Add some property testing, and fix a bug it found.

src/signed/add.rs

@@ -60,16 +60,19 @@ macro_rules! add_impls
                         value: &self.value + &rhs.value
                     }
                 } else {
-                    if self.value > rhs.value {
-                        $bigger {
-                            negative: self.negative,
-                            value: $ubigger::from(&self.value - &rhs.value)
-                        }
-                    } else {
-                        $bigger {
-                            negative: rhs.negative,
-                            value: $ubigger::from(&rhs.value - &self.value)
-                        }
+                    match self.value.cmp(&rhs.value) {
+                        Ordering::Greater =>
+                            $bigger {
+                                negative: self.negative,
+                                value: $ubigger::from(&self.value - &rhs.value)
+                            },
+                        Ordering::Less =>
+                            $bigger {
+                                negative: rhs.negative,
+                                value: $ubigger::from(&rhs.value - &self.value)
+                            },
+                        Ordering::Equal =>
+                            $bigger::zero()
                     }
                 }
             }

src/signed/base.rs

@@ -85,16 +85,18 @@ macro_rules! signed_impls {
             }
         }
 
-//        #[cfg(test)]
-//        impl Arbitrary for $sname {
-//            fn arbitrary<G: Gen>(g: &mut G) -> $name {
-//                let neg = g.gen::<bool>();
-//                let val = g.gen::<$name>();
-//                $sname{ negative: neg, value: val }
-//            }
-//        }
-
         #[cfg(test)]
+        impl Arbitrary for $sname {
+            fn arbitrary<G>(g: &mut G) -> $sname
+              where G: Gen
+            {
+                let neg = bool::arbitrary(g);
+                let val = $name::arbitrary(g);
+                let neg2 = if val.is_zero() { false } else { neg };
+                $sname{ negative: neg2, value: val }
+            }
+        }
+
         impl fmt::Debug for $sname {
             fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                 if self.negative {

src/signed/mod.rs

@@ -26,6 +26,8 @@ mod shift;
 #[macro_use]
 mod subtraction;
 
+#[cfg(test)]
+use quickcheck::{Arbitrary,Gen};
 use std::cmp::{Ord,Ordering,PartialOrd};
 use std::fmt;
 use std::ops::{Add,AddAssign};