path: root/deps/v8/src/objects/string.cc

// Copyright 2019 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/objects/string.h"

#include "src/char-predicates.h"
#include "src/conversions.h"
#include "src/handles-inl.h"
#include "src/heap/heap-inl.h"  // For LooksValid implementation.
#include "src/objects/map.h"
#include "src/objects/oddball.h"
#include "src/objects/string-comparator.h"
#include "src/objects/string-inl.h"
#include "src/ostreams.h"
#include "src/string-builder-inl.h"
#include "src/string-hasher.h"
#include "src/string-search.h"
#include "src/string-stream.h"
#include "src/unicode-inl.h"

namespace v8 {
namespace internal {

Handle<String> String::SlowFlatten(Isolate* isolate, Handle<ConsString> cons,
                                   PretenureFlag pretenure) {
  DCHECK_NE(cons->second()->length(), 0);

  // TurboFan can create cons strings with empty first parts.
  while (cons->first()->length() == 0) {
    // We do not want to call this function recursively. Therefore we call
    // String::Flatten only in those cases where String::SlowFlatten is not
    // called again.
    if (cons->second()->IsConsString() && !cons->second()->IsFlat()) {
      cons = handle(ConsString::cast(cons->second()), isolate);
    } else {
      return String::Flatten(isolate, handle(cons->second(), isolate));
    }
  }

  DCHECK(AllowHeapAllocation::IsAllowed());
  int length = cons->length();
  PretenureFlag tenure = ObjectInYoungGeneration(*cons) ? pretenure : TENURED;
  Handle<SeqString> result;
  if (cons->IsOneByteRepresentation()) {
    Handle<SeqOneByteString> flat = isolate->factory()
                                        ->NewRawOneByteString(length, tenure)
                                        .ToHandleChecked();
    DisallowHeapAllocation no_gc;
    WriteToFlat(*cons, flat->GetChars(no_gc), 0, length);
    result = flat;
  } else {
    Handle<SeqTwoByteString> flat = isolate->factory()
                                        ->NewRawTwoByteString(length, tenure)
                                        .ToHandleChecked();
    DisallowHeapAllocation no_gc;
    WriteToFlat(*cons, flat->GetChars(no_gc), 0, length);
    result = flat;
  }
  cons->set_first(isolate, *result);
  cons->set_second(isolate, ReadOnlyRoots(isolate).empty_string());
  DCHECK(result->IsFlat());
  return result;
}

bool String::MakeExternal(v8::String::ExternalStringResource* resource) {
  DisallowHeapAllocation no_allocation;
  // Externalizing twice leaks the external resource, so it's
  // prohibited by the API.
  DCHECK(this->SupportsExternalization());
  DCHECK(resource->IsCacheable());
#ifdef ENABLE_SLOW_DCHECKS
  if (FLAG_enable_slow_asserts) {
    // Assert that the resource and the string are equivalent.
    DCHECK(static_cast<size_t>(this->length()) == resource->length());
    ScopedVector<uc16> smart_chars(this->length());
    String::WriteToFlat(*this, smart_chars.start(), 0, this->length());
    DCHECK_EQ(0, memcmp(smart_chars.start(), resource->data(),
                        resource->length() * sizeof(smart_chars[0])));
  }
#endif                      // DEBUG
  int size = this->Size();  // Byte size of the original string.
  // Abort if size does not allow in-place conversion.
  if (size < ExternalString::kUncachedSize) return false;
  Isolate* isolate;
  // Read-only strings cannot be made external, since that would mutate the
  // string.
  if (!GetIsolateFromWritableObject(*this, &isolate)) return false;
  Heap* heap = isolate->heap();
  bool is_internalized = this->IsInternalizedString();
  bool has_pointers = StringShape(*this).IsIndirect();
  if (has_pointers) {
    heap->NotifyObjectLayoutChange(*this, size, no_allocation);
  }
  // Morph the string to an external string by replacing the map and
  // reinitializing the fields.  This won't work if the space the existing
  // string occupies is too small for a regular external string.  Instead, we
  // resort to an uncached external string instead, omitting the field caching
  // the address of the backing store.  When we encounter uncached external
  // strings in generated code, we need to bailout to runtime.
  Map new_map;
  ReadOnlyRoots roots(heap);
  if (size < ExternalString::kSize) {
    if (is_internalized) {
      new_map = roots.uncached_external_internalized_string_map();
    } else {
      new_map = roots.uncached_external_string_map();
    }
  } else {
    new_map = is_internalized ? roots.external_internalized_string_map()
                              : roots.external_string_map();
  }

  // Byte size of the external String object.
  int new_size = this->SizeFromMap(new_map);
  heap->CreateFillerObjectAt(this->address() + new_size, size - new_size,
                             ClearRecordedSlots::kNo);
  if (has_pointers) {
    heap->ClearRecordedSlotRange(this->address(), this->address() + new_size);
  }

  // We are storing the new map using release store after creating a filler for
  // the left-over space to avoid races with the sweeper thread.
  this->synchronized_set_map(new_map);

  ExternalTwoByteString self = ExternalTwoByteString::cast(*this);
  self->SetResource(isolate, resource);
  heap->RegisterExternalString(*this);
  if (is_internalized) self->Hash();  // Force regeneration of the hash value.
  return true;
}

bool String::MakeExternal(v8::String::ExternalOneByteStringResource* resource) {
  DisallowHeapAllocation no_allocation;
  // Externalizing twice leaks the external resource, so it's
  // prohibited by the API.
  DCHECK(this->SupportsExternalization());
  DCHECK(resource->IsCacheable());
#ifdef ENABLE_SLOW_DCHECKS
  if (FLAG_enable_slow_asserts) {
    // Assert that the resource and the string are equivalent.
    DCHECK(static_cast<size_t>(this->length()) == resource->length());
    if (this->IsTwoByteRepresentation()) {
      ScopedVector<uint16_t> smart_chars(this->length());
      String::WriteToFlat(*this, smart_chars.start(), 0, this->length());
      DCHECK(String::IsOneByte(smart_chars.start(), this->length()));
    }
    ScopedVector<char> smart_chars(this->length());
    String::WriteToFlat(*this, smart_chars.start(), 0, this->length());
    DCHECK_EQ(0, memcmp(smart_chars.start(), resource->data(),
                        resource->length() * sizeof(smart_chars[0])));
  }
#endif                      // DEBUG
  int size = this->Size();  // Byte size of the original string.
  // Abort if size does not allow in-place conversion.
  if (size < ExternalString::kUncachedSize) return false;
  Isolate* isolate;
  // Read-only strings cannot be made external, since that would mutate the
  // string.
  if (!GetIsolateFromWritableObject(*this, &isolate)) return false;
  Heap* heap = isolate->heap();
  bool is_internalized = this->IsInternalizedString();
  bool has_pointers = StringShape(*this).IsIndirect();

  if (has_pointers) {
    heap->NotifyObjectLayoutChange(*this, size, no_allocation);
  }

  // Morph the string to an external string by replacing the map and
  // reinitializing the fields.  This won't work if the space the existing
  // string occupies is too small for a regular external string.  Instead, we
  // resort to an uncached external string instead, omitting the field caching
  // the address of the backing store.  When we encounter uncached external
  // strings in generated code, we need to bailout to runtime.
  Map new_map;
  ReadOnlyRoots roots(heap);
  if (size < ExternalString::kSize) {
    new_map = is_internalized
                  ? roots.uncached_external_one_byte_internalized_string_map()
                  : roots.uncached_external_one_byte_string_map();
  } else {
    new_map = is_internalized
                  ? roots.external_one_byte_internalized_string_map()
                  : roots.external_one_byte_string_map();
  }

  // Byte size of the external String object.
  int new_size = this->SizeFromMap(new_map);
  heap->CreateFillerObjectAt(this->address() + new_size, size - new_size,
                             ClearRecordedSlots::kNo);
  if (has_pointers) {
    heap->ClearRecordedSlotRange(this->address(), this->address() + new_size);
  }

  // We are storing the new map using release store after creating a filler for
  // the left-over space to avoid races with the sweeper thread.
  this->synchronized_set_map(new_map);

  ExternalOneByteString self = ExternalOneByteString::cast(*this);
  self->SetResource(isolate, resource);
  heap->RegisterExternalString(*this);
  if (is_internalized) self->Hash();  // Force regeneration of the hash value.
  return true;
}

bool String::SupportsExternalization() {
  if (this->IsThinString()) {
    return i::ThinString::cast(*this)->actual()->SupportsExternalization();
  }

  Isolate* isolate;
  // RO_SPACE strings cannot be externalized.
  if (!GetIsolateFromWritableObject(*this, &isolate)) {
    return false;
  }

  // Already an external string.
  if (StringShape(*this).IsExternal()) {
    return false;
  }

#ifdef V8_COMPRESS_POINTERS
  // Small strings may not be in-place externalizable.
  if (this->Size() < ExternalString::kUncachedSize) return false;
#else
  DCHECK_LE(ExternalString::kUncachedSize, this->Size());
#endif

  return !isolate->heap()->IsInGCPostProcessing();
}

void String::StringShortPrint(StringStream* accumulator, bool show_details) {
  const char* internalized_marker = this->IsInternalizedString() ? "#" : "";

  int len = length();
  if (len > kMaxShortPrintLength) {
    accumulator->Add("<Very long string[%s%u]>", internalized_marker, len);
    return;
  }

  if (!LooksValid()) {
    accumulator->Add("<Invalid String>");
    return;
  }

  StringCharacterStream stream(*this);

  bool truncated = false;
  if (len > kMaxShortPrintLength) {
    len = kMaxShortPrintLength;
    truncated = true;
  }
  bool one_byte = true;
  for (int i = 0; i < len; i++) {
    uint16_t c = stream.GetNext();

    if (c < 32 || c >= 127) {
      one_byte = false;
    }
  }
  stream.Reset(*this);
  if (one_byte) {
    if (show_details)
      accumulator->Add("<String[%s%u]: ", internalized_marker, length());
    for (int i = 0; i < len; i++) {
      accumulator->Put(static_cast<char>(stream.GetNext()));
    }
    if (show_details) accumulator->Put('>');
  } else {
    // Backslash indicates that the string contains control
    // characters and that backslashes are therefore escaped.
    if (show_details)
      accumulator->Add("<String[%s%u]\\: ", internalized_marker, length());
    for (int i = 0; i < len; i++) {
      uint16_t c = stream.GetNext();
      if (c == '\n') {
        accumulator->Add("\\n");
      } else if (c == '\r') {
        accumulator->Add("\\r");
      } else if (c == '\\') {
        accumulator->Add("\\\\");
      } else if (c < 32 || c > 126) {
        accumulator->Add("\\x%02x", c);
      } else {
        accumulator->Put(static_cast<char>(c));
      }
    }
    if (truncated) {
      accumulator->Put('.');
      accumulator->Put('.');
      accumulator->Put('.');
    }
    if (show_details) accumulator->Put('>');
  }
  return;
}

void String::PrintUC16(std::ostream& os, int start, int end) {  // NOLINT
  if (end < 0) end = length();
  StringCharacterStream stream(*this, start);
  for (int i = start; i < end && stream.HasMore(); i++) {
    os << AsUC16(stream.GetNext());
  }
}

// static
Handle<String> String::Trim(Isolate* isolate, Handle<String> string,
                            TrimMode mode) {
  string = String::Flatten(isolate, string);
  int const length = string->length();

  // Perform left trimming if requested.
  int left = 0;
  if (mode == kTrim || mode == kTrimStart) {
    while (left < length && IsWhiteSpaceOrLineTerminator(string->Get(left))) {
      left++;
    }
  }

  // Perform right trimming if requested.
  int right = length;
  if (mode == kTrim || mode == kTrimEnd) {
    while (right > left &&
           IsWhiteSpaceOrLineTerminator(string->Get(right - 1))) {
      right--;
    }
  }

  return isolate->factory()->NewSubString(string, left, right);
}

bool String::LooksValid() {
  // TODO(leszeks): Maybe remove this check entirely, Heap::Contains uses
  // basically the same logic as the way we access the heap in the first place.
  MemoryChunk* chunk = MemoryChunk::FromHeapObject(*this);
  // RO_SPACE objects should always be valid.
  if (chunk->owner()->identity() == RO_SPACE) return true;
  if (chunk->heap() == nullptr) return false;
  return chunk->heap()->Contains(*this);
}

namespace {

bool AreDigits(const uint8_t* s, int from, int to) {
  for (int i = from; i < to; i++) {
    if (s[i] < '0' || s[i] > '9') return false;
  }

  return true;
}

int ParseDecimalInteger(const uint8_t* s, int from, int to) {
  DCHECK_LT(to - from, 10);  // Overflow is not possible.
  DCHECK(from < to);
  int d = s[from] - '0';

  for (int i = from + 1; i < to; i++) {
    d = 10 * d + (s[i] - '0');
  }

  return d;
}

}  // namespace

// static
Handle<Object> String::ToNumber(Isolate* isolate, Handle<String> subject) {
  // Flatten {subject} string first.
  subject = String::Flatten(isolate, subject);

  // Fast array index case.
  uint32_t index;
  if (subject->AsArrayIndex(&index)) {
    return isolate->factory()->NewNumberFromUint(index);
  }

  // Fast case: short integer or some sorts of junk values.
  if (subject->IsSeqOneByteString()) {
    int len = subject->length();
    if (len == 0) return handle(Smi::kZero, isolate);

    DisallowHeapAllocation no_gc;
    uint8_t const* data =
        Handle<SeqOneByteString>::cast(subject)->GetChars(no_gc);
    bool minus = (data[0] == '-');
    int start_pos = (minus ? 1 : 0);

    if (start_pos == len) {
      return isolate->factory()->nan_value();
    } else if (data[start_pos] > '9') {
      // Fast check for a junk value. A valid string may start from a
      // whitespace, a sign ('+' or '-'), the decimal point, a decimal digit
      // or the 'I' character ('Infinity'). All of that have codes not greater
      // than '9' except 'I' and &nbsp;.
      if (data[start_pos] != 'I' && data[start_pos] != 0xA0) {
        return isolate->factory()->nan_value();
      }
    } else if (len - start_pos < 10 && AreDigits(data, start_pos, len)) {
      // The maximal/minimal smi has 10 digits. If the string has less digits
      // we know it will fit into the smi-data type.
      int d = ParseDecimalInteger(data, start_pos, len);
      if (minus) {
        if (d == 0) return isolate->factory()->minus_zero_value();
        d = -d;
      } else if (!subject->HasHashCode() && len <= String::kMaxArrayIndexSize &&
                 (len == 1 || data[0] != '0')) {
        // String hash is not calculated yet but all the data are present.
        // Update the hash field to speed up sequential convertions.
        uint32_t hash = StringHasher::MakeArrayIndexHash(d, len);
#ifdef DEBUG
        subject->Hash();  // Force hash calculation.
        DCHECK_EQ(static_cast<int>(subject->hash_field()),
                  static_cast<int>(hash));
#endif
        subject->set_hash_field(hash);
      }
      return handle(Smi::FromInt(d), isolate);
    }
  }

  // Slower case.
  int flags = ALLOW_HEX | ALLOW_OCTAL | ALLOW_BINARY;
  return isolate->factory()->NewNumber(StringToDouble(isolate, subject, flags));
}

String::FlatContent String::GetFlatContent(
    const DisallowHeapAllocation& no_gc) {
  USE(no_gc);
  int length = this->length();
  StringShape shape(*this);
  String string = *this;
  int offset = 0;
  if (shape.representation_tag() == kConsStringTag) {
    ConsString cons = ConsString::cast(string);
    if (cons->second()->length() != 0) {
      return FlatContent();
    }
    string = cons->first();
    shape = StringShape(string);
  } else if (shape.representation_tag() == kSlicedStringTag) {
    SlicedString slice = SlicedString::cast(string);
    offset = slice->offset();
    string = slice->parent();
    shape = StringShape(string);
    DCHECK(shape.representation_tag() != kConsStringTag &&
           shape.representation_tag() != kSlicedStringTag);
  }
  if (shape.representation_tag() == kThinStringTag) {
    ThinString thin = ThinString::cast(string);
    string = thin->actual();
    shape = StringShape(string);
    DCHECK(!shape.IsCons());
    DCHECK(!shape.IsSliced());
  }
  if (shape.encoding_tag() == kOneByteStringTag) {
    const uint8_t* start;
    if (shape.representation_tag() == kSeqStringTag) {
      start = SeqOneByteString::cast(string)->GetChars(no_gc);
    } else {
      start = ExternalOneByteString::cast(string)->GetChars();
    }
    return FlatContent(start + offset, length);
  } else {
    DCHECK_EQ(shape.encoding_tag(), kTwoByteStringTag);
    const uc16* start;
    if (shape.representation_tag() == kSeqStringTag) {
      start = SeqTwoByteString::cast(string)->GetChars(no_gc);
    } else {
      start = ExternalTwoByteString::cast(string)->GetChars();
    }
    return FlatContent(start + offset, length);
  }
}

std::unique_ptr<char[]> String::ToCString(AllowNullsFlag allow_nulls,
                                          RobustnessFlag robust_flag,
                                          int offset, int length,
                                          int* length_return) {
  if (robust_flag == ROBUST_STRING_TRAVERSAL && !LooksValid()) {
    return std::unique_ptr<char[]>();
  }
  // Negative length means the to the end of the string.
  if (length < 0) length = kMaxInt - offset;

  // Compute the size of the UTF-8 string. Start at the specified offset.
  StringCharacterStream stream(*this, offset);
  int character_position = offset;
  int utf8_bytes = 0;
  int last = unibrow::Utf16::kNoPreviousCharacter;
  while (stream.HasMore() && character_position++ < offset + length) {
    uint16_t character = stream.GetNext();
    utf8_bytes += unibrow::Utf8::Length(character, last);
    last = character;
  }

  if (length_return) {
    *length_return = utf8_bytes;
  }

  char* result = NewArray<char>(utf8_bytes + 1);

  // Convert the UTF-16 string to a UTF-8 buffer. Start at the specified offset.
  stream.Reset(*this, offset);
  character_position = offset;
  int utf8_byte_position = 0;
  last = unibrow::Utf16::kNoPreviousCharacter;
  while (stream.HasMore() && character_position++ < offset + length) {
    uint16_t character = stream.GetNext();
    if (allow_nulls == DISALLOW_NULLS && character == 0) {
      character = ' ';
    }
    utf8_byte_position +=
        unibrow::Utf8::Encode(result + utf8_byte_position, character, last);
    last = character;
  }
  result[utf8_byte_position] = 0;
  return std::unique_ptr<char[]>(result);
}

std::unique_ptr<char[]> String::ToCString(AllowNullsFlag allow_nulls,
                                          RobustnessFlag robust_flag,
                                          int* length_return) {
  return ToCString(allow_nulls, robust_flag, 0, -1, length_return);
}

template <typename sinkchar>
void String::WriteToFlat(String src, sinkchar* sink, int f, int t) {
  DisallowHeapAllocation no_gc;
  String source = src;
  int from = f;
  int to = t;
  while (true) {
    DCHECK(0 <= from && from <= to && to <= source->length());
    switch (StringShape(source).full_representation_tag()) {
      case kOneByteStringTag | kExternalStringTag: {
        CopyChars(sink, ExternalOneByteString::cast(source)->GetChars() + from,
                  to - from);
        return;
      }
      case kTwoByteStringTag | kExternalStringTag: {
        const uc16* data = ExternalTwoByteString::cast(source)->GetChars();
        CopyChars(sink, data + from, to - from);
        return;
      }
      case kOneByteStringTag | kSeqStringTag: {
        CopyChars(sink, SeqOneByteString::cast(source)->GetChars(no_gc) + from,
                  to - from);
        return;
      }
      case kTwoByteStringTag | kSeqStringTag: {
        CopyChars(sink, SeqTwoByteString::cast(source)->GetChars(no_gc) + from,
                  to - from);
        return;
      }
      case kOneByteStringTag | kConsStringTag:
      case kTwoByteStringTag | kConsStringTag: {
        ConsString cons_string = ConsString::cast(source);
        String first = cons_string->first();
        int boundary = first->length();
        if (to - boundary >= boundary - from) {
          // Right hand side is longer.  Recurse over left.
          if (from < boundary) {
            WriteToFlat(first, sink, from, boundary);
            if (from == 0 && cons_string->second() == first) {
              CopyChars(sink + boundary, sink, boundary);
              return;
            }
            sink += boundary - from;
            from = 0;
          } else {
            from -= boundary;
          }
          to -= boundary;
          source = cons_string->second();
        } else {
          // Left hand side is longer.  Recurse over right.
          if (to > boundary) {
            String second = cons_string->second();
            // When repeatedly appending to a string, we get a cons string that
            // is unbalanced to the left, a list, essentially.  We inline the
            // common case of sequential one-byte right child.
            if (to - boundary == 1) {
              sink[boundary - from] = static_cast<sinkchar>(second->Get(0));
            } else if (second->IsSeqOneByteString()) {
              CopyChars(sink + boundary - from,
                        SeqOneByteString::cast(second)->GetChars(no_gc),
                        to - boundary);
            } else {
              WriteToFlat(second, sink + boundary - from, 0, to - boundary);
            }
            to = boundary;
          }
          source = first;
        }
        break;
      }
      case kOneByteStringTag | kSlicedStringTag:
      case kTwoByteStringTag | kSlicedStringTag: {
        SlicedString slice = SlicedString::cast(source);
        unsigned offset = slice->offset();
        WriteToFlat(slice->parent(), sink, from + offset, to + offset);
        return;
      }
      case kOneByteStringTag | kThinStringTag:
      case kTwoByteStringTag | kThinStringTag:
        source = ThinString::cast(source)->actual();
        break;
    }
  }
}

template <typename SourceChar>
static void CalculateLineEndsImpl(Isolate* isolate, std::vector<int>* line_ends,
                                  Vector<const SourceChar> src,
                                  bool include_ending_line) {
  const int src_len = src.length();
  for (int i = 0; i < src_len - 1; i++) {
    SourceChar current = src[i];
    SourceChar next = src[i + 1];
    if (IsLineTerminatorSequence(current, next)) line_ends->push_back(i);
  }

  if (src_len > 0 && IsLineTerminatorSequence(src[src_len - 1], 0)) {
    line_ends->push_back(src_len - 1);
  }
  if (include_ending_line) {
    // Include one character beyond the end of script. The rewriter uses that
    // position for the implicit return statement.
    line_ends->push_back(src_len);
  }
}

Handle<FixedArray> String::CalculateLineEnds(Isolate* isolate,
                                             Handle<String> src,
                                             bool include_ending_line) {
  src = Flatten(isolate, src);
  // Rough estimate of line count based on a roughly estimated average
  // length of (unpacked) code.
  int line_count_estimate = src->length() >> 4;
  std::vector<int> line_ends;
  line_ends.reserve(line_count_estimate);
  {
    DisallowHeapAllocation no_allocation;  // ensure vectors stay valid.
    // Dispatch on type of strings.
    String::FlatContent content = src->GetFlatContent(no_allocation);
    DCHECK(content.IsFlat());
    if (content.IsOneByte()) {
      CalculateLineEndsImpl(isolate, &line_ends, content.ToOneByteVector(),
                            include_ending_line);
    } else {
      CalculateLineEndsImpl(isolate, &line_ends, content.ToUC16Vector(),
                            include_ending_line);
    }
  }
  int line_count = static_cast<int>(line_ends.size());
  Handle<FixedArray> array = isolate->factory()->NewFixedArray(line_count);
  for (int i = 0; i < line_count; i++) {
    array->set(i, Smi::FromInt(line_ends[i]));
  }
  return array;
}

bool String::SlowEquals(String other) {
  DisallowHeapAllocation no_gc;
  // Fast check: negative check with lengths.
  int len = length();
  if (len != other->length()) return false;
  if (len == 0) return true;

  // Fast check: if at least one ThinString is involved, dereference it/them
  // and restart.
  if (this->IsThinString() || other->IsThinString()) {
    if (other->IsThinString()) other = ThinString::cast(other)->actual();
    if (this->IsThinString()) {
      return ThinString::cast(*this)->actual()->Equals(other);
    } else {
      return this->Equals(other);
    }
  }

  // Fast check: if hash code is computed for both strings
  // a fast negative check can be performed.
  if (HasHashCode() && other->HasHashCode()) {
#ifdef ENABLE_SLOW_DCHECKS
    if (FLAG_enable_slow_asserts) {
      if (Hash() != other->Hash()) {
        bool found_difference = false;
        for (int i = 0; i < len; i++) {
          if (Get(i) != other->Get(i)) {
            found_difference = true;
            break;
          }
        }
        DCHECK(found_difference);
      }
    }
#endif
    if (Hash() != other->Hash()) return false;
  }

  // We know the strings are both non-empty. Compare the first chars
  // before we try to flatten the strings.
  if (this->Get(0) != other->Get(0)) return false;

  if (IsSeqOneByteString() && other->IsSeqOneByteString()) {
    const uint8_t* str1 = SeqOneByteString::cast(*this)->GetChars(no_gc);
    const uint8_t* str2 = SeqOneByteString::cast(other)->GetChars(no_gc);
    return CompareRawStringContents(str1, str2, len);
  }

  StringComparator comparator;
  return comparator.Equals(*this, other);
}

bool String::SlowEquals(Isolate* isolate, Handle<String> one,
                        Handle<String> two) {
  // Fast check: negative check with lengths.
  int one_length = one->length();
  if (one_length != two->length()) return false;
  if (one_length == 0) return true;

  // Fast check: if at least one ThinString is involved, dereference it/them
  // and restart.
  if (one->IsThinString() || two->IsThinString()) {
    if (one->IsThinString())
      one = handle(ThinString::cast(*one)->actual(), isolate);
    if (two->IsThinString())
      two = handle(ThinString::cast(*two)->actual(), isolate);
    return String::Equals(isolate, one, two);
  }

  // Fast check: if hash code is computed for both strings
  // a fast negative check can be performed.
  if (one->HasHashCode() && two->HasHashCode()) {
#ifdef ENABLE_SLOW_DCHECKS
    if (FLAG_enable_slow_asserts) {
      if (one->Hash() != two->Hash()) {
        bool found_difference = false;
        for (int i = 0; i < one_length; i++) {
          if (one->Get(i) != two->Get(i)) {
            found_difference = true;
            break;
          }
        }
        DCHECK(found_difference);
      }
    }
#endif
    if (one->Hash() != two->Hash()) return false;
  }

  // We know the strings are both non-empty. Compare the first chars
  // before we try to flatten the strings.
  if (one->Get(0) != two->Get(0)) return false;

  one = String::Flatten(isolate, one);
  two = String::Flatten(isolate, two);

  DisallowHeapAllocation no_gc;
  String::FlatContent flat1 = one->GetFlatContent(no_gc);
  String::FlatContent flat2 = two->GetFlatContent(no_gc);

  if (flat1.IsOneByte() && flat2.IsOneByte()) {
    return CompareRawStringContents(flat1.ToOneByteVector().start(),
                                    flat2.ToOneByteVector().start(),
                                    one_length);
  } else {
    for (int i = 0; i < one_length; i++) {
      if (flat1.Get(i) != flat2.Get(i)) return false;
    }
    return true;
  }
}

// static
ComparisonResult String::Compare(Isolate* isolate, Handle<String> x,
                                 Handle<String> y) {
  // A few fast case tests before we flatten.
  if (x.is_identical_to(y)) {
    return ComparisonResult::kEqual;
  } else if (y->length() == 0) {
    return x->length() == 0 ? ComparisonResult::kEqual
                            : ComparisonResult::kGreaterThan;
  } else if (x->length() == 0) {
    return ComparisonResult::kLessThan;
  }

  int const d = x->Get(0) - y->Get(0);
  if (d < 0) {
    return ComparisonResult::kLessThan;
  } else if (d > 0) {
    return ComparisonResult::kGreaterThan;
  }

  // Slow case.
  x = String::Flatten(isolate, x);
  y = String::Flatten(isolate, y);

  DisallowHeapAllocation no_gc;
  ComparisonResult result = ComparisonResult::kEqual;
  int prefix_length = x->length();
  if (y->length() < prefix_length) {
    prefix_length = y->length();
    result = ComparisonResult::kGreaterThan;
  } else if (y->length() > prefix_length) {
    result = ComparisonResult::kLessThan;
  }
  int r;
  String::FlatContent x_content = x->GetFlatContent(no_gc);
  String::FlatContent y_content = y->GetFlatContent(no_gc);
  if (x_content.IsOneByte()) {
    Vector<const uint8_t> x_chars = x_content.ToOneByteVector();
    if (y_content.IsOneByte()) {
      Vector<const uint8_t> y_chars = y_content.ToOneByteVector();
      r = CompareChars(x_chars.start(), y_chars.start(), prefix_length);
    } else {
      Vector<const uc16> y_chars = y_content.ToUC16Vector();
      r = CompareChars(x_chars.start(), y_chars.start(), prefix_length);
    }
  } else {
    Vector<const uc16> x_chars = x_content.ToUC16Vector();
    if (y_content.IsOneByte()) {
      Vector<const uint8_t> y_chars = y_content.ToOneByteVector();
      r = CompareChars(x_chars.start(), y_chars.start(), prefix_length);
    } else {
      Vector<const uc16> y_chars = y_content.ToUC16Vector();
      r = CompareChars(x_chars.start(), y_chars.start(), prefix_length);
    }
  }
  if (r < 0) {
    result = ComparisonResult::kLessThan;
  } else if (r > 0) {
    result = ComparisonResult::kGreaterThan;
  }
  return result;
}

Object String::IndexOf(Isolate* isolate, Handle<Object> receiver,
                       Handle<Object> search, Handle<Object> position) {
  if (receiver->IsNullOrUndefined(isolate)) {
    THROW_NEW_ERROR_RETURN_FAILURE(
        isolate, NewTypeError(MessageTemplate::kCalledOnNullOrUndefined,
                              isolate->factory()->NewStringFromAsciiChecked(
                                  "String.prototype.indexOf")));
  }
  Handle<String> receiver_string;
  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, receiver_string,
                                     Object::ToString(isolate, receiver));

  Handle<String> search_string;
  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, search_string,
                                     Object::ToString(isolate, search));

  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, position,
                                     Object::ToInteger(isolate, position));

  uint32_t index = receiver_string->ToValidIndex(*position);
  return Smi::FromInt(
      String::IndexOf(isolate, receiver_string, search_string, index));
}

namespace {

template <typename T>
int SearchString(Isolate* isolate, String::FlatContent receiver_content,
                 Vector<T> pat_vector, int start_index) {
  if (receiver_content.IsOneByte()) {
    return SearchString(isolate, receiver_content.ToOneByteVector(), pat_vector,
                        start_index);
  }
  return SearchString(isolate, receiver_content.ToUC16Vector(), pat_vector,
                      start_index);
}

}  // namespace

int String::IndexOf(Isolate* isolate, Handle<String> receiver,
                    Handle<String> search, int start_index) {
  DCHECK_LE(0, start_index);
  DCHECK(start_index <= receiver->length());

  uint32_t search_length = search->length();
  if (search_length == 0) return start_index;

  uint32_t receiver_length = receiver->length();
  if (start_index + search_length > receiver_length) return -1;

  receiver = String::Flatten(isolate, receiver);
  search = String::Flatten(isolate, search);

  DisallowHeapAllocation no_gc;  // ensure vectors stay valid
  // Extract flattened substrings of cons strings before getting encoding.
  String::FlatContent receiver_content = receiver->GetFlatContent(no_gc);
  String::FlatContent search_content = search->GetFlatContent(no_gc);

  // dispatch on type of strings
  if (search_content.IsOneByte()) {
    Vector<const uint8_t> pat_vector = search_content.ToOneByteVector();
    return SearchString<const uint8_t>(isolate, receiver_content, pat_vector,
                                       start_index);
  }
  Vector<const uc16> pat_vector = search_content.ToUC16Vector();
  return SearchString<const uc16>(isolate, receiver_content, pat_vector,
                                  start_index);
}

MaybeHandle<String> String::GetSubstitution(Isolate* isolate, Match* match,
                                            Handle<String> replacement,
                                            int start_index) {
  DCHECK_GE(start_index, 0);

  Factory* factory = isolate->factory();

  const int replacement_length = replacement->length();
  const int captures_length = match->CaptureCount();

  replacement = String::Flatten(isolate, replacement);

  Handle<String> dollar_string =
      factory->LookupSingleCharacterStringFromCode('$');
  int next_dollar_ix =
      String::IndexOf(isolate, replacement, dollar_string, start_index);
  if (next_dollar_ix < 0) {
    return replacement;
  }

  IncrementalStringBuilder builder(isolate);

  if (next_dollar_ix > 0) {
    builder.AppendString(factory->NewSubString(replacement, 0, next_dollar_ix));
  }

  while (true) {
    const int peek_ix = next_dollar_ix + 1;
    if (peek_ix >= replacement_length) {
      builder.AppendCharacter('$');
      return builder.Finish();
    }

    int continue_from_ix = -1;
    const uint16_t peek = replacement->Get(peek_ix);
    switch (peek) {
      case '$':  // $$
        builder.AppendCharacter('$');
        continue_from_ix = peek_ix + 1;
        break;
      case '&':  // $& - match
        builder.AppendString(match->GetMatch());
        continue_from_ix = peek_ix + 1;
        break;
      case '`':  // $` - prefix
        builder.AppendString(match->GetPrefix());
        continue_from_ix = peek_ix + 1;
        break;
      case '\'':  // $' - suffix
        builder.AppendString(match->GetSuffix());
        continue_from_ix = peek_ix + 1;
        break;
      case '0':
      case '1':
      case '2':
      case '3':
      case '4':
      case '5':
      case '6':
      case '7':
      case '8':
      case '9': {
        // Valid indices are $1 .. $9, $01 .. $09 and $10 .. $99
        int scaled_index = (peek - '0');
        int advance = 1;

        if (peek_ix + 1 < replacement_length) {
          const uint16_t next_peek = replacement->Get(peek_ix + 1);
          if (next_peek >= '0' && next_peek <= '9') {
            const int new_scaled_index = scaled_index * 10 + (next_peek - '0');
            if (new_scaled_index < captures_length) {
              scaled_index = new_scaled_index;
              advance = 2;
            }
          }
        }

        if (scaled_index == 0 || scaled_index >= captures_length) {
          builder.AppendCharacter('$');
          continue_from_ix = peek_ix;
          break;
        }

        bool capture_exists;
        Handle<String> capture;
        ASSIGN_RETURN_ON_EXCEPTION(
            isolate, capture, match->GetCapture(scaled_index, &capture_exists),
            String);
        if (capture_exists) builder.AppendString(capture);
        continue_from_ix = peek_ix + advance;
        break;
      }
      case '<': {  // $<name> - named capture
        typedef String::Match::CaptureState CaptureState;

        if (!match->HasNamedCaptures()) {
          builder.AppendCharacter('$');
          continue_from_ix = peek_ix;
          break;
        }

        Handle<String> bracket_string =
            factory->LookupSingleCharacterStringFromCode('>');
        const int closing_bracket_ix =
            String::IndexOf(isolate, replacement, bracket_string, peek_ix + 1);

        if (closing_bracket_ix == -1) {
          // No closing bracket was found, treat '$<' as a string literal.
          builder.AppendCharacter('$');
          continue_from_ix = peek_ix;
          break;
        }

        Handle<String> capture_name =
            factory->NewSubString(replacement, peek_ix + 1, closing_bracket_ix);
        Handle<String> capture;
        CaptureState capture_state;
        ASSIGN_RETURN_ON_EXCEPTION(
            isolate, capture,
            match->GetNamedCapture(capture_name, &capture_state), String);

        switch (capture_state) {
          case CaptureState::INVALID:
          case CaptureState::UNMATCHED:
            break;
          case CaptureState::MATCHED:
            builder.AppendString(capture);
            break;
        }

        continue_from_ix = closing_bracket_ix + 1;
        break;
      }
      default:
        builder.AppendCharacter('$');
        continue_from_ix = peek_ix;
        break;
    }

    // Go the the next $ in the replacement.
    // TODO(jgruber): Single-char lookups could be much more efficient.
    DCHECK_NE(continue_from_ix, -1);
    next_dollar_ix =
        String::IndexOf(isolate, replacement, dollar_string, continue_from_ix);

    // Return if there are no more $ characters in the replacement. If we
    // haven't reached the end, we need to append the suffix.
    if (next_dollar_ix < 0) {
      if (continue_from_ix < replacement_length) {
        builder.AppendString(factory->NewSubString(
            replacement, continue_from_ix, replacement_length));
      }
      return builder.Finish();
    }

    // Append substring between the previous and the next $ character.
    if (next_dollar_ix > continue_from_ix) {
      builder.AppendString(
          factory->NewSubString(replacement, continue_from_ix, next_dollar_ix));
    }
  }

  UNREACHABLE();
}

namespace {  // for String.Prototype.lastIndexOf

template <typename schar, typename pchar>
int StringMatchBackwards(Vector<const schar> subject,
                         Vector<const pchar> pattern, int idx) {
  int pattern_length = pattern.length();
  DCHECK_GE(pattern_length, 1);
  DCHECK(idx + pattern_length <= subject.length());

  if (sizeof(schar) == 1 && sizeof(pchar) > 1) {
    for (int i = 0; i < pattern_length; i++) {
      uc16 c = pattern[i];
      if (c > String::kMaxOneByteCharCode) {
        return -1;
      }
    }
  }

  pchar pattern_first_char = pattern[0];
  for (int i = idx; i >= 0; i--) {
    if (subject[i] != pattern_first_char) continue;
    int j = 1;
    while (j < pattern_length) {
      if (pattern[j] != subject[i + j]) {
        break;
      }
      j++;
    }
    if (j == pattern_length) {
      return i;
    }
  }
  return -1;
}

}  // namespace

Object String::LastIndexOf(Isolate* isolate, Handle<Object> receiver,
                           Handle<Object> search, Handle<Object> position) {
  if (receiver->IsNullOrUndefined(isolate)) {
    THROW_NEW_ERROR_RETURN_FAILURE(
        isolate, NewTypeError(MessageTemplate::kCalledOnNullOrUndefined,
                              isolate->factory()->NewStringFromAsciiChecked(
                                  "String.prototype.lastIndexOf")));
  }
  Handle<String> receiver_string;
  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, receiver_string,
                                     Object::ToString(isolate, receiver));

  Handle<String> search_string;
  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, search_string,
                                     Object::ToString(isolate, search));

  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, position,
                                     Object::ToNumber(isolate, position));

  uint32_t start_index;

  if (position->IsNaN()) {
    start_index = receiver_string->length();
  } else {
    ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, position,
                                       Object::ToInteger(isolate, position));
    start_index = receiver_string->ToValidIndex(*position);
  }

  uint32_t pattern_length = search_string->length();
  uint32_t receiver_length = receiver_string->length();

  if (start_index + pattern_length > receiver_length) {
    start_index = receiver_length - pattern_length;
  }

  if (pattern_length == 0) {
    return Smi::FromInt(start_index);
  }

  receiver_string = String::Flatten(isolate, receiver_string);
  search_string = String::Flatten(isolate, search_string);

  int last_index = -1;
  DisallowHeapAllocation no_gc;  // ensure vectors stay valid

  String::FlatContent receiver_content = receiver_string->GetFlatContent(no_gc);
  String::FlatContent search_content = search_string->GetFlatContent(no_gc);

  if (search_content.IsOneByte()) {
    Vector<const uint8_t> pat_vector = search_content.ToOneByteVector();
    if (receiver_content.IsOneByte()) {
      last_index = StringMatchBackwards(receiver_content.ToOneByteVector(),
                                        pat_vector, start_index);
    } else {
      last_index = StringMatchBackwards(receiver_content.ToUC16Vector(),
                                        pat_vector, start_index);
    }
  } else {
    Vector<const uc16> pat_vector = search_content.ToUC16Vector();
    if (receiver_content.IsOneByte()) {
      last_index = StringMatchBackwards(receiver_content.ToOneByteVector(),
                                        pat_vector, start_index);
    } else {
      last_index = StringMatchBackwards(receiver_content.ToUC16Vector(),
                                        pat_vector, start_index);
    }
  }
  return Smi::FromInt(last_index);
}

bool String::IsUtf8EqualTo(Vector<const char> str, bool allow_prefix_match) {
  int slen = length();
  // Can't check exact length equality, but we can check bounds.
  int str_len = str.length();
  if (!allow_prefix_match &&
      (str_len < slen ||
       str_len > slen * static_cast<int>(unibrow::Utf8::kMaxEncodedSize))) {
    return false;
  }

  int i = 0;
  unibrow::Utf8Iterator it = unibrow::Utf8Iterator(str);
  while (i < slen && !it.Done()) {
    if (Get(i++) != *it) return false;
    ++it;
  }

  return (allow_prefix_match || i == slen) && it.Done();
}

template <>
bool String::IsEqualTo(Vector<const uint8_t> str) {
  return IsOneByteEqualTo(str);
}

template <>
bool String::IsEqualTo(Vector<const uc16> str) {
  return IsTwoByteEqualTo(str);
}

bool String::IsOneByteEqualTo(Vector<const uint8_t> str) {
  int slen = length();
  if (str.length() != slen) return false;
  DisallowHeapAllocation no_gc;
  FlatContent content = GetFlatContent(no_gc);
  if (content.IsOneByte()) {
    return CompareChars(content.ToOneByteVector().start(), str.start(), slen) ==
           0;
  }
  return CompareChars(content.ToUC16Vector().start(), str.start(), slen) == 0;
}

bool String::IsTwoByteEqualTo(Vector<const uc16> str) {
  int slen = length();
  if (str.length() != slen) return false;
  DisallowHeapAllocation no_gc;
  FlatContent content = GetFlatContent(no_gc);
  if (content.IsOneByte()) {
    return CompareChars(content.ToOneByteVector().start(), str.start(), slen) ==
           0;
  }
  return CompareChars(content.ToUC16Vector().start(), str.start(), slen) == 0;
}

uint32_t String::ComputeAndSetHash() {
  DisallowHeapAllocation no_gc;
  // Should only be called if hash code has not yet been computed.
  DCHECK(!HasHashCode());

  // Store the hash code in the object.
  uint32_t field =
      IteratingStringHasher::Hash(*this, HashSeed(GetReadOnlyRoots()));
  set_hash_field(field);

  // Check the hash code is there.
  DCHECK(HasHashCode());
  uint32_t result = field >> kHashShift;
  DCHECK_NE(result, 0);  // Ensure that the hash value of 0 is never computed.
  return result;
}

bool String::ComputeArrayIndex(uint32_t* index) {
  int length = this->length();
  if (length == 0 || length > kMaxArrayIndexSize) return false;
  StringCharacterStream stream(*this);
  return StringToArrayIndex(&stream, index);
}

bool String::SlowAsArrayIndex(uint32_t* index) {
  DisallowHeapAllocation no_gc;
  if (length() <= kMaxCachedArrayIndexLength) {
    Hash();  // force computation of hash code
    uint32_t field = hash_field();
    if ((field & kIsNotArrayIndexMask) != 0) return false;
    // Isolate the array index form the full hash field.
    *index = ArrayIndexValueBits::decode(field);
    return true;
  } else {
    return ComputeArrayIndex(index);
  }
}

void String::PrintOn(FILE* file) {
  int length = this->length();
  for (int i = 0; i < length; i++) {
    PrintF(file, "%c", Get(i));
  }
}

Handle<String> SeqString::Truncate(Handle<SeqString> string, int new_length) {
  if (new_length == 0) return string->GetReadOnlyRoots().empty_string_handle();

  int new_size, old_size;
  int old_length = string->length();
  if (old_length <= new_length) return string;

  if (string->IsSeqOneByteString()) {
    old_size = SeqOneByteString::SizeFor(old_length);
    new_size = SeqOneByteString::SizeFor(new_length);
  } else {
    DCHECK(string->IsSeqTwoByteString());
    old_size = SeqTwoByteString::SizeFor(old_length);
    new_size = SeqTwoByteString::SizeFor(new_length);
  }

  int delta = old_size - new_size;

  Address start_of_string = string->address();
  DCHECK(IsAligned(start_of_string, kObjectAlignment));
  DCHECK(IsAligned(start_of_string + new_size, kObjectAlignment));

  Heap* heap = Heap::FromWritableHeapObject(*string);
  // Sizes are pointer size aligned, so that we can use filler objects
  // that are a multiple of pointer size.
  heap->CreateFillerObjectAt(start_of_string + new_size, delta,
                             ClearRecordedSlots::kNo);
  // We are storing the new length using release store after creating a filler
  // for the left-over space to avoid races with the sweeper thread.
  string->synchronized_set_length(new_length);

  return string;
}

void SeqOneByteString::clear_padding() {
  int data_size = SeqString::kHeaderSize + length() * kOneByteSize;
  memset(reinterpret_cast<void*>(address() + data_size), 0,
         SizeFor(length()) - data_size);
}

void SeqTwoByteString::clear_padding() {
  int data_size = SeqString::kHeaderSize + length() * kUC16Size;
  memset(reinterpret_cast<void*>(address() + data_size), 0,
         SizeFor(length()) - data_size);
}

uint16_t ConsString::ConsStringGet(int index) {
  DCHECK(index >= 0 && index < this->length());

  // Check for a flattened cons string
  if (second()->length() == 0) {
    String left = first();
    return left->Get(index);
  }

  String string = String::cast(*this);

  while (true) {
    if (StringShape(string).IsCons()) {
      ConsString cons_string = ConsString::cast(string);
      String left = cons_string->first();
      if (left->length() > index) {
        string = left;
      } else {
        index -= left->length();
        string = cons_string->second();
      }
    } else {
      return string->Get(index);
    }
  }

  UNREACHABLE();
}

uint16_t ThinString::ThinStringGet(int index) { return actual()->Get(index); }

uint16_t SlicedString::SlicedStringGet(int index) {
  return parent()->Get(offset() + index);
}

int ExternalString::ExternalPayloadSize() const {
  int length_multiplier = IsTwoByteRepresentation() ? i::kShortSize : kCharSize;
  return length() * length_multiplier;
}

FlatStringReader::FlatStringReader(Isolate* isolate, Handle<String> str)
    : Relocatable(isolate), str_(str.location()), length_(str->length()) {
  PostGarbageCollection();
}

FlatStringReader::FlatStringReader(Isolate* isolate, Vector<const char> input)
    : Relocatable(isolate),
      str_(nullptr),
      is_one_byte_(true),
      length_(input.length()),
      start_(input.start()) {}

void FlatStringReader::PostGarbageCollection() {
  if (str_ == nullptr) return;
  Handle<String> str(str_);
  DCHECK(str->IsFlat());
  DisallowHeapAllocation no_gc;
  // This does not actually prevent the vector from being relocated later.
  String::FlatContent content = str->GetFlatContent(no_gc);
  DCHECK(content.IsFlat());
  is_one_byte_ = content.IsOneByte();
  if (is_one_byte_) {
    start_ = content.ToOneByteVector().start();
  } else {
    start_ = content.ToUC16Vector().start();
  }
}

void ConsStringIterator::Initialize(ConsString cons_string, int offset) {
  DCHECK(!cons_string.is_null());
  root_ = cons_string;
  consumed_ = offset;
  // Force stack blown condition to trigger restart.
  depth_ = 1;
  maximum_depth_ = kStackSize + depth_;
  DCHECK(StackBlown());
}

String ConsStringIterator::Continue(int* offset_out) {
  DCHECK_NE(depth_, 0);
  DCHECK_EQ(0, *offset_out);
  bool blew_stack = StackBlown();
  String string;
  // Get the next leaf if there is one.
  if (!blew_stack) string = NextLeaf(&blew_stack);
  // Restart search from root.
  if (blew_stack) {
    DCHECK(string.is_null());
    string = Search(offset_out);
  }
  // Ensure future calls return null immediately.
  if (string.is_null()) Reset(ConsString());
  return string;
}

String ConsStringIterator::Search(int* offset_out) {
  ConsString cons_string = root_;
  // Reset the stack, pushing the root string.
  depth_ = 1;
  maximum_depth_ = 1;
  frames_[0] = cons_string;
  const int consumed = consumed_;
  int offset = 0;
  while (true) {
    // Loop until the string is found which contains the target offset.
    String string = cons_string->first();
    int length = string->length();
    int32_t type;
    if (consumed < offset + length) {
      // Target offset is in the left branch.
      // Keep going if we're still in a ConString.
      type = string->map()->instance_type();
      if ((type & kStringRepresentationMask) == kConsStringTag) {
        cons_string = ConsString::cast(string);
        PushLeft(cons_string);
        continue;
      }
      // Tell the stack we're done descending.
      AdjustMaximumDepth();
    } else {
      // Descend right.
      // Update progress through the string.
      offset += length;
      // Keep going if we're still in a ConString.
      string = cons_string->second();
      type = string->map()->instance_type();
      if ((type & kStringRepresentationMask) == kConsStringTag) {
        cons_string = ConsString::cast(string);
        PushRight(cons_string);
        continue;
      }
      // Need this to be updated for the current string.
      length = string->length();
      // Account for the possibility of an empty right leaf.
      // This happens only if we have asked for an offset outside the string.
      if (length == 0) {
        // Reset so future operations will return null immediately.
        Reset(ConsString());
        return String();
      }
      // Tell the stack we're done descending.
      AdjustMaximumDepth();
      // Pop stack so next iteration is in correct place.
      Pop();
    }
    DCHECK_NE(length, 0);
    // Adjust return values and exit.
    consumed_ = offset + length;
    *offset_out = consumed - offset;
    return string;
  }
  UNREACHABLE();
}

String ConsStringIterator::NextLeaf(bool* blew_stack) {
  while (true) {
    // Tree traversal complete.
    if (depth_ == 0) {
      *blew_stack = false;
      return String();
    }
    // We've lost track of higher nodes.
    if (StackBlown()) {
      *blew_stack = true;
      return String();
    }
    // Go right.
    ConsString cons_string = frames_[OffsetForDepth(depth_ - 1)];
    String string = cons_string->second();
    int32_t type = string->map()->instance_type();
    if ((type & kStringRepresentationMask) != kConsStringTag) {
      // Pop stack so next iteration is in correct place.
      Pop();
      int length = string->length();
      // Could be a flattened ConsString.
      if (length == 0) continue;
      consumed_ += length;
      return string;
    }
    cons_string = ConsString::cast(string);
    PushRight(cons_string);
    // Need to traverse all the way left.
    while (true) {
      // Continue left.
      string = cons_string->first();
      type = string->map()->instance_type();
      if ((type & kStringRepresentationMask) != kConsStringTag) {
        AdjustMaximumDepth();
        int length = string->length();
        if (length == 0) break;  // Skip empty left-hand sides of ConsStrings.
        consumed_ += length;
        return string;
      }
      cons_string = ConsString::cast(string);
      PushLeft(cons_string);
    }
  }
  UNREACHABLE();
}

}  // namespace internal
}  // namespace v8