String.swift

// RUN: %empty-directory(%t)
// RUN: if [ %target-runtime == "objc" ]; \
// RUN: then \
// RUN:   %target-clang -fobjc-arc %S/Inputs/NSSlowString/NSSlowString.m -c -o %t/NSSlowString.o && \
// RUN:   %target-build-swift -I %S/Inputs/NSSlowString/ %t/NSSlowString.o %s -Xfrontend -disable-access-control -o %t/String; \
// RUN: else \
// RUN:   %target-build-swift %s -Xfrontend -disable-access-control -o %t/String; \
// RUN: fi

// RUN: %target-codesign %t/String
// RUN: %target-run %t/String
// REQUIRES: executable_test
// XFAIL: interpret

import StdlibUnittest
import StdlibCollectionUnittest

#if _runtime(_ObjC)
import NSSlowString
import Foundation  // For NSRange
#endif

extension Collection {
  internal func index(_nth n: Int) -> Index {
    precondition(n >= 0)
    return index(startIndex, offsetBy: numericCast(n))
  }
  internal func index(_nthLast n: Int) -> Index {
    precondition(n >= 0)
    return index(endIndex, offsetBy: -numericCast(n))
  }
}

extension String {
  var nativeCapacity: Int {
    switch self._classify()._form {
      case ._native: break
      default: preconditionFailure()
    }
    return self._classify()._capacity
  }
  var capacity: Int {
    return self._classify()._capacity
  }
  var unusedCapacity: Int {
    return Swift.max(0, self._classify()._capacity - self._classify()._count)
  }
  var bufferID: ObjectIdentifier? {
    return _rawIdentifier()
  }
  func _rawIdentifier() -> ObjectIdentifier? {
    return self._classify()._objectIdentifier
  }

  var byteWidth: Int {
    return _classify()._isASCII ? 1 : 2
  }
}

extension Substring {
  var bufferID: ObjectIdentifier? {
    return _wholeString.bufferID
  }
}

// A thin wrapper around _StringGuts implementing RangeReplaceableCollection
struct StringGutsCollection: RangeReplaceableCollection, RandomAccessCollection {
  typealias Element = UTF16.CodeUnit
  typealias Index = Int
  typealias Indices = CountableRange<Int>

  init(_ guts: _StringGuts) {
    self._guts = guts
  }

  init() {
    self.init(_StringGuts())
  }

  var _guts: _StringGuts

  var startIndex: Index { return 0 }
  var endIndex: Index { return _guts.count }
  var indices: Indices { return startIndex..<endIndex }

  subscript(position: Index) -> Element {
    return _guts.codeUnit(atCheckedOffset: position)
  }

  mutating func replaceSubrange<C>(
    _ subrange: Range<Index>,
    with newElements: C
  ) where C : Collection, C.Element == Element {
    _guts.replaceSubrange(subrange, with: newElements)
  }

  mutating func reserveCapacity(_ n: Int) {
    _guts.reserveCapacity(n)
  }
}

var StringTests = TestSuite("StringTests")

StringTests.test("sizeof") {
#if arch(i386) || arch(arm)
  expectEqual(12, MemoryLayout<String>.size)
#else
  expectEqual(16, MemoryLayout<String>.size)
#endif
}

StringTests.test("AssociatedTypes-UTF8View") {
  typealias View = String.UTF8View
  expectCollectionAssociatedTypes(
    collectionType: View.self,
    iteratorType: View.Iterator.self,
    subSequenceType: Substring.UTF8View.self,
    indexType: View.Index.self,
    indicesType: DefaultIndices<View>.self)
}

StringTests.test("AssociatedTypes-UTF16View") {
  typealias View = String.UTF16View
  expectCollectionAssociatedTypes(
    collectionType: View.self,
    iteratorType: IndexingIterator<View>.self,
    subSequenceType: Substring.UTF16View.self,
    indexType: View.Index.self,
    indicesType: View.Indices.self)
}

StringTests.test("AssociatedTypes-UnicodeScalarView") {
  typealias View = String.UnicodeScalarView
  expectCollectionAssociatedTypes(
    collectionType: View.self,
    iteratorType: View.Iterator.self,
    subSequenceType: Substring.UnicodeScalarView.self,
    indexType: View.Index.self,
    indicesType: DefaultIndices<View>.self)
}

StringTests.test("AssociatedTypes-CharacterView") {
  expectCollectionAssociatedTypes(
    collectionType: String.self,
    iteratorType: IndexingIterator<String>.self,
    subSequenceType: Substring.self,
    indexType: String.Index.self,
    indicesType: DefaultIndices<String>.self)
}

func checkUnicodeScalarViewIteration(
    _ expectedScalars: [UInt32], _ str: String
) {
  do {
    let us = str.unicodeScalars
    var i = us.startIndex
    let end = us.endIndex
    var decoded: [UInt32] = []
    while i != end {
      expectTrue(i < us.index(after: i)) // Check for Comparable conformance
      decoded.append(us[i].value)
      i = us.index(after: i)
    }
    expectEqual(expectedScalars, decoded)
  }
  do {
    let us = str.unicodeScalars
    let start = us.startIndex
    var i = us.endIndex
    var decoded: [UInt32] = []
    while i != start {
      i = us.index(before: i)
      decoded.append(us[i].value)
    }
    expectEqual(expectedScalars, decoded)
  }
}

StringTests.test("unicodeScalars") {
  checkUnicodeScalarViewIteration([], "")
  checkUnicodeScalarViewIteration([ 0x0000 ], "\u{0000}")
  checkUnicodeScalarViewIteration([ 0x0041 ], "A")
  checkUnicodeScalarViewIteration([ 0x007f ], "\u{007f}")
  checkUnicodeScalarViewIteration([ 0x0080 ], "\u{0080}")
  checkUnicodeScalarViewIteration([ 0x07ff ], "\u{07ff}")
  checkUnicodeScalarViewIteration([ 0x0800 ], "\u{0800}")
  checkUnicodeScalarViewIteration([ 0xd7ff ], "\u{d7ff}")
  checkUnicodeScalarViewIteration([ 0x8000 ], "\u{8000}")
  checkUnicodeScalarViewIteration([ 0xe000 ], "\u{e000}")
  checkUnicodeScalarViewIteration([ 0xfffd ], "\u{fffd}")
  checkUnicodeScalarViewIteration([ 0xffff ], "\u{ffff}")
  checkUnicodeScalarViewIteration([ 0x10000 ], "\u{00010000}")
  checkUnicodeScalarViewIteration([ 0x10ffff ], "\u{0010ffff}")
}

StringTests.test("Index/Comparable") {
  let empty = ""
  expectTrue(empty.startIndex == empty.endIndex)
  expectFalse(empty.startIndex != empty.endIndex)
  expectTrue(empty.startIndex <= empty.endIndex)
  expectTrue(empty.startIndex >= empty.endIndex)
  expectFalse(empty.startIndex > empty.endIndex)
  expectFalse(empty.startIndex < empty.endIndex)

  let nonEmpty = "borkus biqualificated"
  expectFalse(nonEmpty.startIndex == nonEmpty.endIndex)
  expectTrue(nonEmpty.startIndex != nonEmpty.endIndex)
  expectTrue(nonEmpty.startIndex <= nonEmpty.endIndex)
  expectFalse(nonEmpty.startIndex >= nonEmpty.endIndex)
  expectFalse(nonEmpty.startIndex > nonEmpty.endIndex)
  expectTrue(nonEmpty.startIndex < nonEmpty.endIndex)
}

StringTests.test("Index/Hashable") {
  let s = "abcdef"
  let t = Set(s.indices)
  expectEqual(s.count, t.count)
  expectTrue(t.contains(s.startIndex))
}

StringTests.test("ForeignIndexes/Valid") {
  // It is actually unclear what the correct behavior is.  This test is just a
  // change detector.
  //
  // <rdar://problem/18037897> Design, document, implement invalidation model
  // for foreign String indexes
  do {
    let donor = "abcdef"
    let acceptor = "uvwxyz"
    expectEqual("u", acceptor[donor.startIndex])
    expectEqual("wxy",
      acceptor[donor.index(_nth: 2)..<donor.index(_nth: 5)])
  }
  do {
    let donor = "abcdef"
    let acceptor = "\u{1f601}\u{1f602}\u{1f603}"
    expectEqual("\u{1f601}", acceptor[donor.startIndex])
    expectEqual("\u{fffd}", acceptor[donor.index(after: donor.startIndex)])
  }
}

StringTests.test("ForeignIndexes/UnexpectedCrash")
  .xfail(
    .always("<rdar://problem/18029290> String.Index caches the grapheme " +
      "cluster size, but it is not always correct to use"))
  .code {

  let donor = "\u{1f601}\u{1f602}\u{1f603}"
  let acceptor = "abcdef"

  // Adjust donor.startIndex to ensure it caches a stride
  let start = donor.index(before: donor.index(after: donor.startIndex))
  
  // FIXME: this traps right now when trying to construct Character("ab").
  expectEqual("a", acceptor[start])
}

StringTests.test("ForeignIndexes/subscript(Index)/OutOfBoundsTrap") {
  let donor = "abcdef"
  let acceptor = "uvw"

  expectEqual("u", acceptor[donor.index(_nth: 0)])
  expectEqual("v", acceptor[donor.index(_nth: 1)])
  expectEqual("w", acceptor[donor.index(_nth: 2)])

  let i = donor.index(_nth: 3)
  expectCrashLater()
  _ = acceptor[i]
}

StringTests.test("String/subscript(_:Range)") {
  let s = "foobar"
  let from = s.startIndex
  let to = s.index(before: s.endIndex)
  let actual = s[from..<to]
  expectEqual("fooba", actual)
}

StringTests.test("String/subscript(_:ClosedRange)") {
  let s = "foobar"
  let from = s.startIndex
  let to = s.index(before: s.endIndex)
  let actual = s[from...to]
  expectEqual(s, actual)
}

StringTests.test("ForeignIndexes/subscript(Range)/OutOfBoundsTrap/1") {
  let donor = "abcdef"
  let acceptor = "uvw"

  expectEqual("uvw", acceptor[donor.startIndex..<donor.index(_nth: 3)])

  let r = donor.startIndex..<donor.index(_nth: 4)
  expectCrashLater()
  _ = acceptor[r]
}

StringTests.test("ForeignIndexes/subscript(Range)/OutOfBoundsTrap/2") {
  let donor = "abcdef"
  let acceptor = "uvw"

  expectEqual("uvw", acceptor[donor.startIndex..<donor.index(_nth: 3)])

  let r = donor.index(_nth: 4)..<donor.index(_nth: 5)
  expectCrashLater()
  _ = acceptor[r]
}

StringTests.test("ForeignIndexes/subscript(ClosedRange)/OutOfBoundsTrap/1") {
  let donor = "abcdef"
  let acceptor = "uvw"

  expectEqual("uvw", acceptor[donor.startIndex...donor.index(_nth: 2)])

  let r = donor.startIndex...donor.index(_nth: 3)
  expectCrashLater()
  _ = acceptor[r]
}

StringTests.test("ForeignIndexes/subscript(ClosedRange)/OutOfBoundsTrap/2") {
  let donor = "abcdef"
  let acceptor = "uvw"

  expectEqual("uvw", acceptor[donor.startIndex...donor.index(_nth: 2)])

  let r = donor.index(_nth: 3)...donor.index(_nth: 5)
  expectCrashLater()
  _ = acceptor[r]
}

StringTests.test("ForeignIndexes/replaceSubrange/OutOfBoundsTrap/1") {
  let donor = "abcdef"
  var acceptor = "uvw"

  acceptor.replaceSubrange(
    donor.startIndex..<donor.index(_nth: 1), with: "u")
  expectEqual("uvw", acceptor)

  let r = donor.startIndex..<donor.index(_nth: 4)
  expectCrashLater()
  acceptor.replaceSubrange(r, with: "")
}

StringTests.test("ForeignIndexes/replaceSubrange/OutOfBoundsTrap/2") {
  let donor = "abcdef"
  var acceptor = "uvw"

  acceptor.replaceSubrange(
    donor.startIndex..<donor.index(_nth: 1), with: "u")
  expectEqual("uvw", acceptor)

  let r = donor.index(_nth: 4)..<donor.index(_nth: 5)
  expectCrashLater()
  acceptor.replaceSubrange(r, with: "")
}

StringTests.test("ForeignIndexes/removeAt/OutOfBoundsTrap") {
  do {
    let donor = "abcdef"
    var acceptor = "uvw"

    let removed = acceptor.remove(at: donor.startIndex)
    expectEqual("u", removed)
    expectEqual("vw", acceptor)
  }

  let donor = "abcdef"
  var acceptor = "uvw"

  let i = donor.index(_nth: 4)
  expectCrashLater()
  acceptor.remove(at: i)
}

StringTests.test("ForeignIndexes/removeSubrange/OutOfBoundsTrap/1") {
  do {
    let donor = "abcdef"
    var acceptor = "uvw"

    acceptor.removeSubrange(
      donor.startIndex..<donor.index(after: donor.startIndex))
    expectEqual("vw", acceptor)
  }

  let donor = "abcdef"
  var acceptor = "uvw"

  let r = donor.startIndex..<donor.index(_nth: 4)
  expectCrashLater()
  acceptor.removeSubrange(r)
}

StringTests.test("ForeignIndexes/removeSubrange/OutOfBoundsTrap/2") {
  let donor = "abcdef"
  var acceptor = "uvw"

  let r = donor.index(_nth: 4)..<donor.index(_nth: 5)
  expectCrashLater()
  acceptor.removeSubrange(r)
}

StringTests.test("hasPrefix")
  .skip(.nativeRuntime("String.hasPrefix undefined without _runtime(_ObjC)"))
  .code {
#if _runtime(_ObjC)
  expectTrue("".hasPrefix(""))
  expectFalse("".hasPrefix("a"))
  expectTrue("a".hasPrefix(""))
  expectTrue("a".hasPrefix("a"))

  // U+0301 COMBINING ACUTE ACCENT
  // U+00E1 LATIN SMALL LETTER A WITH ACUTE
  expectFalse("abc".hasPrefix("a\u{0301}"))
  expectFalse("a\u{0301}bc".hasPrefix("a"))
  expectTrue("\u{00e1}bc".hasPrefix("a\u{0301}"))
  expectTrue("a\u{0301}bc".hasPrefix("\u{00e1}"))
#else
  expectUnreachable()
#endif
}

StringTests.test("literalConcatenation") {
  do {
    // UnicodeScalarLiteral + UnicodeScalarLiteral
    var s = "1" + "2"
    expectType(String.self, &s)
    expectEqual("12", s)
  }
  do {
    // UnicodeScalarLiteral + ExtendedGraphemeClusterLiteral
    var s = "1" + "a\u{0301}"
    expectType(String.self, &s)
    expectEqual("1a\u{0301}", s)
  }
  do {
    // UnicodeScalarLiteral + StringLiteral
    var s = "1" + "xyz"
    expectType(String.self, &s)
    expectEqual("1xyz", s)
  }

  do {
    // ExtendedGraphemeClusterLiteral + UnicodeScalar
    var s = "a\u{0301}" + "z"
    expectType(String.self, &s)
    expectEqual("a\u{0301}z", s)
  }
  do {
    // ExtendedGraphemeClusterLiteral + ExtendedGraphemeClusterLiteral
    var s = "a\u{0301}" + "e\u{0302}"
    expectType(String.self, &s)
    expectEqual("a\u{0301}e\u{0302}", s)
  }
  do {
    // ExtendedGraphemeClusterLiteral + StringLiteral
    var s = "a\u{0301}" + "xyz"
    expectType(String.self, &s)
    expectEqual("a\u{0301}xyz", s)
  }

  do {
    // StringLiteral + UnicodeScalar
    var s = "xyz" + "1"
    expectType(String.self, &s)
    expectEqual("xyz1", s)
  }
  do {
    // StringLiteral + ExtendedGraphemeClusterLiteral
    var s = "xyz" + "a\u{0301}"
    expectType(String.self, &s)
    expectEqual("xyza\u{0301}", s)
  }
  do {
    // StringLiteral + StringLiteral
    var s = "xyz" + "abc"
    expectType(String.self, &s)
    expectEqual("xyzabc", s)
  }
}

StringTests.test("substringDoesNotCopy/Swift3")
  .xfail(.always("Swift 3 compatibility: Self-sliced Strings are copied"))
  .code {

  let size = 16
  for sliceStart in [0, 2, 8, size] {
    for sliceEnd in [0, 2, 8, sliceStart + 1] {
      if sliceStart > size || sliceEnd > size || sliceEnd < sliceStart {
        continue
      }
      var s0 = String(repeating: "x", count: size)
      let originalIdentity = s0.bufferID
      s0 = String(s0[s0.index(_nth: sliceStart)..<s0.index(_nth: sliceEnd)])
      expectEqual(originalIdentity, s0.bufferID)
    }
  }
}

StringTests.test("substringDoesNotCopy/Swift4") {

  let size = 16
  for sliceStart in [0, 2, 8, size] {
    for sliceEnd in [0, 2, 8, sliceStart + 1] {
      if sliceStart > size || sliceEnd > size || sliceEnd < sliceStart {
        continue
      }
      let s0 = String(repeating: "x", count: size)
      let originalIdentity = s0.bufferID
      let s1 = s0[s0.index(_nth: sliceStart)..<s0.index(_nth: sliceEnd)]
      expectEqual(s1.bufferID, originalIdentity)
    }
  }
}

StringTests.test("appendToEmptyString") {
  let x = "Bumfuzzle"
  expectNil(x.bufferID)

  // Appending to empty string literal should replace it.
  var a1 = ""
  a1 += x
  expectNil(a1.bufferID)

  // Appending to native string should keep the existing buffer.
  var b1 = ""
  b1.reserveCapacity(20)
  let b1ID = b1.bufferID
  b1 += x
  expectEqual(b1.bufferID, b1ID)

  // .append(_:) should have the same behavior as +=
  var a2 = ""
  a2.append(x)
  expectNil(a2.bufferID)

  var b2 = ""
  b2.reserveCapacity(20)
  let b2ID = b2.bufferID
  b2.append(x)
  expectEqual(b2.bufferID, b2ID)
}

StringTests.test("Swift3Slice/Empty") {
  let size = 16
  let s = String(repeating: "x", count: size)
  expectNotNil(s.bufferID)
  for i in 0 ... size {
    let slice = s[s.index(_nth: i)..<s.index(_nth: i)]
    // Empty substrings still have indices relative to their base and can refer
    // to the whole string. If the whole string has storage, so should its
    // substring.
    expectNotNil(slice.bufferID)
  }
}

StringTests.test("Swift3Slice/Full") {
  let size = 16
  let s = String(repeating: "x", count: size)
  let slice = s[s.startIndex..<s.endIndex]
  // Most Swift 3 substrings are extracted into their own buffer,
  // but if the substring covers the full original string, it is used instead.
  expectEqual(slice.bufferID, s.bufferID)
}

StringTests.test("appendToSubstring") {
  for initialSize in 1..<16 {
    for sliceStart in [0, 2, 8, initialSize] {
      for sliceEnd in [0, 2, 8, sliceStart + 1] {
        if sliceStart > initialSize || sliceEnd > initialSize ||
          sliceEnd < sliceStart {
          continue
        }
        var s0 = String(repeating: "x", count: initialSize)
        s0 = String(s0[s0.index(_nth: sliceStart)..<s0.index(_nth: sliceEnd)])
        s0 += "x"
        expectEqual(
          String(
            repeating: "x",
            count: sliceEnd - sliceStart + 1),
          s0)
      }
    }
  }
}

StringTests.test("appendToSubstringBug")
  .xfail(.always("Swift 3 compatibility: Self-sliced Strings are copied"))
  .code {
  // String used to have a heap overflow bug when one attempted to append to a
  // substring that pointed to the end of a string buffer.
  //
  //                           Unused capacity
  //                           VVV
  // String buffer [abcdefghijk   ]
  //                      ^    ^
  //                      +----+
  // Substring -----------+
  //
  // In the example above, there are only three elements of unused capacity.
  // The bug was that the implementation mistakenly assumed 9 elements of
  // unused capacity (length of the prefix "abcdef" plus truly unused elements
  // at the end).

  func stringWithUnusedCapacity() -> (String, Int) {
    var s0 = String(repeating: "x", count: 17)
    if s0.unusedCapacity == 0 { s0 += "y" }
    let cap = s0.unusedCapacity
    expectNotEqual(0, cap)
    
    // This sorta checks for the original bug
    expectEqual(
      cap, String(s0[s0.index(_nth: 1)..<s0.endIndex]).unusedCapacity)
    
    return (s0, cap)
  }

  do {
    var (s, _) = { ()->(String, Int) in
      let (s0, unused) = stringWithUnusedCapacity()
      return (String(s0[s0.index(_nth: 5)..<s0.endIndex]), unused)
    }()
    let originalID = s.bufferID
    // Appending to a String always results in storage that 
    // starts at the beginning of its native buffer
    s += "z"
    expectNotEqual(originalID, s.bufferID)
  }

  do {
    var (s, _) = { ()->(Substring, Int) in
      let (s0, unused) = stringWithUnusedCapacity()
      return (s0[s0.index(_nth: 5)..<s0.endIndex], unused)
    }()
    let originalID = s.bufferID
    // FIXME: Ideally, appending to a Substring with a unique buffer reference
    // does not reallocate unless necessary.  Today, however, it appears to do
    // so unconditionally unless the slice falls at the beginning of its buffer.
    s += "z"
    expectNotEqual(originalID, s.bufferID)
  }

  // Try again at the beginning of the buffer
  do {
    var (s, unused) = { ()->(Substring, Int) in
      let (s0, unused) = stringWithUnusedCapacity()
      return (s0[...], unused)
    }()
    let originalID = s.bufferID
    s += "z"
    expectEqual(originalID, s.bufferID)
    s += String(repeating: "z", count: unused - 1)
    expectEqual(originalID, s.bufferID)
    s += "."
    expectNotEqual(originalID, s.bufferID)
    unused += 0 // warning suppression
  }
}

StringTests.test("COW/removeSubrange/start") {
  var str = "12345678"
  let literalIdentity = str.bufferID

  // Check literal-to-heap reallocation.
  do {
    let slice = str
    expectEqual(literalIdentity, str.bufferID)
    expectEqual(literalIdentity, slice.bufferID)
    expectEqual("12345678", str)
    expectEqual("12345678", slice)

    // This mutation should reallocate the string.
    str.removeSubrange(str.startIndex..<str.index(_nth: 1))
    expectNotEqual(literalIdentity, str.bufferID)
    expectEqual(literalIdentity, slice.bufferID)
    let heapStrIdentity = str.bufferID
    expectEqual("2345678", str)
    expectEqual("12345678", slice)

    // No more reallocations are expected.
    str.removeSubrange(str.startIndex..<str.index(_nth: 1))
    expectEqual(heapStrIdentity, str.bufferID)
    expectEqual(literalIdentity, slice.bufferID)
    expectEqual("345678", str)
    expectEqual("12345678", slice)
  }

  // Check heap-to-heap reallocation.
  expectEqual("345678", str)
  do {
    let heapStrIdentity1 = str.bufferID

    let slice = str
    expectEqual(heapStrIdentity1, str.bufferID)
    expectEqual(heapStrIdentity1, slice.bufferID)
    expectEqual("345678", str)
    expectEqual("345678", slice)

    // This mutation should reallocate the string.
    str.removeSubrange(str.startIndex..<str.index(_nth: 1))
    expectNotEqual(heapStrIdentity1, str.bufferID)
    expectEqual(heapStrIdentity1, slice.bufferID)
    let heapStrIdentity2 = str.bufferID
    expectEqual("45678", str)
    expectEqual("345678", slice)

    // No more reallocations are expected.
    str.removeSubrange(str.startIndex..<str.index(_nth: 1))
    expectEqual(heapStrIdentity2, str.bufferID)
    expectEqual(heapStrIdentity1, slice.bufferID)
    expectEqual("5678", str)
    expectEqual("345678", slice)
  }
}

StringTests.test("COW/removeSubrange/end") {
  var str = "12345678"
  let literalIdentity = str.bufferID

  // Check literal-to-heap reallocation.
  expectEqual("12345678", str)
  do {
    let slice = str
    expectEqual(literalIdentity, str.bufferID)
    expectEqual(literalIdentity, slice.bufferID)
    expectEqual("12345678", str)
    expectEqual("12345678", slice)

    // This mutation should reallocate the string.
    str.removeSubrange(str.index(_nthLast: 1)..<str.endIndex)
    expectNotEqual(literalIdentity, str.bufferID)
    expectEqual(literalIdentity, slice.bufferID)
    let heapStrIdentity = str.bufferID
    expectEqual("1234567", str)
    expectEqual("12345678", slice)

    // No more reallocations are expected.
    str.append("x")
    str.removeSubrange(str.index(_nthLast: 1)..<str.endIndex)
    expectEqual(heapStrIdentity, str.bufferID)
    expectEqual(literalIdentity, slice.bufferID)
    expectEqual("1234567", str)
    expectEqual("12345678", slice)

    str.removeSubrange(str.index(_nthLast: 1)..<str.endIndex)
    str.append("x")
    str.removeSubrange(str.index(_nthLast: 1)..<str.endIndex)
    expectEqual(heapStrIdentity, str.bufferID)
    expectEqual(literalIdentity, slice.bufferID)
    expectEqual("123456", str)
    expectEqual("12345678", slice)
  }

  // Check heap-to-heap reallocation.
  expectEqual("123456", str)
  do {
    let heapStrIdentity1 = str.bufferID

    let slice = str
    expectEqual(heapStrIdentity1, str.bufferID)
    expectEqual(heapStrIdentity1, slice.bufferID)
    expectEqual("123456", str)
    expectEqual("123456", slice)

    // This mutation should reallocate the string.
    str.removeSubrange(str.index(_nthLast: 1)..<str.endIndex)
    expectNotEqual(heapStrIdentity1, str.bufferID)
    expectEqual(heapStrIdentity1, slice.bufferID)
    let heapStrIdentity = str.bufferID
    expectEqual("12345", str)
    expectEqual("123456", slice)

    // No more reallocations are expected.
    str.append("x")
    str.removeSubrange(str.index(_nthLast: 1)..<str.endIndex)
    expectEqual(heapStrIdentity, str.bufferID)
    expectEqual(heapStrIdentity1, slice.bufferID)
    expectEqual("12345", str)
    expectEqual("123456", slice)

    str.removeSubrange(str.index(_nthLast: 1)..<str.endIndex)
    str.append("x")
    str.removeSubrange(str.index(_nthLast: 1)..<str.endIndex)
    expectEqual(heapStrIdentity, str.bufferID)
    expectEqual(heapStrIdentity1, slice.bufferID)
    expectEqual("1234", str)
    expectEqual("123456", slice)
  }
}

StringTests.test("COW/replaceSubrange/end") {
  // Check literal-to-heap reallocation.
  do {
    let str = "12345678"
    let literalIdentity = str.bufferID

    var slice = str[str.startIndex..<str.index(_nth: 7)]
    expectEqual(literalIdentity, str.bufferID)
    expectEqual(literalIdentity, slice.bufferID)
    expectEqual("12345678", str)
    expectEqual("1234567", slice)

    // This mutation should reallocate the string.
    slice.replaceSubrange(slice.endIndex..<slice.endIndex, with: "a")
    expectNotEqual(literalIdentity, slice.bufferID)
    expectEqual(literalIdentity, str.bufferID)
    let heapStrIdentity = slice.bufferID
    expectEqual("1234567a", slice)
    expectEqual("12345678", str)

    // No more reallocations are expected.
    slice.replaceSubrange(
      slice.index(_nthLast: 1)..<slice.endIndex, with: "b")
    expectEqual(heapStrIdentity, slice.bufferID)
    expectEqual(literalIdentity, str.bufferID)

    expectEqual("1234567b", slice)
    expectEqual("12345678", str)
  }

  // Check literal-to-heap reallocation.
  do {
    var str = "12345678"
    let literalIdentity = str.bufferID

    // Move the string to the heap.
    str.reserveCapacity(32)
    expectNotEqual(literalIdentity, str.bufferID)
    let heapStrIdentity1 = str.bufferID

    // FIXME: We have to use Swift 4's Substring to get the desired storage
    // semantics; in Swift 3 mode, self-sliced strings get allocated a new
    // buffer immediately.
    var slice = str[str.startIndex..<str.index(_nth: 7)]
    expectEqual(heapStrIdentity1, str.bufferID)
    expectEqual(heapStrIdentity1, slice.bufferID)

    // This mutation should reallocate the string.
    slice.replaceSubrange(slice.endIndex..<slice.endIndex, with: "a")
    expectNotEqual(heapStrIdentity1, slice.bufferID)
    expectEqual(heapStrIdentity1, str.bufferID)
    let heapStrIdentity2 = slice.bufferID
    expectEqual("1234567a", slice)
    expectEqual("12345678", str)

    // No more reallocations are expected.
    slice.replaceSubrange(
      slice.index(_nthLast: 1)..<slice.endIndex, with: "b")
    expectEqual(heapStrIdentity2, slice.bufferID)
    expectEqual(heapStrIdentity1, str.bufferID)

    expectEqual("1234567b", slice)
    expectEqual("12345678", str)
  }
}

func asciiString<
  S: Sequence
>(_ content: S) -> String
where S.Iterator.Element == Character {
  var s = String()
  s.append(contentsOf: content)
  expectTrue(s._classify()._isASCII)
  return s
}

StringTests.test("stringGutsExtensibility")
  .skip(.nativeRuntime("Foundation dependency"))
  .code {
#if _runtime(_ObjC)
  let ascii = UTF16.CodeUnit(UnicodeScalar("X").value)
  let nonAscii = UTF16.CodeUnit(UnicodeScalar("é").value)

  for k in 0..<3 {
    for count in 1..<16 {
      for boundary in 0..<count {
        
        var x = (
            k == 0 ? asciiString("b")
          : k == 1 ? ("b" as NSString as String)
          : ("b" as NSMutableString as String)
        )._guts

        if k == 0 { expectTrue(x.isSingleByte) }
        
        for i in 0..<count {
          x.append(contentsOf:
            repeatElement(i < boundary ? ascii : nonAscii, count: 3))
        }
        // Make sure we can append pure ASCII to wide storage
        x.append(contentsOf: repeatElement(ascii, count: 2))
        
        expectEqualSequence(
          [UTF16.CodeUnit(UnicodeScalar("b").value)]
          + Array(repeatElement(ascii, count: 3*boundary))
          + repeatElement(nonAscii, count: 3*(count - boundary))
          + repeatElement(ascii, count: 2),
          StringGutsCollection(x)
        )
      }
    }
  }
#else
  expectUnreachable()
#endif
}

StringTests.test("stringGutsReserve")
  .skip(.nativeRuntime("Foundation dependency"))
  .code {
#if _runtime(_ObjC)
  guard #available(macOS 10.13, iOS 11.0, tvOS 11.0, *) else { return }
  for k in 0...7 {
    var base: String
    var startedNative: Bool
    let shared: String = "X"

    // Managed native, unmanaged native, or small
    func isSwiftNative(_ s: String) -> Bool {
      switch s._classify()._form {
        case ._native: return true
        case ._small: return true
        case ._immortal: return true
        default: return false
      }
    }

    switch k {
    case 0: (base, startedNative) = (String(), true)
    case 1: (base, startedNative) = (asciiString("x"), true)
    case 2: (base, startedNative) = ("Ξ", true)
#if arch(i386) || arch(arm)
    case 3: (base, startedNative) = ("x" as NSString as String, false)
    case 4: (base, startedNative) = ("x" as NSMutableString as String, false)
#else
    case 3: (base, startedNative) = ("x" as NSString as String, true)
    case 4: (base, startedNative) = ("x" as NSMutableString as String, true)
#endif
    case 5: (base, startedNative) = (shared, true)
    case 6: (base, startedNative) = ("xá" as NSString as String, false)
    case 7: (base, startedNative) = ("xá" as NSMutableString as String, false)
    default:
      fatalError("case unhandled!")
    }
    expectEqual(isSwiftNative(base), startedNative)
    
    let originalBuffer = base.bufferID
    let isUnique = base._guts._isUniqueNative()
    let startedUnique =
      startedNative &&
      base._guts._objectIdentifier != nil &&
      isUnique
    
    base.reserveCapacity(16)
    // Now it's unique
    
    // If it was already native and unique, no reallocation
    if startedUnique && startedNative {
      expectEqual(originalBuffer, base.bufferID)
    }
    else {
      expectNotEqual(originalBuffer, base.bufferID)
    }

    // Reserving up to the capacity in a unique native buffer is a no-op
    let nativeBuffer = base.bufferID
    let currentCapacity = base.capacity
    base.reserveCapacity(currentCapacity)
    expectEqual(nativeBuffer, base.bufferID)

    // Reserving more capacity should reallocate
    base.reserveCapacity(currentCapacity + 1)
    expectNotEqual(nativeBuffer, base.bufferID)

    // None of this should change the string contents
    var expected: String
    switch k {
    case 0: expected = ""
    case 1,3,4: expected = "x"
    case 2: expected = "Ξ"
    case 5: expected = shared
    case 6,7: expected = "xá"
    default:
      fatalError("case unhandled!")
    }
    expectEqual(expected, base)
  }
#else
  expectUnreachable()
#endif
}

func makeStringGuts(_ base: String) -> _StringGuts {
  var x = _StringGuts()
  // make sure some - but not all - replacements will have to grow the buffer
  x.reserveCapacity(base._classify()._count * 3 / 2)
  let capacity = x.capacity
  x.append(base._guts)
  // Widening the guts should not make it lose its capacity,
  // but the allocator may decide to get more storage.
  expectGE(x.capacity, capacity)
  return x
}

StringTests.test("StringGutsReplace") {
  let narrow = "01234567890"
  let wide = "ⅠⅡⅢⅣⅤⅥⅦⅧⅨⅩⅪ"
  for s1 in [narrow, wide] {
    for s2 in [narrow, wide] {
      let g1 = makeStringGuts(s1)
      let g2 = makeStringGuts(s2 + s2)
      checkRangeReplaceable(
        { StringGutsCollection(g1) },
        { StringGutsCollection(g2)[0..<$0] }
      )
      checkRangeReplaceable(
        { StringGutsCollection(g1) },
        { Array(StringGutsCollection(g2))[0..<$0] }
      )
    }
  }
}

StringTests.test("UnicodeScalarViewReplace") {
  let narrow = "01234567890"
  let wide = "ⅠⅡⅢⅣⅤⅥⅦⅧⅨⅩⅪ"
  for s1 in [narrow, wide] {
    for s2 in [narrow, wide] {
      checkRangeReplaceable(
        { String(makeStringGuts(s1)).unicodeScalars },
        { String(makeStringGuts(s2 + s2)._extractSlice(0..<$0)).unicodeScalars }
      )
      checkRangeReplaceable(
        { String(makeStringGuts(s1)).unicodeScalars },
        { Array(String(makeStringGuts(s2 + s2)).unicodeScalars)[0..<$0] }
      )
    }
  }
}

StringTests.test("reserveCapacity") {
  var s = ""
  let id0 = s.bufferID
  let oldCap = s.capacity
  let x: Character = "x" // Help the typechecker - <rdar://problem/17128913>
  s.insert(contentsOf: repeatElement(x, count: s.capacity + 1), at: s.endIndex)
  expectNotEqual(id0, s.bufferID)
  s = ""
  print("empty capacity \(s.capacity)")
  s.reserveCapacity(oldCap + 18)
  print("reserving \(oldCap + 18) -> \(s.capacity), width = \(s.byteWidth)")
  let id1 = s.bufferID
  s.insert(contentsOf: repeatElement(x, count: oldCap + 18), at: s.endIndex)
  print("extending by \(oldCap + 18) -> \(s.capacity), width = \(s.byteWidth)")
  expectEqual(id1, s.bufferID)
  s.insert(contentsOf: repeatElement(x, count: s.capacity + 100), at: s.endIndex)
  expectNotEqual(id1, s.bufferID)
}

StringTests.test("toInt") {
  expectNil(Int(""))
  expectNil(Int("+"))
  expectNil(Int("-"))
  expectEqual(20, Int("+20"))
  expectEqual(0, Int("0"))
  expectEqual(-20, Int("-20"))
  expectNil(Int("-cc20"))
  expectNil(Int("  -20"))
  expectNil(Int("  \t 20ddd"))

  expectEqual(Int.min, Int("\(Int.min)"))
  expectEqual(Int.min + 1, Int("\(Int.min + 1)"))
  expectEqual(Int.max, Int("\(Int.max)"))
  expectEqual(Int.max - 1, Int("\(Int.max - 1)"))

  expectNil(Int("\(Int.min)0"))
  expectNil(Int("\(Int.max)0"))

  // Make a String from an Int, mangle the String's characters,
  // then print if the new String is or is not still an Int.
  func testConvertabilityOfStringWithModification(
    _ initialValue: Int,
    modification: (_ chars: inout [UTF8.CodeUnit]) -> Void
  ) {
    var chars = Array(String(initialValue).utf8)
    modification(&chars)
    let str = String(decoding: chars, as: UTF8.self)
    expectNil(Int(str))
  }

  testConvertabilityOfStringWithModification(Int.min) {
    $0[2] += 1; ()  // underflow by lots
  }

  testConvertabilityOfStringWithModification(Int.max) {
    $0[1] += 1; ()  // overflow by lots
  }

  // Test values lower than min.
  do {
    let base = UInt(Int.max)
    expectEqual(Int.min + 1, Int("-\(base)"))
    expectEqual(Int.min, Int("-\(base + 1)"))
    for i in 2..<20 {
      expectNil(Int("-\(base + UInt(i))"))
    }
  }

  // Test values greater than min.
  do {
    let base = UInt(Int.max)
    for i in UInt(0)..<20 {
      expectEqual(-Int(base - i) , Int("-\(base - i)"))
    }
  }

  // Test values greater than max.
  do {
    let base = UInt(Int.max)
    expectEqual(Int.max, Int("\(base)"))
    for i in 1..<20 {
      expectNil(Int("\(base + UInt(i))"))
    }
  }

  // Test values lower than max.
  do {
    let base = Int.max
    for i in 0..<20 {
      expectEqual(base - i, Int("\(base - i)"))
    }
  }
}

// Make sure strings don't grow unreasonably quickly when appended-to
StringTests.test("growth") {
  var s = ""
  var s2 = s

  for _ in 0..<20 {
    s += "x"
    s2 = s
  }
  expectEqual(s2, s)
  expectLE(s.nativeCapacity, 40)
}

StringTests.test("Construction") {
  expectEqual("abc", String(["a", "b", "c"] as [Character]))
}

StringTests.test("Conversions") {
  // Whether we are natively ASCII or small ASCII
  func isKnownASCII(_ s: String) -> Bool {
    return s.byteWidth == 1
  }
  do {
    let c: Character = "a"
    let x = String(c)
    expectTrue(isKnownASCII(x))

    let s: String = "a"
    expectEqual(s, x)
  }

  do {
    let c: Character = "\u{B977}"
    let x = String(c)
    expectFalse(isKnownASCII(x))

    let s: String = "\u{B977}"
    expectEqual(s, x)
  }
}


#if os(Linux) || os(FreeBSD) || os(PS4) || os(Android)
import Glibc
#endif

StringTests.test("lowercased()") {
  // Use setlocale so tolower() is correct on ASCII.
  setlocale(LC_ALL, "C")

  // Check the ASCII domain.
  let asciiDomain: [Int32] = Array(0..<128)
  expectEqualFunctionsForDomain(
    asciiDomain,
    { String(UnicodeScalar(Int(tolower($0)))!) },
    { String(UnicodeScalar(Int($0))!).lowercased() })

  expectEqual("", "".lowercased())
  expectEqual("abcd", "abCD".lowercased())
  expectEqual("абвг", "абВГ".lowercased())
  expectEqual("たちつてと", "たちつてと".lowercased())

  //
  // Special casing.
  //

  // U+0130 LATIN CAPITAL LETTER I WITH DOT ABOVE
  // to lower case:
  // U+0069 LATIN SMALL LETTER I
  // U+0307 COMBINING DOT ABOVE
  expectEqual("\u{0069}\u{0307}", "\u{0130}".lowercased())

  // U+0049 LATIN CAPITAL LETTER I
  // U+0307 COMBINING DOT ABOVE
  // to lower case:
  // U+0069 LATIN SMALL LETTER I
  // U+0307 COMBINING DOT ABOVE
  expectEqual("\u{0069}\u{0307}", "\u{0049}\u{0307}".lowercased())
}

StringTests.test("uppercased()") {
  // Use setlocale so toupper() is correct on ASCII.
  setlocale(LC_ALL, "C")

  // Check the ASCII domain.
  let asciiDomain: [Int32] = Array(0..<128)
  expectEqualFunctionsForDomain(
    asciiDomain,
    { String(UnicodeScalar(Int(toupper($0)))!) },
    { String(UnicodeScalar(Int($0))!).uppercased() })

  expectEqual("", "".uppercased())
  expectEqual("ABCD", "abCD".uppercased())
  expectEqual("АБВГ", "абВГ".uppercased())
  expectEqual("たちつてと", "たちつてと".uppercased())

  //
  // Special casing.
  //

  // U+0069 LATIN SMALL LETTER I
  // to upper case:
  // U+0049 LATIN CAPITAL LETTER I
  expectEqual("\u{0049}", "\u{0069}".uppercased())

  // U+00DF LATIN SMALL LETTER SHARP S
  // to upper case:
  // U+0053 LATIN CAPITAL LETTER S
  // U+0073 LATIN SMALL LETTER S
  // But because the whole string is converted to uppercase, we just get two
  // U+0053.
  expectEqual("\u{0053}\u{0053}", "\u{00df}".uppercased())

  // U+FB01 LATIN SMALL LIGATURE FI
  // to upper case:
  // U+0046 LATIN CAPITAL LETTER F
  // U+0069 LATIN SMALL LETTER I
  // But because the whole string is converted to uppercase, we get U+0049
  // LATIN CAPITAL LETTER I.
  expectEqual("\u{0046}\u{0049}", "\u{fb01}".uppercased())
}

StringTests.test("unicodeViews") {
  // Check the UTF views work with slicing

  // U+FFFD REPLACEMENT CHARACTER
  // U+1F3C2 SNOWBOARDER
  // U+2603 SNOWMAN
  let winter = "\u{1F3C2}\u{2603}"

  // slices
  // First scalar is 4 bytes long, so this should be invalid
  expectNil(
    String(winter.utf8[
        winter.utf8.startIndex
        ..<
        winter.utf8.index(after: winter.utf8.index(after: winter.utf8.startIndex))
      ]))

  /*
  // FIXME: note changed String(describing:) results
  expectEqual(
    "\u{FFFD}",
    String(describing: 
      winter.utf8[
        winter.utf8.startIndex
        ..<
        winter.utf8.index(after: winter.utf8.index(after: winter.utf8.startIndex))
      ]))
  */
  
  expectEqual(
    "\u{1F3C2}", String(
      winter.utf8[winter.utf8.startIndex..<winter.utf8.index(_nth: 4)]))

  expectEqual(
    "\u{1F3C2}", String(
      winter.utf16[winter.utf16.startIndex..<winter.utf16.index(_nth: 2)]))
  
  expectEqual(
    "\u{1F3C2}", String(
      winter.unicodeScalars[
        winter.unicodeScalars.startIndex..<winter.unicodeScalars.index(_nth: 1)
      ]))

  // views
  expectEqual(
    winter, String(
      winter.utf8[winter.utf8.startIndex..<winter.utf8.index(_nth: 7)]))

  expectEqual(
    winter, String(
      winter.utf16[winter.utf16.startIndex..<winter.utf16.index(_nth: 3)]))

  expectEqual(
    winter, String(
      winter.unicodeScalars[
        winter.unicodeScalars.startIndex..<winter.unicodeScalars.index(_nth: 2)
      ]))

  let ga = "\u{304b}\u{3099}"
  expectEqual(ga, String(ga.utf8[ga.utf8.startIndex..<ga.utf8.index(_nth: 6)]))
}

// Validate that index conversion does something useful for Cocoa
// programmers.
StringTests.test("indexConversion")
  .skip(.nativeRuntime("Foundation dependency"))
  .code {
#if _runtime(_ObjC)
  let re : NSRegularExpression
  do {
    re = try NSRegularExpression(
      pattern: "([^ ]+)er", options: NSRegularExpression.Options())
  } catch { fatalError("couldn't build regexp: \(error)") }

  let s = "go further into the larder to barter."

  var matches: [String] = []
  
  re.enumerateMatches(
    in: s, options: NSRegularExpression.MatchingOptions(), range: NSRange(0..<s.utf16.count)
  ) {
    result, flags, stop
  in
    let r = result!.range(at: 1)
    let start = String.UTF16Index(encodedOffset: r.location)
    let end = String.UTF16Index(encodedOffset: r.location + r.length)
    matches.append(String(s.utf16[start..<end])!)
  }

  expectEqual(["furth", "lard", "bart"], matches)
#else
  expectUnreachable()
#endif
}

StringTests.test("String.append(_: UnicodeScalar)") {
  var s = ""

  do {
    // U+0061 LATIN SMALL LETTER A
    let input: UnicodeScalar = "\u{61}"
    s.append(String(input))
    expectEqual(["\u{61}"], Array(s.unicodeScalars))
  }
  do {
    // U+304B HIRAGANA LETTER KA
    let input: UnicodeScalar = "\u{304b}"
    s.append(String(input))
    expectEqual(["\u{61}", "\u{304b}"], Array(s.unicodeScalars))
  }
  do {
    // U+3099 COMBINING KATAKANA-HIRAGANA VOICED SOUND MARK
    let input: UnicodeScalar = "\u{3099}"
    s.append(String(input))
    expectEqual(["\u{61}", "\u{304b}", "\u{3099}"], Array(s.unicodeScalars))
  }
  do {
    // U+1F425 FRONT-FACING BABY CHICK
    let input: UnicodeScalar = "\u{1f425}"
    s.append(String(input))
    expectEqual(
      ["\u{61}", "\u{304b}", "\u{3099}", "\u{1f425}"],
      Array(s.unicodeScalars))
  }
}

StringTests.test("String.append(_: Character)") {
  let baseCharacters: [Character] = [
    // U+0061 LATIN SMALL LETTER A
    "\u{61}",

    // U+304B HIRAGANA LETTER KA
    // U+3099 COMBINING KATAKANA-HIRAGANA VOICED SOUND MARK
    "\u{304b}\u{3099}",

    // U+3072 HIRAGANA LETTER HI
    // U+309A COMBINING KATAKANA-HIRAGANA SEMI-VOICED SOUND MARK
    "\u{3072}\u{309A}",

    // U+1F425 FRONT-FACING BABY CHICK
    "\u{1f425}",

    // U+0061 LATIN SMALL LETTER A
    // U+0300 COMBINING GRAVE ACCENT
    // U+0301 COMBINING ACUTE ACCENT
    "\u{61}\u{0300}\u{0301}",

    // U+0061 LATIN SMALL LETTER A
    // U+0300 COMBINING GRAVE ACCENT
    // U+0301 COMBINING ACUTE ACCENT
    // U+0302 COMBINING CIRCUMFLEX ACCENT
    "\u{61}\u{0300}\u{0301}\u{0302}",

    // U+0061 LATIN SMALL LETTER A
    // U+0300 COMBINING GRAVE ACCENT
    // U+0301 COMBINING ACUTE ACCENT
    // U+0302 COMBINING CIRCUMFLEX ACCENT
    // U+0303 COMBINING TILDE
    "\u{61}\u{0300}\u{0301}\u{0302}\u{0303}",
  ]
  let baseStrings = [""] + baseCharacters.map { String($0) }

  for baseIdx in baseStrings.indices {
    for prefix in ["", " "] {
      let base = baseStrings[baseIdx]
      for inputIdx in baseCharacters.indices {
        let input = (prefix + String(baseCharacters[inputIdx])).last!
        var s = base
        s.append(input)
        expectEqualSequence(
          Array(base) + [input],
          Array(s),
          "baseIdx=\(baseIdx) inputIdx=\(inputIdx)")
      }
    }
  }
}

internal func decodeCString<
  C : UnicodeCodec
>(_ s: String, as codec: C.Type)
-> (result: String, repairsMade: Bool)? {
  let units = s.unicodeScalars.map({ $0.value }) + [0]
  return units.map({ C.CodeUnit($0) }).withUnsafeBufferPointer {
    String.decodeCString($0.baseAddress, as: C.self)
  }
}

StringTests.test("String.decodeCString/UTF8") {
  let actual = decodeCString("foobar", as: UTF8.self)
  expectFalse(actual!.repairsMade)
  expectEqual("foobar", actual!.result)
}

StringTests.test("String.decodeCString/UTF16") {
  let actual = decodeCString("foobar", as: UTF16.self)
  expectFalse(actual!.repairsMade)
  expectEqual("foobar", actual!.result)
}

StringTests.test("String.decodeCString/UTF32") {
  let actual = decodeCString("foobar", as: UTF32.self)
  expectFalse(actual!.repairsMade)
  expectEqual("foobar", actual!.result)
}

internal struct ReplaceSubrangeTest {
  let original: String
  let newElements: String
  let rangeSelection: RangeSelection
  let expected: String
  let closedExpected: String?
  let loc: SourceLoc

  internal init(
    original: String, newElements: String,
    rangeSelection: RangeSelection, expected: String, closedExpected: String? = nil,
    file: String = #file, line: UInt = #line
  ) {
    self.original = original
    self.newElements = newElements
    self.rangeSelection = rangeSelection
    self.expected = expected
    self.closedExpected = closedExpected
    self.loc = SourceLoc(file, line, comment: "replaceSubrange() test data")
  }
}

internal struct RemoveSubrangeTest {
  let original: String
  let rangeSelection: RangeSelection
  let expected: String
  let closedExpected: String
  let loc: SourceLoc

  internal init(
    original: String, rangeSelection: RangeSelection, expected: String,
    closedExpected: String? = nil,
    file: String = #file, line: UInt = #line
  ) {
    self.original = original
    self.rangeSelection = rangeSelection
    self.expected = expected
    self.closedExpected = closedExpected ?? expected
    self.loc = SourceLoc(file, line, comment: "replaceSubrange() test data")
  }
}

let replaceSubrangeTests = [
  ReplaceSubrangeTest(
    original: "",
    newElements: "",
    rangeSelection: .emptyRange,
    expected: ""
  ),
  ReplaceSubrangeTest(
    original: "",
    newElements: "meela",
    rangeSelection: .emptyRange,
    expected: "meela"
  ),
  ReplaceSubrangeTest(
    original: "eela",
    newElements: "m",
    rangeSelection: .leftEdge,
    expected: "meela",
    closedExpected: "mela"
  ),
  ReplaceSubrangeTest(
    original: "meel",
    newElements: "a",
    rangeSelection: .rightEdge,
    expected: "meela",
    closedExpected: "meea"
  ),
  ReplaceSubrangeTest(
    original: "a",
    newElements: "meel",
    rangeSelection: .leftEdge,
    expected: "meela",
    closedExpected: "meel"
  ),
  ReplaceSubrangeTest(
    original: "m",
    newElements: "eela",
    rangeSelection: .rightEdge,
    expected: "meela",
    closedExpected: "eela"
  ),
  ReplaceSubrangeTest(
    original: "alice",
    newElements: "bob",
    rangeSelection: .offsets(1, 1),
    expected: "aboblice",
    closedExpected: "abobice"
  ),
  ReplaceSubrangeTest(
    original: "alice",
    newElements: "bob",
    rangeSelection: .offsets(1, 2),
    expected: "abobice",
    closedExpected: "abobce"
  ),
  ReplaceSubrangeTest(
    original: "alice",
    newElements: "bob",
    rangeSelection: .offsets(1, 3),
    expected: "abobce",
    closedExpected: "abobe"
  ),
  ReplaceSubrangeTest(
    original: "alice",
    newElements: "bob",
    rangeSelection: .offsets(1, 4),
    expected: "abobe",
    closedExpected: "abob"
  ),
  ReplaceSubrangeTest(
    original: "alice",
    newElements: "bob",
    rangeSelection: .offsets(1, 5),
    expected: "abob"
  ),
  ReplaceSubrangeTest(
    original: "bob",
    newElements: "meela",
    rangeSelection: .offsets(1, 2),
    expected: "bmeelab",
    closedExpected: "bmeela"
  ),
]

let removeSubrangeTests = [
  RemoveSubrangeTest(
    original: "",
    rangeSelection: .emptyRange,
    expected: ""
  ),
  RemoveSubrangeTest(
    original: "a",
    rangeSelection: .middle,
    expected: ""
  ),
  RemoveSubrangeTest(
    original: "perdicus",
    rangeSelection: .leftHalf,
    expected: "icus"
  ),
  RemoveSubrangeTest(
    original: "perdicus",
    rangeSelection: .rightHalf,
    expected: "perd"
  ),
  RemoveSubrangeTest(
    original: "alice",
    rangeSelection: .middle,
    expected: "ae"
  ),
  RemoveSubrangeTest(
    original: "perdicus",
    rangeSelection: .middle,
    expected: "pes"
  ),
  RemoveSubrangeTest(
    original: "perdicus",
    rangeSelection: .offsets(1, 2),
    expected: "prdicus",
    closedExpected: "pdicus"
  ),
  RemoveSubrangeTest(
    original: "perdicus",
    rangeSelection: .offsets(3, 6),
    expected: "perus",
    closedExpected: "pers"
  )
]

StringTests.test("String.replaceSubrange()/characters/range") {
  for test in replaceSubrangeTests {
    var theString = test.original
    let c = test.original
    let rangeToReplace = test.rangeSelection.range(in: c)
    let newCharacters : [Character] = Array(test.newElements)
    theString.replaceSubrange(rangeToReplace, with: newCharacters)
    expectEqual(
      test.expected,
      theString,
      stackTrace: SourceLocStack().with(test.loc))
  }
}

StringTests.test("String.replaceSubrange()/string/range") {
  for test in replaceSubrangeTests {
    var theString = test.original
    let c = test.original
    let rangeToReplace = test.rangeSelection.range(in: c)
    theString.replaceSubrange(rangeToReplace, with: test.newElements)
    expectEqual(
      test.expected,
      theString,
      stackTrace: SourceLocStack().with(test.loc))
  }
}

StringTests.test("String.replaceSubrange()/characters/closedRange") {
  for test in replaceSubrangeTests {
    guard let closedExpected = test.closedExpected else {
      continue
    }
    var theString = test.original
    let c = test.original
    let rangeToReplace = test.rangeSelection.closedRange(in: c)
    let newCharacters = Array(test.newElements)
    theString.replaceSubrange(rangeToReplace, with: newCharacters)
    expectEqual(
      closedExpected,
      theString,
      stackTrace: SourceLocStack().with(test.loc))
  }
}

StringTests.test("String.replaceSubrange()/string/closedRange") {
  for test in replaceSubrangeTests {
    guard let closedExpected = test.closedExpected else {
      continue
    }
    var theString = test.original
    let c = test.original
    let rangeToReplace = test.rangeSelection.closedRange(in: c)
    theString.replaceSubrange(rangeToReplace, with: test.newElements)
    expectEqual(
      closedExpected,
      theString,
      stackTrace: SourceLocStack().with(test.loc))
  }
}

StringTests.test("String.removeSubrange()/range") {
  for test in removeSubrangeTests {
    var theString = test.original
    let c = test.original
    let rangeToRemove = test.rangeSelection.range(in: c)
    theString.removeSubrange(rangeToRemove)
    expectEqual(
      test.expected,
      theString,
      stackTrace: SourceLocStack().with(test.loc))
  }
}

StringTests.test("String.removeSubrange()/closedRange") {
  for test in removeSubrangeTests {
    switch test.rangeSelection {
      case .emptyRange: continue
      default: break
    }
    var theString = test.original
    let c = test.original
    let rangeToRemove = test.rangeSelection.closedRange(in: c)
    theString.removeSubrange(rangeToRemove)
    expectEqual(
      test.closedExpected,
      theString,
      stackTrace: SourceLocStack().with(test.loc))
  }
}

//===----------------------------------------------------------------------===//
// COW(🐄) tests
//===----------------------------------------------------------------------===//

public let testSuffix = "z"
StringTests.test("COW.Smoke") {
  var s1 = "COW Smoke Cypseloides" + testSuffix
  let identity1 = s1._rawIdentifier()
  
  var s2 = s1
  expectEqual(identity1, s2._rawIdentifier())
  
  s2.append(" cryptus")
  expectTrue(identity1 != s2._rawIdentifier())
  
  s1.remove(at: s1.startIndex)
  expectEqual(identity1, s1._rawIdentifier())
  
  _fixLifetime(s1)
  _fixLifetime(s2)  
}

struct COWStringTest {
  let test: String
  let name: String
}

var testCases: [COWStringTest] {
  return [ COWStringTest(test: "abcdefghijklmnopqrxtuvwxyz", name: "ASCII"),
           COWStringTest(test: "🐮🐄🤠", name: "Unicode") 
         ]
}

for test in testCases {
  StringTests.test("COW.\(test.name).IndexesDontAffectUniquenessCheck") {
    let s = test.test + testSuffix
    let identity1 = s._rawIdentifier()
  
    let startIndex = s.startIndex
    let endIndex = s.endIndex
    expectNotEqual(startIndex, endIndex)
    expectLT(startIndex, endIndex)
    expectLE(startIndex, endIndex)
    expectGT(endIndex, startIndex)
    expectGE(endIndex, startIndex)
  
    expectEqual(identity1, s._rawIdentifier())
  
    // Keep indexes alive during the calls above
    _fixLifetime(startIndex)
    _fixLifetime(endIndex)
  }
}

for test in testCases {
  StringTests.test("COW.\(test.name).SubscriptWithIndexDoesNotReallocate") {
    let s = test.test + testSuffix
    let identity1 = s._rawIdentifier()

    let startIndex = s.startIndex
    let empty = startIndex == s.endIndex
    expectNotEqual((s.startIndex < s.endIndex), empty)
    expectLE(s.startIndex, s.endIndex)
    expectEqual((s.startIndex >= s.endIndex), empty)
    expectGT(s.endIndex, s.startIndex)
    expectEqual(identity1, s._rawIdentifier())
  }
}

for test in testCases {
  StringTests.test("COW.\(test.name).RemoveAtDoesNotReallocate") {
    do {
      var s = test.test + testSuffix
      let identity1 = s._rawIdentifier()

      let index1 = s.startIndex
      expectEqual(identity1, s._rawIdentifier())

      let _ = s.remove(at: index1)
      expectEqual(identity1, s._rawIdentifier())
    }

    do {
      let s1 = test.test + testSuffix
      let identity1 = s1._rawIdentifier()

      var s2 = s1
      expectEqual(identity1, s1._rawIdentifier())
      expectEqual(identity1, s2._rawIdentifier())

      let index1 = s1.startIndex
      expectEqual(identity1, s1._rawIdentifier())
      expectEqual(identity1, s2._rawIdentifier())

      let _ = s2.remove(at: index1)

      expectEqual(identity1, s1._rawIdentifier())
      expectTrue(identity1 == s2._rawIdentifier())
    }
  }
}

for test in testCases {
  StringTests.test("COW.\(test.name).RemoveAtDoesNotReallocate") {
    do {
      var s = test.test + testSuffix
      expectGT(s.count, 0)

      s.removeAll()
      let identity1 = s._rawIdentifier()
      expectEqual(0, s.count)
      expectEqual(identity1, s._rawIdentifier())
    }

    do {
      var s = test.test + testSuffix
      let identity1 = s._rawIdentifier()
      expectGT(s.count, 3)

      s.removeAll(keepingCapacity: true)
      expectEqual(identity1, s._rawIdentifier())
      expectEqual(0, s.count)
    }

    do {
      let s1 = test.test + testSuffix
      let identity1 = s1._rawIdentifier()
      expectGT(s1.count, 0)

      var s2 = s1
      s2.removeAll()
      let identity2 = s2._rawIdentifier()
      expectEqual(identity1, s1._rawIdentifier())
      expectTrue(identity2 != identity1)
      expectGT(s1.count, 0)
      expectEqual(0, s2.count)

      // Keep variables alive.
      _fixLifetime(s1)
      _fixLifetime(s2)
    }

    do {
      let s1 = test.test + testSuffix
      let identity1 = s1._rawIdentifier()
      expectGT(s1.count, 0)

      var s2 = s1
      s2.removeAll(keepingCapacity: true)
      let identity2 = s2._rawIdentifier()
      expectEqual(identity1, s1._rawIdentifier())
      expectTrue(identity2 != identity1)
      expectGT(s1.count, 0)
      expectEqual(0, s2.count)

      // Keep variables alive.
      _fixLifetime(s1)
      _fixLifetime(s2)
    }
  }
}

for test in testCases {
  StringTests.test("COW.\(test.name).CountDoesNotReallocate") {
    let s = test.test + testSuffix
    let identity1 = s._rawIdentifier()

    expectGT(s.count, 0)
    expectEqual(identity1, s._rawIdentifier())
  } 
}

for test in testCases {
  StringTests.test("COW.\(test.name).GenerateDoesNotReallocate") {
    let s = test.test + testSuffix
    let identity1 = s._rawIdentifier()

    var iter = s.makeIterator()
    var copy = String()
    while let value = iter.next() {
      copy.append(value)
    }
    expectEqual(copy, s)
    expectEqual(identity1, s._rawIdentifier())
  }
}

for test in testCases {
  StringTests.test("COW.\(test.name).EqualityTestDoesNotReallocate") {
    let s1 = test.test + testSuffix
    let identity1 = s1._rawIdentifier()

    var s2 = test.test + testSuffix
    let identity2 = s2._rawIdentifier()

    expectEqual(s1, s2)
    expectEqual(identity1, s1._rawIdentifier())
    expectEqual(identity2, s2._rawIdentifier())

    s2.remove(at: s2.startIndex)
    expectNotEqual(s1, s2)
    expectEqual(identity1, s1._rawIdentifier())
    expectEqual(identity2, s2._rawIdentifier())
  }
}

struct ComparisonTestCase {  
  var strings: [String]
  // var test: (String, String) -> Void
  var comparison: _Ordering
  
  init(_ strings: [String], _ comparison: _Ordering) {
    self.strings = strings
    self.comparison = comparison
  }
  
  func test() {
    for pair in zip(strings, strings[1...]) {
      switch comparison {
      case .less:
        expectLT(pair.0, pair.1)
      case .greater:
        expectGT(pair.0, pair.1)
      case .equal:
        expectEqual(pair.0, pair.1)
      }
    }
  }
  
  func testOpaqueStrings() {
#if _runtime(_ObjC)
    let opaqueStrings = strings.map { NSSlowString(string: $0) as String }
    for pair in zip(opaqueStrings, opaqueStrings[1...]) {
      switch comparison {
      case .less:
        expectLT(pair.0, pair.1)
      case .greater:
        expectGT(pair.0, pair.1)
      case .equal:
        expectEqual(pair.0, pair.1)
      }
    }
#endif
  }
  
  func testOpaqueSubstrings() {
#if _runtime(_ObjC)
    for pair in zip(strings, strings[1...]) {
      let string1 = pair.0.dropLast()
      let string2 = pair.1
      let opaqueString = (NSSlowString(string: pair.0) as String).dropLast()
      
      guard string1.count > 0 else { return }
      
      let expectedResult: _Ordering = string1 < string2 ? .less : (string1 > string2 ? .greater : .equal)
      let opaqueResult: _Ordering = opaqueString < string2 ? .less : (opaqueString > string2 ? .greater : .equal)
      
      expectEqual(opaqueResult, expectedResult)
    }
#endif
  }
}

let comparisonTestCases = [
  ComparisonTestCase(["a", "a"], .equal),
  ComparisonTestCase(["abcdefg", "abcdefg"], .equal),
  ComparisonTestCase(["", "Z", "a", "b", "c", "\u{00c5}", "á"], .less),

  ComparisonTestCase(["ábcdefg", "ábcdefgh", "ábcdefghi"], .less),
  ComparisonTestCase(["abcdefg", "abcdefgh", "abcdefghi"], .less),

  ComparisonTestCase(["á", "\u{0061}\u{0301}"], .equal),
  ComparisonTestCase(["à", "\u{0061}\u{0301}", "â", "\u{e3}", "a\u{0308}"], .less),
  
  // Exploding scalars AND exploding segments
  ComparisonTestCase(["\u{fa2}", "\u{fa1}\u{fb7}"], .equal),
  ComparisonTestCase([
    "\u{fa2}\u{fa2}\u{fa2}\u{fa2}",
    "\u{fa1}\u{fb7}\u{fa1}\u{fb7}\u{fa1}\u{fb7}\u{fa1}\u{fb7}"
    ], .equal),
  ComparisonTestCase([
    "\u{fa2}\u{fa2}\u{fa2}\u{fa2}\u{fa2}\u{fa2}\u{fa2}\u{fa2}",
    "\u{fa1}\u{fb7}\u{fa1}\u{fb7}\u{fa1}\u{fb7}\u{fa1}\u{fb7}\u{fa1}\u{fb7}\u{fa1}\u{fb7}\u{fa1}\u{fb7}\u{fa1}\u{fb7}"
    ], .equal),
  ComparisonTestCase([
    "a\u{fa2}\u{fa2}a\u{fa2}\u{fa2}\u{fa2}\u{fa2}\u{fa2}\u{fa2}",
    "a\u{fa1}\u{fb7}\u{fa1}\u{fb7}a\u{fa1}\u{fb7}\u{fa1}\u{fb7}\u{fa1}\u{fb7}\u{fa1}\u{fb7}\u{fa1}\u{fb7}\u{fa1}\u{fb7}"
    ], .equal),

  ComparisonTestCase(["a\u{301}\u{0301}", "\u{e1}"], .greater),
  ComparisonTestCase(["😀", "😀"], .equal),
  ComparisonTestCase(["\u{2f9df}", "\u{8f38}"], .equal),
  ComparisonTestCase([
    "a",
    "\u{2f9df}", // D87E DDDF as written, but normalizes to 8f38
    "\u{2f9df}\u{2f9df}", // D87E DDDF as written, but normalizes to 8f38
    "👨🏻", // D83D DC68 D83C DFFB
    "👨🏻‍⚕️", // D83D DC68 D83C DFFB 200D 2695 FE0F
    "👩‍⚕️", // D83D DC69 200D 2695 FE0F
    "👩🏾", // D83D DC69 D83C DFFE
    "👩🏾‍⚕", // D83D DC69 D83C DFFE 200D 2695 FE0F
    "😀", // D83D DE00
    "😅", // D83D DE05
    "🧀" // D83E DDC0 -- aka a really big scalar
  ], .less),

  
  ComparisonTestCase(["f̛̗̘̙̜̹̺̻̼͇͈͉͍͎̽̾̿̀́͂̓̈́͆͊͋͌̚ͅ͏͓͔͕͖͙͚͐͑͒͗͛ͣͤͥͦ͘͜͟͢͝͞͠͡", "ơ̗̘̙̜̹̺̻̼͇͈͉͍͎̽̾̿̀́͂̓̈́͆͊͋͌̚ͅ͏͓͔͕͖͙͚͐͑͒͗͛ͥͦͧͨͩͪͫͬͭͮ͘"], .less),
  ComparisonTestCase(["\u{f90b}", "\u{5587}"], .equal),
  
  ComparisonTestCase(["a\u{1D160}a", "a\u{1D158}\u{1D1C7}"], .less),
  
  ComparisonTestCase(["\u{212b}", "\u{00c5}"], .equal),
  ComparisonTestCase([
    "A",
    "a",
    "aa",
    "ae",
    "ae🧀",
    "az",
    "aze\u{300}",
    "ae\u{301}",
    "ae\u{301}ae\u{301}",
    "ae\u{301}ae\u{301}ae\u{301}",
    "ae\u{301}ae\u{301}ae\u{301}ae\u{301}",
    "ae\u{301}ae\u{301}ae\u{301}ae\u{301}ae\u{301}",
    "ae\u{301}ae\u{301}ae\u{301}ae\u{301}ae\u{301}ae\u{301}",
    "ae\u{301}ae\u{301}ae\u{301}ae\u{301}ae\u{301}ae\u{301}ae\u{301}",
    "ae\u{301}ae\u{301}ae\u{301}ae\u{301}ae\u{301}ae\u{301}ae\u{301}ae\u{301}",
    "ae\u{302}",
    "ae\u{302}{303}",
    "ae\u{302}🧀",
    "ae\u{303}",
    "\u{f90b}\u{f90c}\u{f90d}", // Normalizes to BMP scalars
    "🧀", // D83E DDC0 -- aka a really big scalar
    "\u{FFEE}" // half width CJK dot
  ], .less),
  
  ComparisonTestCase(["ư̴̵̶̷̸̗̘̙̜̹̺̻̼͇͈͉͍͎̽̾̿̀́͂̓̈́͆͊͋͌̚ͅ͏͓͔͕͖͙͚͐͑͒͗͛ͣͤͥͦͧͨͩͪͫͬͭͮ͘͜͟͢͝͞͠͡", "ì̡̢̧̨̝̞̟̠̣̤̥̦̩̪̫̬̭̮̯̰̹̺̻̼͇͈͉͍͎́̂̃̄̉̊̋̌̍̎̏̐̑̒̓̽̾̿̀́͂̓̈́͆͊͋͌ͅ͏͓͔͕͖͙͐͑͒͗ͬͭͮ͘"], .greater),
  ComparisonTestCase(["ư̴̵̶̷̸̗̘̙̜̹̺̻̼͇͈͉͍͎̽̾̿̀́͂̓̈́͆͊͋͌̚ͅ͏͓͔͕͖͙͚͐͑͒͗͛ͣͤͥͦͧͨͩͪͫͬͭͮ͘͜͟͢͝͞͠͡", "aì̡̢̧̨̝̞̟̠̣̤̥̦̩̪̫̬̭̮̯̰̹̺̻̼͇͈͉͍͎́̂̃̄̉̊̋̌̍̎̏̐̑̒̓̽̾̿̀́͂̓̈́͆͊͋͌ͅ͏͓͔͕͖͙͐͑͒͗ͬͭͮ͘"], .greater),
  ComparisonTestCase(["ì̡̢̧̨̝̞̟̠̣̤̥̦̩̪̫̬̭̮̯̰̹̺̻̼͇͈͉͍͎́̂̃̄̉̊̋̌̍̎̏̐̑̒̓̽̾̿̀́͂̓̈́͆͊͋͌ͅ͏͓͔͕͖͙͐͑͒͗ͬͭͮ͘", "ì̡̢̧̨̝̞̟̠̣̤̥̦̩̪̫̬̭̮̯̰̹̺̻̼͇͈͉͍͎́̂̃̄̉̊̋̌̍̎̏̐̑̒̓̽̾̿̀́͂̓̈́͆͊͋͌ͅ͏͓͔͕͖͙͐͑͒͗ͬͭͮ͘"], .equal)
]

for test in comparisonTestCases {
  StringTests.test("Comparison.\(test.strings)") {
    test.test()
  }

  StringTests.test("Comparison.OpaqueString.\(test.strings)")
    .skip(.linuxAny(reason: "NSSlowString requires ObjC interop"))
    .code {
    test.testOpaqueStrings()
  }

  StringTests.test("Comparison.OpaqueSubstring.\(test.strings)")
    .skip(.linuxAny(reason: "NSSlowString requires ObjC interop"))
    .code {
    test.testOpaqueSubstrings()
  }
}

runAllTests()