From 4e0b573f5563c054d2df82aec4b1be052dd0d7c8 Mon Sep 17 00:00:00 2001
From: Manna Harbour <51143715+manna-harbour@users.noreply.github.com>
Date: Tue, 22 Sep 2020 15:03:46 +1000
Subject: [PATCH] Import manna-harbour/bilateral-combinations.

https://github.com/manna-harbour/qmk_firmware/issues/29

Perhaps this will tame my home row mods?
---
 docs/tap_hold.md | 24 +++++++++++++
 quantum/action.c | 92 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 116 insertions(+)

diff --git a/docs/tap_hold.md b/docs/tap_hold.md
index 094a10753a1a..b9f26d6d5238 100644
--- a/docs/tap_hold.md
+++ b/docs/tap_hold.md
@@ -417,6 +417,30 @@ 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`.
 
+## Bilateral Combinations
+
+The last mod-tap hold will be converted to the corresponding mod-tap tap if another key on the same hand is tapped during the hold, unless a key on the other hand is tapped first.
+
+This option can be used to prevent accidental modifier combinations with mod-tap, in particular those caused by rollover on home row mods.  As only the last mod-tap hold is affected, it should be enabled after adjusting settings and typing style so that accidental mods happen only occasionally, e.g. with a long enough tapping term, ignore mod tap interrupt, and deliberately brief keypresses.
+
+To enable bilateral combinations, add the following to your `config.h`:
+
+```c
+#define BILATERAL_COMBINATIONS
+```
+
+If `BILATERAL_COMBINATIONS` is defined to a value, hold times greater than that value will permit same hand combinations.  For example:
+
+```c
+#define BILATERAL_COMBINATIONS 500
+```
+
+To monitor activations in the background, enable debugging, enable the console, enable terminal bell, add `#define DEBUG_ACTION` to `config.h`, and use something like the following shell command line:
+
+```sh
+hid_listen | sed -u 's/BILATERAL_COMBINATIONS: change/&\a/g'
+```
+
 ## Retro Tapping
 
 To enable `retro tapping`, add the following to your `config.h`:
diff --git a/quantum/action.c b/quantum/action.c
index 6368f7398c61..5e0e1d75787c 100644
--- a/quantum/action.c
+++ b/quantum/action.c
@@ -60,6 +60,14 @@ __attribute__((weak)) bool get_hold_on_other_key_press(uint16_t keycode, keyreco
 }
 #endif
 
+#if (BILATERAL_COMBINATIONS + 0)
+#    include "quantum.h"
+#endif
+
+#ifdef IGNORE_MOD_TAP_INTERRUPT_PER_KEY
+__attribute__((weak)) bool get_ignore_mod_tap_interrupt(uint16_t keycode, keyrecord_t *record) { return false; }
+#endif
+
 #ifdef RETRO_TAPPING_PER_KEY
 __attribute__((weak)) bool get_retro_tapping(uint16_t keycode, keyrecord_t *record) {
     return false;
@@ -358,6 +366,68 @@ void register_mouse(uint8_t mouse_keycode, bool pressed) {
 #endif
 }
 
+#ifdef BILATERAL_COMBINATIONS
+static struct {
+    bool active;
+    uint8_t code;
+    uint8_t tap;
+    uint8_t mods;
+    bool left;
+#    if (BILATERAL_COMBINATIONS + 0)
+    uint16_t time;
+#    endif
+} bilateral_combinations = { false };
+
+static bool bilateral_combinations_left(keypos_t key) {
+#    ifdef SPLIT_KEYBOARD
+    return key.row < MATRIX_ROWS / 2;
+#    else
+    if (MATRIX_COLS > MATRIX_ROWS) {
+        return key.col < MATRIX_COLS / 2;
+    } else {
+        return key.row < MATRIX_ROWS / 2;
+    }
+#    endif
+}
+
+static void bilateral_combinations_hold(action_t action, keyevent_t event) {
+    dprint("BILATERAL_COMBINATIONS: hold\n");
+    bilateral_combinations.active = true;
+    bilateral_combinations.code = action.key.code;
+    bilateral_combinations.tap = action.layer_tap.code;
+    bilateral_combinations.mods = (action.kind.id == ACT_LMODS_TAP) ? action.key.mods : action.key.mods << 4;
+    bilateral_combinations.left = bilateral_combinations_left(event.key);
+#    if (BILATERAL_COMBINATIONS + 0)
+    bilateral_combinations.time = event.time;
+#    endif
+}
+
+static void bilateral_combinations_release(uint8_t code) {
+    dprint("BILATERAL_COMBINATIONS: release\n");
+    if (bilateral_combinations.active && (code == bilateral_combinations.code)) {
+        bilateral_combinations.active = false;
+    }
+}
+
+static void bilateral_combinations_tap(keyevent_t event) {
+    dprint("BILATERAL_COMBINATIONS: tap\n");
+    if (bilateral_combinations.active) {
+        if (bilateral_combinations_left(event.key) == bilateral_combinations.left) {
+#    if (BILATERAL_COMBINATIONS + 0)
+            if (TIMER_DIFF_16(event.time, bilateral_combinations.time) > BILATERAL_COMBINATIONS) {
+                dprint("BILATERAL_COMBINATIONS: timeout\n");
+                return;
+            }
+#    endif
+            dprint("BILATERAL_COMBINATIONS: change\n");
+            unregister_mods(bilateral_combinations.mods);
+            tap_code(bilateral_combinations.tap);
+        }
+        bilateral_combinations.active = false;
+    }
+}
+#endif
+
 /** \brief Take an action and processes it.
  *
  * FIXME: Needs documentation.
@@ -403,6 +473,12 @@ void process_action(keyrecord_t *record, action_t action) {
                     }
                     send_keyboard_report();
                 }
+#ifdef BILATERAL_COMBINATIONS
+                if (!(IS_MODIFIER_KEYCODE(action.key.code) || action.key.code == KC_NO)) {
+                    // regular keycode tap during mod-tap hold
+                    bilateral_combinations_tap(event);
+                }
+#endif
                 register_code(action.key.code);
             } else {
                 unregister_code(action.key.code);
@@ -510,12 +586,20 @@ void process_action(keyrecord_t *record, action_t action) {
                             } else
 #    endif
                             {
+#    ifdef BILATERAL_COMBINATIONS
+                                // mod-tap tap
+                                bilateral_combinations_tap(event);
+#    endif
                                 ac_dprintf("MODS_TAP: Tap: register_code\n");
                                 register_code(action.key.code);
                             }
                         } else {
                             ac_dprintf("MODS_TAP: No tap: add_mods\n");
                             register_mods(mods);
+#    ifdef BILATERAL_COMBINATIONS
+                            // mod-tap hold
+                            bilateral_combinations_hold(action, event);
+#    endif
                         }
                     } else {
                         if (tap_count > 0) {
@@ -536,6 +620,10 @@ void process_action(keyrecord_t *record, action_t action) {
                             }
 #    endif
                             unregister_mods(mods);
+#    ifdef BILATERAL_COMBINATIONS
+                            // mod-tap release
+                            bilateral_combinations_release(action.key.code);
+#    endif
                         }
                     }
                     break;
@@ -693,6 +781,10 @@ void process_action(keyrecord_t *record, action_t action) {
 #    ifndef NO_ACTION_TAPPING /* tap key */
                     if (event.pressed) {
                         if (tap_count > 0) {
+#        ifdef BILATERAL_COMBINATIONS
+                            // layer-tap tap
+                            bilateral_combinations_tap(event);
+#        endif
                             ac_dprintf("KEYMAP_TAP_KEY: Tap: register_code\n");
                             register_code(action.layer_tap.code);
                         } else {