[stdlib] StringProtocol._toUTF16Indices: Speed up conversion of short ranges

lorentey · lorentey · commit 2423b8ba3c63 · 2022-12-27T20:22:38.000-08:00
Instead of calling `_toUTF16Index` twice, call it once and then use
`index(_:offsetBy:)` to potentially avoid another breadcrumbs lookup.
diff --git a/stdlib/public/core/StringBridge.swift b/stdlib/public/core/StringBridge.swift
@@ -807,7 +807,7 @@ extension StringProtocol {
   public // SPI(Foundation)
   func _toUTF16Indices(_ range: Range<Int>) -> Range<Index> {
     let lowerbound = _toUTF16Index(range.lowerBound)
-    let upperbound = _toUTF16Index(range.lowerBound + range.count)
+    let upperbound = self.utf16.index(lowerbound, offsetBy: range.count)
     return Range(uncheckedBounds: (lower: lowerbound, upper: upperbound))
   }
 }

Original file line number	Diff line number	Diff line change
`@@ -807,7 +807,7 @@ extension StringProtocol {`
`807`	`807`	`public // SPI(Foundation)`
`808`	`808`	`func _toUTF16Indices(_ range: Range<Int>) -> Range<Index> {`
`809`	`809`	`let lowerbound = _toUTF16Index(range.lowerBound)`
`810`		`- let upperbound = _toUTF16Index(range.lowerBound + range.count)`
	`810`	`+ let upperbound = self.utf16.index(lowerbound, offsetBy: range.count)`
`811`	`811`	`return Range(uncheckedBounds: (lower: lowerbound, upper: upperbound))`
`812`	`812`	`}`
`813`	`813`	`}`