simplify rgb to hsv conversion

By inlining some values, the starting hue calculation can be reduced from using three subtractions and two divisions to using one subtraction and one division.
"fmod(n+1,1)" can be replaced with "n - floor(n)" due to the second parameter being 1.
The saturation calculation can be done without floating point values.
The "CLIP8"s are unnecessary because the values are already in the correct range.

src/libImaging/Convert.c

@@ -310,40 +310,46 @@ rgb2bgr24(UINT8 *out, const UINT8 *in, int xsize) {
 }
 
 static void
-rgb2hsv_row(UINT8 *out, const UINT8 *in) {  // following colorsys.py
-    float h, s, rc, gc, bc, cr;
-    UINT8 maxc, minc;
-    UINT8 r, g, b;
-    UINT8 uh, us, uv;
-
-    r = in[0];
-    g = in[1];
-    b = in[2];
-    maxc = MAX(r, MAX(g, b));
-    minc = MIN(r, MIN(g, b));
-    uv = maxc;
+rgb2hsv_row(UINT8 *out, const UINT8 *in) {
+    // based on Python's colorsys module
+
+    const UINT8 r = in[0];
+    const UINT8 g = in[1];
+    const UINT8 b = in[2];
+
+    const UINT8 maxc = MAX(r, MAX(g, b));
+    const UINT8 minc = MIN(r, MIN(g, b));
+
+    UINT8 uh, us;
+    const UINT8 uv = maxc;
+
     if (minc == maxc) {
         uh = 0;
         us = 0;
     } else {
-        cr = (float)(maxc - minc);
-        s = cr / (float)maxc;
-        rc = ((float)(maxc - r)) / cr;
-        gc = ((float)(maxc - g)) / cr;
-        bc = ((float)(maxc - b)) / cr;
+        const UINT8 color_range = maxc - minc;
+        double h;
+
+        const double cr = (double)color_range;
         if (r == maxc) {
-            h = bc - gc;
+            h = (g - b) / cr;
         } else if (g == maxc) {
-            h = 2.0 + rc - bc;
+            h = 2.0 + (b - r) / cr;
         } else {
-            h = 4.0 + gc - rc;
+            h = 4.0 + (r - g) / cr;
         }
-        // incorrect hue happens if h/6 is negative.
-        h = fmod((h / 6.0 + 1.0), 1.0);
 
-        uh = (UINT8)CLIP8((int)(h * 255.0));
-        us = (UINT8)CLIP8((int)(s * 255.0));
+        // the modulus operator in Python does not exactly match
+        // the modulus operator or the fmod function in C
+        // https://stackoverflow.com/a/3883019/3878168
+        // "h = (h/6.0) % 1.0" in Python can be computed as:
+        h = h / 6.0;
+        h = h - floor(h);
+
+        uh = (UINT8)(255.0 * h);
+        us = 255 * color_range / maxc;
     }
+
     out[0] = uh;
     out[1] = us;
     out[2] = uv;
@@ -359,7 +365,8 @@ rgb2hsv(UINT8 *out, const UINT8 *in, int xsize) {
 }
 
 static void
-hsv2rgb(UINT8 *out, const UINT8 *in, int xsize) {  // following colorsys.py
+hsv2rgb(UINT8 *out, const UINT8 *in, int xsize) {
+    // based on Python's colorsys module
 
     int p, q, t;
     UINT8 up, uq, ut;