[stdlib] Optimize StringProtocol._toUTF16Indices/_toUTF16Offsets

Speed up conversion between UTF-16 offset ranges
and string index ranges, by carefully switching
between absolute and relative index calculations,
depending on the distance we need to go.

It is a surprisingly tricky puzzle to do this
correctly while avoiding redundant calculations.
Offset ranges within substrings add the additional
complication of having to bias offset values with
the absolute offset of the substring’s start index.

Author: lorentey

stdlib/public/core/StringBridge.swift

@@ -795,19 +795,33 @@ extension StringProtocol {
   @_specialize(where Self == Substring)
   public // SPI(Foundation)
   func _toUTF16Offsets(_ indices: Range<Index>) -> Range<Int> {
-    let lowerbound = _toUTF16Offset(indices.lowerBound)
-    let length = self.utf16.distance(
-      from: indices.lowerBound, to: indices.upperBound)
-    return Range(
-      uncheckedBounds: (lower: lowerbound, upper: lowerbound + length))
+    if Self.self == String.self {
+      let s = unsafeBitCast(self, to: String.self)
+      return s.utf16._offsetRange(for: indices, from: s.startIndex)
+    }
+    if Self.self == Substring.self {
+      let s = unsafeBitCast(self, to: Substring.self)
+      return s._slice._base.utf16._offsetRange(for: indices, from: s.startIndex)
+    }
+    let startOffset = _toUTF16Offset(indices.lowerBound)
+    let endOffset = _toUTF16Offset(indices.upperBound)
+    return Range(uncheckedBounds: (lower: startOffset, upper: endOffset))
   }
 
   @_specialize(where Self == String)
   @_specialize(where Self == Substring)
   public // SPI(Foundation)
   func _toUTF16Indices(_ range: Range<Int>) -> Range<Index> {
+    if Self.self == String.self {
+      let s = unsafeBitCast(self, to: String.self)
+      return s.utf16._indexRange(for: range, from: s.startIndex)
+    }
+    if Self.self == Substring.self {
+      let s = unsafeBitCast(self, to: Substring.self)
+      return s._slice._base.utf16._indexRange(for: range, from: s.startIndex)
+    }
     let lowerbound = _toUTF16Index(range.lowerBound)
-    let upperbound = self.utf16.index(lowerbound, offsetBy: range.count)
+    let upperbound = _toUTF16Index(range.upperBound)
     return Range(uncheckedBounds: (lower: lowerbound, upper: upperbound))
   }
 }

stdlib/public/core/StringUTF16View.swift

@@ -299,6 +299,70 @@ extension String.UTF16View: BidirectionalCollection {
     return _nativeGetOffset(for: endIndex)
   }
 
+  internal func _indexRange(
+    for offsets: Range<Int>,
+    from start: Index
+  ) -> Range<Index> {
+    _internalInvariant(_guts.hasMatchingEncoding(start))
+    if _slowPath(_guts.isForeign) {
+      let lower = self.index(start, offsetBy: offsets.lowerBound)
+      let upper = _foreignIndex(lower, offsetBy: offsets.count)
+      return Range(uncheckedBounds: (lower, upper))
+    }
+    if offsets.count < _breadcrumbStride / 2, !_guts.isASCII {
+      let lower = self.index(start, offsetBy: offsets.lowerBound)
+      let upper = _index(lower, offsetBy: offsets.count)._knownUTF8
+      return Range(uncheckedBounds: (lower, upper))
+    }
+
+    let bias = _nativeGetOffset(for: start)
+    let lower = (
+      offsets.lowerBound - bias <= _breadcrumbStride / 2
+      ? _index(start, offsetBy: offsets.lowerBound)
+      : _nativeGetIndex(for: bias + offsets.lowerBound))
+    let upper = _nativeGetIndex(for: bias + offsets.upperBound)
+    return Range(uncheckedBounds: (lower, upper))
+  }
+
+  internal func _offsetRange(
+    for range: Range<Index>,
+    from start: Index
+  ) -> Range<Int> {
+    let lower = _guts.ensureMatchingEncoding(range.lowerBound)
+    let upper = _guts.ensureMatchingEncoding(range.upperBound)
+    _internalInvariant(_guts.hasMatchingEncoding(start))
+
+    _precondition(
+      ifLinkedOnOrAfter: .v5_7_0,
+      lower._encodedOffset <= _guts.count,
+      "String index is out of bounds")
+    _precondition(
+      ifLinkedOnOrAfter: .v5_7_0,
+      upper._encodedOffset <= _guts.count,
+      "String index is out of bounds")
+
+    if _slowPath(_guts.isForeign) {
+      let lowerOffset = _foreignDistance(from: start, to: lower)
+      let distance = _foreignDistance(from: lower, to: upper)
+      return Range(uncheckedBounds: (lowerOffset, lowerOffset + distance))
+    }
+
+    let utf8Distance = upper._encodedOffset - lower._encodedOffset
+    if utf8Distance.magnitude <= _breadcrumbStride / 2, !_guts.isASCII {
+      let lowerOffset = distance(from: start, to: lower)
+      let distance = _utf16Distance(from: lower, to: upper)
+      return Range(uncheckedBounds: (lowerOffset, lowerOffset + distance))
+    }
+    let bias = _nativeGetOffset(for: start)
+    let utf8StartOffset = lower._encodedOffset - start._encodedOffset
+    let lowerOffset = (
+      utf8StartOffset <= _breadcrumbStride / 2
+      ? _utf16Distance(from: start, to: lower)
+      : _nativeGetOffset(for: lower) - bias)
+    let upperOffset = _nativeGetOffset(for: upper) - bias
+    return Range(uncheckedBounds: (lowerOffset, upperOffset))
+  }
+
   /// Accesses the code unit at the given position.
   ///
   /// The following example uses the subscript to print the value of a