try again with achordion

ferris/achordion.c

@@ -0,0 +1,220 @@
+// 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.
+
+/**
+ * @file achordion.c
+ * @brief Achordion implementation
+ *
+ * For full documentation, see
+ * <https://getreuer.info/posts/keyboards/achordion>
+ */
+
+#include "achordion.h"
+
+// Copy of the `record` and `keycode` args for the current active tap-hold key.
+static keyrecord_t tap_hold_record;
+static uint16_t tap_hold_keycode = KC_NO;
+// Timeout timer. When it expires, the key is considered held.
+static uint16_t hold_timer = 0;
+// Eagerly applied mods, if any.
+static uint8_t eager_mods = 0;
+
+// Achordion's current state.
+enum {
+  // A tap-hold key is pressed, but hasn't yet been settled as tapped or held.
+  STATE_UNSETTLED,
+  // Achordion is inactive.
+  STATE_RELEASED,
+  // Active tap-hold key has been settled as tapped.
+  STATE_TAPPING,
+  // Active tap-hold key has been settled as held.
+  STATE_HOLDING,
+  // This state is set while calling `process_record()`, which will recursively
+  // call `process_achordion()`. This state is checked so that we don't process
+  // events generated by Achordion and potentially create an infinite loop.
+  STATE_RECURSING,
+};
+static uint8_t achordion_state = STATE_RELEASED;
+
+// Calls `process_record()` with state set to RECURSING.
+static void recursively_process_record(keyrecord_t* record, uint8_t state) {
+  achordion_state = STATE_RECURSING;
+  process_record(record);
+  achordion_state = state;
+}
+
+// Clears eagerly-applied mods.
+static void clear_eager_mods(void) {
+  unregister_mods(eager_mods);
+  eager_mods = 0;
+}
+
+// Sends hold press event and settles the active tap-hold key as held.
+static void settle_as_hold(void) {
+  clear_eager_mods();
+  // Create hold press event.
+  recursively_process_record(&tap_hold_record, STATE_HOLDING);
+}
+
+bool process_achordion(uint16_t keycode, keyrecord_t* record) {
+  // Don't process events that Achordion generated.
+  if (achordion_state == STATE_RECURSING) {
+    return true;
+  }
+
+  // Determine whether the current event is for a mod-tap or layer-tap key.
+  const bool is_mt = IS_QK_MOD_TAP(keycode);
+  const bool is_tap_hold = is_mt || IS_QK_LAYER_TAP(keycode);
+  // Check that this is a normal key event, don't act on combos.
+#ifdef IS_KEYEVENT
+  const bool is_key_event = IS_KEYEVENT(record->event);
+#else
+  const bool is_key_event = (record->event.key.row < 254 &&
+                             record->event.key.col < 254);
+#endif
+
+  if (achordion_state == STATE_RELEASED) {
+    if (is_tap_hold && record->tap.count == 0 && record->event.pressed &&
+        is_key_event) {
+      // A tap-hold key is pressed and considered by QMK as "held".
+      const uint16_t timeout = achordion_timeout(keycode);
+      if (timeout > 0) {
+        achordion_state = STATE_UNSETTLED;
+        // Save info about this key.
+        tap_hold_keycode = keycode;
+        tap_hold_record = *record;
+        hold_timer = record->event.time + timeout;
+
+        if (is_mt) {  // Apply mods immediately if they are "eager."
+          uint8_t mod = mod_config(QK_MOD_TAP_GET_MODS(tap_hold_keycode));
+          if (achordion_eager_mod(mod)) {
+            eager_mods = ((mod & 0x10) == 0) ? mod : (mod << 4);
+            register_mods(eager_mods);
+          }
+        }
+
+        dprintf("Achordion: Key 0x%04X pressed.%s\n", keycode,
+                eager_mods ? " Set eager mods." : "");
+        return false;  // Skip default handling.
+      }
+    }
+
+    return true;  // Otherwise, continue with default handling.
+  }
+
+  if (keycode == tap_hold_keycode && !record->event.pressed) {
+    // The active tap-hold key is being released.
+    if (achordion_state == STATE_HOLDING) {
+      dprintln("Achordion: Key released. Plumbing hold release.");
+      tap_hold_record.event.pressed = false;
+      // Plumb hold release event.
+      recursively_process_record(&tap_hold_record, STATE_RELEASED);
+    } else {
+      dprintf("Achordion: Key released.%s\n",
+              eager_mods ? " Clearing eager mods." : "");
+      if (is_mt) {
+        clear_eager_mods();
+      }
+    }
+
+    achordion_state = STATE_RELEASED;
+    return false;
+  }
+
+  if (achordion_state == STATE_UNSETTLED && record->event.pressed) {
+    // Press event occurred on a key other than the active tap-hold key.
+
+    // If the other key is *also* a tap-hold key and considered by QMK to be
+    // held, then we settle the active key as held. This way, things like
+    // chording multiple home row modifiers will work, but let's our logic
+    // consider simply a single tap-hold key as "active" at a time.
+    //
+    // Otherwise, we call `achordion_chord()` to determine whether to settle the
+    // tap-hold key as tapped vs. held. We implement the tap or hold by plumbing
+    // events back into the handling pipeline so that QMK features and other
+    // user code can see them. This is done by calling `process_record()`, which
+    // in turn calls most handlers including `process_record_user()`.
+    if (!is_key_event || (is_tap_hold && record->tap.count == 0) ||
+        achordion_chord(tap_hold_keycode, &tap_hold_record, keycode, record)) {
+      dprintln("Achordion: Plumbing hold press.");
+      settle_as_hold();
+    } else {
+      clear_eager_mods();  // Clear in case eager mods were set.
+
+      dprintln("Achordion: Plumbing tap press.");
+      tap_hold_record.tap.count = 1;  // Revise event as a tap.
+      tap_hold_record.tap.interrupted = true;
+      // Plumb tap press event.
+      recursively_process_record(&tap_hold_record, STATE_TAPPING);
+
+      send_keyboard_report();
+#if TAP_CODE_DELAY > 0
+      wait_ms(TAP_CODE_DELAY);
+#endif  // TAP_CODE_DELAY > 0
+
+      dprintln("Achordion: Plumbing tap release.");
+      tap_hold_record.event.pressed = false;
+      // Plumb tap release event.
+      recursively_process_record(&tap_hold_record, STATE_TAPPING);
+    }
+
+    recursively_process_record(record, achordion_state);  // Re-process event.
+    return false;  // Block the original event.
+  }
+
+  return true;
+}
+
+void achordion_task(void) {
+  if (achordion_state == STATE_UNSETTLED &&
+      timer_expired(timer_read(), hold_timer)) {
+    dprintln("Achordion: Timeout. Plumbing hold press.");
+    settle_as_hold();  // Timeout expired, settle the key as held.
+  }
+}
+
+// Returns true if `pos` on the left hand of the keyboard, false if right.
+static bool on_left_hand(keypos_t pos) {
+#ifdef SPLIT_KEYBOARD
+  return pos.row < MATRIX_ROWS / 2;
+#else
+  return (MATRIX_COLS > MATRIX_ROWS) ? pos.col < MATRIX_COLS / 2
+                                     : pos.row < MATRIX_ROWS / 2;
+#endif
+}
+
+bool achordion_opposite_hands(const keyrecord_t* tap_hold_record,
+                              const keyrecord_t* other_record) {
+  return on_left_hand(tap_hold_record->event.key) !=
+         on_left_hand(other_record->event.key);
+}
+
+// By default, use the BILATERAL_COMBINATIONS rule to consider the tap-hold key
+// "held" only when it and the other key are on opposite hands.
+__attribute__((weak)) bool achordion_chord(uint16_t tap_hold_keycode,
+                                           keyrecord_t* tap_hold_record,
+                                           uint16_t other_keycode,
+                                           keyrecord_t* other_record) {
+  return achordion_opposite_hands(tap_hold_record, other_record);
+}
+
+// By default, the timeout is 1000 ms for all keys.
+__attribute__((weak)) uint16_t achordion_timeout(uint16_t tap_hold_keycode) {
+  return 1000;
+}
+
+// By default, Shift and Ctrl mods are eager, and Alt and GUI are not.
+__attribute__((weak)) bool achordion_eager_mod(uint8_t mod) {
+  return (mod & (MOD_LALT | MOD_LGUI)) == 0;
+}

ferris/achordion.h

@@ -0,0 +1,164 @@
+// 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.
+
+/**
+ * @file achordion.h
+ * @brief Achordion: Customizing the tap-hold decision.
+ *
+ * Overview
+ * --------
+ *
+ * This library customizes when tap-hold keys are considered held vs. tapped
+ * based on the next pressed key, like Manna Harbour's Bilateral Combinations or
+ * ZMK's positional hold. The library works on top of QMK's existing tap-hold
+ * implementation. You define mod-tap and layer-tap keys as usual and use
+ * Achordion to fine-tune the behavior.
+ *
+ * When QMK settles a tap-hold key as held, Achordion intercepts the event.
+ * Achordion then revises the event as a tap or passes it along as a hold:
+ *
+ *  * Chord condition: On the next key press, a customizable `achordion_chord()`
+ *    function is called, which takes the tap-hold key and the next key pressed
+ *    as args. When the function returns true, the tap-hold key is settled as
+ *    held, and otherwise as tapped.
+ *
+ *  * Timeout: If no other key press occurs within a timeout, the tap-hold key
+ *    is settled as held. This is customizable with `achordion_timeout()`.
+ *
+ * Achordion only changes the behavior when QMK considered the key held. It
+ * changes some would-be holds to taps, but no taps to holds.
+ *
+ * @note Some QMK features handle events before the point where Achordion can
+ * intercept them, particularly: Combos, Key Lock, and Dynamic Macros. It's
+ * still possible to use these features and Achordion in your keymap, but beware
+ * they might behave poorly when used simultaneously with tap-hold keys.
+ *
+ *
+ * For full documentation, see
+ * <https://getreuer.info/posts/keyboards/achordion>
+ */
+
+#pragma once
+
+#include "quantum.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Handler function for Achordion.
+ *
+ * Call this function from `process_record_user()` as
+ *
+ *     #include "features/achordion.h"
+ *
+ *     bool process_record_user(uint16_t keycode, keyrecord_t* record) {
+ *       if (!process_achordion(keycode, record)) { return false; }
+ *       // Your macros...
+ *       return true;
+ *     }
+ */
+bool process_achordion(uint16_t keycode, keyrecord_t* record);
+
+/**
+ * Matrix task function for Achordion.
+ *
+ * Call this function from `matrix_scan_user()` as
+ *
+ *     void matrix_scan_user(void) {
+ *       achordion_task();
+ *     }
+ */
+void achordion_task(void);
+
+/**
+ * Optional callback to customize which key chords are considered "held".
+ *
+ * In your keymap.c, define the callback
+ *
+ *     bool achordion_chord(uint16_t tap_hold_keycode,
+ *                          keyrecord_t* tap_hold_record,
+ *                          uint16_t other_keycode,
+ *                          keyrecord_t* other_record) {
+ *        // Conditions...
+ *     }
+ *
+ * This callback is called if while `tap_hold_keycode` is pressed,
+ * `other_keycode` is pressed. Return true if the tap-hold key should be
+ * considered held, or false to consider it tapped.
+ *
+ * @param tap_hold_keycode Keycode of the tap-hold key.
+ * @param tap_hold_record keyrecord_t from the tap-hold press event.
+ * @param other_keycode Keycode of the other key.
+ * @param other_record keyrecord_t from the other key's press event.
+ * @return True if the tap-hold key should be considered held.
+ */
+bool achordion_chord(uint16_t tap_hold_keycode, keyrecord_t* tap_hold_record,
+                     uint16_t other_keycode, keyrecord_t* other_record);
+
+/**
+ * Optional callback to define a timeout duration per keycode.
+ *
+ * In your keymap.c, define the callback
+ *
+ *     uint16_t achordion_timeout(uint16_t tap_hold_keycode) {
+ *       // ...
+ *     }
+ *
+ * The callback determines Achordion's timeout duration for `tap_hold_keycode`
+ * in units of milliseconds. The timeout be in the range 0 to 32767 ms (upper
+ * bound is due to 16-bit timer limitations). Use a timeout of 0 to bypass
+ * Achordion.
+ *
+ * @param tap_hold_keycode Keycode of the tap-hold key.
+ * @return Timeout duration in milliseconds in the range 0 to 32767.
+ */
+uint16_t achordion_timeout(uint16_t tap_hold_keycode);
+
+/**
+ * Optional callback defining which mods are "eagerly" applied.
+ *
+ * This callback defines  which mods are "eagerly" applied while a mod-tap
+ * key is still being settled. This is helpful to reduce delay particularly when
+ * using mod-tap keys with an external mouse.
+ *
+ * Define this callback in your keymap.c. The default callback is eager for
+ * Shift and Ctrl, and not for Alt and GUI:
+ *
+ *     bool achordion_eager_mod(uint8_t mod) {
+ *       return (mod & (MOD_LALT | MOD_LGUI)) == 0;
+ *     }
+ *
+ * @note `mod` should be compared with `MOD_` prefixed codes, not `KC_` codes,
+ * described at <https://docs.qmk.fm/#/mod_tap>.
+ *
+ * @param mod Modifier `MOD_` code.
+ * @return True if the modifier should be eagerly applied.
+ */
+bool achordion_eager_mod(uint8_t mod);
+
+/**
+ * Returns true if the args come from keys on opposite hands.
+ *
+ * @param tap_hold_record keyrecord_t from the tap-hold key's event.
+ * @param other_record keyrecord_t from the other key's event.
+ * @return True if the keys are on opposite hands.
+ */
+bool achordion_opposite_hands(const keyrecord_t* tap_hold_record,
+                              const keyrecord_t* other_record);
+
+#ifdef __cplusplus
+}
+#endif

ferris/config.h

@@ -30,10 +30,10 @@
 // details see https://precondition.github.io/home-row-mods
 
 // the default of 200
-#define TAPPING_TERM 180
+#define TAPPING_TERM 200
 
 // Enable rapid switch from tap to hold, disables double tap hold auto-repeat.
 #define QUICK_TAP_TERM 0
 
-// try to be more permissive with holds for layer switch keys
+// try to be more permissive with holds
-#define PERMISSIVE_HOLD_PER_KEY
+#define PERMISSIVE_HOLD

ferris/keymap.c

@@ -89,3 +89,23 @@ bool get_permissive_hold(uint16_t keycode, keyrecord_t *record) {
             return false;
     }
 }
+
+//
+// improve home row modifiers via achordion
+//
+
+#include "achordion.h"
+
+bool process_record_user(uint16_t keycode, keyrecord_t* record)
+{
+    if (!process_achordion(keycode, record)) {
+        return false;
+    }
+
+    return true;
+}
+
+void matrix_scan_user(void)
+{
+    achordion_task();
+}

ferris/rules.mk

@@ -17,6 +17,9 @@ MAGIC_ENABLE = no
 # more responsive debouncing: https://michael.stapelberg.ch/posts/2021-05-08-keyboard-input-latency-qmk-kinesis/
 DEBOUNCE_TYPE = sym_eager_pk
 
+# add achordion to improve home row modifiers
+SRC += achordion.c
+
 # build
 # qmk compile -kb ferris/sweep -km christoph-cullmann
 # qmk flash -kb ferris/sweep -km christoph-cullmann

planck/achordion.c

@@ -0,0 +1,220 @@
+// 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.
+
+/**
+ * @file achordion.c
+ * @brief Achordion implementation
+ *
+ * For full documentation, see
+ * <https://getreuer.info/posts/keyboards/achordion>
+ */
+
+#include "achordion.h"
+
+// Copy of the `record` and `keycode` args for the current active tap-hold key.
+static keyrecord_t tap_hold_record;
+static uint16_t tap_hold_keycode = KC_NO;
+// Timeout timer. When it expires, the key is considered held.
+static uint16_t hold_timer = 0;
+// Eagerly applied mods, if any.
+static uint8_t eager_mods = 0;
+
+// Achordion's current state.
+enum {
+  // A tap-hold key is pressed, but hasn't yet been settled as tapped or held.
+  STATE_UNSETTLED,
+  // Achordion is inactive.
+  STATE_RELEASED,
+  // Active tap-hold key has been settled as tapped.
+  STATE_TAPPING,
+  // Active tap-hold key has been settled as held.
+  STATE_HOLDING,
+  // This state is set while calling `process_record()`, which will recursively
+  // call `process_achordion()`. This state is checked so that we don't process
+  // events generated by Achordion and potentially create an infinite loop.
+  STATE_RECURSING,
+};
+static uint8_t achordion_state = STATE_RELEASED;
+
+// Calls `process_record()` with state set to RECURSING.
+static void recursively_process_record(keyrecord_t* record, uint8_t state) {
+  achordion_state = STATE_RECURSING;
+  process_record(record);
+  achordion_state = state;
+}
+
+// Clears eagerly-applied mods.
+static void clear_eager_mods(void) {
+  unregister_mods(eager_mods);
+  eager_mods = 0;
+}
+
+// Sends hold press event and settles the active tap-hold key as held.
+static void settle_as_hold(void) {
+  clear_eager_mods();
+  // Create hold press event.
+  recursively_process_record(&tap_hold_record, STATE_HOLDING);
+}
+
+bool process_achordion(uint16_t keycode, keyrecord_t* record) {
+  // Don't process events that Achordion generated.
+  if (achordion_state == STATE_RECURSING) {
+    return true;
+  }
+
+  // Determine whether the current event is for a mod-tap or layer-tap key.
+  const bool is_mt = IS_QK_MOD_TAP(keycode);
+  const bool is_tap_hold = is_mt || IS_QK_LAYER_TAP(keycode);
+  // Check that this is a normal key event, don't act on combos.
+#ifdef IS_KEYEVENT
+  const bool is_key_event = IS_KEYEVENT(record->event);
+#else
+  const bool is_key_event = (record->event.key.row < 254 &&
+                             record->event.key.col < 254);
+#endif
+
+  if (achordion_state == STATE_RELEASED) {
+    if (is_tap_hold && record->tap.count == 0 && record->event.pressed &&
+        is_key_event) {
+      // A tap-hold key is pressed and considered by QMK as "held".
+      const uint16_t timeout = achordion_timeout(keycode);
+      if (timeout > 0) {
+        achordion_state = STATE_UNSETTLED;
+        // Save info about this key.
+        tap_hold_keycode = keycode;
+        tap_hold_record = *record;
+        hold_timer = record->event.time + timeout;
+
+        if (is_mt) {  // Apply mods immediately if they are "eager."
+          uint8_t mod = mod_config(QK_MOD_TAP_GET_MODS(tap_hold_keycode));
+          if (achordion_eager_mod(mod)) {
+            eager_mods = ((mod & 0x10) == 0) ? mod : (mod << 4);
+            register_mods(eager_mods);
+          }
+        }
+
+        dprintf("Achordion: Key 0x%04X pressed.%s\n", keycode,
+                eager_mods ? " Set eager mods." : "");
+        return false;  // Skip default handling.
+      }
+    }
+
+    return true;  // Otherwise, continue with default handling.
+  }
+
+  if (keycode == tap_hold_keycode && !record->event.pressed) {
+    // The active tap-hold key is being released.
+    if (achordion_state == STATE_HOLDING) {
+      dprintln("Achordion: Key released. Plumbing hold release.");
+      tap_hold_record.event.pressed = false;
+      // Plumb hold release event.
+      recursively_process_record(&tap_hold_record, STATE_RELEASED);
+    } else {
+      dprintf("Achordion: Key released.%s\n",
+              eager_mods ? " Clearing eager mods." : "");
+      if (is_mt) {
+        clear_eager_mods();
+      }
+    }
+
+    achordion_state = STATE_RELEASED;
+    return false;
+  }
+
+  if (achordion_state == STATE_UNSETTLED && record->event.pressed) {
+    // Press event occurred on a key other than the active tap-hold key.
+
+    // If the other key is *also* a tap-hold key and considered by QMK to be
+    // held, then we settle the active key as held. This way, things like
+    // chording multiple home row modifiers will work, but let's our logic
+    // consider simply a single tap-hold key as "active" at a time.
+    //
+    // Otherwise, we call `achordion_chord()` to determine whether to settle the
+    // tap-hold key as tapped vs. held. We implement the tap or hold by plumbing
+    // events back into the handling pipeline so that QMK features and other
+    // user code can see them. This is done by calling `process_record()`, which
+    // in turn calls most handlers including `process_record_user()`.
+    if (!is_key_event || (is_tap_hold && record->tap.count == 0) ||
+        achordion_chord(tap_hold_keycode, &tap_hold_record, keycode, record)) {
+      dprintln("Achordion: Plumbing hold press.");
+      settle_as_hold();
+    } else {
+      clear_eager_mods();  // Clear in case eager mods were set.
+
+      dprintln("Achordion: Plumbing tap press.");
+      tap_hold_record.tap.count = 1;  // Revise event as a tap.
+      tap_hold_record.tap.interrupted = true;
+      // Plumb tap press event.
+      recursively_process_record(&tap_hold_record, STATE_TAPPING);
+
+      send_keyboard_report();
+#if TAP_CODE_DELAY > 0
+      wait_ms(TAP_CODE_DELAY);
+#endif  // TAP_CODE_DELAY > 0
+
+      dprintln("Achordion: Plumbing tap release.");
+      tap_hold_record.event.pressed = false;
+      // Plumb tap release event.
+      recursively_process_record(&tap_hold_record, STATE_TAPPING);
+    }
+
+    recursively_process_record(record, achordion_state);  // Re-process event.
+    return false;  // Block the original event.
+  }
+
+  return true;
+}
+
+void achordion_task(void) {
+  if (achordion_state == STATE_UNSETTLED &&
+      timer_expired(timer_read(), hold_timer)) {
+    dprintln("Achordion: Timeout. Plumbing hold press.");
+    settle_as_hold();  // Timeout expired, settle the key as held.
+  }
+}
+
+// Returns true if `pos` on the left hand of the keyboard, false if right.
+static bool on_left_hand(keypos_t pos) {
+#ifdef SPLIT_KEYBOARD
+  return pos.row < MATRIX_ROWS / 2;
+#else
+  return (MATRIX_COLS > MATRIX_ROWS) ? pos.col < MATRIX_COLS / 2
+                                     : pos.row < MATRIX_ROWS / 2;
+#endif
+}
+
+bool achordion_opposite_hands(const keyrecord_t* tap_hold_record,
+                              const keyrecord_t* other_record) {
+  return on_left_hand(tap_hold_record->event.key) !=
+         on_left_hand(other_record->event.key);
+}
+
+// By default, use the BILATERAL_COMBINATIONS rule to consider the tap-hold key
+// "held" only when it and the other key are on opposite hands.
+__attribute__((weak)) bool achordion_chord(uint16_t tap_hold_keycode,
+                                           keyrecord_t* tap_hold_record,
+                                           uint16_t other_keycode,
+                                           keyrecord_t* other_record) {
+  return achordion_opposite_hands(tap_hold_record, other_record);
+}
+
+// By default, the timeout is 1000 ms for all keys.
+__attribute__((weak)) uint16_t achordion_timeout(uint16_t tap_hold_keycode) {
+  return 1000;
+}
+
+// By default, Shift and Ctrl mods are eager, and Alt and GUI are not.
+__attribute__((weak)) bool achordion_eager_mod(uint8_t mod) {
+  return (mod & (MOD_LALT | MOD_LGUI)) == 0;
+}

planck/achordion.h

@@ -0,0 +1,164 @@
+// 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.
+
+/**
+ * @file achordion.h
+ * @brief Achordion: Customizing the tap-hold decision.
+ *
+ * Overview
+ * --------
+ *
+ * This library customizes when tap-hold keys are considered held vs. tapped
+ * based on the next pressed key, like Manna Harbour's Bilateral Combinations or
+ * ZMK's positional hold. The library works on top of QMK's existing tap-hold
+ * implementation. You define mod-tap and layer-tap keys as usual and use
+ * Achordion to fine-tune the behavior.
+ *
+ * When QMK settles a tap-hold key as held, Achordion intercepts the event.
+ * Achordion then revises the event as a tap or passes it along as a hold:
+ *
+ *  * Chord condition: On the next key press, a customizable `achordion_chord()`
+ *    function is called, which takes the tap-hold key and the next key pressed
+ *    as args. When the function returns true, the tap-hold key is settled as
+ *    held, and otherwise as tapped.
+ *
+ *  * Timeout: If no other key press occurs within a timeout, the tap-hold key
+ *    is settled as held. This is customizable with `achordion_timeout()`.
+ *
+ * Achordion only changes the behavior when QMK considered the key held. It
+ * changes some would-be holds to taps, but no taps to holds.
+ *
+ * @note Some QMK features handle events before the point where Achordion can
+ * intercept them, particularly: Combos, Key Lock, and Dynamic Macros. It's
+ * still possible to use these features and Achordion in your keymap, but beware
+ * they might behave poorly when used simultaneously with tap-hold keys.
+ *
+ *
+ * For full documentation, see
+ * <https://getreuer.info/posts/keyboards/achordion>
+ */
+
+#pragma once
+
+#include "quantum.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Handler function for Achordion.
+ *
+ * Call this function from `process_record_user()` as
+ *
+ *     #include "features/achordion.h"
+ *
+ *     bool process_record_user(uint16_t keycode, keyrecord_t* record) {
+ *       if (!process_achordion(keycode, record)) { return false; }
+ *       // Your macros...
+ *       return true;
+ *     }
+ */
+bool process_achordion(uint16_t keycode, keyrecord_t* record);
+
+/**
+ * Matrix task function for Achordion.
+ *
+ * Call this function from `matrix_scan_user()` as
+ *
+ *     void matrix_scan_user(void) {
+ *       achordion_task();
+ *     }
+ */
+void achordion_task(void);
+
+/**
+ * Optional callback to customize which key chords are considered "held".
+ *
+ * In your keymap.c, define the callback
+ *
+ *     bool achordion_chord(uint16_t tap_hold_keycode,
+ *                          keyrecord_t* tap_hold_record,
+ *                          uint16_t other_keycode,
+ *                          keyrecord_t* other_record) {
+ *        // Conditions...
+ *     }
+ *
+ * This callback is called if while `tap_hold_keycode` is pressed,
+ * `other_keycode` is pressed. Return true if the tap-hold key should be
+ * considered held, or false to consider it tapped.
+ *
+ * @param tap_hold_keycode Keycode of the tap-hold key.
+ * @param tap_hold_record keyrecord_t from the tap-hold press event.
+ * @param other_keycode Keycode of the other key.
+ * @param other_record keyrecord_t from the other key's press event.
+ * @return True if the tap-hold key should be considered held.
+ */
+bool achordion_chord(uint16_t tap_hold_keycode, keyrecord_t* tap_hold_record,
+                     uint16_t other_keycode, keyrecord_t* other_record);
+
+/**
+ * Optional callback to define a timeout duration per keycode.
+ *
+ * In your keymap.c, define the callback
+ *
+ *     uint16_t achordion_timeout(uint16_t tap_hold_keycode) {
+ *       // ...
+ *     }
+ *
+ * The callback determines Achordion's timeout duration for `tap_hold_keycode`
+ * in units of milliseconds. The timeout be in the range 0 to 32767 ms (upper
+ * bound is due to 16-bit timer limitations). Use a timeout of 0 to bypass
+ * Achordion.
+ *
+ * @param tap_hold_keycode Keycode of the tap-hold key.
+ * @return Timeout duration in milliseconds in the range 0 to 32767.
+ */
+uint16_t achordion_timeout(uint16_t tap_hold_keycode);
+
+/**
+ * Optional callback defining which mods are "eagerly" applied.
+ *
+ * This callback defines  which mods are "eagerly" applied while a mod-tap
+ * key is still being settled. This is helpful to reduce delay particularly when
+ * using mod-tap keys with an external mouse.
+ *
+ * Define this callback in your keymap.c. The default callback is eager for
+ * Shift and Ctrl, and not for Alt and GUI:
+ *
+ *     bool achordion_eager_mod(uint8_t mod) {
+ *       return (mod & (MOD_LALT | MOD_LGUI)) == 0;
+ *     }
+ *
+ * @note `mod` should be compared with `MOD_` prefixed codes, not `KC_` codes,
+ * described at <https://docs.qmk.fm/#/mod_tap>.
+ *
+ * @param mod Modifier `MOD_` code.
+ * @return True if the modifier should be eagerly applied.
+ */
+bool achordion_eager_mod(uint8_t mod);
+
+/**
+ * Returns true if the args come from keys on opposite hands.
+ *
+ * @param tap_hold_record keyrecord_t from the tap-hold key's event.
+ * @param other_record keyrecord_t from the other key's event.
+ * @return True if the keys are on opposite hands.
+ */
+bool achordion_opposite_hands(const keyrecord_t* tap_hold_record,
+                              const keyrecord_t* other_record);
+
+#ifdef __cplusplus
+}
+#endif

planck/config.h

@@ -30,13 +30,13 @@
 // details see https://precondition.github.io/home-row-mods
 
 // the default of 200
-#define TAPPING_TERM 180
+#define TAPPING_TERM 200
 
 // Enable rapid switch from tap to hold, disables double tap hold auto-repeat.
 #define QUICK_TAP_TERM 0
 
-// try to be more permissive with holds for layer switch keys
+// try to be more permissive with holds
-#define PERMISSIVE_HOLD_PER_KEY
+#define PERMISSIVE_HOLD
 
 //
 // audio configuration

planck/keymap.c

@@ -155,3 +155,37 @@ layer_state_t layer_state_set_user(layer_state_t state)
     rgblight_set_layer_state(_NAV, layer_state_cmp(state, _NAV));
     return state;
 }
+
+//
+// improve home row modifiers via achordion
+//
+
+#include "achordion.h"
+
+bool process_record_user(uint16_t keycode, keyrecord_t* record)
+{
+    if (!process_achordion(keycode, record)) {
+        return false;
+    }
+
+    return true;
+}
+
+void matrix_scan_user(void)
+{
+    achordion_task();
+}
+
+static bool planck_on_left_hand(keypos_t pos)
+{
+    // planck is like a split keyboard, beside for the last row
+    return (pos.row < 3) || (pos.row == 3 && pos.col < 3) || (pos.row == 7 && pos.col > 2);
+}
+
+bool achordion_chord(uint16_t tap_hold_keycode,
+                     keyrecord_t* tap_hold_record,
+                     uint16_t other_keycode,
+                     keyrecord_t* other_record)
+{
+    return planck_on_left_hand(tap_hold_record->event.key) != planck_on_left_hand(other_record->event.key);
+}

planck/rules.mk

@@ -17,6 +17,9 @@ MAGIC_ENABLE = no
 # more responsive debouncing: https://michael.stapelberg.ch/posts/2021-05-08-keyboard-input-latency-qmk-kinesis/
 DEBOUNCE_TYPE = sym_eager_pk
 
+# add achordion to improve home row modifiers
+SRC += achordion.c
+
 # build
 # qmk compile -kb planck/rev6_drop -km christoph-cullmann
 # qmk flash -kb planck/rev6_drop -km christoph-cullmann