//===----------------------------------------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//

/// The core implementation of a highly-optimizable String that
/// can store both ASCII and UTF-16, and can wrap native Swift
/// _StringBuffer or NSString instances.
///
/// Usage note: when elements are 8 bits wide, this code may
/// dereference one past the end of the byte array that it owns, so
/// make sure that storage is allocated!  You want a null terminator
/// anyway, so it shouldn't be a burden.
//
// Implementation note: We try hard to avoid branches in this code, so
// for example we use integer math to avoid switching on the element
// size with the ternary operator.  This is also the cause of the
// extra element requirement for 8 bit elements.  See the
// implementation of subscript(Int) -> UTF16.CodeUnit below for details.
public struct _StringCore {
  //===--------------------------------------------------------------------===//
  // Internals
  public var _baseAddress: COpaquePointer
  var _countAndFlags: UWord
  public var _owner: AnyObject?

  /// (private) create the implementation of a string from its component parts.
  init(
    baseAddress: COpaquePointer,
    _countAndFlags: UWord,
    owner: AnyObject?
  ) {
    self._baseAddress = baseAddress
    self._countAndFlags = _countAndFlags
    self._owner = owner
    _invariantCheck()
  }

  func _invariantCheck() {
    // Note: this code is intentionally #if'ed out.  It unconditionally
    // accesses lazily initialized globals, and thus it is a performance burden
    // in non-checked builds.
#if INTERNAL_CHECKS_ENABLED
    _sanityCheck(count >= 0)
    
    if _baseAddress == .null() {
#if _runtime(_ObjC)
      _sanityCheck(cocoaBuffer != nil,
        "Only opaque cocoa strings may have a null base pointer")
#endif
      _sanityCheck(elementWidth == 2,
        "Opaque cocoa strings should have an elementWidth of 2")
    }
    else if _baseAddress == _emptyStringBase {
      _sanityCheck(count == 0, "Empty string storage with non-zero length")
      _sanityCheck(_owner == nil, "String pointing at empty storage has owner")
    }
    else if let buffer = nativeBuffer {
      _sanityCheck(elementWidth == buffer.elementWidth,
        "_StringCore elementWidth doesn't match its buffer's")
      _sanityCheck(UnsafeMutablePointer(_baseAddress) >= buffer.start)
      _sanityCheck(UnsafeMutablePointer(_baseAddress) <= buffer.usedEnd)
      _sanityCheck(UnsafeMutablePointer(_pointerToNth(count)) <= buffer.usedEnd)
    }
#endif
  }

  /// Bitmask for the count part of _countAndFlags
  var _countMask: UWord {
    return UWord.max >> 2
  }
  
  /// Bitmask for the flags part of _countAndFlags
  var _flagMask: UWord {
    return ~_countMask
  }

  /// Value by which to multiply a 2nd byte fetched in order to
  /// assemble a UTF-16 code unit from our contiguous storage.  If we
  /// store ASCII, this will be zero.  Otherwise, it will be 0x100
  var _highByteMultiplier: UTF16.CodeUnit {
    return UTF16.CodeUnit(elementShift) << 8
  }

  /// Return a pointer to the Nth element of contiguous
  /// storage.  Caveats: The string must have contiguous storage; the
  /// element may be 1 or 2 bytes wide, depending on elementWidth; the
  /// result may be null if the string is empty.
  func _pointerToNth(n: Int) -> COpaquePointer {
    _sanityCheck(hasContiguousStorage && n >= 0 && n <= count)
    return COpaquePointer(
      UnsafeMutablePointer<RawByte>(_baseAddress) + (n << elementShift))
  }

  static func _copyElements(
    srcStart: COpaquePointer, srcElementWidth: Int,
    dstStart: COpaquePointer, dstElementWidth: Int,
    count: Int
  ) {
    // Copy the old stuff into the new storage
    if _fastPath(srcElementWidth == dstElementWidth) {
      // No change in storage width; we can use memcpy
      _memcpy(
        dest: UnsafeMutablePointer(dstStart),
        src: UnsafeMutablePointer(srcStart),
        size: UInt(count << (srcElementWidth - 1)))
    }
    else if (srcElementWidth < dstElementWidth) {
      // Widening ASCII to UTF-16; we need to copy the bytes manually
      var dest = UnsafeMutablePointer<UTF16.CodeUnit>(dstStart)
      var src = UnsafeMutablePointer<UTF8.CodeUnit>(srcStart)
      let srcEnd = src + count
      while (src != srcEnd) {
        dest++.memory = UTF16.CodeUnit(src++.memory)
      }
    }
    else {
      // Narrowing UTF-16 to ASCII; we need to copy the bytes manually
      var dest = UnsafeMutablePointer<UTF8.CodeUnit>(dstStart)
      var src = UnsafeMutablePointer<UTF16.CodeUnit>(srcStart)
      let srcEnd = src + count
      while (src != srcEnd) {
        dest++.memory = UTF8.CodeUnit(src++.memory)
      }
    }
  }
  
  //===--------------------------------------------------------------------===//
  // Initialization
  public init(
    baseAddress: COpaquePointer,
    count: Int,
    elementShift: Int,
    hasCocoaBuffer: Bool,
    owner: AnyObject?
  ) {
    _sanityCheck(elementShift == 0 || elementShift == 1)
    self._baseAddress = baseAddress
   
    self._countAndFlags
      = (UWord(elementShift) << (UWord._sizeInBits - 1))
      | ((hasCocoaBuffer ? 1 : 0) << (UWord._sizeInBits - 2))
      | UWord(count)
    
    self._owner = owner
    _sanityCheck(UWord(count) & _flagMask == 0, "String too long to represent")
    _invariantCheck()
  }

  /// Create a _StringCore that covers the entire length of the _StringBuffer.
  init(_ buffer: _StringBuffer) {
    self = _StringCore(
      baseAddress: COpaquePointer(buffer.start),
      count: buffer.usedCount,
      elementShift: buffer.elementShift,
      hasCocoaBuffer: false,
      owner: buffer._anyObject
    )
  }
  
  /// Create the implementation of an empty string.
  /// NOTE: there is no null terminator in an empty string!
  public init() {
    self._baseAddress = _emptyStringBase
    self._countAndFlags = 0
    self._owner = .None
    _invariantCheck()
  }

  //===--------------------------------------------------------------------===//
  // Properties
  
  /// The number of elements stored
  /// Complexity: O(1).
  public var count: Int {
    get {
      return Int(_countAndFlags & _countMask)
    }
    set(newValue) {
      _sanityCheck(UWord(newValue) & _flagMask == 0)
      _countAndFlags = (_countAndFlags & _flagMask) | UWord(newValue)
    }
  }

  /// left shift amount to apply to an offset N so that when
  /// added to a UnsafeMutablePointer<RawByte>, it traverses N elements
  var elementShift: Int {
    return Int(_countAndFlags >> (UWord._sizeInBits - 1))
  }
  
  /// the number of bytes per element
  /// If the string does not have an ASCII buffer available (including the case
  /// when we don't have a utf16 buffer) then it equals 2.
  public var elementWidth: Int {
    return elementShift + 1
  }

  public var hasContiguousStorage: Bool {
#if _runtime(_ObjC)
    return _fastPath(_baseAddress != .null())
#else
    return true
#endif
  }

  /// are we using an NSString for storage?
  public var hasCocoaBuffer: Bool {
    return Word((_countAndFlags << 1).value) < 0
  }

  public var startASCII: UnsafeMutablePointer<UTF8.CodeUnit> {
    _sanityCheck(elementWidth == 1, "String does not contain contiguous ASCII")
    return UnsafeMutablePointer(_baseAddress)
  }

  /// True iff a contiguous ASCII buffer available.
  public var isASCII: Bool {
    return elementWidth == 1
  }

  public var startUTF16: UnsafeMutablePointer<UTF16.CodeUnit> {
    _sanityCheck(
      count == 0 || elementWidth == 2,
      "String does not contain contiguous UTF-16")
    return UnsafeMutablePointer(_baseAddress)
  }

  /// the native _StringBuffer, if any, or .None.
  public var nativeBuffer: _StringBuffer? {
    if !hasCocoaBuffer {
      return _owner.map {
        unsafeBitCast($0, _StringBuffer.self)
      }
    }
    return nil
  }

#if _runtime(_ObjC)
  /// the Cocoa String buffer, if any, or .None.
  public var cocoaBuffer: _CocoaStringType? {
    if hasCocoaBuffer {
      return _owner.map {
        unsafeBitCast($0, _CocoaStringType.self)
      }
    }
    return nil
  }
#endif
  
  //===--------------------------------------------------------------------===//
  // slicing
  
  /// Return the given sub-_StringCore
  public subscript(subRange: Range<Int>) -> _StringCore {
    _precondition(
      subRange.startIndex >= 0,
      "subscript: subRange start precedes String start")

    _precondition(
      subRange.endIndex <= count,
      "subscript: subRange extends past String end")

    let newCount = subRange.endIndex - subRange.startIndex
    _sanityCheck(UWord(newCount) & _flagMask == 0)

    if hasContiguousStorage {
      return _StringCore(
        baseAddress: _pointerToNth(subRange.startIndex),
        _countAndFlags: (_countAndFlags & _flagMask) | UWord(newCount),
        owner: _owner)
    }
#if _runtime(_ObjC)
    return _cocoaStringSlice(target: self, subRange: subRange)
#else
    _sanityCheckFailure("subscript: non-native string without objc runtime")
#endif
  }

  /// Get the Nth UTF-16 Code Unit stored
  func _nthContiguous(position: Int) -> UTF16.CodeUnit {
    let p = UnsafeMutablePointer<UInt8>(_pointerToNth(position)._rawValue)
    // Always dereference two bytes, but when elements are 8 bits we
    // multiply the high byte by 0.
    // FIXME(performance): use masking instead of multiplication.
    return UTF16.CodeUnit(p.memory)
      + UTF16.CodeUnit((p + 1).memory) * _highByteMultiplier
  }

  /// Get the Nth UTF-16 Code Unit stored
  public subscript(position: Int) -> UTF16.CodeUnit {
    _precondition(
      position >= 0,
      "subscript: index precedes String start")

    _precondition(
      position <= count,
      "subscript: index points past String end")

    if _fastPath(_baseAddress != .null()) {
      return _nthContiguous(position)
    }
#if _runtime(_ObjC)
    return _cocoaStringSubscript(target: self, position: position)
#else
    _sanityCheckFailure("subscript: non-native string without objc runtime")
#endif
  }

  /// Write the string, in the given encoding, to output.
  func encode<
    Encoding: UnicodeCodecType,
    Output: SinkType
    where Encoding.CodeUnit == Output.Element
  >(encoding: Encoding.Type, inout output: Output)
  {
    if _fastPath(_baseAddress != .null()) {
      if _fastPath(elementWidth == 1) {
        var out = output
        for x in UnsafeBufferPointer(
          start: UnsafeMutablePointer<UTF8.CodeUnit>(_baseAddress),
          count: count
        ) {
          Encoding.encode(UnicodeScalar(UInt32(x)), output: &out)
        }
      }
      else {
        let hadError = transcode(UTF16.self, encoding,
          UnsafeBufferPointer(
            start: UnsafeMutablePointer<UTF16.CodeUnit>(_baseAddress),
            count: count
          ).generate(),
          &output,
          stopOnError: true
        )
        _sanityCheck(!hadError, "Swift.String with native storage should not have unpaired surrogates")
      }
    }
    else if (hasCocoaBuffer) {
#if _runtime(_ObjC)
      _StringCore(
        _cocoaStringToContiguous(source: cocoaBuffer!, range: 0..<count,
                                 minimumCapacity: 0)
      ).encode(encoding, output: &output)
#else
      _sanityCheckFailure("encode: non-native string without objc runtime")
#endif
    }
  }

  /// Attempt to claim unused capacity in the String's existing
  /// native buffer, if any.  Return zero and a pointer to the claimed
  /// storage if successful. Otherwise, returns a suggested new
  /// capacity and a null pointer.  
  ///
  /// Note: If successful, effectively appends garbage to the String
  /// until it has newSize UTF-16 code units; you must immediately copy
  /// valid UTF-16 into that storage.
  ///
  /// Note: if unsuccessful because of insufficient space in an
  /// existing buffer, the suggested new capacity will at least double
  /// the existing buffer's storage
  mutating func _claimCapacity(
    newSize: Int, minElementWidth: Int) -> (Int, COpaquePointer) {
    if _fastPath((nativeBuffer != nil) && elementWidth >= minElementWidth) {
      var buffer = nativeBuffer!

      // In order to grow the substring in place, this _StringCore should point
      // at the substring at the end of a _StringBuffer.  Otherwise, some other
      // String is using parts of the buffer beyond our last byte.
      let usedStart = _pointerToNth(0)
      let usedEnd = _pointerToNth(count)

      // Attempt to claim unused capacity in the buffer
      if _fastPath(
        buffer.grow(
          UnsafePointer(usedStart)..<UnsafePointer(usedEnd),
          newUsedCount: newSize)
      ) {
        count = newSize
        return (0, usedEnd)
      }
      else if newSize > buffer.capacity {
        // Growth failed because of insufficient storage; double the size
        return (max(_growArrayCapacity(buffer.capacity), newSize), .null())
      }
    }
    return (newSize, .null())
  }

  /// Ensure that this String references a _StringBuffer having
  /// a capacity of at least newSize elements of at least the given width.
  /// Effectively appends garbage to the String until it has newSize
  /// UTF-16 code units.  Returns a pointer to the garbage code units;
  /// you must immediately copy valid data into that storage.
  mutating func _growBuffer(
    newSize: Int, minElementWidth: Int
  ) -> COpaquePointer {
    let (newCapacity, existingStorage)
      = _claimCapacity(newSize, minElementWidth: minElementWidth)

    if _fastPath(existingStorage != nil) {
      return existingStorage
    }
    
    let oldCount = count
    
    _copyInPlace(
      newSize: newSize,
      newCapacity: newCapacity,
      minElementWidth: minElementWidth)
    
    return _pointerToNth(oldCount)
  }

  /// Replace the storage of self with a native _StringBuffer having a
  /// capacity of at least newCapacity elements of at least the given
  /// width.  Effectively appends garbage to the String until it has
  /// newSize UTF-16 code units.
  mutating func _copyInPlace(
    #newSize: Int, newCapacity: Int, minElementWidth: Int
  ) {
    _sanityCheck(newCapacity >= newSize)
    var oldCount = count
    
    // Allocate storage.
    let newElementWidth =
      minElementWidth >= elementWidth
      ? minElementWidth
      : representableAsASCII() ? 1 : 2

    var newStorage = _StringBuffer(capacity: newCapacity, initialSize: newSize,
                                   elementWidth: newElementWidth)

    if hasContiguousStorage {
      _StringCore._copyElements(
        _baseAddress, srcElementWidth: elementWidth,
        dstStart: COpaquePointer(newStorage.start), 
        dstElementWidth: newElementWidth, count: oldCount)
    }
    else {
#if _runtime(_ObjC)
      // Opaque cocoa buffers might not store ASCII, so assert that
      // we've allocated for 2-byte elements.
      // FIXME: can we get Cocoa to tell us quickly that an opaque
      // string is ASCII?  Do we care much about that edge case?
      _sanityCheck(newStorage.elementShift == 1)
      _cocoaStringReadAll(source: cocoaBuffer!, 
                          destination: UnsafeMutablePointer(newStorage.start))
#else
      _sanityCheckFailure("_copyInPlace: non-native string without objc runtime")
#endif
    }
    
    self = _StringCore(newStorage)
  }

  /// Append `c` to `self`.
  ///
  /// Complexity: O(1) when amortized over repeated appends of equal
  /// character values
  mutating func append(c: UnicodeScalar) {
    let width = UTF16.width(c)
    append(
      width == 2 ? UTF16.leadSurrogate(c) : UTF16.CodeUnit(c.value),
      width == 2 ? UTF16.trailSurrogate(c) : nil
    )
  }

  /// Append `u` to `self`.
  ///
  /// Complexity: amortized O(1).
  public mutating func append(u: UTF16.CodeUnit) {
    append(u, nil)
  }
  
  mutating func append(u0: UTF16.CodeUnit, _ u1: UTF16.CodeUnit?) {
    _invariantCheck()
    let minBytesPerCodeUnit = u0 <= 0x7f ? 1 : 2
    let utf16Width = u1 == nil ? 1 : 2
    
    let destination = _growBuffer(
      count + utf16Width, minElementWidth: minBytesPerCodeUnit)

    if _fastPath(elementWidth == 1) {
      _sanityCheck(
        _pointerToNth(count) 
        == COpaquePointer(UnsafeMutablePointer<RawByte>(destination) + 1))

      UnsafeMutablePointer<UTF8.CodeUnit>(destination)[0] = UTF8.CodeUnit(u0)
    }
    else {
      let destination16
        = UnsafeMutablePointer<UTF16.CodeUnit>(destination._rawValue)

      destination16[0] = u0
      if u1 != nil {
        destination16[1] = u1!
      }
    }
    _invariantCheck()
  }

  mutating func append(rhs: _StringCore) {
    _invariantCheck()
    let minElementWidth
    = elementWidth >= rhs.elementWidth
      ? elementWidth
      : rhs.representableAsASCII() ? 1 : 2

    let destination = _growBuffer(
      count + rhs.count, minElementWidth: minElementWidth)

    if _fastPath(rhs.hasContiguousStorage) {
      _StringCore._copyElements(
        rhs._baseAddress, srcElementWidth: rhs.elementWidth, 
        dstStart: destination, dstElementWidth:elementWidth, count: rhs.count)
    }
    else {
#if _runtime(_ObjC)
      _sanityCheck(elementWidth == 2)
      _cocoaStringReadAll(source: rhs.cocoaBuffer!, 
                          destination: UnsafeMutablePointer(destination))
#else
      _sanityCheckFailure("subscript: non-native string without objc runtime")
#endif
    }
    _invariantCheck()
  }

  /// Return true iff the contents of this string can be
  /// represented as pure ASCII. O(N) in the worst case
  func representableAsASCII() -> Bool {
    if _slowPath(!hasContiguousStorage) {
      return false
    }
    if _fastPath(elementWidth == 1) {
      return true
    }
    return !contains(
      UnsafeBufferPointer(
        start: UnsafeMutablePointer<UTF16.CodeUnit>(_baseAddress), count: count)
    ) { $0 > 0x7f }
  }
}

extension _StringCore : CollectionType {
  public
  var startIndex: Int {
    return 0
  }

  public
  var endIndex: Int {
    return count
  }

  public
  func generate() -> IndexingGenerator<_StringCore> {
    return IndexingGenerator(self)
  }
}

extension _StringCore : Sliceable {}

extension _StringCore : ExtensibleCollectionType {
  
  public mutating func reserveCapacity(n: Int) {
    if _fastPath(!hasCocoaBuffer) {
      if _fastPath(isUniquelyReferencedNonObjC(&_owner)) {
        
        let subRange: Range<UnsafePointer<RawByte>>
          = UnsafePointer(_pointerToNth(0))..<UnsafePointer(_pointerToNth(count))

        if _fastPath(nativeBuffer!.hasCapacity(n, forSubRange: subRange)) {
          return
        }
      }
    }
    _copyInPlace(newSize: count, newCapacity: max(count, n), minElementWidth: 1)
  }

  public mutating func extend<
    S : SequenceType
      where S.Generator.Element == UTF16.CodeUnit
  >(s: S) {
    var width = elementWidth
    if width == 1 {
      if let hasNonAscii = s~>_preprocessingPass({
          s in contains(s) { $0 > 0x7f }
        }) {
        width = hasNonAscii ? 2 : 1
      }
    }
    
    let growth = s~>_underestimateCount()
    var g = s.generate()

    if _fastPath(growth > 0) {
      let newSize = count + growth
      let destination = _growBuffer(newSize, minElementWidth: width)
      if elementWidth == 1 {
        let destination8 = UnsafeMutablePointer<UTF8.CodeUnit>(destination)
        for i in 0..<growth {
          destination8[i] = UTF8.CodeUnit(g.next()!)
        }
      }
      else {
        let destination16 = UnsafeMutablePointer<UTF16.CodeUnit>(destination)
        for i in 0..<growth {
          destination16[i] = g.next()!
        }
      }
    }
    // Append any remaining elements
    for u in GeneratorSequence(g) {
      self.append(u)
    }
  }
}

// Used to support a tighter invariant: all strings with contiguous
// storage have a non-NULL base address.
var _emptyStringStorage: UInt32 = 0

var _emptyStringBase: COpaquePointer {
  return COpaquePointer(
    UnsafeMutablePointer<UInt16>(Builtin.addressof(&_emptyStringStorage)))
}

extension _StringCore : RangeReplaceableCollectionType {
  /// Replace the given `subRange` of elements with `newElements`.
  ///
  /// Complexity: O(\ `count(subRange)`\ ) if `subRange.endIndex
  /// == self.endIndex` and `isEmpty(newElements)`\ , O(N) otherwise.
  public mutating func replaceRange<
    C: CollectionType where C.Generator.Element == UTF16.CodeUnit
  >(
    subRange: Range<Int>, with newElements: C
  ) {
    _precondition(
      subRange.startIndex >= 0,
      "replaceRange: subRange start precedes String start")
    
    _precondition(
      subRange.endIndex <= count,
      "replaceRange: subRange extends past String end")
    
    let width = elementWidth == 2 || contains(newElements) { $0 > 0x7f } ? 2 : 1
    let replacementCount = numericCast(Swift.count(newElements)) as Int
    let replacedCount = Swift.count(subRange)
    let tailCount = count - subRange.endIndex
    let growth = replacementCount - replacedCount
    let newCount = count + growth

    // Successfully claiming capacity only ensures that we can modify
    // the newly-claimed storage without observably mutating other
    // strings, i.e., when we're appending.  Already-used characters
    // can only be mutated when we have a unique reference to the
    // buffer.
    let appending = subRange.startIndex == endIndex

    let existingStorage = !hasCocoaBuffer && (
      appending || isUniquelyReferencedNonObjC(&_owner)
    ) ? _claimCapacity(newCount, minElementWidth: width).1 : nil

    if _fastPath(existingStorage != nil) {
      let rangeStart = UnsafeMutablePointer<UInt8>(
        _pointerToNth(subRange.startIndex))
      let tailStart = rangeStart + (replacedCount << elementShift)
      
      if growth > 0 {
        (tailStart + (growth << elementShift)).assignBackwardFrom(
          tailStart, count: tailCount << elementShift)
      }
      
      if _fastPath(elementWidth == 1) {
        var dst = rangeStart
        for u in newElements {
          dst++.memory = UInt8(u & 0xFF)
        }
      }
      else {
        var dst = UnsafeMutablePointer<UTF16.CodeUnit>(rangeStart)
        for u in newElements {
          dst++.memory = u
        }
      }
      
      if growth < 0 {
        (tailStart + (growth << elementShift)).assignFrom(
          tailStart, count: tailCount << elementShift)
      }
    }
    else {
      var r = _StringCore(
        _StringBuffer(
          capacity: newCount,
          initialSize: 0,
          elementWidth:
            width == 1 ? 1
            : representableAsASCII() && !contains(newElements) { $0 > 0x7f } ? 1
            : 2
        ))
      r.extend(self[0..<subRange.startIndex])
      r.extend(newElements)
      r.extend(self[subRange.endIndex..<count])
      self = r
    }
  }

  public mutating func insert(newElement: UTF16.CodeUnit, atIndex i: Int) {
    Swift.insert(&self, newElement, atIndex: i)
  }
  
  public mutating func splice<
    S : CollectionType where S.Generator.Element == UTF16.CodeUnit
  >(newElements: S, atIndex i: Int) {
    Swift.splice(&self, newElements, atIndex: i)
  }

  public mutating func removeAtIndex(i: Int) -> UTF16.CodeUnit {
    return Swift.removeAtIndex(&self, i)
  }
  
  public mutating func removeRange(subRange: Range<Int>) {
    Swift.removeRange(&self, subRange)
  }

  public mutating func removeAll(keepCapacity: Bool = false) {
    Swift.removeAll(&self, keepCapacity: keepCapacity)
  }
}