How to implement "SNAP TAP" like feature on QMK keyboard

keymap.c

// Not for copypaste! It is only part of keymap.c!
// You should adapt the code above for your keyboard especially for your layers.
// WARNING! You could be banned in Valve games because the feautre treated by Valve as a cheat.
//
// It uses two user hooks: layer_state_set_user() and process_record_user() as defined by QMK.

// Example enum with layers.
enum layers {
    DEFAULT_LAYER, 
    GAME_LAYER,
    // ...
}


// global var that keeps current layer
uint8_t cur_layer = YOUR_DEFAULT_LAYER;

// Hook that triggered on a layer state change.
layer_state_t layer_state_set_user(layer_state_t state) {
    static uint8_t old_layer = 0xff;
    cur_layer = get_highest_layer(state);

    // ... handle other things on layer change
}



// Hook on the each key press/release
bool process_record_user(uint16_t keycode, keyrecord_t *record) {

    // SNAP TAP (separate A/D key press in games)
    static bool apressed = false;
    static bool dpressed = false;
    // Apply only for GAME_LAYER and not mess A/D keys for other layers.
    if (cur_layer == GAME_LAYER) {
	    switch(keycode) {
	    case KC_A:
	    if (record->event.pressed) {
		apressed = true;
		if (dpressed) {
		    SEND_STRING(SS_UP(X_D));
		    dpressed = false;
		};
	    } else {
		// to prevent double key release
		if (!apressed) {
		    return false;
		}
		apressed = false;
	    }
	    return true;
	    case KC_D:
	    if (record->event.pressed) {
		dpressed = true;
		if (apressed) {
		    SEND_STRING(SS_UP(X_A));
		};
	    } else {
		// to prevent double key release
		if (!dpressed) {
		    return false;
		}
		dpressed = false;
	    }
	    if (!record->event.pressed) {
		// to prevent double release
		if (dpressed) {
		    return false;
		}
	    }
	    return true;
	    }
    }
    
    // ... handle other key presses
}