From a5b727a1560568d881430b34b7f0057d33e827b0 Mon Sep 17 00:00:00 2001
From: Timothy Warren <twarren@nabancard.com>
Date: Wed, 11 Mar 2020 13:58:56 -0400
Subject: [PATCH] Test zeros for addition

---
 src/bigint.rs | 75 +++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 55 insertions(+), 20 deletions(-)

diff --git a/src/bigint.rs b/src/bigint.rs
index 551ecc6..c47bf88 100644
--- a/src/bigint.rs
+++ b/src/bigint.rs
@@ -77,6 +77,11 @@ impl BigInt {
             trailing_zeros += 1;
         }
 
+        // Always keep at least one digit
+        if trailing_zeros == current_len {
+            trailing_zeros -= 1;
+        }
+
         let new_len = current_len - trailing_zeros;
 
         self.inner.truncate(new_len);
@@ -94,6 +99,23 @@ impl BigInt {
     pub fn from_str_radix<T: ToString>(s: T, radix: usize) -> BigInt {
         todo!();
     }
+
+    fn get_digit_count(a: &Self, b: &Self) -> usize {
+        let a_digits = a.inner.len();
+        let b_digits = b.inner.len();
+
+        let digits = if b_digits > a_digits {
+            b_digits
+        } else {
+            a_digits
+        };
+
+        if digits > 0 {
+            digits
+        } else {
+            1
+        }
+    }
 }
 
 impl Add for BigInt {
@@ -107,21 +129,20 @@ impl Add for BigInt {
 
         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 digits = Self::get_digit_count(&self, &rhs) + 1;
 
         let mut carry = 0usize;
-        for i in 0..=digits {
+        for i in 0..digits {
             let a = *self.inner.get(i).unwrap_or(&0usize);
             let b = *rhs.inner.get(i).unwrap_or(&0usize);
 
             let (res, overflowed) = a.overflowing_add(b);
+            if res == 0 && !overflowed {
+                out.inner.push(res + carry);
+                carry = 0;
+                continue;
+            }
+
             if overflowed {
                 out.inner.push(res + carry);
                 carry = 1;
@@ -134,6 +155,8 @@ impl Add for BigInt {
             }
         }
 
+        out.trim_zeros();
+
         out
     }
 }
@@ -145,14 +168,7 @@ impl Sub for BigInt {
         // @TODO: handle signs
         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 digits = Self::get_digit_count(&self, &rhs);
 
         let mut borrow = 0usize;
         for i in 0..digits {
@@ -423,19 +439,38 @@ mod tests {
             diff.inner[0],
             core::usize::MAX - 1
         );
+    }
 
-        let a = BigInt {
+    #[test]
+    fn test_sub_assign() {
+        let mut a = BigInt {
             inner: vec![1,0,1],
             sign: Positive
         };
         let b = BigInt::from(2);
-        let diff = a - b;
+
+        a -= b;
+
         assert_eq!(
-            diff.inner,
+            a.inner,
             vec![core::usize::MAX, core::usize::MAX]
         );
     }
 
+    #[test]
+    fn test_zeros() {
+        let a = BigInt::new();
+        let b = BigInt::new();
+
+        // let c = a.clone() - b.clone();
+        // assert_eq!(a.clone(), b.clone());
+        // assert_eq!(c, a.clone());
+
+        let c = a.clone() + b.clone();
+        assert_eq!(a.clone(), b.clone());
+        assert_eq!(c, a.clone());
+    }
+
     #[test]
     fn test_not() {
         let a = BigInt::from(0u8);