Improve performance of Collection.removeFirst(_:) where Self == SubSequence

stephencelis · web-flow · commit 099ecbe0b03a · 2020-06-18T15:29:45.000-04:00
diff --git a/stdlib/public/core/Collection.swift b/stdlib/public/core/Collection.swift
@@ -1664,7 +1664,7 @@ extension Collection where SubSequence == Self {
   public mutating func removeFirst(_ k: Int) {
     if k == 0 { return }
     _precondition(k >= 0, "Number of elements to remove should be non-negative")
-    _precondition(count >= k,
+    _precondition(index(startIndex, offsetBy: k, limitedBy: endIndex) != nil,
       "Can't remove more items from a collection than it contains")
     self = self[index(startIndex, offsetBy: k)..<endIndex]
   }

Original file line number	Diff line number	Diff line change
`@@ -1664,7 +1664,7 @@ extension Collection where SubSequence == Self {`
`1664`	`1664`	`public mutating func removeFirst(_ k: Int) {`
`1665`	`1665`	`if k == 0 { return }`
`1666`	`1666`	`_precondition(k >= 0, "Number of elements to remove should be non-negative")`
`1667`		`- _precondition(count >= k,`
	`1667`	`+ _precondition(index(startIndex, offsetBy: k, limitedBy: endIndex) != nil,`
`1668`	`1668`	`"Can't remove more items from a collection than it contains")`
`1669`	`1669`	`self = self[index(startIndex, offsetBy: k)..<endIndex]`
`1670`	`1670`	`}`