QMK Custom Layer Backspace

/*  QMK code to create a custom keycode that allows access to a target layer within a short time frame,
 *  but otherwise acts as a backspace key.
 */


// config.h -------------------------------------------------------------------

#define LT_BSPC_LAYER      _NUMPAD  // set your target layer here
#define LT_BSPC_HOLD_TIME  500      // set the amount of time in ms that the target layer is available



// keymap.c ----------------------------------------------------------------

enum custom_keycodes {
    LT_BSPC = SAFE_RANGE,
};


uint16_t last_keypress_time = 0;    // timer variable to track when the last key was pressed
uint16_t lt_bspc_pressed_time = 0;  // timer variable to track when LT_BSPC was pressed
bool     bspc_held = false;         // tracks whether KC_BACKSPACE is currently registered


static bool key_was_pressed_after_lt_bspc(void) {
    return (lt_bspc_pressed_time != 0)  &&  (last_keypress_time != lt_bspc_pressed_time);
}

bool process_record_user(uint16_t keycode, keyrecord_t *record) {
    if (record->event.pressed)
        last_keypress_time = record->event.time;  // update on every keypress with the time of the keypress

    switch (keycode) {
        case LT_BSPC:
            if (record->event.pressed) {                    // on LT_BSPC keypress
                layer_on(LT_BSPC_LAYER);                    //   activate the target layer
                lt_bspc_pressed_time = record->event.time;  //   mark the time that LT_BSPC was pressed
            } else {                                        // on LT_BSPC keyrelease
                layer_off(LT_BSPC_LAYER);                   //   deactivate the target layer

                // if no other key was pressed after LT_BSPC, and LT_BSPC has not been held for longer than LT_BSPC_HOLD_TIME
                if (!key_was_pressed_after_lt_bspc()  &&  timer_elapsed(lt_bspc_pressed_time) < LT_BSPC_HOLD_TIME) {
                    tap_code(KC_BACKSPACE);
                }
                // unregister KC_BACKSPACE if it is registered
                if (bspc_held) {
                    unregister_code(KC_BACKSPACE);
                    bspc_held = false;
                }
                lt_bspc_pressed_time = 0;  // reset the pressed time
            }
            break;
    }
    return true;
}


void housekeeping_task_user(void) {
    if (!bspc_held  &&  lt_bspc_pressed_time != 0) {
        if (!key_was_pressed_after_lt_bspc()  &&  timer_elapsed(lt_bspc_pressed_time) >= LT_BSPC_HOLD_TIME) {
            register_code(KC_BACKSPACE);
            bspc_held = true;
            layer_off(LT_BSPC_LAYER);
        }
    }
}