From 881972b2a5cecdf68f9a766632999dddfb4df5a0 Mon Sep 17 00:00:00 2001
From: Igor Aleksanov <12111581+popzxc@users.noreply.github.com>
Date: Mon, 28 May 2018 21:41:07 +0300
Subject: [PATCH] Fix multiplication for uint256 (#88)

---
 src/util/uint.rs | 47 +++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 43 insertions(+), 4 deletions(-)

diff --git a/src/util/uint.rs b/src/util/uint.rs
index 6b7bbd8..c6b5650 100644
--- a/src/util/uint.rs
+++ b/src/util/uint.rs
@@ -61,12 +61,17 @@ macro_rules! construct_uint {
                 let mut carry = [0u64; $n_words];
                 let mut ret = [0u64; $n_words];
                 for i in 0..$n_words {
+                    let not_last_word = i < $n_words - 1;
                     let upper = other as u64 * (arr[i] >> 32);
                     let lower = other as u64 * (arr[i] & 0xFFFFFFFF);
-                    if i < 3 {
+                    if not_last_word {
                         carry[i + 1] += upper >> 32;
                     }
-                    ret[i] = lower + (upper << 32);
+                    let (sum, overflow) = lower.overflowing_add(upper << 32);
+                    ret[i] = sum;
+                    if overflow && not_last_word {
+                        carry[i + 1] += 1;
+                    }
                 }
                 $name(ret) + $name(carry)
             }
@@ -118,10 +123,11 @@ macro_rules! construct_uint {
             type Output = $name;
 
             fn mul(self, other: $name) -> $name {
-                let mut me = self;
+                let mut me = $name::zero();
                 // TODO: be more efficient about this
                 for i in 0..(2 * $n_words) {
-                    me = (me + me.mul_u32((other >> (32 * i)).low_u32())) << (32 * i);
+                    let to_mul = (other >> (32 * i)).low_u32();
+                    me = me + (self.mul_u32(to_mul) << (32 * i));
                 }
                 me
             }
@@ -468,6 +474,39 @@ mod tests {
         // TODO: bit inversion
     }
 
+    #[test]
+    pub fn mul_u32_test() {
+        let u64_val = Uint256::from_u64(0xDEADBEEFDEADBEEF).unwrap();
+
+        let u96_res = u64_val.mul_u32(0xFFFFFFFF);
+        let u128_res = u96_res.mul_u32(0xFFFFFFFF);
+        let u160_res = u128_res.mul_u32(0xFFFFFFFF);
+        let u192_res = u160_res.mul_u32(0xFFFFFFFF);
+        let u224_res = u192_res.mul_u32(0xFFFFFFFF);
+        let u256_res = u224_res.mul_u32(0xFFFFFFFF);
+
+        assert_eq!(u96_res, Uint256([0xffffffff21524111u64, 0xDEADBEEE, 0, 0]));
+        assert_eq!(u128_res, Uint256([0x21524111DEADBEEFu64, 0xDEADBEEE21524110, 0, 0]));
+        assert_eq!(u160_res, Uint256([0xBD5B7DDD21524111u64, 0x42A4822200000001, 0xDEADBEED, 0]));
+        assert_eq!(u192_res, Uint256([0x63F6C333DEADBEEFu64, 0xBD5B7DDFBD5B7DDB, 0xDEADBEEC63F6C334, 0]));
+        assert_eq!(u224_res, Uint256([0x7AB6FBBB21524111u64, 0xFFFFFFFBA69B4558, 0x854904485964BAAA, 0xDEADBEEB]));
+        assert_eq!(u256_res, Uint256([0xA69B4555DEADBEEFu64, 0xA69B455CD41BB662, 0xD41BB662A69B4550, 0xDEADBEEAA69B455C]));
+    }
+
+    #[test]
+    pub fn multiplication_test() {
+        let u64_val = Uint256::from_u64(0xDEADBEEFDEADBEEF).unwrap();
+
+        let u128_res = u64_val * u64_val;
+
+        assert_eq!(u128_res, Uint256([0x048D1354216DA321u64, 0xC1B1CD13A4D13D46, 0, 0]));
+
+        let u256_res = u128_res * u128_res;
+
+        assert_eq!(u256_res, Uint256([0xF4E166AAD40D0A41u64, 0xF5CF7F3618C2C886u64,
+                                      0x4AFCFF6F0375C608u64, 0x928D92B4D7F5DF33u64]));
+    }
+
     #[test]
     pub fn uint256_bitslice_test() {
         let init = Uint256::from_u64(0xDEADBEEFDEADBEEF).unwrap();