//===--- StringUTF16.swift ------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2015 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See http://swift.org/LICENSE.txt for license information
// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//

extension String {
  /// A collection of UTF-16 code units that encodes a `String` value.
  public struct UTF16View : Sliceable, Reflectable {
    public struct Index {
      // Foundation needs access to these fields so it can expose
      // random access
      public // SPI(Foundation)
      init(_offset: Int) { self._offset = _offset }
      internal init(_ offset: Int) { _offset = offset }
      public let _offset: Int
    }
    
    /// The position of the first code unit if the `String` is
    /// non-empty; identical to `endIndex` otherwise.
    public var startIndex: Index {
      return Index(0)
    }
    
    /// The "past the end" position.
    ///
    /// `endIndex` is not a valid argument to `subscript`, and is always
    /// reachable from `startIndex` by zero or more applications of
    /// `successor()`.
    public var endIndex: Index {
      return Index(_length)
    }

    func _toInternalIndex(i: Int) -> Int {
      return _core.startIndex + _offset + i
    }

    // This is to avoid printing "func generate() -> GeneratorOf<UInt16>"
    public typealias _GeneratorType = GeneratorOf<UInt16>

    /// A type whose instances can produce the elements of this
    /// sequence, in order.
    public typealias Generator = _GeneratorType

    /// Return a *generator* over the code points that comprise this
    /// *sequence*.
    ///
    /// Complexity: O(1)
    public func generate() -> Generator {
      var index = startIndex
      return GeneratorOf<UInt16> {
        if _fastPath(index != self.endIndex) {
          return self[index++]
        }
        return .None
      }
    }

    /// Access the element at `position`.
    ///
    /// Requires: `position` is a valid position in `self` and
    /// `position != endIndex`.
    public subscript(i: Index) -> Generator.Element {
      let position = i._offset
      _precondition(position >= 0 && position < _length,
          "out-of-range access on a UTF16View")

      var index = _toInternalIndex(position)
      let u = _core[index]
      if _fastPath((u >> 11) != 0b1101_1) {
        // Neither high-surrogate, nor low-surrogate -- well-formed sequence
        // of 1 code unit.
        return u
      }

      if (u >> 10) == 0b1101_10 {
        // `u` is a high-surrogate.  SequenceType is well-formed if it
        // is followed by a low-surrogate.
        if _fastPath(
               index + 1 < _core.count &&
               (_core[index + 1] >> 10) == 0b1101_11) {
          return u
        }
        return 0xfffd
      }

      // `u` is a low-surrogate.  SequenceType is well-formed if
      // previous code unit is a high-surrogate.
      if _fastPath(index != 0 && (_core[index - 1] >> 10) == 0b1101_10) {
        return u
      }
      return 0xfffd
    }

#if _runtime(_ObjC)
    // These may become less important once <rdar://problem/19255291> is addressed.

    @availability(
      *, unavailable,
      message="Indexing a String's UTF16View requires a String.UTF16View.Index, which can be constructed from Int when Foundation is imported")
    public subscript(i: Int) -> Generator.Element {
      return self[Index(_offset: i)]
    }

    @availability(
      *, unavailable,
      message="Slicing a String's UTF16View requires a Range<String.UTF16View.Index>, String.UTF16View.Index can be constructed from Int when Foundation is imported")
    public subscript(subRange: Range<Int>) -> UTF16View {
      return self[Index(_offset: subRange.startIndex)..<Index(_offset: subRange.endIndex)]
    }
#endif
    
    /// Access the elements delimited by the given half-open range of
    /// indices.
    ///
    /// Complexity: O(1) unless bridging from Objective-C requires an
    /// O(N) conversion.
    public subscript(subRange: Range<Index>) -> UTF16View {
      return UTF16View(
        _core, offset: _toInternalIndex(subRange.startIndex._offset),
          length: subRange.endIndex._offset - subRange.startIndex._offset)
    }

    internal init(_ _core: _StringCore) {
      self._offset = 0
      self._length = _core.count
      self._core = _core
    }

    internal init(_ _core: _StringCore, offset: Int, length: Int) {
      self._offset = offset
      self._length = length
      self._core = _core
    }
    
    /// Returns a mirror that reflects `self`.
    public func getMirror() -> MirrorType {
      return _UTF16ViewMirror(self)
    }

    var _offset: Int
    var _length: Int
    let _core: _StringCore
  }

  /// A UTF-16 encoding of `self`.
  public var utf16: UTF16View {
    return UTF16View(_core)
  }
}

extension String.UTF16View.Index : BidirectionalIndexType {
  public func successor() -> String.UTF16View.Index {
    return String.UTF16View.Index(_offset.successor())
  }
  public func predecessor() -> String.UTF16View.Index {
    return String.UTF16View.Index(_offset.predecessor())
  }
}

public func == (
  lhs: String.UTF16View.Index, rhs: String.UTF16View.Index
) -> Bool {
  return lhs._offset == rhs._offset
}