Added MathUtil.floorDiv, and tests.

src/main/java/pythagoras/i/MathUtil.java

@@ -17,4 +17,20 @@ public class MathUtil
         if (value > high) return high;
         return value;
     }
+
+    /**
+     * Computes the floored division {@code dividend/divisor} which is useful when dividing
+     * potentially negative numbers into bins.
+     *
+     * <p> For example, the following numbers {@code floorDiv} 10 are:
+     * <pre>
+     * -15 -10 -8 -2 0 2 8 10 15
+     *  -2  -1 -1 -1 0 0 0  1  1
+     * </pre>
+     */
+    public static int floorDiv (int dividend, int divisor) {
+        boolean numpos = dividend >= 0, denpos = divisor >= 0;
+        if (numpos == denpos) return dividend / divisor;
+        return denpos ? (dividend - divisor + 1) / divisor : (dividend - divisor - 1) / divisor;
+    }
 }