1 files changed, 63 insertions, 0 deletions
diff --git a/tmk_core/common/action_layer.c b/tmk_core/common/action_layer.c
index 76164adb5d..fc721a7323 100644
--- a/tmk_core/common/action_layer.c
+++ b/tmk_core/common/action_layer.c
@@ -110,6 +110,69 @@ void layer_debug(void)
 }
 #endif
 
+#if !defined(NO_ACTION_LAYER) && defined(PREVENT_STUCK_MODIFIERS)
+uint8_t source_layers_cache[MAX_LAYER_BITS][(MATRIX_ROWS * MATRIX_COLS + 7) / 8] = {0};
+
+void update_source_layers_cache(keypos_t key, uint8_t layer)
+{
+    const uint8_t key_number = key.col + (key.row * MATRIX_COLS);
+    const uint8_t storage_row = key_number / 8;
+    const uint8_t storage_bit = key_number % 8;
+
+    for (uint8_t bit_number = 0; bit_number < MAX_LAYER_BITS; bit_number++) {
+        source_layers_cache[bit_number][storage_row] ^=
+            (-((layer & (1U << bit_number)) != 0)
+             ^ source_layers_cache[bit_number][storage_row])
+            & (1U << storage_bit);
+    }
+}
+
+uint8_t read_source_layers_cache(keypos_t key)
+{
+    const uint8_t key_number = key.col + (key.row * MATRIX_COLS);
+    const uint8_t storage_row = key_number / 8;
+    const uint8_t storage_bit = key_number % 8;
+    uint8_t layer = 0;
+
+    for (uint8_t bit_number = 0; bit_number < MAX_LAYER_BITS; bit_number++) {
+        layer |=
+            ((source_layers_cache[bit_number][storage_row]
+              & (1U << storage_bit)) != 0)
+            << bit_number;
+    }
+
+    return layer;
+}
+#endif
+
+/*
+ * Make sure the action triggered when the key is released is the same
+ * one as the one triggered on press. It's important for the mod keys
+ * when the layer is switched after the down event but before the up
+ * event as they may get stuck otherwise.
+ */
+action_t store_or_get_action(bool pressed, keypos_t key)
+{
+#if !defined(NO_ACTION_LAYER) && defined(PREVENT_STUCK_MODIFIERS)
+    if (disable_action_cache) {
+        return layer_switch_get_action(key);
+    }
+
+    uint8_t layer;
+
+    if (pressed) {
+        layer = layer_switch_get_layer(key);
+        update_source_layers_cache(key, layer);
+    }
+    else {
+        layer = read_source_layers_cache(key);
+    }
+    return action_for_key(layer, key);
+#else
+    return layer_switch_get_action(key);
+#endif
+}
+
 
 int8_t layer_switch_get_layer(keypos_t key)
 {