From cc0adf544208a36786e0ffa7f219ff79983d380b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tomasz=20Drwi=C4=99ga?= <tomasz@ethcore.io>
Date: Mon, 7 Mar 2016 17:06:08 +0100
Subject: [PATCH] Optimizing and simplifying add and shl

---
 util/bigint/src/uint.rs | 89 ++++++++++++++++-------------------------
 1 file changed, 34 insertions(+), 55 deletions(-)

diff --git a/util/bigint/src/uint.rs b/util/bigint/src/uint.rs
index ad4f0a99c..47a975d5f 100644
--- a/util/bigint/src/uint.rs
+++ b/util/bigint/src/uint.rs
@@ -71,29 +71,19 @@ macro_rules! uint_overflowing_add_reg {
 	($name:ident, $n_words:expr, $self_expr: expr, $other: expr) => ({
 		let $name(ref me) = $self_expr;
 		let $name(ref you) = $other;
+
 		let mut ret = [0u64; $n_words];
-		let mut carry = [0u64; $n_words];
-		let mut b_carry = false;
-		let mut overflow = false;
+		let mut carry = [0u64; $n_words + 1];
 
 		for i in 0..$n_words {
-			ret[i] = me[i].wrapping_add(you[i]);
+			let (res1, overflow1) = me[i].overflowing_add(you[i]);
+			let (res2, overflow2) = res1.overflowing_add(carry[i]);
 
-			if ret[i] < me[i] {
-				if i < $n_words - 1 {
-					carry[i + 1] = 1;
-					b_carry = true;
-				} else {
-					overflow = true;
-				}
-			}
-		}
-		if b_carry {
-			let ret = overflowing!($name(ret).overflowing_add($name(carry)), overflow);
-			(ret, overflow)
-		} else {
-			($name(ret), overflow)
+			ret[i] = res2;
+			carry[i+1] = overflow1 as u64 + overflow2 as u64;
 		}
+
+		($name(ret), carry[$n_words] > 0)
 	})
 }
 
@@ -673,37 +663,10 @@ macro_rules! construct_uint {
 			}
 
 			fn overflowing_shl(self, shift32: u32) -> ($name, bool) {
-				let $name(ref original) = self;
-				let mut ret = [0u64; $n_words];
 				let shift = shift32 as usize;
-				let word_shift = shift / 64;
-				let bit_shift = shift % 64;
-				for i in 0..$n_words {
-					// Shift
-					if i + word_shift < $n_words {
-						ret[i + word_shift] += original[i] << bit_shift;
-					}
-					// Carry
-					if bit_shift > 0 && i + word_shift + 1 < $n_words {
-						ret[i + word_shift + 1] += original[i] >> (64 - bit_shift);
-					}
-				}
-				// Detecting overflow
-				let last = $n_words - word_shift - if bit_shift > 0 { 1 } else { 0 };
-				let overflow = if bit_shift > 0 {
-					(original[last] >> (64 - bit_shift)) > 0
-				} else if word_shift > 0 {
-					original[last] > 0
-				} else {
-					false
-				};
 
-				for i in last+1..$n_words-1 {
-					if original[i] > 0 {
-						return ($name(ret), true);
-					}
-				}
-				($name(ret), overflow)
+				let res = self << shift;
+				(res, self != (res >> shift))
 			}
 		}
 
@@ -987,14 +950,15 @@ macro_rules! construct_uint {
 				let mut ret = [0u64; $n_words];
 				let word_shift = shift / 64;
 				let bit_shift = shift % 64;
-				for i in 0..$n_words {
-					// Shift
-					if i + word_shift < $n_words {
-						ret[i + word_shift] += original[i] << bit_shift;
-					}
-					// Carry
-					if bit_shift > 0 && i + word_shift + 1 < $n_words {
-						ret[i + word_shift + 1] += original[i] >> (64 - bit_shift);
+
+				// shift
+				for i in word_shift..$n_words {
+					ret[i] += original[i - word_shift] << bit_shift;
+				}
+				// carry
+				if bit_shift > 0 {
+					for i in word_shift+1..$n_words {
+						ret[i] += original[i - 1 - word_shift] >> (64 - bit_shift);
 					}
 				}
 				$name(ret)
@@ -1672,6 +1636,11 @@ mod tests {
 
 	#[test]
 	pub fn uint256_shl_overflow() {
+		assert_eq!(
+			U256::from_str("7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff").unwrap()
+			<< 4,
+			U256::from_str("fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff0").unwrap()
+		);
 		assert_eq!(
 			U256::from_str("7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff").unwrap()
 			.overflowing_shl(4),
@@ -1681,6 +1650,16 @@ mod tests {
 
 	#[test]
 	pub fn uint256_shl_overflow_words() {
+		assert_eq!(
+			U256::from_str("0000000000000001ffffffffffffffffffffffffffffffffffffffffffffffff").unwrap()
+			<< 64,
+			U256::from_str("ffffffffffffffffffffffffffffffffffffffffffffffff0000000000000000").unwrap()
+		);
+		assert_eq!(
+			U256::from_str("0000000000000000ffffffffffffffffffffffffffffffffffffffffffffffff").unwrap()
+			<< 64,
+			U256::from_str("ffffffffffffffffffffffffffffffffffffffffffffffff0000000000000000").unwrap()
+		);
 		assert_eq!(
 			U256::from_str("0000000000000001ffffffffffffffffffffffffffffffffffffffffffffffff").unwrap()
 			.overflowing_shl(64),