Add addition and subtraction impls to Decimal

src/util/decimal.rs

@@ -21,6 +21,8 @@
 //! altcoins with different granularity may require a wider type.
 //!
 
+use std::ops;
+
 use serde::{ser, de};
 use strason::Json;
 
@@ -47,6 +49,37 @@ impl PartialOrd<Decimal> for Decimal {
     }
 }
 
+impl ops::Add for Decimal {
+    type Output = Decimal;
+
+    #[inline]
+    fn add(self, other: Decimal) -> Decimal {
+        if self.exponent > other.exponent {
+            Decimal {
+                mantissa: other.mantissa * 10i64.pow((self.exponent - other.exponent) as u32) + self.mantissa,
+                exponent: self.exponent
+            }
+        } else {
+            Decimal {
+                mantissa: self.mantissa * 10i64.pow((other.exponent - self.exponent) as u32) + other.mantissa,
+                exponent: other.exponent
+            }
+        }
+    }
+}
+
+impl ops::Neg for Decimal {
+    type Output = Decimal;
+    #[inline]
+    fn neg(self) -> Decimal { Decimal { mantissa: -self.mantissa, exponent: self.exponent } }
+}
+
+impl ops::Sub for Decimal {
+    type Output = Decimal;
+    #[inline]
+    fn sub(self, other: Decimal) -> Decimal { self + (-other) }
+}
+
 impl Decimal {
     /// Creates a new Decimal
     pub fn new(mantissa: i64, exponent: usize) -> Decimal {
@@ -190,6 +223,20 @@ mod tests {
         assert!(d3 > d2);
     }
 
+    #[test]
+    fn arithmetic() {
+        let d1 = Decimal::new(5, 1);   //  0.5
+        let d2 = Decimal::new(-2, 2);  // -0.02
+        let d3 = Decimal::new(3, 0);   //  3.0
+
+        assert_eq!(d1 + d2, Decimal::new(48, 2));
+        assert_eq!(d1 - d2, Decimal::new(52, 2));
+        assert_eq!(d1 + d3, Decimal::new(35, 1));
+        assert_eq!(d1 - d3, Decimal::new(-25, 1));
+        assert_eq!(d2 + d3, Decimal::new(298, 2));
+        assert_eq!(d2 - d3, Decimal::new(-302, 2));
+    }
+
     #[test]
     fn json_parse() {
         let json = Json::from_str("0.00980000").unwrap();