Smoother mouse acceleration function

The cubic acceleration function used to approximate a sine wave produced some
noticeable jumps when beginning to accelerate the mouse cursor movement (several
1s, followed by several 4s, then 7s, with no intermediate values). This
parabolic function produces smoother mouse cursor motion, without any sudden
jumps in speed.

Signed-off-by: Michael Richters <gedankenexperimenter@gmail.com>

src/kaleidoscope/plugin/MouseKeys/MouseWrapper.cpp

@@ -114,22 +114,17 @@ void MouseWrapper_::warp(uint8_t warp_cmd) {
   warp_jump(section_left, section_top, next_height, next_width);
 }
 
-// cubic wave function based on code from FastLED
-// produces a shape similar to a sine curve from 0 to 255
-// (slow growth at 0, fast growth in the middle, slow growth at 255)
-// http://www.wolframalpha.com/input/?i=((3((x)**2)%2F256)+-+((2((x)(x)(x%2F256))%2F256)))+%2B+1
+// To approximate a sine wave, this uses two parabolas. Acceleration begins
+// slowly, grows rapidly in the middle, and slows again near the top.
 uint8_t MouseWrapper_::acceleration(uint8_t cycles) {
-  uint16_t i = cycles;
-
-  uint16_t ii = (i * i) >> 8;
-  uint16_t iii = (ii * i) >> 8;
-
-  i = ((3 * ii) - (2 * iii)) + 1;
-
-  // Just in case (may go up to 256 at peak)
-  if (i > 255) i = 255;
-
-  return i;
+  if (cycles < 128) {
+    uint16_t c2 = cycles * cycles;
+    return 1 + (c2 >> 7);
+  } else {
+    uint16_t remaining_cycles = 256 - cycles;
+    uint16_t c2 = remaining_cycles * remaining_cycles;
+    return 255 - (c2 >> 7);
+  }
 }
 
 // Get the diagonalized version of a value, i.e. value * sqrt(2) / 2. If the