valueCellIndex() logic optimization

Meter.c

@@ -322,20 +322,22 @@ static void GraphMeterMode_reallocateGraphBuffer(Meter* this, const GraphDrawCon
 }
 
 static size_t GraphMeterMode_valueCellIndex(unsigned int graphHeight, bool isPercentChart, int deltaExp, unsigned int y, unsigned int* scaleFactor, unsigned int* increment) {
+   assert(deltaExp >= 0);
+   assert(deltaExp < UINT16_WIDTH);
+
    if (scaleFactor)
       *scaleFactor = 1;
+   if (increment)
+      *increment = isPercentChart ? 1 : (2U << deltaExp);
 
-   assert(deltaExp >= 0);
-   assert(deltaExp < UINT16_WIDTH);
    unsigned int yTop = (graphHeight - 1) >> deltaExp;
+   if (y > yTop) {
+      assert(increment == NULL);
+      return (size_t)-1;
+   }
+
    if (isPercentChart) {
       assert(deltaExp == 0);
-      if (increment)
-         *increment = 1;
-
-      if (y > yTop)
-         return (size_t)-1;
-
       return y;
    }
    // A record may be rendered in different scales depending on the largest
@@ -355,24 +357,33 @@ static size_t GraphMeterMode_valueCellIndex(unsigned int graphHeight, bool isPer
    //           [5] [10]    X    X    X | computed from cells of a
    //           [3]  [6] (12)    X    X | lower scale and not stored in
    //           [1]  [2]  [4]  [8] (16) | the array.
-   if (increment)
-      *increment = 2U << deltaExp;
-
-   if (y > yTop)
-      return (size_t)-1;
 
    // "b" is the "base" offset or the upper bits of offset
    unsigned int b = (y * 2) << deltaExp;
    unsigned int offset = 1U << deltaExp;
-   if (y == yTop) {
-      assert(((2 * graphHeight - 1) & b) == b);
-      unsigned int offsetTop = powerOf2Floor(2 * graphHeight - 1 - b);
-      if (scaleFactor && offsetTop) {
-         *scaleFactor = offset / offsetTop;
+
+   if (!scaleFactor) {
+      // This function is called for writing. The "increment" argument is
+      // optional, but the caller should assert the index is in bounds.
+      if (b + offset > 2 * graphHeight - 1) {
+         assert(increment == NULL);
+         return (size_t)-1;
       }
-      return b + offsetTop;
+      return b + offset;
+   }
+
+   // This function is called for reading.
+   assert(increment == NULL);
+
+   if (y < yTop)
+      return b + offset;
+
+   assert(((2 * graphHeight - 1) & b) == b);
+   unsigned int offsetTop = powerOf2Floor(2 * graphHeight - 1 - b);
+   if (offsetTop) {
+      *scaleFactor = offset / offsetTop;
    }
-   return b + offset;
+   return b + offsetTop;
 }
 
 static uint8_t GraphMeterMode_findTopCellItem(const Meter* this, double scaledTotal, unsigned int topCell) {
@@ -915,8 +926,7 @@ static void GraphMeterMode_recordNewValue(Meter* this, const GraphDrawContext* c
    // Clear cells
    unsigned int y = ((unsigned int)numDots + 8 - 1) / 8; // Round up
    size_t i = GraphMeterMode_valueCellIndex(graphHeight, isPercentChart, 0, y, NULL, NULL);
-   if (i != (size_t)-1) {
-      assert(i < nCellsPerValue);
+   if (i < nCellsPerValue) {
       memset(&valueStart[i], 0, (nCellsPerValue - i) * sizeof(*valueStart));
    }
 
@@ -1049,7 +1059,7 @@ static int GraphMeterMode_lookupCell(const Meter* this, const GraphDrawContext*
 
       unsigned int scaleFactor;
       size_t i = GraphMeterMode_valueCellIndex(graphHeight, isPercentChart, deltaExpArg, y, &scaleFactor, NULL);
-      if (i == (size_t)-1)
+      if (i >= nCellsPerValue)
          goto cellIsEmpty;
 
       if (deltaExp >= UINT16_WIDTH) {