behaviors(hold-tap): Implement quick_tap_ms (TAPPING_FORCE_HOLD)

Tap-and-hold a hold-tap to hold the tap behavior so it can repeat.
After a tap, if the same key is pressed within `quick_tap_ms`, the
tap behavior is always picked.

This is useful for things like `&ht LSHFT BACKSPACE` where holding
the backspace is required.

Implements #288.

app/dts/bindings/behaviors/zmk,behavior-hold-tap.yaml

@@ -13,6 +13,9 @@ properties:
     required: true
   tapping_term_ms:
     type: int
+  quick_tap_ms:
+    type: int
+    default: -1
   flavor:
     type: string
     required: false

app/src/behaviors/behavior_hold_tap.c

@@ -40,6 +40,7 @@ struct behavior_hold_tap_config {
     int tapping_term_ms;
     char *hold_behavior_dev;
     char *tap_behavior_dev;
+    int quick_tap_ms;
     enum flavor flavor;
 };
 
@@ -67,6 +68,24 @@ struct active_hold_tap active_hold_taps[ZMK_BHV_HOLD_TAP_MAX_HELD] = {};
 // We capture most position_state_changed events and some modifiers_state_changed events.
 const zmk_event_t *captured_events[ZMK_BHV_HOLD_TAP_MAX_CAPTURED_EVENTS] = {};
 
+// Keep track of which key was tapped most recently for 'quick_tap_ms'
+struct last_tapped {
+    int32_t position;
+    int64_t tap_deadline;
+};
+
+struct last_tapped last_tapped;
+
+static void store_last_tapped(struct active_hold_tap *hold_tap) {
+    last_tapped.position = hold_tap->position;
+    last_tapped.tap_deadline = hold_tap->timestamp + hold_tap->config->quick_tap_ms;
+}
+
+static bool is_quick_tap(struct active_hold_tap *hold_tap) {
+    return last_tapped.position == hold_tap->position &&
+           last_tapped.tap_deadline > hold_tap->timestamp;
+}
+
 static int capture_event(const zmk_event_t *event) {
     for (int i = 0; i < ZMK_BHV_HOLD_TAP_MAX_CAPTURED_EVENTS; i++) {
         if (captured_events[i] == NULL) {
@@ -191,6 +210,7 @@ enum decision_moment {
     HT_OTHER_KEY_DOWN = 1,
     HT_OTHER_KEY_UP = 2,
     HT_TIMER_EVENT = 3,
+    HT_QUICK_TAP = 4,
 };
 
 static void decide_balanced(struct active_hold_tap *hold_tap, enum decision_moment event) {
@@ -204,6 +224,10 @@ static void decide_balanced(struct active_hold_tap *hold_tap, enum decision_mome
         hold_tap->is_hold = 1;
         hold_tap->is_decided = true;
         break;
+    case HT_QUICK_TAP:
+        hold_tap->is_hold = 0;
+        hold_tap->is_decided = true;
+        break;
     default:
         return;
     }
@@ -219,6 +243,10 @@ static void decide_tap_preferred(struct active_hold_tap *hold_tap, enum decision
         hold_tap->is_hold = 1;
         hold_tap->is_decided = true;
         break;
+    case HT_QUICK_TAP:
+        hold_tap->is_hold = 0;
+        hold_tap->is_decided = true;
+        break;
     default:
         return;
     }
@@ -235,6 +263,10 @@ static void decide_hold_preferred(struct active_hold_tap *hold_tap, enum decisio
         hold_tap->is_hold = 1;
         hold_tap->is_decided = true;
         break;
+    case HT_QUICK_TAP:
+        hold_tap->is_hold = 0;
+        hold_tap->is_decided = true;
+        break;
     default:
         return;
     }
@@ -293,6 +325,7 @@ static void decide_hold_tap(struct active_hold_tap *hold_tap, enum decision_mome
         binding.behavior_dev = hold_tap->config->tap_behavior_dev;
         binding.param1 = hold_tap->param_tap;
         binding.param2 = 0;
+        store_last_tapped(hold_tap);
     }
     behavior_keymap_binding_pressed(&binding, event);
     release_captured_events();
@@ -320,6 +353,10 @@ static int on_hold_tap_binding_pressed(struct zmk_behavior_binding *binding,
     LOG_DBG("%d new undecided hold_tap", event.position);
     undecided_hold_tap = hold_tap;
 
+    if (is_quick_tap(hold_tap)) {
+        decide_hold_tap(hold_tap, HT_QUICK_TAP);
+    }
+
     // if this behavior was queued we have to adjust the timer to only
     // wait for the remaining time.
     int32_t tapping_term_ms_left = (hold_tap->timestamp + cfg->tapping_term_ms) - k_uptime_get();
@@ -492,6 +529,7 @@ static struct behavior_hold_tap_data behavior_hold_tap_data;
         .tapping_term_ms = DT_INST_PROP(n, tapping_term_ms),                                       \
         .hold_behavior_dev = DT_LABEL(DT_INST_PHANDLE_BY_IDX(n, bindings, 0)),                     \
         .tap_behavior_dev = DT_LABEL(DT_INST_PHANDLE_BY_IDX(n, bindings, 1)),                      \
+        .quick_tap_ms = DT_INST_PROP(n, quick_tap_ms),                                             \
         .flavor = DT_ENUM_IDX(DT_DRV_INST(n), flavor),                                             \
     };                                                                                             \
     DEVICE_AND_API_INIT(behavior_hold_tap_##n, DT_INST_LABEL(n), behavior_hold_tap_init,           \

app/tests/hold-tap/balanced/5-quick-tap/events.patterns

@@ -0,0 +1,4 @@
+s/.*hid_listener_keycode/kp/p
+s/.*mo_keymap_binding/mo/p
+s/.*on_hold_tap_binding/ht_binding/p
+s/.*decide_hold_tap/ht_decide/p

app/tests/hold-tap/balanced/5-quick-tap/keycode_events.snapshot

@@ -0,0 +1,10 @@
+ht_binding_pressed: 0 new undecided hold_tap
+ht_decide: 0 decided tap (balanced event 0)
+kp_pressed: usage_page 0x07 keycode 0x09 implicit_mods 0x00 explicit_mods 0x00
+kp_released: usage_page 0x07 keycode 0x09 implicit_mods 0x00 explicit_mods 0x00
+ht_binding_released: 0 cleaning up hold-tap
+ht_binding_pressed: 0 new undecided hold_tap
+ht_decide: 0 decided tap (balanced event 4)
+kp_pressed: usage_page 0x07 keycode 0x09 implicit_mods 0x00 explicit_mods 0x00
+kp_released: usage_page 0x07 keycode 0x09 implicit_mods 0x00 explicit_mods 0x00
+ht_binding_released: 0 cleaning up hold-tap

app/tests/hold-tap/balanced/5-quick-tap/native_posix.keymap

@@ -0,0 +1,14 @@
+#include <dt-bindings/zmk/keys.h>
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/kscan_mock.h>
+#include "../behavior_keymap.dtsi"
+
+
+&kscan {
+	events = <
+		ZMK_MOCK_PRESS(0,0,10) 
+		ZMK_MOCK_RELEASE(0,0,10) 
+		ZMK_MOCK_PRESS(0,0,400)
+		ZMK_MOCK_RELEASE(0,0,10) 
+	>;
+};

app/tests/hold-tap/balanced/behavior_keymap.dtsi

@@ -10,6 +10,7 @@
 			#binding-cells = <2>;
 			flavor = "balanced";
 			tapping_term_ms = <300>;
+			quick_tap_ms = <200>;
 			bindings = <&kp>, <&kp>;
 		};
 	};

app/tests/hold-tap/hold-preferred/5-quick-tap/events.patterns

@@ -0,0 +1,4 @@
+s/.*hid_listener_keycode/kp/p
+s/.*mo_keymap_binding/mo/p
+s/.*on_hold_tap_binding/ht_binding/p
+s/.*decide_hold_tap/ht_decide/p

app/tests/hold-tap/hold-preferred/5-quick-tap/keycode_events.snapshot

@@ -0,0 +1,10 @@
+ht_binding_pressed: 0 new undecided hold_tap
+ht_decide: 0 decided tap (hold-preferred event 0)
+kp_pressed: usage_page 0x07 keycode 0x09 implicit_mods 0x00 explicit_mods 0x00
+kp_released: usage_page 0x07 keycode 0x09 implicit_mods 0x00 explicit_mods 0x00
+ht_binding_released: 0 cleaning up hold-tap
+ht_binding_pressed: 0 new undecided hold_tap
+ht_decide: 0 decided tap (hold-preferred event 4)
+kp_pressed: usage_page 0x07 keycode 0x09 implicit_mods 0x00 explicit_mods 0x00
+kp_released: usage_page 0x07 keycode 0x09 implicit_mods 0x00 explicit_mods 0x00
+ht_binding_released: 0 cleaning up hold-tap

app/tests/hold-tap/hold-preferred/5-quick-tap/native_posix.keymap

@@ -0,0 +1,14 @@
+#include <dt-bindings/zmk/keys.h>
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/kscan_mock.h>
+#include "../behavior_keymap.dtsi"
+
+
+&kscan {
+	events = <
+		ZMK_MOCK_PRESS(0,0,10) 
+		ZMK_MOCK_RELEASE(0,0,10) 
+		ZMK_MOCK_PRESS(0,0,400)
+		ZMK_MOCK_RELEASE(0,0,10) 
+	>;
+};

app/tests/hold-tap/hold-preferred/behavior_keymap.dtsi

@@ -12,6 +12,7 @@
 			#binding-cells = <2>;
 			flavor = "hold-preferred";
 			tapping_term_ms = <300>;
+			quick_tap_ms = <200>;
 			bindings = <&kp>, <&kp>;
 		};
 	};

app/tests/hold-tap/tap-preferred/5-quick-tap/events.patterns

@@ -0,0 +1,4 @@
+s/.*hid_listener_keycode/kp/p
+s/.*mo_keymap_binding/mo/p
+s/.*on_hold_tap_binding/ht_binding/p
+s/.*decide_hold_tap/ht_decide/p

app/tests/hold-tap/tap-preferred/5-quick-tap/keycode_events.snapshot

@@ -0,0 +1,10 @@
+ht_binding_pressed: 0 new undecided hold_tap
+ht_decide: 0 decided tap (tap-preferred event 0)
+kp_pressed: usage_page 0x07 keycode 0x09 implicit_mods 0x00 explicit_mods 0x00
+kp_released: usage_page 0x07 keycode 0x09 implicit_mods 0x00 explicit_mods 0x00
+ht_binding_released: 0 cleaning up hold-tap
+ht_binding_pressed: 0 new undecided hold_tap
+ht_decide: 0 decided tap (tap-preferred event 4)
+kp_pressed: usage_page 0x07 keycode 0x09 implicit_mods 0x00 explicit_mods 0x00
+kp_released: usage_page 0x07 keycode 0x09 implicit_mods 0x00 explicit_mods 0x00
+ht_binding_released: 0 cleaning up hold-tap

app/tests/hold-tap/tap-preferred/5-quick-tap/native_posix.keymap

@@ -0,0 +1,14 @@
+#include <dt-bindings/zmk/keys.h>
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/kscan_mock.h>
+#include "../behavior_keymap.dtsi"
+
+
+&kscan {
+	events = <
+		ZMK_MOCK_PRESS(0,0,10) 
+		ZMK_MOCK_RELEASE(0,0,10) 
+		ZMK_MOCK_PRESS(0,0,400)
+		ZMK_MOCK_RELEASE(0,0,10) 
+	>;
+};

app/tests/hold-tap/tap-preferred/behavior_keymap.dtsi

@@ -10,6 +10,7 @@
 			#binding-cells = <2>;
 			flavor = "tap-preferred";
 			tapping_term_ms = <300>;
+			quick_tap_ms = <200>;
 			bindings = <&kp>, <&kp>;
 		};
 	};

docs/docs/behaviors/hold-tap.md

@@ -11,7 +11,7 @@ Simply put, the hold-tap key will output the 'hold' behavior if it's held for a
 
 ### Hold-Tap
 
-The `tapping_term_ms` parameter decides between a 'tap' and a 'hold'.
+The graph below shows how the hold-tap decides between a 'tap' and a 'hold'.
 
 ![Simple behavior](../assets/hold-tap/case1_2.png)
 
@@ -37,6 +37,18 @@ For basic usage, please see [mod-tap](./mod-tap.md) and [layer-tap](./layers.md)
 
 ### Advanced Configuration
 
+#### `tapping_term_ms`
+
+Defines how long a key must be pressed to trigger Hold behavior.
+
+#### `quick_tap_ms`
+
+If you press a tapped hold-tap again within `quick_tap_ms` milliseconds, it will always trigger the tap behavior. This is useful for things like a backspace, where a quick tap+hold holds backspace pressed. Set this to a negative value to disable. The default is -1 (disabled).
+
+In QMK, unlike ZMK, this functionality is enabled by default, and you turn it off using `TAPPING_FORCE_HOLD`.
+
+#### Home row mods
+
 This example configures a hold-tap that works well for homerow mods:
 
 ```
@@ -50,6 +62,7 @@ This example configures a hold-tap that works well for homerow mods:
 			label = "HOMEROW_MODS";
 			#binding-cells = <2>;
 			tapping_term_ms = <150>;
+			quick_tap_ms = <0>;
 			flavor = "tap-preferred";
 			bindings = <&kp>, <&kp>;
 		};