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+// 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"
+
+/**
+ * Suppress tap-hold mods within a *typing streak* by defining
+ * ACHORDION_STREAK. This can help preventing accidental mod
+ * activation when performing a fast tapping sequence.
+ * This is inspired by https://sunaku.github.io/home-row-mods.html#typing-streaks
+ *
+ * Enable with:
+ *
+ *    #define ACHORDION_STREAK
+ *
+ * Adjust the maximum time between key events before modifiers can be enabled
+ * by defining the following callback in your keymap.c:
+ *
+ *    uint16_t achordion_streak_timeout(uint16_t tap_hold_keycode) {
+ *      return 100;  // Default of 100 ms.
+ *    }
+ */
+#ifdef ACHORDION_STREAK
+uint16_t achordion_streak_timeout(uint16_t tap_hold_keycode);
+#endif
+
+#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