src/runtime/runtime-strings.cc

// Copyright 2014 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "src/runtime/runtime-utils.h"

#include "src/arguments.h"
#include "src/regexp/jsregexp-inl.h"
#include "src/string-builder.h"
#include "src/string-search.h"

namespace v8 {
namespace internal {

// This may return an empty MaybeHandle if an exception is thrown or
// we abort due to reaching the recursion limit.
MaybeHandle<String> StringReplaceOneCharWithString(
    Isolate* isolate, Handle<String> subject, Handle<String> search,
    Handle<String> replace, bool* found, int recursion_limit) {
  StackLimitCheck stackLimitCheck(isolate);
  if (stackLimitCheck.HasOverflowed() || (recursion_limit == 0)) {
    return MaybeHandle<String>();
  }
  recursion_limit--;
  if (subject->IsConsString()) {
    ConsString* cons = ConsString::cast(*subject);
    Handle<String> first = Handle<String>(cons->first());
    Handle<String> second = Handle<String>(cons->second());
    Handle<String> new_first;
    if (!StringReplaceOneCharWithString(isolate, first, search, replace, found,
                                        recursion_limit).ToHandle(&new_first)) {
      return MaybeHandle<String>();
    }
    if (*found) return isolate->factory()->NewConsString(new_first, second);

    Handle<String> new_second;
    if (!StringReplaceOneCharWithString(isolate, second, search, replace, found,
                                        recursion_limit)
             .ToHandle(&new_second)) {
      return MaybeHandle<String>();
    }
    if (*found) return isolate->factory()->NewConsString(first, new_second);

    return subject;
  } else {
    int index = String::IndexOf(isolate, subject, search, 0);
    if (index == -1) return subject;
    *found = true;
    Handle<String> first = isolate->factory()->NewSubString(subject, 0, index);
    Handle<String> cons1;
    ASSIGN_RETURN_ON_EXCEPTION(
        isolate, cons1, isolate->factory()->NewConsString(first, replace),
        String);
    Handle<String> second =
        isolate->factory()->NewSubString(subject, index + 1, subject->length());
    return isolate->factory()->NewConsString(cons1, second);
  }
}


RUNTIME_FUNCTION(Runtime_StringReplaceOneCharWithString) {
  HandleScope scope(isolate);
  DCHECK(args.length() == 3);
  CONVERT_ARG_HANDLE_CHECKED(String, subject, 0);
  CONVERT_ARG_HANDLE_CHECKED(String, search, 1);
  CONVERT_ARG_HANDLE_CHECKED(String, replace, 2);

  // If the cons string tree is too deep, we simply abort the recursion and
  // retry with a flattened subject string.
  const int kRecursionLimit = 0x1000;
  bool found = false;
  Handle<String> result;
  if (StringReplaceOneCharWithString(isolate, subject, search, replace, &found,
                                     kRecursionLimit).ToHandle(&result)) {
    return *result;
  }
  if (isolate->has_pending_exception()) return isolate->heap()->exception();

  subject = String::Flatten(subject);
  if (StringReplaceOneCharWithString(isolate, subject, search, replace, &found,
                                     kRecursionLimit).ToHandle(&result)) {
    return *result;
  }
  if (isolate->has_pending_exception()) return isolate->heap()->exception();
  // In case of empty handle and no pending exception we have stack overflow.
  return isolate->StackOverflow();
}


RUNTIME_FUNCTION(Runtime_StringIndexOf) {
  HandleScope scope(isolate);
  DCHECK(args.length() == 3);
  return String::IndexOf(isolate, args.at<Object>(0), args.at<Object>(1),
                         args.at<Object>(2));
}

RUNTIME_FUNCTION(Runtime_StringLastIndexOf) {
  HandleScope handle_scope(isolate);
  return String::LastIndexOf(isolate, args.at<Object>(0), args.at<Object>(1),
                             isolate->factory()->undefined_value());
}

RUNTIME_FUNCTION(Runtime_SubString) {
  HandleScope scope(isolate);
  DCHECK(args.length() == 3);

  CONVERT_ARG_HANDLE_CHECKED(String, string, 0);
  int start, end;
  // We have a fast integer-only case here to avoid a conversion to double in
  // the common case where from and to are Smis.
  if (args[1]->IsSmi() && args[2]->IsSmi()) {
    CONVERT_SMI_ARG_CHECKED(from_number, 1);
    CONVERT_SMI_ARG_CHECKED(to_number, 2);
    start = from_number;
    end = to_number;
  } else if (args[1]->IsNumber() && args[2]->IsNumber()) {
    CONVERT_DOUBLE_ARG_CHECKED(from_number, 1);
    CONVERT_DOUBLE_ARG_CHECKED(to_number, 2);
    start = FastD2IChecked(from_number);
    end = FastD2IChecked(to_number);
  } else {
    return isolate->ThrowIllegalOperation();
  }
  // The following condition is intentionally robust because the SubStringStub
  // delegates here and we test this in cctest/test-strings/RobustSubStringStub.
  if (end < start || start < 0 || end > string->length()) {
    return isolate->ThrowIllegalOperation();
  }
  isolate->counters()->sub_string_runtime()->Increment();

  return *isolate->factory()->NewSubString(string, start, end);
}


RUNTIME_FUNCTION(Runtime_StringAdd) {
  HandleScope scope(isolate);
  DCHECK(args.length() == 2);
  CONVERT_ARG_HANDLE_CHECKED(Object, obj1, 0);
  CONVERT_ARG_HANDLE_CHECKED(Object, obj2, 1);
  isolate->counters()->string_add_runtime()->Increment();
  MaybeHandle<String> maybe_str1(Object::ToString(isolate, obj1));
  MaybeHandle<String> maybe_str2(Object::ToString(isolate, obj2));
  Handle<String> str1;
  Handle<String> str2;
  maybe_str1.ToHandle(&str1);
  maybe_str2.ToHandle(&str2);
  RETURN_RESULT_OR_FAILURE(isolate,
                           isolate->factory()->NewConsString(str1, str2));
}


RUNTIME_FUNCTION(Runtime_InternalizeString) {
  HandleScope handles(isolate);
  DCHECK(args.length() == 1);
  CONVERT_ARG_HANDLE_CHECKED(String, string, 0);
  return *isolate->factory()->InternalizeString(string);
}


RUNTIME_FUNCTION(Runtime_StringCharCodeAtRT) {
  HandleScope handle_scope(isolate);
  DCHECK(args.length() == 2);

  CONVERT_ARG_HANDLE_CHECKED(String, subject, 0);
  CONVERT_NUMBER_CHECKED(uint32_t, i, Uint32, args[1]);

  // Flatten the string.  If someone wants to get a char at an index
  // in a cons string, it is likely that more indices will be
  // accessed.
  subject = String::Flatten(subject);

  if (i >= static_cast<uint32_t>(subject->length())) {
    return isolate->heap()->nan_value();
  }

  return Smi::FromInt(subject->Get(i));
}


RUNTIME_FUNCTION(Runtime_StringCompare) {
  HandleScope handle_scope(isolate);
  DCHECK_EQ(2, args.length());
  CONVERT_ARG_HANDLE_CHECKED(String, x, 0);
  CONVERT_ARG_HANDLE_CHECKED(String, y, 1);
  isolate->counters()->string_compare_runtime()->Increment();
  switch (String::Compare(x, y)) {
    case ComparisonResult::kLessThan:
      return Smi::FromInt(LESS);
    case ComparisonResult::kEqual:
      return Smi::FromInt(EQUAL);
    case ComparisonResult::kGreaterThan:
      return Smi::FromInt(GREATER);
    case ComparisonResult::kUndefined:
      break;
  }
  UNREACHABLE();
  return Smi::kZero;
}


RUNTIME_FUNCTION(Runtime_StringBuilderConcat) {
  HandleScope scope(isolate);
  DCHECK(args.length() == 3);
  CONVERT_ARG_HANDLE_CHECKED(JSArray, array, 0);
  int32_t array_length;
  if (!args[1]->ToInt32(&array_length)) {
    THROW_NEW_ERROR_RETURN_FAILURE(isolate, NewInvalidStringLengthError());
  }
  CONVERT_ARG_HANDLE_CHECKED(String, special, 2);

  size_t actual_array_length = 0;
  CHECK(TryNumberToSize(array->length(), &actual_array_length));
  CHECK(array_length >= 0);
  CHECK(static_cast<size_t>(array_length) <= actual_array_length);

  // This assumption is used by the slice encoding in one or two smis.
  DCHECK(Smi::kMaxValue >= String::kMaxLength);

  CHECK(array->HasFastElements());
  JSObject::EnsureCanContainHeapObjectElements(array);

  int special_length = special->length();
  if (!array->HasFastObjectElements()) {
    return isolate->Throw(isolate->heap()->illegal_argument_string());
  }

  int length;
  bool one_byte = special->HasOnlyOneByteChars();

  {
    DisallowHeapAllocation no_gc;
    FixedArray* fixed_array = FixedArray::cast(array->elements());
    if (fixed_array->length() < array_length) {
      array_length = fixed_array->length();
    }

    if (array_length == 0) {
      return isolate->heap()->empty_string();
    } else if (array_length == 1) {
      Object* first = fixed_array->get(0);
      if (first->IsString()) return first;
    }
    length = StringBuilderConcatLength(special_length, fixed_array,
                                       array_length, &one_byte);
  }

  if (length == -1) {
    return isolate->Throw(isolate->heap()->illegal_argument_string());
  }

  if (one_byte) {
    Handle<SeqOneByteString> answer;
    ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
        isolate, answer, isolate->factory()->NewRawOneByteString(length));
    StringBuilderConcatHelper(*special, answer->GetChars(),
                              FixedArray::cast(array->elements()),
                              array_length);
    return *answer;
  } else {
    Handle<SeqTwoByteString> answer;
    ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
        isolate, answer, isolate->factory()->NewRawTwoByteString(length));
    StringBuilderConcatHelper(*special, answer->GetChars(),
                              FixedArray::cast(array->elements()),
                              array_length);
    return *answer;
  }
}


RUNTIME_FUNCTION(Runtime_StringBuilderJoin) {
  HandleScope scope(isolate);
  DCHECK(args.length() == 3);
  CONVERT_ARG_HANDLE_CHECKED(JSArray, array, 0);
  int32_t array_length;
  if (!args[1]->ToInt32(&array_length)) {
    THROW_NEW_ERROR_RETURN_FAILURE(isolate, NewInvalidStringLengthError());
  }
  CONVERT_ARG_HANDLE_CHECKED(String, separator, 2);
  CHECK(array->HasFastObjectElements());
  CHECK(array_length >= 0);

  Handle<FixedArray> fixed_array(FixedArray::cast(array->elements()));
  if (fixed_array->length() < array_length) {
    array_length = fixed_array->length();
  }

  if (array_length == 0) {
    return isolate->heap()->empty_string();
  } else if (array_length == 1) {
    Object* first = fixed_array->get(0);
    CHECK(first->IsString());
    return first;
  }

  int separator_length = separator->length();
  CHECK(separator_length > 0);
  int max_nof_separators =
      (String::kMaxLength + separator_length - 1) / separator_length;
  if (max_nof_separators < (array_length - 1)) {
    THROW_NEW_ERROR_RETURN_FAILURE(isolate, NewInvalidStringLengthError());
  }
  int length = (array_length - 1) * separator_length;
  for (int i = 0; i < array_length; i++) {
    Object* element_obj = fixed_array->get(i);
    CHECK(element_obj->IsString());
    String* element = String::cast(element_obj);
    int increment = element->length();
    if (increment > String::kMaxLength - length) {
      STATIC_ASSERT(String::kMaxLength < kMaxInt);
      length = kMaxInt;  // Provoke exception;
      break;
    }
    length += increment;
  }

  Handle<SeqTwoByteString> answer;
  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
      isolate, answer, isolate->factory()->NewRawTwoByteString(length));

  DisallowHeapAllocation no_gc;

  uc16* sink = answer->GetChars();
#ifdef DEBUG
  uc16* end = sink + length;
#endif

  CHECK(fixed_array->get(0)->IsString());
  String* first = String::cast(fixed_array->get(0));
  String* separator_raw = *separator;

  int first_length = first->length();
  String::WriteToFlat(first, sink, 0, first_length);
  sink += first_length;

  for (int i = 1; i < array_length; i++) {
    DCHECK(sink + separator_length <= end);
    String::WriteToFlat(separator_raw, sink, 0, separator_length);
    sink += separator_length;

    CHECK(fixed_array->get(i)->IsString());
    String* element = String::cast(fixed_array->get(i));
    int element_length = element->length();
    DCHECK(sink + element_length <= end);
    String::WriteToFlat(element, sink, 0, element_length);
    sink += element_length;
  }
  DCHECK(sink == end);

  // Use %_FastOneByteArrayJoin instead.
  DCHECK(!answer->IsOneByteRepresentation());
  return *answer;
}

template <typename sinkchar>
static void WriteRepeatToFlat(String* src, Vector<sinkchar> buffer, int cursor,
                              int repeat, int length) {
  if (repeat == 0) return;

  sinkchar* start = &buffer[cursor];
  String::WriteToFlat<sinkchar>(src, start, 0, length);

  int done = 1;
  sinkchar* next = start + length;

  while (done < repeat) {
    int block = Min(done, repeat - done);
    int block_chars = block * length;
    CopyChars(next, start, block_chars);
    next += block_chars;
    done += block;
  }
}

template <typename Char>
static void JoinSparseArrayWithSeparator(FixedArray* elements,
                                         int elements_length,
                                         uint32_t array_length,
                                         String* separator,
                                         Vector<Char> buffer) {
  DisallowHeapAllocation no_gc;
  int previous_separator_position = 0;
  int separator_length = separator->length();
  DCHECK_LT(0, separator_length);
  int cursor = 0;
  for (int i = 0; i < elements_length; i += 2) {
    int position = NumberToInt32(elements->get(i));
    String* string = String::cast(elements->get(i + 1));
    int string_length = string->length();
    if (string->length() > 0) {
      int repeat = position - previous_separator_position;
      WriteRepeatToFlat<Char>(separator, buffer, cursor, repeat,
                              separator_length);
      cursor += repeat * separator_length;
      previous_separator_position = position;
      String::WriteToFlat<Char>(string, &buffer[cursor], 0, string_length);
      cursor += string->length();
    }
  }

  int last_array_index = static_cast<int>(array_length - 1);
  // Array length must be representable as a signed 32-bit number,
  // otherwise the total string length would have been too large.
  DCHECK(array_length <= 0x7fffffff);  // Is int32_t.
  int repeat = last_array_index - previous_separator_position;
  WriteRepeatToFlat<Char>(separator, buffer, cursor, repeat, separator_length);
  cursor += repeat * separator_length;
  DCHECK(cursor <= buffer.length());
}


RUNTIME_FUNCTION(Runtime_SparseJoinWithSeparator) {
  HandleScope scope(isolate);
  DCHECK(args.length() == 3);
  CONVERT_ARG_HANDLE_CHECKED(JSArray, elements_array, 0);
  CONVERT_NUMBER_CHECKED(uint32_t, array_length, Uint32, args[1]);
  CONVERT_ARG_HANDLE_CHECKED(String, separator, 2);
  // elements_array is fast-mode JSarray of alternating positions
  // (increasing order) and strings.
  CHECK(elements_array->HasFastSmiOrObjectElements());
  // array_length is length of original array (used to add separators);
  // separator is string to put between elements. Assumed to be non-empty.
  CHECK(array_length > 0);

  // Find total length of join result.
  int string_length = 0;
  bool is_one_byte = separator->IsOneByteRepresentation();
  bool overflow = false;
  CONVERT_NUMBER_CHECKED(int, elements_length, Int32, elements_array->length());
  CHECK(elements_length <= elements_array->elements()->length());
  CHECK((elements_length & 1) == 0);  // Even length.
  FixedArray* elements = FixedArray::cast(elements_array->elements());
  {
    DisallowHeapAllocation no_gc;
    for (int i = 0; i < elements_length; i += 2) {
      String* string = String::cast(elements->get(i + 1));
      int length = string->length();
      if (is_one_byte && !string->IsOneByteRepresentation()) {
        is_one_byte = false;
      }
      if (length > String::kMaxLength ||
          String::kMaxLength - length < string_length) {
        overflow = true;
        break;
      }
      string_length += length;
    }
  }

  int separator_length = separator->length();
  if (!overflow && separator_length > 0) {
    if (array_length <= 0x7fffffffu) {
      int separator_count = static_cast<int>(array_length) - 1;
      int remaining_length = String::kMaxLength - string_length;
      if ((remaining_length / separator_length) >= separator_count) {
        string_length += separator_length * (array_length - 1);
      } else {
        // Not room for the separators within the maximal string length.
        overflow = true;
      }
    } else {
      // Nonempty separator and at least 2^31-1 separators necessary
      // means that the string is too large to create.
      STATIC_ASSERT(String::kMaxLength < 0x7fffffff);
      overflow = true;
    }
  }
  if (overflow) {
    // Throw an exception if the resulting string is too large. See
    // https://code.google.com/p/chromium/issues/detail?id=336820
    // for details.
    THROW_NEW_ERROR_RETURN_FAILURE(isolate, NewInvalidStringLengthError());
  }

  if (is_one_byte) {
    Handle<SeqOneByteString> result = isolate->factory()
                                          ->NewRawOneByteString(string_length)
                                          .ToHandleChecked();
    JoinSparseArrayWithSeparator<uint8_t>(
        FixedArray::cast(elements_array->elements()), elements_length,
        array_length, *separator,
        Vector<uint8_t>(result->GetChars(), string_length));
    return *result;
  } else {
    Handle<SeqTwoByteString> result = isolate->factory()
                                          ->NewRawTwoByteString(string_length)
                                          .ToHandleChecked();
    JoinSparseArrayWithSeparator<uc16>(
        FixedArray::cast(elements_array->elements()), elements_length,
        array_length, *separator,
        Vector<uc16>(result->GetChars(), string_length));
    return *result;
  }
}


// Copies Latin1 characters to the given fixed array looking up
// one-char strings in the cache. Gives up on the first char that is
// not in the cache and fills the remainder with smi zeros. Returns
// the length of the successfully copied prefix.
static int CopyCachedOneByteCharsToArray(Heap* heap, const uint8_t* chars,
                                         FixedArray* elements, int length) {
  DisallowHeapAllocation no_gc;
  FixedArray* one_byte_cache = heap->single_character_string_cache();
  Object* undefined = heap->undefined_value();
  int i;
  WriteBarrierMode mode = elements->GetWriteBarrierMode(no_gc);
  for (i = 0; i < length; ++i) {
    Object* value = one_byte_cache->get(chars[i]);
    if (value == undefined) break;
    elements->set(i, value, mode);
  }
  if (i < length) {
    DCHECK(Smi::kZero == 0);
    memset(elements->data_start() + i, 0, kPointerSize * (length - i));
  }
#ifdef DEBUG
  for (int j = 0; j < length; ++j) {
    Object* element = elements->get(j);
    DCHECK(element == Smi::kZero ||
           (element->IsString() && String::cast(element)->LooksValid()));
  }
#endif
  return i;
}


// Converts a String to JSArray.
// For example, "foo" => ["f", "o", "o"].
RUNTIME_FUNCTION(Runtime_StringToArray) {
  HandleScope scope(isolate);
  DCHECK(args.length() == 2);
  CONVERT_ARG_HANDLE_CHECKED(String, s, 0);
  CONVERT_NUMBER_CHECKED(uint32_t, limit, Uint32, args[1]);

  s = String::Flatten(s);
  const int length = static_cast<int>(Min<uint32_t>(s->length(), limit));

  Handle<FixedArray> elements;
  int position = 0;
  if (s->IsFlat() && s->IsOneByteRepresentation()) {
    // Try using cached chars where possible.
    elements = isolate->factory()->NewUninitializedFixedArray(length);

    DisallowHeapAllocation no_gc;
    String::FlatContent content = s->GetFlatContent();
    if (content.IsOneByte()) {
      Vector<const uint8_t> chars = content.ToOneByteVector();
      // Note, this will initialize all elements (not only the prefix)
      // to prevent GC from seeing partially initialized array.
      position = CopyCachedOneByteCharsToArray(isolate->heap(), chars.start(),
                                               *elements, length);
    } else {
      MemsetPointer(elements->data_start(), isolate->heap()->undefined_value(),
                    length);
    }
  } else {
    elements = isolate->factory()->NewFixedArray(length);
  }
  for (int i = position; i < length; ++i) {
    Handle<Object> str =
        isolate->factory()->LookupSingleCharacterStringFromCode(s->Get(i));
    elements->set(i, *str);
  }

#ifdef DEBUG
  for (int i = 0; i < length; ++i) {
    DCHECK(String::cast(elements->get(i))->length() == 1);
  }
#endif

  return *isolate->factory()->NewJSArrayWithElements(elements);
}


static inline bool ToUpperOverflows(uc32 character) {
  // y with umlauts and the micro sign are the only characters that stop
  // fitting into one-byte when converting to uppercase.
  static const uc32 yuml_code = 0xff;
  static const uc32 micro_code = 0xb5;
  return (character == yuml_code || character == micro_code);
}


template <class Converter>
MUST_USE_RESULT static Object* ConvertCaseHelper(
    Isolate* isolate, String* string, SeqString* result, int result_length,
    unibrow::Mapping<Converter, 128>* mapping) {
  DisallowHeapAllocation no_gc;
  // We try this twice, once with the assumption that the result is no longer
  // than the input and, if that assumption breaks, again with the exact
  // length.  This may not be pretty, but it is nicer than what was here before
  // and I hereby claim my vaffel-is.
  //
  // NOTE: This assumes that the upper/lower case of an ASCII
  // character is also ASCII.  This is currently the case, but it
  // might break in the future if we implement more context and locale
  // dependent upper/lower conversions.
  bool has_changed_character = false;

  // Convert all characters to upper case, assuming that they will fit
  // in the buffer
  StringCharacterStream stream(string);
  unibrow::uchar chars[Converter::kMaxWidth];
  // We can assume that the string is not empty
  uc32 current = stream.GetNext();
  bool ignore_overflow = Converter::kIsToLower || result->IsSeqTwoByteString();
  for (int i = 0; i < result_length;) {
    bool has_next = stream.HasMore();
    uc32 next = has_next ? stream.GetNext() : 0;
    int char_length = mapping->get(current, next, chars);
    if (char_length == 0) {
      // The case conversion of this character is the character itself.
      result->Set(i, current);
      i++;
    } else if (char_length == 1 &&
               (ignore_overflow || !ToUpperOverflows(current))) {
      // Common case: converting the letter resulted in one character.
      DCHECK(static_cast<uc32>(chars[0]) != current);
      result->Set(i, chars[0]);
      has_changed_character = true;
      i++;
    } else if (result_length == string->length()) {
      bool overflows = ToUpperOverflows(current);
      // We've assumed that the result would be as long as the
      // input but here is a character that converts to several
      // characters.  No matter, we calculate the exact length
      // of the result and try the whole thing again.
      //
      // Note that this leaves room for optimization.  We could just
      // memcpy what we already have to the result string.  Also,
      // the result string is the last object allocated we could
      // "realloc" it and probably, in the vast majority of cases,
      // extend the existing string to be able to hold the full
      // result.
      int next_length = 0;
      if (has_next) {
        next_length = mapping->get(next, 0, chars);
        if (next_length == 0) next_length = 1;
      }
      int current_length = i + char_length + next_length;
      while (stream.HasMore()) {
        current = stream.GetNext();
        overflows |= ToUpperOverflows(current);
        // NOTE: we use 0 as the next character here because, while
        // the next character may affect what a character converts to,
        // it does not in any case affect the length of what it convert
        // to.
        int char_length = mapping->get(current, 0, chars);
        if (char_length == 0) char_length = 1;
        current_length += char_length;
        if (current_length > String::kMaxLength) {
          AllowHeapAllocation allocate_error_and_return;
          THROW_NEW_ERROR_RETURN_FAILURE(isolate,
                                         NewInvalidStringLengthError());
        }
      }
      // Try again with the real length.  Return signed if we need
      // to allocate a two-byte string for to uppercase.
      return (overflows && !ignore_overflow) ? Smi::FromInt(-current_length)
                                             : Smi::FromInt(current_length);
    } else {
      for (int j = 0; j < char_length; j++) {
        result->Set(i, chars[j]);
        i++;
      }
      has_changed_character = true;
    }
    current = next;
  }
  if (has_changed_character) {
    return result;
  } else {
    // If we didn't actually change anything in doing the conversion
    // we simple return the result and let the converted string
    // become garbage; there is no reason to keep two identical strings
    // alive.
    return string;
  }
}


static const uintptr_t kOneInEveryByte = kUintptrAllBitsSet / 0xFF;
static const uintptr_t kAsciiMask = kOneInEveryByte << 7;

// Given a word and two range boundaries returns a word with high bit
// set in every byte iff the corresponding input byte was strictly in
// the range (m, n). All the other bits in the result are cleared.
// This function is only useful when it can be inlined and the
// boundaries are statically known.
// Requires: all bytes in the input word and the boundaries must be
// ASCII (less than 0x7F).
static inline uintptr_t AsciiRangeMask(uintptr_t w, char m, char n) {
  // Use strict inequalities since in edge cases the function could be
  // further simplified.
  DCHECK(0 < m && m < n);
  // Has high bit set in every w byte less than n.
  uintptr_t tmp1 = kOneInEveryByte * (0x7F + n) - w;
  // Has high bit set in every w byte greater than m.
  uintptr_t tmp2 = w + kOneInEveryByte * (0x7F - m);
  return (tmp1 & tmp2 & (kOneInEveryByte * 0x80));
}


#ifdef DEBUG
static bool CheckFastAsciiConvert(char* dst, const char* src, int length,
                                  bool changed, bool is_to_lower) {
  bool expected_changed = false;
  for (int i = 0; i < length; i++) {
    if (dst[i] == src[i]) continue;
    expected_changed = true;
    if (is_to_lower) {
      DCHECK('A' <= src[i] && src[i] <= 'Z');
      DCHECK(dst[i] == src[i] + ('a' - 'A'));
    } else {
      DCHECK('a' <= src[i] && src[i] <= 'z');
      DCHECK(dst[i] == src[i] - ('a' - 'A'));
    }
  }
  return (expected_changed == changed);
}
#endif


template <class Converter>
static bool FastAsciiConvert(char* dst, const char* src, int length,
                             bool* changed_out) {
#ifdef DEBUG
  char* saved_dst = dst;
  const char* saved_src = src;
#endif
  DisallowHeapAllocation no_gc;
  // We rely on the distance between upper and lower case letters
  // being a known power of 2.
  DCHECK('a' - 'A' == (1 << 5));
  // Boundaries for the range of input characters than require conversion.
  static const char lo = Converter::kIsToLower ? 'A' - 1 : 'a' - 1;
  static const char hi = Converter::kIsToLower ? 'Z' + 1 : 'z' + 1;
  bool changed = false;
  uintptr_t or_acc = 0;
  const char* const limit = src + length;

  // dst is newly allocated and always aligned.
  DCHECK(IsAligned(reinterpret_cast<intptr_t>(dst), sizeof(uintptr_t)));
  // Only attempt processing one word at a time if src is also aligned.
  if (IsAligned(reinterpret_cast<intptr_t>(src), sizeof(uintptr_t))) {
    // Process the prefix of the input that requires no conversion one aligned
    // (machine) word at a time.
    while (src <= limit - sizeof(uintptr_t)) {
      const uintptr_t w = *reinterpret_cast<const uintptr_t*>(src);
      or_acc |= w;
      if (AsciiRangeMask(w, lo, hi) != 0) {
        changed = true;
        break;
      }
      *reinterpret_cast<uintptr_t*>(dst) = w;
      src += sizeof(uintptr_t);
      dst += sizeof(uintptr_t);
    }
    // Process the remainder of the input performing conversion when
    // required one word at a time.
    while (src <= limit - sizeof(uintptr_t)) {
      const uintptr_t w = *reinterpret_cast<const uintptr_t*>(src);
      or_acc |= w;
      uintptr_t m = AsciiRangeMask(w, lo, hi);
      // The mask has high (7th) bit set in every byte that needs
      // conversion and we know that the distance between cases is
      // 1 << 5.
      *reinterpret_cast<uintptr_t*>(dst) = w ^ (m >> 2);
      src += sizeof(uintptr_t);
      dst += sizeof(uintptr_t);
    }
  }
  // Process the last few bytes of the input (or the whole input if
  // unaligned access is not supported).
  while (src < limit) {
    char c = *src;
    or_acc |= c;
    if (lo < c && c < hi) {
      c ^= (1 << 5);
      changed = true;
    }
    *dst = c;
    ++src;
    ++dst;
  }

  if ((or_acc & kAsciiMask) != 0) return false;

  DCHECK(CheckFastAsciiConvert(saved_dst, saved_src, length, changed,
                               Converter::kIsToLower));

  *changed_out = changed;
  return true;
}


template <class Converter>
MUST_USE_RESULT static Object* ConvertCase(
    Handle<String> s, Isolate* isolate,
    unibrow::Mapping<Converter, 128>* mapping) {
  s = String::Flatten(s);
  int length = s->length();
  // Assume that the string is not empty; we need this assumption later
  if (length == 0) return *s;

  // Simpler handling of ASCII strings.
  //
  // NOTE: This assumes that the upper/lower case of an ASCII
  // character is also ASCII.  This is currently the case, but it
  // might break in the future if we implement more context and locale
  // dependent upper/lower conversions.
  if (s->IsOneByteRepresentationUnderneath()) {
    // Same length as input.
    Handle<SeqOneByteString> result =
        isolate->factory()->NewRawOneByteString(length).ToHandleChecked();
    DisallowHeapAllocation no_gc;
    String::FlatContent flat_content = s->GetFlatContent();
    DCHECK(flat_content.IsFlat());
    bool has_changed_character = false;
    bool is_ascii = FastAsciiConvert<Converter>(
        reinterpret_cast<char*>(result->GetChars()),
        reinterpret_cast<const char*>(flat_content.ToOneByteVector().start()),
        length, &has_changed_character);
    // If not ASCII, we discard the result and take the 2 byte path.
    if (is_ascii) return has_changed_character ? *result : *s;
  }

  Handle<SeqString> result;  // Same length as input.
  if (s->IsOneByteRepresentation()) {
    result = isolate->factory()->NewRawOneByteString(length).ToHandleChecked();
  } else {
    result = isolate->factory()->NewRawTwoByteString(length).ToHandleChecked();
  }

  Object* answer = ConvertCaseHelper(isolate, *s, *result, length, mapping);
  if (answer->IsException(isolate) || answer->IsString()) return answer;

  DCHECK(answer->IsSmi());
  length = Smi::cast(answer)->value();
  if (s->IsOneByteRepresentation() && length > 0) {
    ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
        isolate, result, isolate->factory()->NewRawOneByteString(length));
  } else {
    if (length < 0) length = -length;
    ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
        isolate, result, isolate->factory()->NewRawTwoByteString(length));
  }
  return ConvertCaseHelper(isolate, *s, *result, length, mapping);
}


RUNTIME_FUNCTION(Runtime_StringToLowerCase) {
  HandleScope scope(isolate);
  DCHECK_EQ(args.length(), 1);
  CONVERT_ARG_HANDLE_CHECKED(String, s, 0);
  return ConvertCase(s, isolate, isolate->runtime_state()->to_lower_mapping());
}


RUNTIME_FUNCTION(Runtime_StringToUpperCase) {
  HandleScope scope(isolate);
  DCHECK_EQ(args.length(), 1);
  CONVERT_ARG_HANDLE_CHECKED(String, s, 0);
  return ConvertCase(s, isolate, isolate->runtime_state()->to_upper_mapping());
}

RUNTIME_FUNCTION(Runtime_StringLessThan) {
  HandleScope handle_scope(isolate);
  DCHECK_EQ(2, args.length());
  CONVERT_ARG_HANDLE_CHECKED(String, x, 0);
  CONVERT_ARG_HANDLE_CHECKED(String, y, 1);
  switch (String::Compare(x, y)) {
    case ComparisonResult::kLessThan:
      return isolate->heap()->true_value();
    case ComparisonResult::kEqual:
    case ComparisonResult::kGreaterThan:
      return isolate->heap()->false_value();
    case ComparisonResult::kUndefined:
      break;
  }
  UNREACHABLE();
  return Smi::kZero;
}

RUNTIME_FUNCTION(Runtime_StringLessThanOrEqual) {
  HandleScope handle_scope(isolate);
  DCHECK_EQ(2, args.length());
  CONVERT_ARG_HANDLE_CHECKED(String, x, 0);
  CONVERT_ARG_HANDLE_CHECKED(String, y, 1);
  switch (String::Compare(x, y)) {
    case ComparisonResult::kEqual:
    case ComparisonResult::kLessThan:
      return isolate->heap()->true_value();
    case ComparisonResult::kGreaterThan:
      return isolate->heap()->false_value();
    case ComparisonResult::kUndefined:
      break;
  }
  UNREACHABLE();
  return Smi::kZero;
}

RUNTIME_FUNCTION(Runtime_StringGreaterThan) {
  HandleScope handle_scope(isolate);
  DCHECK_EQ(2, args.length());
  CONVERT_ARG_HANDLE_CHECKED(String, x, 0);
  CONVERT_ARG_HANDLE_CHECKED(String, y, 1);
  switch (String::Compare(x, y)) {
    case ComparisonResult::kGreaterThan:
      return isolate->heap()->true_value();
    case ComparisonResult::kEqual:
    case ComparisonResult::kLessThan:
      return isolate->heap()->false_value();
    case ComparisonResult::kUndefined:
      break;
  }
  UNREACHABLE();
  return Smi::kZero;
}

RUNTIME_FUNCTION(Runtime_StringGreaterThanOrEqual) {
  HandleScope handle_scope(isolate);
  DCHECK_EQ(2, args.length());
  CONVERT_ARG_HANDLE_CHECKED(String, x, 0);
  CONVERT_ARG_HANDLE_CHECKED(String, y, 1);
  switch (String::Compare(x, y)) {
    case ComparisonResult::kEqual:
    case ComparisonResult::kGreaterThan:
      return isolate->heap()->true_value();
    case ComparisonResult::kLessThan:
      return isolate->heap()->false_value();
    case ComparisonResult::kUndefined:
      break;
  }
  UNREACHABLE();
  return Smi::kZero;
}

RUNTIME_FUNCTION(Runtime_StringEqual) {
  HandleScope handle_scope(isolate);
  DCHECK_EQ(2, args.length());
  CONVERT_ARG_HANDLE_CHECKED(String, x, 0);
  CONVERT_ARG_HANDLE_CHECKED(String, y, 1);
  return isolate->heap()->ToBoolean(String::Equals(x, y));
}

RUNTIME_FUNCTION(Runtime_StringNotEqual) {
  HandleScope handle_scope(isolate);
  DCHECK_EQ(2, args.length());
  CONVERT_ARG_HANDLE_CHECKED(String, x, 0);
  CONVERT_ARG_HANDLE_CHECKED(String, y, 1);
  return isolate->heap()->ToBoolean(!String::Equals(x, y));
}

RUNTIME_FUNCTION(Runtime_FlattenString) {
  HandleScope scope(isolate);
  DCHECK(args.length() == 1);
  CONVERT_ARG_HANDLE_CHECKED(String, str, 0);
  return *String::Flatten(str);
}


RUNTIME_FUNCTION(Runtime_StringCharFromCode) {
  HandleScope handlescope(isolate);
  DCHECK_EQ(1, args.length());
  if (args[0]->IsNumber()) {
    CONVERT_NUMBER_CHECKED(uint32_t, code, Uint32, args[0]);
    code &= 0xffff;
    return *isolate->factory()->LookupSingleCharacterStringFromCode(code);
  }
  return isolate->heap()->empty_string();
}

RUNTIME_FUNCTION(Runtime_ExternalStringGetChar) {
  SealHandleScope shs(isolate);
  DCHECK_EQ(2, args.length());
  CONVERT_ARG_CHECKED(ExternalString, string, 0);
  CONVERT_INT32_ARG_CHECKED(index, 1);
  return Smi::FromInt(string->Get(index));
}

RUNTIME_FUNCTION(Runtime_StringCharCodeAt) {
  SealHandleScope shs(isolate);
  DCHECK(args.length() == 2);
  if (!args[0]->IsString()) return isolate->heap()->undefined_value();
  if (!args[1]->IsNumber()) return isolate->heap()->undefined_value();
  if (std::isinf(args.number_at(1))) return isolate->heap()->nan_value();
  return __RT_impl_Runtime_StringCharCodeAtRT(args, isolate);
}

}  // namespace internal
}  // namespace v8