From c614e91e23d4d3dea80bc78886a1b7e96456596b Mon Sep 17 00:00:00 2001
From: Raghavendra Nagaraj <jamdagni86@gmail.com>
Date: Fri, 26 Oct 2018 17:40:49 +0530
Subject: [PATCH] container/list: combining insert and remove operations while
 moving elements within a list.

Methods MoveToFront, MoveToBack, MoveBefore and MoveAfter remove the given element from the list and later add the same element at a desired position. This has resulted in some inefficiencies, with remove method making 4 unncessary operations and insert method making 2. The commit merges these two methods into a method called move.

Fixes #27747
---
 src/container/list/list.go | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/src/container/list/list.go b/src/container/list/list.go
index dc4260e1316..b8b599aabb1 100644
--- a/src/container/list/list.go
+++ b/src/container/list/list.go
@@ -116,6 +116,23 @@ func (l *List) remove(e *Element) *Element {
 	return e
 }
 
+// move moves e to next to at and returns e.
+func (l *List) move(e, at *Element) *Element {
+	if e == at {
+		return e
+	}
+	e.prev.next = e.next
+	e.next.prev = e.prev
+
+	n := at.next
+	at.next = e
+	e.prev = at
+	e.next = n
+	n.prev = e
+
+	return e
+}
+
 // Remove removes e from l if e is an element of list l.
 // It returns the element value e.Value.
 // The element must not be nil.
@@ -170,7 +187,7 @@ func (l *List) MoveToFront(e *Element) {
 		return
 	}
 	// see comment in List.Remove about initialization of l
-	l.insert(l.remove(e), &l.root)
+	l.move(e, &l.root)
 }
 
 // MoveToBack moves element e to the back of list l.
@@ -181,7 +198,7 @@ func (l *List) MoveToBack(e *Element) {
 		return
 	}
 	// see comment in List.Remove about initialization of l
-	l.insert(l.remove(e), l.root.prev)
+	l.move(e, l.root.prev)
 }
 
 // MoveBefore moves element e to its new position before mark.
@@ -191,7 +208,7 @@ func (l *List) MoveBefore(e, mark *Element) {
 	if e.list != l || e == mark || mark.list != l {
 		return
 	}
-	l.insert(l.remove(e), mark.prev)
+	l.move(e, mark.prev)
 }
 
 // MoveAfter moves element e to its new position after mark.
@@ -201,7 +218,7 @@ func (l *List) MoveAfter(e, mark *Element) {
 	if e.list != l || e == mark || mark.list != l {
 		return
 	}
-	l.insert(l.remove(e), mark)
+	l.move(e, mark)
 }
 
 // PushBackList inserts a copy of an other list at the back of list l.