Refactor of the debounce change calculation

firmware/debounce.h

@@ -2,6 +2,12 @@
 
 #include <stdint.h>
 
+/*
+each of these 8 bit variables are storing the state for 8 keys
+
+so for key 0, the counter is represented by db0[0] and db1[0] 
+and the state in state[0].
+*/
 typedef struct {
     uint8_t db0;    // counter bit 0
     uint8_t db1;    // counter bit 1
@@ -15,24 +21,45 @@ typedef struct {
  *    will increment that bit's counter. When it overflows, the change is
  *    comitted to the final debounced state and the changed bit returned.
  *
+ *    Because each key's counter and state are stored in this stacked way,
+ *    8 keys are processed in parallel at each operation.
+ *
  * args:
  *    sample - the current state
  *    debouncer - the state variables of the debouncer
  *
  * returns: bits that have changed in the final debounced state
+ *
+ * handy XOR truth table:   A  B  O
+ *                          0  0  0
+ *                          0  1  1
+ *                          1  0  1
+ *                          1  1  0
+ * 
+ * This is used below as a difference detector:
+ *   if A ^ B is true, A and B are different.
+ *
+ * And a way to flip selected bits in a variable or register:
+ *   Set B to 1, then A ^ B = !A
  */
 static inline uint8_t debounce(uint8_t sample, debounce_t *debouncer) {
     uint8_t delta, changes;
 
-    // Set delta to changes from last stable state
+    // Use xor to detect changes from last stable state:
+    // if a key has changed, it's bit will be 1, otherwise 0
     delta = sample ^ debouncer->state;
 
-    // Increment counters and reset any unchanged bits
+    // Increment counters and reset any unchanged bits:
+    // increment bit 1 for all changed keys
     debouncer->db1 = ((debouncer->db1) ^ (debouncer->db0)) & delta;
-    debouncer->db0  = ~(debouncer->db0) & delta;
+    // increment bit 0 for all changed keys
+    debouncer->db0 = ~(debouncer->db0) & delta;
+
+    // Calculate returned change set: if delta is still true
+    // and the counter has wrapped back to 0, the key is changed.
+    changes = delta & ~debouncer->db0 & ~debouncer->db1;
 
-    // update state & calculate returned change set
-    changes = ~(~delta | (debouncer->db0) | (debouncer->db1));
+    // Update state: in this case use xor to flip any bit that is true in changes.
     debouncer->state ^= changes;
 
     return changes;