Use a timer instead of a loop counter

Loop counters are an unreliable way to track time, use a timer instead.

Fixes #2.

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

README.md

@@ -40,15 +40,10 @@ The plugin provides two methods on the `HeatmapEffect` object:
 
 ### `.configure(updateFrequency)`
 
-> Sets up the update frequency of the heatmap. The smaller this value is, the
-> more often the heatmap gets updated, but that comes with a cost: updating the
-> heatmap takes a fairly large amount of computation, and is not fast. Doing it
-> often will considerably slow down the keyboard, and that is rarely a desirable
-> thing.
->
-> Nevertheless, the frequency preferred is a very subjective thing, which is why
-> this function exists. Give it a number, and it will wait that many scan cycles
-> between updates.
+> Sets up the update frequency of the heatmap, to update every `updateFrequency`
+> milliseconds. Updating the heatmap incurs a significant performance penalty,
+> and should not be done too often. Doing it too rarely, on the other hand, make
+> it much less useful. One has to strike a reasonable balance.
 >
 > Defaults to *500*.
 

src/Akela/Heatmap.cpp

@@ -22,8 +22,8 @@ namespace Akela {
   uint8_t Heatmap::heatmap[ROWS][COLS];
   uint16_t Heatmap::totalKeys;
   uint8_t Heatmap::highestCount;
-  uint16_t Heatmap::updateFrequency = 50;
-  uint16_t Heatmap::loopCount;
+  uint16_t Heatmap::updateFrequency = 500;
+  uint32_t Heatmap::startTime;
 
   const float Heatmap::heatColors[][3] = {{0.0, 0.0, 0.0}, {0.1, 1, 0.1}, {1, 1, 0.1}, {1, 0.1, 0.1}};
 
@@ -104,12 +104,11 @@ namespace Akela {
 
   void
   Heatmap::update (void) {
-    loopCount++;
-
-    if (loopCount < updateFrequency)
+    if (startTime && (millis () - startTime) < updateFrequency)
       return;
 
-    loopCount = 0;
+    startTime = millis ();
+
     for (uint8_t r = 0; r < ROWS; r++) {
       for (uint8_t c = 0; c < COLS; c++) {
         uint8_t cap = max(totalKeys, 1);

src/Akela/Heatmap.h

@@ -36,7 +36,7 @@ namespace Akela {
     static uint16_t totalKeys;
     static uint8_t highestCount;
     static uint16_t updateFrequency;
-    static uint16_t loopCount;
+    static uint32_t startTime;
 
     static const float heatColors[][3];