layers: Implement a two-stage cache

With the new implementation, there are two lookup functions, because we have two
caches, and different parts of the firmware will want to use either this or
that (or perhaps both, in rare cases).

First of all, we use caches because looking up a key through all the layers
is costy, and the cost increases dramatically the more layers we have.

Then, we have the `effectiveKeymapCache`, because to have layer behaviours
we want, that is, if you hold a key on a layer, release the layer key but
continue holding the other, we want for the layered keycode to continue
repeating. At the same time, we want other keys to not be affected by the
now-turned-off layer. So we update the keycode in the cache on-demand, when
the key is pressed or released. (see the top of `handleKeyswitchEvent`).

On the other hand, we also have plugins that scan the whole keymap, and do
things based on that information, such as highlighting keys that changed
between layers. These need to be able to look at a state of where the
keymap *should* be, not necessarily where it is. The `effectiveKeymapCache`
is not useful here. So we use a `keymapCache` which we update whenever
layers change (see `Layer.on` and `Layer.off`), and it updates the cache to
show how the keymap should look, without the `effectiveKeymapCache`-induced
behaviour.

Thus, if we are curious about what a given key will do, use `lookup`. If we
are curious what the active layer state describes the key as, use
`lookupUncached`.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

src/key_events.cpp

@@ -44,8 +44,10 @@ static bool handleKeyswitchEventDefault(Key mappedKey, byte row, byte col, uint8
 }
 
 void handleKeyswitchEvent(Key mappedKey, byte row, byte col, uint8_t keyState) {
+  /* If a key had an on or off event, we update the effective keymap cache. See
+   * layers.h for an explanation about the different caches we have. */
   if (keyToggledOn(keyState) || keyToggledOff(keyState))
-    Layer.updateKeymapCache(row, col);
+    Layer.updateEffectiveKeymapCache(row, col);
 
   /* If the key we are dealing with is masked, ignore it until it is released.
    * When releasing it, clear the mask, so future key events can be handled

src/layers.cpp

@@ -4,7 +4,8 @@ static uint8_t DefaultLayer;
 static uint32_t LayerState;
 
 uint8_t Layer_::highestLayer;
-Key Layer_::keyMap[ROWS][COLS];
+Key Layer_::effectiveKeymapCache[ROWS][COLS];
+uint8_t Layer_::keymapCache[ROWS][COLS];
 Key(*Layer_::getKey)(uint8_t layer, byte row, byte col) = Layer.getKeyFromPROGMEM;
 
 static void handleKeymapKeyswitchEvent(Key keymapEntry, uint8_t keyState) {
@@ -62,29 +63,29 @@ Layer_::getKeyFromPROGMEM(uint8_t layer, byte row, byte col) {
 }
 
 void
-Layer_::updateKeymapCache(byte row, byte col) {
-  int8_t layer = highestLayer;
-
-  if (row >= ROWS || col >= COLS)
-    return;
-
-  for (layer = highestLayer; layer >= DefaultLayer; layer--) {
-    if (Layer.isOn(layer)) {
-      Key mappedKey = (*getKey)(layer, row, col);
-
-      if (mappedKey != Key_Transparent) {
-        keyMap[row][col] = mappedKey;
-        break;
-      }
-    }
-  }
+Layer_::updateEffectiveKeymapCache(byte row, byte col) {
+  int8_t layer = keymapCache[row][col];
+  effectiveKeymapCache[row][col] = (*getKey)(layer, row, col);
 }
 
 void
 Layer_::updateKeymapCache(void) {
+  memset(keymapCache, DefaultLayer, ROWS * COLS);
   for (byte row = 0; row < ROWS; row++) {
     for (byte col = 0; col < COLS; col++) {
-      updateKeymapCache(row, col);
+      int8_t layer = highestLayer;
+
+      while (layer > DefaultLayer) {
+        if (Layer.isOn(layer)) {
+          Key mappedKey = (*getKey)(layer, row, col);
+
+          if (mappedKey != Key_Transparent) {
+            keymapCache[row][col] = layer;
+            break;
+          }
+        }
+        layer--;
+      }
     }
   }
 }
@@ -109,8 +110,8 @@ void Layer_::on(uint8_t layer) {
   if (layer > highestLayer)
     highestLayer = layer;
 
-  // Update the key cache, so that if anything depends on knowing the active
-  // layout, the layout will be in sync.
+  /* If the layer did turn on, update the keymap cache. See layers.h for an
+   * explanation about the caches we have. */
   if (!wasOn)
     updateKeymapCache();
 }
@@ -122,8 +123,8 @@ void Layer_::off(uint8_t layer) {
   if (layer == highestLayer)
     highestLayer = top();
 
-  // Update the key cache, so that if anything depends on knowing the active
-  // layout, the layout will be in sync.
+  /* If the layer did turn off, update the keymap cache. See layers.h for an
+   * explanation about the caches we have. */
   if (wasOn)
     updateKeymapCache();
 }

src/layers.h

@@ -8,9 +8,41 @@ class Layer_ {
  public:
   Layer_(void);
 
+  /* There are two lookup functions, because we have two caches, and different
+   * parts of the firmware will want to use either this or that (or perhaps
+   * both, in rare cases).
+   *
+   * First of all, we use caches because looking up a key through all the layers
+   * is costy, and the cost increases dramatically the more layers we have.
+   *
+   * Then, we have the `effectiveKeymapCache`, because to have layer behaviours
+   * we want, that is, if you hold a key on a layer, release the layer key but
+   * continue holding the other, we want for the layered keycode to continue
+   * repeating. At the same time, we want other keys to not be affected by the
+   * now-turned-off layer. So we update the keycode in the cache on-demand, when
+   * the key is pressed or released. (see the top of `handleKeyswitchEvent`).
+   *
+   * On the other hand, we also have plugins that scan the whole keymap, and do
+   * things based on that information, such as highlighting keys that changed
+   * between layers. These need to be able to look at a state of where the
+   * keymap *should* be, not necessarily where it is. The `effectiveKeymapCache`
+   * is not useful here. So we use a `keymapCache` which we update whenever
+   * layers change (see `Layer.on` and `Layer.off`), and it updates the cache to
+   * show how the keymap should look, without the `effectiveKeymapCache`-induced
+   * behaviour.
+   *
+   * Thus, if we are curious about what a given key will do, use `lookup`. If we
+   * are curious what the active layer state describes the key as, use
+   * `lookupUncached`.
+   */
   static Key lookup(byte row, byte col) {
-    return keyMap[row][col];
+    return effectiveKeymapCache[row][col];
   }
+  static Key lookupUncached(byte row, byte col) {
+    uint8_t layer = keymapCache[row][col];
+    return (*getKey)(layer, row, col);
+  }
+
   static void on(uint8_t layer);
   static void off(uint8_t layer);
   static void move(uint8_t layer);
@@ -32,12 +64,13 @@ class Layer_ {
 
   static Key getKeyFromPROGMEM(uint8_t layer, byte row, byte col);
 
-  static void updateKeymapCache(byte row, byte col);
+  static void updateEffectiveKeymapCache(byte row, byte col);
   static void updateKeymapCache(void);
 
  private:
   static uint8_t highestLayer;
-  static Key keyMap[ROWS][COLS];
+  static Key effectiveKeymapCache[ROWS][COLS];
+  static uint8_t keymapCache[ROWS][COLS];
 };
 
 Key layer_getKey(uint8_t layer, uint8_t r, uint8_t c);