use integer math for the rgb to hsv conversion

src/libImaging/Convert.c

@@ -310,45 +310,33 @@ rgb2bgr24(UINT8 *out, const UINT8 *in, int xsize) {
 }
 
 static void
-rgb2hsv_row(UINT8 *out, const UINT8 *in) {  // following colorsys.py
-    double 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;
-    if (minc == maxc) {
-        uh = 0;
-        us = 0;
-    } else {
-        cr = (double)(maxc - minc);
-        s = cr / (double)maxc;
-        rc = ((double)(maxc - r)) / cr;
-        gc = ((double)(maxc - g)) / cr;
-        bc = ((double)(maxc - b)) / cr;
+rgb2hsv_row(UINT8 *out, const UINT8 *in) {
+    // following https://literateprograms.org/rgb_to_hsv_color_space_conversion__c_.html
+
+    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 h = 0, s = 0;
+    const UINT8 v = maxc;
+
+    if (v != 0 && minc != maxc) {
         if (r == maxc) {
-            h = bc - gc;
+            h = 0 + (UINT16)43 * (g - b) / (maxc - minc);
         } else if (g == maxc) {
-            h = 2.0 + rc - bc;
+            h = 85 + (UINT16)43 * (b - r) / (maxc - minc);
         } else {
-            h = 4.0 + gc - rc;
+            h = 171 + (UINT16)43 * (r - g) / (maxc - minc);
         }
-        // incorrect hue happens if h/6 is negative.
-        //h = fmod((h / 6.0 + 1.0), 1.0);
-        h = h / 6.0 + 1.0;
-        h = h - (int)h;
 
-        uh = (UINT8)CLIP8((int)(h * 255.0));
-        us = (UINT8)CLIP8((int)(s * 255.0));
+        s = (UINT16)255 * (maxc - minc) / v;
     }
-    out[0] = uh;
-    out[1] = us;
-    out[2] = uv;
+
+    out[0] = h;
+    out[1] = s;
+    out[2] = v;
 }
 
 static void