Partially implement subtraction

src/bigint.rs

@@ -137,7 +137,33 @@ impl Sub for BigInt {
     type Output = Self;
 
     fn sub(self, rhs: Self) -> Self::Output {
-        todo!()
+        let mut out = BigInt::new_empty();
+
+        let u_digits = self.inner.len();
+        let v_digits = rhs.inner.len();
+
+        let digits = if v_digits > u_digits {
+            v_digits
+        } else {
+            u_digits
+        };
+
+        let mut borrow = 0usize;
+        for i in 0..digits {
+            let a = *self.inner.get(i).unwrap_or(&0usize);
+            let b = *rhs.inner.get(i).unwrap_or(&0usize);
+
+            if a > b {
+                out.inner.push(a - b - borrow);
+                borrow = 0;
+            } else if a < b {
+                todo!();
+            } else {
+                todo!();
+            }
+        }
+
+        out
     }
 }
 
@@ -335,6 +361,18 @@ mod tests {
         assert_eq!(sum.inner[1], 1usize);
     }
 
+    #[test]
+    fn test_sub() {
+        let a = BigInt::from(core::usize::MAX);
+        let b = BigInt::from(core::u16::MAX);
+
+        let diff = a - b;
+        assert_eq!(
+            diff.inner[0],
+            core::usize::MAX - core::u16::MAX as usize
+        );
+    }
+
     #[test]
     fn test_not() {
         let a = BigInt::from(0u8);