Add integer division with rounding up/down.

While these functions can be found in rational number libraries,
support for rational numbers is an extra complexity that we do not
wish to tackle at this stage.  We'll use these to keep things simple.

src/intdiv.rs

@@ -0,0 +1,77 @@
+//! Integer division.
+
+use std::i32;
+
+pub trait IntDiv {
+    fn div_round_up(self, b: Self) -> Self;
+    fn div_round_down(self, b: Self) -> Self;
+}
+
+impl IntDiv for i32 {
+    fn div_round_up(self, b: Self) -> Self {
+        if b == 0 { panic!("attempt to divide by zero"); }
+        if b == -1 && self == i32::MIN { panic!("attempt to divide with overflow"); }
+
+        let aa = (self as i64).abs();
+        let bb = (b as i64).abs();
+        if (self > 0) == (b > 0) {
+            ((aa + bb - 1) / bb) as i32
+        } else {
+            -(aa / bb) as i32
+        }
+    }
+
+    fn div_round_down(self, b: Self) -> Self {
+        if b == 0 { panic!("attempt to divide by zero"); }
+        if b == -1 && self == i32::MIN { panic!("attempt to divide with overflow"); }
+
+        let aa = (self as i64).abs();
+        let bb = (b as i64).abs();
+        if (self > 0) == (b > 0) {
+            (aa / bb) as i32
+        } else {
+            -((aa + bb - 1) / bb) as i32
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::i32;
+    use super::IntDiv;
+
+    #[test]
+    fn test_div_round_up() {
+        // 7.0 / 5.0 = 1.4
+        assert_eq!(( 0).div_round_up( 5),  0);
+        assert_eq!(( 0).div_round_up(-5),  0);
+        assert_eq!(( 7).div_round_up( 5),  2);
+        assert_eq!(( 7).div_round_up(-5), -1);
+        assert_eq!((-7).div_round_up( 5), -1);
+        assert_eq!((-7).div_round_up(-5),  2);
+        assert_eq!((i32::MIN).div_round_up(1), i32::MIN);
+    }
+
+    #[test]
+    fn test_div_round_down() {
+        // 7.0 / 5.0 = 1.4
+        assert_eq!(( 0).div_round_down( 5),  0);
+        assert_eq!(( 0).div_round_down(-5),  0);
+        assert_eq!(( 7).div_round_down( 5),  1);
+        assert_eq!(( 7).div_round_down(-5), -2);
+        assert_eq!((-7).div_round_down( 5), -2);
+        assert_eq!((-7).div_round_down(-5),  1);
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_div_round_up_panic() {
+        (i32::MIN).div_round_up(-1);
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_div_round_down_panic() {
+        (i32::MIN).div_round_down(-1);
+    }
+}

src/lib.rs

@@ -43,6 +43,7 @@ pub struct Solution {
 
 pub mod constraint;
 
+mod intdiv;
 mod linexpr;
 mod puzzle;