Merge pull request #89 from Quuxplusone/ajo-unstable-remove

Micro-optimize `unstable_remove`.
WG21-SG14 · May 10, 2017 · ee37914 · ee37914
2 parents f3f466d + 201a4c8
commit ee37914
Showing 1 changed file with 57 additions and 27 deletions.
diff --git a/SG14/algorithm_ext.h b/SG14/algorithm_ext.h
@@ -33,7 +33,6 @@ namespace stdext
 			destruct(Dst, current);
 			throw;
 		}
-
 	}
 
 	template<class FwdIt, class Sentinel>
@@ -54,9 +53,8 @@ namespace stdext
 			destruct(first, current);
 			throw;
 		}
-
 	}
-	
+
 	template<class FwdIt, class Sentinel>
 	FwdIt uninitialized_default_construct(FwdIt first, Sentinel last)
 	{
@@ -80,52 +78,80 @@ namespace stdext
 	template<class BidirIt, class UnaryPredicate>
 	BidirIt unstable_remove_if(BidirIt first, BidirIt last, UnaryPredicate p)
 	{
-		while (1) {
-			while ((first != last) && p(*first)) {
+		while (true) {
+			// Find the first instance of "p"...
+			while (true) {
+				if (first == last) {
+					return first;
+				}
+				if (p(*first)) {
+					break;
+				}
 				++first;
 			}
-			if (first == last--) break;
-			while ((first != last) && !p(*last)) {
+			// ...and the last instance of "not p"...
+			while (true) {
 				--last;
+				if (first == last) {
+					return first;
+				}
+				if (!p(*last)) {
+					break;
+				}
 			}
-			if (first == last) break;
-			*first++ = std::move(*last);
+			// ...and move the latter over top of the former.
+			*first = std::move(*last);
+			++first;
 		}
-		return first;
 	}
+
 	template<class BidirIt, class Val>
-	BidirIt unstable_remove(BidirIt first, BidirIt last, Val v)
+	BidirIt unstable_remove(BidirIt first, BidirIt last, const Val& v)
 	{
-		while (1) {
-			while ((first != last) && (*first == v)) {
+		while (true) {
+			// Find the first instance of "v"...
+			while (true) {
+				if (first == last) {
+					return first;
+				}
+				if (*first == v) {
+					break;
+				}
 				++first;
 			}
-			if (first == last--) break;
-			while ((first != last) && !(*last == v)) {
+			// ...and the last instance of "not v"...
+			while (true) {
 				--last;
+				if (first == last) {
+					return first;
+				}
+				if (!(*last == v)) {
+					break;
+				}
 			}
-			if (first == last) break;
-			*first++ = std::move(*last);
+			// ...and move the latter over top of the former.
+			*first = std::move(*last);
+			++first;
 		}
-		return first;
 	}
 
 
-
 	//this exists as a point of reference for providing a stable comparison vs unstable_remove_if
 	template<class BidirIt, class UnaryPredicate>
 	BidirIt partition(BidirIt first, BidirIt last, UnaryPredicate p)
 	{
-		while (1) {
+		while (true) {
 			while ((first != last) && p(*first)) {
 				++first;
 			}
-			if (first == last--) break;
+			if (first == last) break;
+			--last;
 			while ((first != last) && !p(*last)) {
 				--last;
 			}
 			if (first == last) break;
-			std::iter_swap(first++, last);
+			std::iter_swap(first, last);
+			++first;
 		}
 		return first;
 	}
@@ -135,10 +161,14 @@ namespace stdext
 	ForwardIt remove_if(ForwardIt first, ForwardIt last, UnaryPredicate p)
 	{
 		first = std::find_if(first, last, p);
-		if (first != last)
-			for (ForwardIt i = first; ++i != last; )
-				if (!p(*i))
-					*first++ = std::move(*i);
+		if (first != last) {
+			for (ForwardIt i = first; ++i != last; ) {
+				if (!p(*i)) {
+					*first = std::move(*i);
+					++first;
+				}
+			}
+		}
 		return first;
 	}
-}
+}