *
+ * Alternate Repeat performs this keycode based on the last eligible pressed key
+ * and mods, get_last_keycode() and get_last_mods(). For example, when the last
+ * key was KC_UP, this function returns KC_DOWN. The function returns KC_NO if
+ * the last key doesn't have a defined alternate.
+ */
+uint16_t get_alt_repeat_key_keycode(void);
+
+/**
+ * @brief Calls `process_record()` to alternate repeat the last key.
+ * @param event Event information in the generated record.
+ */
+void alt_repeat_key_invoke(const keyevent_t* event);
+
+/**
+ * @brief Optional user callback to define additional alternate keys.
+ *
+ * When `get_alt_repeat_key_keycode()` is called, it first calls this callback.
+ * It should return a keycode representing the "alternate" of the given keycode
+ * and mods. Returning KC_NO defers to the default definitions in
+ * `get_alt_repeat_key_keycode()`.
+ */
+uint16_t get_alt_repeat_key_keycode_user(uint16_t keycode, uint8_t mods);
+
+#endif // NO_ALT_REPEAT_KEY

tests/repeat_key/alt_repeat_key/config.h

@@ -0,0 +1,18 @@
+// Copyright 2023 Google LLC
+//
+// 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"

tests/repeat_key/alt_repeat_key/test.mk

@@ -0,0 +1,18 @@
+# Copyright 2023 Google LLC
+#
+# 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/>.
+
+REPEAT_KEY_ENABLE = yes
+
+EXTRAKEY_ENABLE = yes

tests/repeat_key/config.h

@@ -0,0 +1,20 @@
+// Copyright 2023 Google LLC
+//
+// 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 NO_ALT_REPEAT_KEY

tests/repeat_key/repeat_key_combo/config.h

@@ -0,0 +1,18 @@
+// Copyright 2023 Google LLC
+//
+// 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"

tests/repeat_key/repeat_key_combo/test.mk

@@ -0,0 +1,18 @@
+# Copyright 2023 Google LLC
+#
+# 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/>.
+
+REPEAT_KEY_ENABLE = yes
+
+COMBO_ENABLE = yes

tests/repeat_key/repeat_key_combo/test_repeat_key_combo.cpp

@@ -0,0 +1,67 @@
+// Copyright 2023 Google LLC
+//
+// 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/>.
+
+#include "keyboard_report_util.hpp"
+#include "keycode.h"
+#include "test_common.hpp"
+#include "test_fixture.hpp"
+#include "test_keymap_key.hpp"
+
+using ::testing::AnyNumber;
+using ::testing::InSequence;
+
+namespace {
+
+extern "C" {
+// Define a combo: KC_X + KC_Y = KC_Q.
+const uint16_t xy_combo[] PROGMEM = {KC_X, KC_Y, COMBO_END};
+combo_t        key_combos[]       = {COMBO(xy_combo, KC_Q)};
+uint16_t       COMBO_LEN          = sizeof(key_combos) / sizeof(*key_combos);
+} // extern "C"
+
+class RepeatKey : public TestFixture {};
+
+// Tests repeating a combo, KC_X + KC_Y = KC_Q, by typing
+// "X, Repeat, Repeat, {X Y}, Repeat, Repeat". This produces "xxxqqq".
+TEST_F(RepeatKey, Combo) {
+    TestDriver driver;
+    KeymapKey  key_x(0, 0, 0, KC_X);
+    KeymapKey  key_y(0, 1, 0, KC_Y);
+    KeymapKey  key_repeat(0, 2, 0, QK_REP);
+    set_keymap({key_x, key_y, key_repeat});
+
+    // Allow any number of empty reports.
+    EXPECT_EMPTY_REPORT(driver).Times(AnyNumber());
+    {
+        InSequence seq;
+        EXPECT_REPORT(driver, (KC_X));
+        EXPECT_REPORT(driver, (KC_X));
+        EXPECT_REPORT(driver, (KC_X));
+        EXPECT_REPORT(driver, (KC_Q));
+        EXPECT_REPORT(driver, (KC_Q));
+        EXPECT_REPORT(driver, (KC_Q));
+    }
+
+    tap_keys(key_x, key_repeat, key_repeat);
+    tap_combo({key_x, key_y});
+
+    EXPECT_KEYCODE_EQ(get_last_keycode(), KC_Q);
+
+    tap_keys(key_repeat, key_repeat);
+
+    testing::Mock::VerifyAndClearExpectations(&driver);
+}
+
+} // namespace

tests/repeat_key/test.mk

@@ -0,0 +1,18 @@
+# Copyright 2023 Google LLC
+#
+# 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/>.
+
+REPEAT_KEY_ENABLE = yes
+
+AUTO_SHIFT_ENABLE = yes

tests/test_common/keycode_table.cpp

@@ -663,6 +663,8 @@ std::map<uint16_t, std::string> KEYCODE_ID_TABLE = {
     {QK_AUTOCORRECT_TOGGLE, "QK_AUTOCORRECT_TOGGLE"},
     {QK_TRI_LAYER_LOWER, "QK_TRI_LAYER_LOWER"},
     {QK_TRI_LAYER_UPPER, "QK_TRI_LAYER_UPPER"},
+    {QK_REPEAT_KEY, "QK_REPEAT_KEY"},
+    {QK_ALT_REPEAT_KEY, "QK_ALT_REPEAT_KEY"},
     {QK_KB_0, "QK_KB_0"},
     {QK_KB_1, "QK_KB_1"},
     {QK_KB_2, "QK_KB_2"},

tests/test_common/test_driver.hpp

@@ -20,6 +20,7 @@
 #include <stdint.h>
 #include "host.h"
 #include "keyboard_report_util.hpp"
+#include "keycode_util.hpp"
 #include "test_logger.hpp"
 
 class TestDriver {
@@ -98,6 +99,17 @@ class TestDriver {
  */
 #define EXPECT_NO_REPORT(driver) EXPECT_ANY_REPORT(driver).Times(0)
 
+/** @brief Tests whether keycode `actual` is equal to `expected`. */
+#define EXPECT_KEYCODE_EQ(actual, expected) EXPECT_THAT((actual), KeycodeEq((expected)))
+
+MATCHER_P(KeycodeEq, expected_keycode, "is equal to " + testing::PrintToString(expected_keycode) + ", keycode " + get_keycode_identifier_or_default(expected_keycode)) {
+    if (arg == expected_keycode) {
+        return true;
+    }
+    *result_listener << "keycode " << get_keycode_identifier_or_default(arg);
+    return false;
+}
+
 /**
  * @brief Verify and clear all gmock expectations that have been setup until
  * this point.