[stdlib] Fix string index sharing (#4896)

* [stdlib] Fix String.UTF16View index sharing

* [stdlib] Fix String.UnicodeScalarView index sharing

* [stdlib] Fix String.CharacterView index sharing

* [stdlib] Test advancing string indices past their ends

* [stdlib] Simplify CharacterView ranged subscript

stdlib/public/core/StringCharacterView.swift

@@ -54,14 +54,22 @@ extension String {
   public struct CharacterView {
     internal var _core: _StringCore
 
+    /// The offset of this view's `_core` from an original core. This works
+    /// around the fact that `_StringCore` is always zero-indexed.
+    /// `_coreOffset` should be subtracted from `UnicodeScalarIndex._position`
+    /// before that value is used as a `_core` index.
+    internal var _coreOffset: Int
+
     /// Creates a view of the given string.
     public init(_ text: String) {
       self._core = text._core
+      self._coreOffset = 0
     }
     
     public // @testable
-    init(_ _core: _StringCore) {
+    init(_ _core: _StringCore, coreOffset: Int = 0) {
       self._core = _core
+      self._coreOffset = coreOffset
     }
   }
 
@@ -139,7 +147,7 @@ extension String {
 extension String.CharacterView : BidirectionalCollection {
   internal typealias UnicodeScalarView = String.UnicodeScalarView
   internal var unicodeScalars: UnicodeScalarView {
-    return UnicodeScalarView(_core)
+    return UnicodeScalarView(_core, coreOffset: _coreOffset)
   }
   
   /// A position in a string's `CharacterView` instance.
@@ -246,7 +254,7 @@ extension String.CharacterView : BidirectionalCollection {
     from start: UnicodeScalarView.Index
   ) -> Int {
     var start = start
-    let end = UnicodeScalarView.Index(_position: _core.count)
+    let end = unicodeScalars.endIndex
     if start == end {
       return 0
     }
@@ -288,7 +296,7 @@ extension String.CharacterView : BidirectionalCollection {
   internal func _measureExtendedGraphemeClusterBackward(
     from end: UnicodeScalarView.Index
   ) -> Int {
-    let start = UnicodeScalarView.Index(_position: 0)
+    let start = unicodeScalars.startIndex
     if start == end {
       return 0
     }
@@ -363,8 +371,8 @@ extension String.CharacterView : RangeReplaceableCollection {
     with newElements: C
   ) where C : Collection, C.Iterator.Element == Character {
     let rawSubRange: Range<Int> =
-      bounds.lowerBound._base._position
-      ..< bounds.upperBound._base._position
+      bounds.lowerBound._base._position - _coreOffset
+      ..< bounds.upperBound._base._position - _coreOffset
     let lazyUTF16 = newElements.lazy.flatMap { $0.utf16 }
     _core.replaceSubrange(rawSubRange, with: lazyUTF16)
   }
@@ -436,10 +444,9 @@ extension String.CharacterView {
   /// - Complexity: O(*n*) if the underlying string is bridged from
   ///   Objective-C, where *n* is the length of the string; otherwise, O(1).
   public subscript(bounds: Range<Index>) -> String.CharacterView {
-    let unicodeScalarRange =
-      bounds.lowerBound._base..<bounds.upperBound._base
-    return String.CharacterView(
-      String(_core).unicodeScalars[unicodeScalarRange]._core)
+    let unicodeScalarRange = bounds.lowerBound._base..<bounds.upperBound._base
+    return String.CharacterView(unicodeScalars[unicodeScalarRange]._core,
+      coreOffset: unicodeScalarRange.lowerBound._position)
   }
 }
 

stdlib/public/core/StringUTF16.swift

@@ -145,7 +145,7 @@ extension String {
     /// The position of the first code unit if the `String` is
     /// nonempty; identical to `endIndex` otherwise.
     public var startIndex: Index {
-      return Index(_offset: 0)
+      return Index(_offset: _offset)
     }
 
     /// The "past the end" position---that is, the position one greater than
@@ -153,7 +153,7 @@ extension String {
     ///
     /// In an empty UTF-16 view, `endIndex` is equal to `startIndex`.
     public var endIndex: Index {
-      return Index(_offset: _length)
+      return Index(_offset: _offset + _length)
     }
 
     public struct Indices {
@@ -204,7 +204,7 @@ extension String {
     }
 
     func _internalIndex(at i: Int) -> Int {
-      return _core.startIndex + _offset + i
+      return _core.startIndex + i
     }
 
     /// Accesses the code unit at the given position.
@@ -220,11 +220,10 @@ extension String {
     /// - Parameter position: A valid index of the view. `position` must be
     ///   less than the view's end index.
     public subscript(i: Index) -> UTF16.CodeUnit {
-      let position = i._offset
-      _precondition(position >= 0 && position < _length,
+      _precondition(i >= startIndex && i < endIndex,
           "out-of-range access on a UTF16View")
 
-      let index = _internalIndex(at: position)
+      let index = _internalIndex(at: i._offset)
       let u = _core[index]
       if _fastPath((u >> 11) != 0b1101_1) {
         // Neither high-surrogate, nor low-surrogate -- well-formed sequence
@@ -292,8 +291,8 @@ extension String {
     }
 
     public var description: String {
-      let start = _internalIndex(at: 0)
-      let end = _internalIndex(at: _length)
+      let start = _internalIndex(at: _offset)
+      let end = _internalIndex(at: _offset + _length)
       return String(_core[start..<end])
     }
 
@@ -337,18 +336,17 @@ extension String {
   public init?(_ utf16: UTF16View) {
     let wholeString = String(utf16._core)
 
-    if let start = UTF16Index(
-      _offset: utf16._offset
-    ).samePosition(in: wholeString) {
-      if let end = UTF16Index(
-        _offset: utf16._offset + utf16._length
-      ).samePosition(in: wholeString) {
-        self = wholeString[start..<end]
-        return
-      }
-    }
+    guard
+      let start = UTF16Index(_offset: utf16._offset)
+        .samePosition(in: wholeString),
+      let end = UTF16Index(_offset: utf16._offset + utf16._length)
+        .samePosition(in: wholeString)
+      else
+    {
         return nil
     }
+    self = wholeString[start..<end]
+  }
 
   /// The index type for subscripting a string's `utf16` view.
   public typealias UTF16Index = UTF16View.Index

stdlib/public/core/StringUnicodeScalarView.swift

@@ -62,8 +62,9 @@ extension String {
     CustomStringConvertible,
     CustomDebugStringConvertible
   {
-    internal init(_ _core: _StringCore) {
+    internal init(_ _core: _StringCore, coreOffset: Int = 0) {
       self._core = _core
+      self._coreOffset = coreOffset
     }
 
     internal struct _ScratchIterator : IteratorProtocol {
@@ -109,12 +110,24 @@ extension String {
       @_versioned internal var _position: Int
     }
 
+    /// Translates a `_core` index into a `UnicodeScalarIndex` using this view's
+    /// `_coreOffset`.
+    internal func _fromCoreIndex(_ i: Int) -> Index {
+      return Index(_position: i + _coreOffset)
+    }
+    
+    /// Translates a `UnicodeScalarIndex` into a `_core` index using this view's
+    /// `_coreOffset`.
+    internal func _toCoreIndex(_ i: Index) -> Int {
+      return i._position - _coreOffset
+    }
+    
     /// The position of the first Unicode scalar value if the string is
     /// nonempty.
     ///
     /// If the string is empty, `startIndex` is equal to `endIndex`.
     public var startIndex: Index {
-      return Index(_position: _core.startIndex)
+      return _fromCoreIndex(_core.startIndex)
     }
 
     /// The "past the end" position---that is, the position one greater than
@@ -122,31 +135,32 @@ extension String {
     ///
     /// In an empty Unicode scalars view, `endIndex` is equal to `startIndex`.
     public var endIndex: Index {
-      return Index(_position: _core.endIndex)
+      return _fromCoreIndex(_core.endIndex)
     }
 
     /// Returns the next consecutive location after `i`.
     ///
     /// - Precondition: The next location exists.
     public func index(after i: Index) -> Index {
-      var scratch = _ScratchIterator(_core, i._position)
+      let i = _toCoreIndex(i)
+      var scratch = _ScratchIterator(_core, i)
       var decoder = UTF16()
       let (_, length) = decoder._decodeOne(&scratch)
-      return Index(_position: i._position + length)
+      return _fromCoreIndex(i + length)
     }
 
     /// Returns the previous consecutive location before `i`.
     ///
     /// - Precondition: The previous location exists.
     public func index(before i: Index) -> Index {
-      var i = i._position - 1
+      var i = _toCoreIndex(i) - 1
       let codeUnit = _core[i]
       if _slowPath((codeUnit >> 10) == 0b1101_11) {
         if i != 0 && (_core[i - 1] >> 10) == 0b1101_10 {
           i -= 1
         }
       }
-      return Index(_position: i)
+      return _fromCoreIndex(i)
     }
 
     /// Accesses the Unicode scalar value at the given position.
@@ -166,7 +180,7 @@ extension String {
     /// - Parameter position: A valid index of the character view. `position`
     ///   must be less than the view's end index.
     public subscript(position: Index) -> UnicodeScalar {
-      var scratch = _ScratchIterator(_core, position._position)
+      var scratch = _ScratchIterator(_core, _toCoreIndex(position))
       var decoder = UTF16()
       switch decoder.decode(&scratch) {
       case .scalarValue(let us):
@@ -192,8 +206,9 @@ extension String {
     /// - Complexity: O(*n*) if the underlying string is bridged from
     ///   Objective-C, where *n* is the length of the string; otherwise, O(1).
     public subscript(r: Range<Index>) -> UnicodeScalarView {
-      return UnicodeScalarView(
-        _core[r.lowerBound._position..<r.upperBound._position])
+      let rawSubRange = _toCoreIndex(r.lowerBound)..<_toCoreIndex(r.upperBound)
+      return UnicodeScalarView(_core[rawSubRange],
+        coreOffset: r.lowerBound._position)
     }
 
     /// An iterator over the Unicode scalars that make up a `UnicodeScalarView`
@@ -270,7 +285,7 @@ extension String {
     }
 
     public var description: String {
-      return String(_core[startIndex._position..<endIndex._position])
+      return String(_core)
     }
 
     public var debugDescription: String {
@@ -278,6 +293,12 @@ extension String {
     }
 
     internal var _core: _StringCore
+    
+    /// The offset of this view's `_core` from an original core. This works
+    /// around the fact that `_StringCore` is always zero-indexed.
+    /// `_coreOffset` should be subtracted from `UnicodeScalarIndex._position`
+    /// before that value is used as a `_core` index.
+    internal var _coreOffset: Int
   }
 
   /// Creates a string corresponding to the given collection of Unicode
@@ -391,9 +412,8 @@ extension String.UnicodeScalarView : RangeReplaceableCollection {
     _ bounds: Range<Index>,
     with newElements: C
   ) where C : Collection, C.Iterator.Element == UnicodeScalar {
-    let rawSubRange: Range<Int> =
-      bounds.lowerBound._position
-      ..< bounds.upperBound._position
+    let rawSubRange: Range<Int> = _toCoreIndex(bounds.lowerBound) ..<
+      _toCoreIndex(bounds.upperBound)
     let lazyUTF16 = newElements.lazy.flatMap { $0.utf16 }
     _core.replaceSubrange(rawSubRange, with: lazyUTF16)
   }

test/stdlib/subString.swift

@@ -5,11 +5,10 @@ import StdlibUnittest
 
 var SubstringTests = TestSuite("SubstringTests")
 
-func checkMatch<S: Collection, T: Collection
+func checkMatch<S: Collection, T: Collection>(_ x: S, _ y: T, _ i: S.Index)
   where S.Index == T.Index, S.Iterator.Element == T.Iterator.Element,
-  S.Iterator.Element: Equatable>(
-  _ x: S, _ y: T, _ i: S.Index) {
-  
+  S.Iterator.Element: Equatable
+{
   expectEqual(x[i], y[i])
 }
 
@@ -25,9 +24,7 @@ SubstringTests.test("String") {
   expectEqual(s3, "cd")
 }
 
-SubstringTests.test("CharacterView")
-  .xfail(.always("CharacterView slices don't share indices"))
-  .code {
+SubstringTests.test("CharacterView") {
   let s = "abcdefg"
   var t = s.characters.dropFirst(2)
   var u = t.dropFirst(2)
@@ -41,6 +38,11 @@ SubstringTests.test("CharacterView")
   checkMatch(t, u, u.index(after: u.startIndex))
   checkMatch(t, u, u.index(before: u.endIndex))
   
+  expectEqual("", String(t.dropFirst(10)))
+  expectEqual("", String(t.dropLast(10)))
+  expectEqual("", String(u.dropFirst(10)))
+  expectEqual("", String(u.dropLast(10)))
+  
   t.replaceSubrange(t.startIndex...t.startIndex, with: ["C"])
   u.replaceSubrange(u.startIndex...u.startIndex, with: ["E"])
   expectEqual(String(u), "Efg")
@@ -48,9 +50,7 @@ SubstringTests.test("CharacterView")
   expectEqual(s, "abcdefg")
 }
 
-SubstringTests.test("UnicodeScalars")
-  .xfail(.always("UnicodeScalarsView slices don't share indices"))
-  .code {
+SubstringTests.test("UnicodeScalars") {
   let s = "abcdefg"
   var t = s.unicodeScalars.dropFirst(2)
   var u = t.dropFirst(2)
@@ -64,6 +64,11 @@ SubstringTests.test("UnicodeScalars")
   checkMatch(t, u, u.index(after: u.startIndex))
   checkMatch(t, u, u.index(before: u.endIndex))
   
+  expectEqual("", String(t.dropFirst(10)))
+  expectEqual("", String(t.dropLast(10)))
+  expectEqual("", String(u.dropFirst(10)))
+  expectEqual("", String(u.dropLast(10)))
+  
   t.replaceSubrange(t.startIndex...t.startIndex, with: ["C"])
   u.replaceSubrange(u.startIndex...u.startIndex, with: ["E"])
   expectEqual(String(u), "Efg")
@@ -71,9 +76,7 @@ SubstringTests.test("UnicodeScalars")
   expectEqual(s, "abcdefg")
 }
 
-SubstringTests.test("UTF16View")
-  .xfail(.always("UTF16View slices don't share indices"))
-  .code {
+SubstringTests.test("UTF16View") {
   let s = "abcdefg"
   let t = s.utf16.dropFirst(2)
   let u = t.dropFirst(2)
@@ -86,6 +89,11 @@ SubstringTests.test("UTF16View")
   checkMatch(t, u, u.startIndex)
   checkMatch(t, u, u.index(after: u.startIndex))
   checkMatch(t, u, u.index(before: u.endIndex))
+  
+  expectEqual("", String(t.dropFirst(10))!)
+  expectEqual("", String(t.dropLast(10))!)
+  expectEqual("", String(u.dropFirst(10))!)
+  expectEqual("", String(u.dropLast(10))!)
 }
 
 SubstringTests.test("UTF8View") {
@@ -99,6 +107,11 @@ SubstringTests.test("UTF8View") {
   checkMatch(s.utf8, t, u.startIndex)
   checkMatch(t, u, u.startIndex)
   checkMatch(t, u, u.index(after: u.startIndex))
+
+  expectEqual("", String(t.dropFirst(10))!)
+  expectEqual("", String(t.dropLast(10))!)
+  expectEqual("", String(u.dropFirst(10))!)
+  expectEqual("", String(u.dropLast(10))!)
 }
 
 runAllTests()