Updates for acceptance of SE-0180

https://github.com/apple/swift-evolution/blob/master/proposals/0180-string-index-overhaul.md

Author: Dave Abrahams

Foundation/ExtraStringAPIs.swift

@@ -17,15 +17,15 @@ extension String.UTF16View.Index : Strideable {
     /// Construct from an integer offset.
     public init(_ offset: Int) {
         _precondition(offset >= 0, "Negative UTF16 index offset not allowed")
-        self.init(_offset: offset)
+        self.init(encodedOffset: offset)
     }
 
     public func distance(to other: String.UTF16View.Index) -> Int {
-        return _offset.distance(to: other._offset)
+        return encodedOffset.distance(to: other.encodedOffset)
     }
 
     public func advanced(by n: Int) -> String.UTF16View.Index {
-        return String.UTF16View.Index(_offset.advanced(by: n))
+        return String.UTF16View.Index(encodedOffset.advanced(by: n))
     }
 }
 

Foundation/NSRange.swift

@@ -261,8 +261,8 @@ extension NSRange {
         where R.Bound == S.Index, S.Index == String.Index {
             let r = region.relative(to: target)
             self = NSRange(
-                location: r.lowerBound._utf16Index - target.startIndex._utf16Index,
-                length: r.upperBound._utf16Index - r.lowerBound._utf16Index
+                location: r.lowerBound.encodedOffset - target.startIndex.encodedOffset,
+                length: r.upperBound.encodedOffset - r.lowerBound.encodedOffset
             )
     }
 

Foundation/NSStringAPI.swift

@@ -30,8 +30,8 @@ func _toNSArray<T, U : AnyObject>(_ a: [T], f: (T) -> U) -> NSArray {
 
 func _toNSRange(_ r: Range<String.Index>) -> NSRange {
   return NSRange(
-    location: r.lowerBound._utf16Index,
-    length: r.upperBound._utf16Index - r.lowerBound._utf16Index)
+    location: r.lowerBound.encodedOffset,
+    length: r.upperBound.encodedOffset - r.lowerBound.encodedOffset)
 }
 
 extension String {
@@ -47,10 +47,7 @@ extension String {
   /// Return an `Index` corresponding to the given offset in our UTF-16
   /// representation.
   func _index(_ utf16Index: Int) -> Index {
-    return Index(
-      _base: String.UnicodeScalarView.Index(_position: utf16Index),
-      in: characters
-    )
+    return Index(encodedOffset: utf16Index)
   }
 
   /// Return a `Range<Index>` corresponding to the given `NSRange` of
@@ -1076,7 +1073,7 @@ extension String {
   public
   func rangeOfComposedCharacterSequence(at anIndex: Index) -> Range<Index> {
     return _range(
-      _ns.rangeOfComposedCharacterSequence(at: anIndex._utf16Index))
+      _ns.rangeOfComposedCharacterSequence(at: anIndex.encodedOffset))
   }
 
   // - (NSRange)rangeOfComposedCharacterSequencesForRange:(NSRange)range
@@ -1392,15 +1389,15 @@ extension String {
   /// Returns a new string containing the characters of the
   /// `String` from the one at a given index to the end.
   public func substring(from index: Index) -> String {
-    return _ns.substring(from: index._utf16Index)
+    return _ns.substring(from: index.encodedOffset)
   }
 
   // - (NSString *)substringToIndex:(NSUInteger)anIndex
 
   /// Returns a new string containing the characters of the
   /// `String` up to, but not including, the one at a given index.
   public func substring(to index: Index) -> String {
-    return _ns.substring(to: index._utf16Index)
+    return _ns.substring(to: index.encodedOffset)
   }
 
   // - (NSString *)substringWithRange:(NSRange)aRange

Foundation/URLComponents.swift

@@ -188,8 +188,8 @@ public struct URLComponents : ReferenceConvertible, Hashable, Equatable, _Mutabl
     private func _toStringRange(_ r : NSRange) -> Range<String.Index>? {
         guard r.location != NSNotFound else { return nil }
 
-        let utf16Start = String.UTF16View.Index(_offset: r.location)
-        let utf16End = String.UTF16View.Index(_offset: r.location + r.length)
+        let utf16Start = String.UTF16View.Index(encodedOffset: r.location)
+        let utf16End = String.UTF16View.Index(encodedOffset: r.location + r.length)
 
         guard let s = self.string else { return nil }
         guard let start = String.Index(utf16Start, within: s) else { return nil }