actOnMatrixScan: Optimize the idle case

When there are no state changes, and no keys pressed on a row, instead of
iterating through a byte bit-by-bit, just fire idle events without checking the
bits. In all other cases, do the bit-walking like we did before.

The reason this is useful is because bit-walking is costly, and slow. If we can
avoid that, we win quite a lot of performance. Since rows being idle is the most
common case on a keyboard, this is a huge net win. Even in the worst case, where
no rows are idle, this is just one byte comparison and a branch slower than our
previous implementation.

As part of this optimization, `actOnHalfRow` was lifted out into its own
function, to reduce code duplication.

Many thanks to @gedankenexperimenter for the original idea!

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

src/Kaleidoscope-Hardware-Model01.cpp

@@ -157,23 +157,31 @@ void Model01::readMatrix() {
   }
 }
 
-
+void Model01::actOnHalfRow(byte row, byte colState, byte colPrevState, byte startPos) {
+  if ((colState == colPrevState) && (colState == 0)) {
+    for (byte col = 0; col < 8; col++) {
+      handleKeyswitchEvent(Key_NoKey, row, startPos - col, 0);
+    }
+  } else {
+    for (byte col = 0; col < 8; col++) {
+      uint8_t keyState = (bitRead(colPrevState, col) << 0) |
+                         (bitRead(colState, col) << 1);
+      handleKeyswitchEvent(Key_NoKey, row, startPos - col, keyState);
+    }
+  }
+}
 
 void Model01::actOnMatrixScan() {
   for (byte row = 0; row < 4; row++) {
-    for (byte col = 0; col < 8; col++) {
+    uint8_t colState = leftHandState.rows[row];
+    uint8_t colPrevState = previousLeftHandState.rows[row];
 
-      uint8_t keynum = (row * 8) + (col);
+    actOnHalfRow(row, colState, colPrevState, 7);
 
-      uint8_t keyState = (bitRead(previousLeftHandState.all, keynum) << 0) |
+    colState = rightHandState.rows[row];
-                         (bitRead(leftHandState.all, keynum) << 1);
+    colPrevState = previousRightHandState.rows[row];
-      handleKeyswitchEvent(Key_NoKey, row, 7 - col, keyState);
 
-      keyState = (bitRead(previousRightHandState.all, keynum) << 0) |
+    actOnHalfRow(row, colState, colPrevState, 15);
-                 (bitRead(rightHandState.all, keynum) << 1);
-
-      handleKeyswitchEvent(Key_NoKey, row, (15 - col), keyState);
-    }
   }
 }
 

src/Kaleidoscope-Hardware-Model01.h

@@ -55,6 +55,8 @@ class Model01 {
   keydata_t previousRightHandState;
 
  private:
+  static void actOnHalfRow(byte row, byte colState, byte colPrevState, byte startPos);
+
   static bool isLEDChanged;
   static KeyboardioScanner leftHand;
   static KeyboardioScanner rightHand;