conversions.cc

Go to the documentation of this file.

// Copyright 2011 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/numbers/conversions.h"
 
#include <limits.h>
#include <stdarg.h>
 
#include <cmath>
#include <optional>
 
#include "src/base/fpu.h"
#include "src/base/numbers/dtoa.h"
#include "src/base/numbers/strtod.h"
#include "src/base/small-vector.h"
#include "src/bigint/bigint.h"
#include "src/common/assert-scope.h"
#include "src/handles/handles.h"
#include "src/heap/factory.h"
#include "src/objects/bigint.h"
#include "src/objects/objects-inl.h"
#include "src/objects/string-inl.h"
#include "src/strings/char-predicates-inl.h"
#include "src/utils/allocation.h"
#include "third_party/fast_float/src/include/fast_float/fast_float.h"
#include "third_party/fast_float/src/include/fast_float/float_common.h"
 
#if defined(_STLP_VENDOR_CSTD)
// STLPort doesn't import fpclassify into the std namespace.
#define FPCLASSIFY_NAMESPACE
#else
#define FPCLASSIFY_NAMESPACE std
#endif
 
namespace v8 {
namespace internal {
 
// Helper class for building result strings in a character buffer. The
// purpose of the class is to use safe operations that checks the
// buffer bounds on all operations in debug mode.
// This simple base class does not allow formatted output.
class SimpleStringBuilder final {
 public:
  // Create a string builder with a buffer of the given size. The
  // buffer is allocated through NewArray<char> and must be
  // deallocated by the caller of Finalize().
  explicit SimpleStringBuilder(size_t size) {
    buffer_ = base::Vector<char>::New(size);
    cursor_ = buffer_.begin();
  }
 
  SimpleStringBuilder(char* buffer, size_t size)
      : buffer_(buffer, size), cursor_(buffer) {}
 
  ~SimpleStringBuilder() {
    if (V8_UNLIKELY(!is_finalized())) Finalize();
  }
 
  // Get the current position in the builder.
  size_t position() const {
    DCHECK(!is_finalized());
    return cursor_ - buffer_.begin();
  }
 
  // Add a single character to the builder. It is not allowed to add
  // 0-characters; use the Finalize() method to terminate the string
  // instead.
  V8_INLINE void AddCharacter(char c) {
    DCHECK_NE(c, '\0');
    DCHECK(!is_finalized());
    DCHECK_LT(position(), buffer_.size());
    *cursor_++ = c;
  }
 
  // Add an entire string to the builder. 'len' must be equal to strlen().
  V8_INLINE void AddString(const char* s, size_t len) {
    DCHECK_EQ(len, strlen(s));
    AddSubstring(s, len);
  }
 
  // Add a string literal to the builder.
  template <size_t N>
  V8_INLINE void AddStringLiteral(const char (&s)[N]) {
    AddSubstring(s, N - 1);
  }
 
  // Add the first 'n' characters of the given 0-terminated string 's' to the
  // builder. The input string must have enough characters.
  V8_INLINE void AddSubstring(const char* s, size_t n) {
    DCHECK(!is_finalized());
    DCHECK_LE(position() + n, buffer_.size());
    DCHECK_LE(n, strlen(s));
    MemCopy(cursor_, s, n * kCharSize);
    cursor_ += n;
  }
 
  // Add character padding to the builder. If count is non-positive,
  // nothing is added to the builder.
  V8_INLINE void AddPadding(char c, int count) {
    DCHECK(!is_finalized());
    DCHECK_LE(position() + std::max(0, count), buffer_.size());
    cursor_ = std::fill_n(cursor_, count, c);
  }
 
  // Add the decimal representation of the value.
  void AddDecimalInteger(int value) {
    uint32_t number = static_cast<uint32_t>(value);
    if (value < 0) {
      AddCharacter('-');
      number = static_cast<uint32_t>(-value);
    }
    int digits = 1;
    for (uint32_t factor = 10; digits < 10; digits++, factor *= 10) {
      if (factor > number) break;
    }
    cursor_ += digits;
    for (int i = 1; i <= digits; i++) {
      *(cursor_ - i) = '0' + static_cast<char>(number % 10);
      number /= 10;
    }
  }
 
  // Finalize the string by, checking that there is no null-character in the
  // content. Returns a pointer one past the last character.
  char* Finalize() {
    DCHECK(!is_finalized());
    DCHECK_LE(position(), buffer_.size());
#ifdef DEBUG
    // Make sure nobody managed to add a 0-character to the
    // buffer while building the string.
    for (const char* buf = buffer_.begin(); buf != cursor_; buf++) {
      DCHECK_NE(*buf, '\0');
    }
#endif
    char* ret = cursor_;
    cursor_ = nullptr;
    DCHECK(is_finalized());
    return ret;
  }
 
 private:
  base::Vector<char> buffer_;
  char* cursor_;
 
  bool is_finalized() const { return cursor_ == nullptr; }
 
  DISALLOW_IMPLICIT_CONSTRUCTORS(SimpleStringBuilder);
};
 
inline double JunkStringValue() {
  return base::bit_cast<double, uint64_t>(kQuietNaNMask);
}
 
inline double SignedZero(bool negative) {
  return negative ? base::uint64_to_double(base::Double::kSignMask) : 0.0;
}
 
inline bool isDigit(int x, int radix) {
  return (x >= '0' && x <= '9' && x < '0' + radix) ||
         (radix > 10 && x >= 'a' && x < 'a' + radix - 10) ||
         (radix > 10 && x >= 'A' && x < 'A' + radix - 10);
}
 
inline bool isBinaryDigit(int x) { return x == '0' || x == '1'; }
 
template <class Char>
bool SubStringEquals(const Char** current, const Char* end,
                     const char* substring) {
  DCHECK(**current == *substring);
  for (substring++; *substring != '\0'; substring++) {
    ++*current;
    if (*current == end || **current != *substring) return false;
  }
  ++*current;
  return true;
}
 
// Returns true if a nonspace character has been found and false if the
// end was been reached before finding a nonspace character.
template <class Char>
inline bool AdvanceToNonspace(const Char** current, const Char* end) {
  while (*current != end) {
    if (!IsWhiteSpaceOrLineTerminator(**current)) return true;
    ++*current;
  }
  return false;
}
 
// Parsing integers with radix 2, 4, 8, 16, 32. Assumes current != end.
template <int radix_log_2, class Char>
double InternalStringToIntDouble(const Char* start, const Char* end,
                                 bool negative, bool allow_trailing_junk) {
  const Char* current = start;
  DCHECK_NE(current, end);
 
  // Skip leading 0s.
  while (*current == '0') {
    ++current;
    if (current == end) return SignedZero(negative);
  }
 
  int64_t number = 0;
  int exponent = 0;
  constexpr int radix = (1 << radix_log_2);
 
  constexpr int lim_0 = '0' + (radix < 10 ? radix : 10);
  constexpr int lim_a = 'a' + (radix - 10);
  constexpr int lim_A = 'A' + (radix - 10);
 
  do {
    int digit;
    if (*current >= '0' && *current < lim_0) {
      digit = static_cast<char>(*current) - '0';
    } else if (*current >= 'a' && *current < lim_a) {
      digit = static_cast<char>(*current) - 'a' + 10;
    } else if (*current >= 'A' && *current < lim_A) {
      digit = static_cast<char>(*current) - 'A' + 10;
    } else {
      // We've not found any digits, this must be junk.
      if (current == start) return JunkStringValue();
      if (allow_trailing_junk || !AdvanceToNonspace(&current, end)) break;
      return JunkStringValue();
    }
 
    number = number * radix + digit;
    int overflow = static_cast<int>(number >> 53);
    if (overflow != 0) {
      // Overflow occurred. Need to determine which direction to round the
      // result.
      int overflow_bits_count = 1;
      while (overflow > 1) {
        overflow_bits_count++;
        overflow >>= 1;
      }
 
      int dropped_bits_mask = ((1 << overflow_bits_count) - 1);
      int dropped_bits = static_cast<int>(number) & dropped_bits_mask;
      number >>= overflow_bits_count;
      exponent = overflow_bits_count;
 
      bool zero_tail = true;
      while (true) {
        ++current;
        if (current == end || !isDigit(*current, radix)) break;
        zero_tail = zero_tail && *current == '0';
        exponent += radix_log_2;
      }
 
      if (!allow_trailing_junk && AdvanceToNonspace(&current, end)) {
        return JunkStringValue();
      }
 
      int middle_value = (1 << (overflow_bits_count - 1));
      if (dropped_bits > middle_value) {
        number++;  // Rounding up.
      } else if (dropped_bits == middle_value) {
        // Rounding to even to consistency with decimals: half-way case rounds
        // up if significant part is odd and down otherwise.
        if ((number & 1) != 0 || !zero_tail) {
          number++;  // Rounding up.
        }
      }
 
      // Rounding up may cause overflow.
      if ((number & (static_cast<int64_t>(1) << 53)) != 0) {
        exponent++;
        number >>= 1;
      }
      break;
    }
    ++current;
  } while (current != end);
 
  DCHECK(number < (int64_t{1} << 53));
  DCHECK(static_cast<int64_t>(static_cast<double>(number)) == number);
 
  if (exponent == 0) {
    if (negative) {
      if (number == 0) return -0.0;
      number = -number;
    }
    return static_cast<double>(number);
  }
 
  DCHECK_NE(number, 0);
  return std::ldexp(static_cast<double>(negative ? -number : number), exponent);
}
 
namespace {
 
// Subclasses of StringToIntHelper get access to internal state:
enum class State { kRunning, kError, kJunk, kEmpty, kZero, kDone };
 
enum class Sign { kNegative, kPositive, kNone };
 
}  // namespace
 
// ES6 18.2.5 parseInt(string, radix) (with NumberParseIntHelper subclass);
// and BigInt parsing cases from https://tc39.github.io/proposal-bigint/
// (with StringToBigIntHelper subclass).
class StringToIntHelper {
 public:
  StringToIntHelper(DirectHandle<String> subject, int radix)
      : subject_(subject), radix_(radix) {
    DCHECK(subject->IsFlat());
  }
 
  // Used for the NumberParseInt operation
  StringToIntHelper(const uint8_t* subject, int radix, size_t length)
      : raw_one_byte_subject_(subject), radix_(radix), length_(length) {}
 
  StringToIntHelper(const base::uc16* subject, int radix, size_t length)
      : raw_two_byte_subject_(subject), radix_(radix), length_(length) {}
 
  // Used for the StringToBigInt operation.
  explicit StringToIntHelper(DirectHandle<String> subject) : subject_(subject) {
    DCHECK(subject->IsFlat());
  }
 
  // Used for parsing BigInt literals, where the input is a Zone-allocated
  // buffer of one-byte digits, along with an optional radix prefix.
  StringToIntHelper(const uint8_t* subject, size_t length)
      : raw_one_byte_subject_(subject), length_(length) {}
  virtual ~StringToIntHelper() = default;
 
 protected:
  // Subclasses must implement these:
  virtual void ParseOneByte(const uint8_t* start) = 0;
  virtual void ParseTwoByte(const base::uc16* start) = 0;
 
  // Subclasses must call this to do all the work.
  void ParseInt();
 
  // Subclass constructors should call these for configuration before calling
  // ParseInt().
  void set_allow_binary_and_octal_prefixes() {
    allow_binary_and_octal_prefixes_ = true;
  }
  void set_disallow_trailing_junk() { allow_trailing_junk_ = false; }
  bool allow_trailing_junk() { return allow_trailing_junk_; }
 
  bool IsOneByte() const {
    if (raw_two_byte_subject_ != nullptr) return false;
    return raw_one_byte_subject_ != nullptr ||
           String::IsOneByteRepresentationUnderneath(*subject_);
  }
 
  base::Vector<const uint8_t> GetOneByteVector(
      const DisallowGarbageCollection& no_gc) {
    if (raw_one_byte_subject_ != nullptr) {
      return base::Vector<const uint8_t>(raw_one_byte_subject_, length_);
    }
    return subject_->GetFlatContent(no_gc).ToOneByteVector();
  }
 
  base::Vector<const base::uc16> GetTwoByteVector(
      const DisallowGarbageCollection& no_gc) {
    if (raw_two_byte_subject_ != nullptr) {
      return base::Vector<const base::uc16>(raw_two_byte_subject_, length_);
    }
    return subject_->GetFlatContent(no_gc).ToUC16Vector();
  }
 
  int radix() { return radix_; }
  size_t cursor() { return cursor_; }
  size_t length() { return length_; }
  bool negative() { return sign_ == Sign::kNegative; }
  Sign sign() { return sign_; }
  State state() { return state_; }
  void set_state(State state) { state_ = state; }
 
 private:
  template <class Char>
  void DetectRadixInternal(const Char* current, size_t length);
 
  DirectHandle<String> subject_;
  const uint8_t* raw_one_byte_subject_ = nullptr;
  const base::uc16* raw_two_byte_subject_ = nullptr;
  int radix_ = 0;
  size_t cursor_ = 0;
  size_t length_ = 0;
  Sign sign_ = Sign::kNone;
  bool leading_zero_ = false;
  bool allow_binary_and_octal_prefixes_ = false;
  bool allow_trailing_junk_ = true;
  State state_ = State::kRunning;
};
 
void StringToIntHelper::ParseInt() {
  DisallowGarbageCollection no_gc;
  if (IsOneByte()) {
    base::Vector<const uint8_t> vector = GetOneByteVector(no_gc);
    DetectRadixInternal(vector.begin(), vector.size());
    if (state_ != State::kRunning) return;
    ParseOneByte(vector.begin());
  } else {
    base::Vector<const base::uc16> vector = GetTwoByteVector(no_gc);
    DetectRadixInternal(vector.begin(), vector.size());
    if (state_ != State::kRunning) return;
    ParseTwoByte(vector.begin());
  }
}
 
template <class Char>
void StringToIntHelper::DetectRadixInternal(const Char* current,
                                            size_t length) {
  const Char* start = current;
  length_ = length;
  const Char* end = start + length;
 
  if (!AdvanceToNonspace(&current, end)) {
    return set_state(State::kEmpty);
  }
 
  if (*current == '+') {
    // Ignore leading sign; skip following spaces.
    ++current;
    if (current == end) {
      return set_state(State::kJunk);
    }
    sign_ = Sign::kPositive;
  } else if (*current == '-') {
    ++current;
    if (current == end) {
      return set_state(State::kJunk);
    }
    sign_ = Sign::kNegative;
  }
 
  if (radix_ == 0) {
    // Radix detection.
    radix_ = 10;
    if (*current == '0') {
      ++current;
      if (current == end) return set_state(State::kZero);
      if (*current == 'x' || *current == 'X') {
        radix_ = 16;
        ++current;
        if (current == end) return set_state(State::kJunk);
      } else if (allow_binary_and_octal_prefixes_ &&
                 (*current == 'o' || *current == 'O')) {
        radix_ = 8;
        ++current;
        if (current == end) return set_state(State::kJunk);
      } else if (allow_binary_and_octal_prefixes_ &&
                 (*current == 'b' || *current == 'B')) {
        radix_ = 2;
        ++current;
        if (current == end) return set_state(State::kJunk);
      } else {
        leading_zero_ = true;
      }
    }
  } else if (radix_ == 16) {
    if (*current == '0') {
      // Allow "0x" prefix.
      ++current;
      if (current == end) return set_state(State::kZero);
      if (*current == 'x' || *current == 'X') {
        ++current;
        if (current == end) return set_state(State::kJunk);
      } else {
        leading_zero_ = true;
      }
    }
  }
  // Skip leading zeros.
  while (*current == '0') {
    leading_zero_ = true;
    ++current;
    if (current == end) return set_state(State::kZero);
  }
  // Detect leading zeros with junk after them, if allowed.
  if (leading_zero_ && allow_trailing_junk_ && !isDigit(*current, radix_)) {
    return set_state(State::kZero);
  }
 
  if (!leading_zero_ && !isDigit(*current, radix_)) {
    return set_state(State::kJunk);
  }
 
  DCHECK(radix_ >= 2 && radix_ <= 36);
  cursor_ = current - start;
}
 
class NumberParseIntHelper : public StringToIntHelper {
 public:
  NumberParseIntHelper(DirectHandle<String> string, int radix)
      : StringToIntHelper(string, radix) {}
 
  NumberParseIntHelper(const uint8_t* string, int radix, size_t length)
      : StringToIntHelper(string, radix, length) {}
 
  NumberParseIntHelper(const base::uc16* string, int radix, size_t length)
      : StringToIntHelper(string, radix, length) {}
 
  template <class Char>
  void ParseInternal(const Char* start) {
    const Char* current = start + cursor();
    const Char* end = start + length();
 
    if (radix() == 10) return HandleBaseTenCase(current, end);
    if (base::bits::IsPowerOfTwo(radix())) {
      result_ = HandlePowerOfTwoCase(current, end);
      set_state(State::kDone);
      return;
    }
    return HandleGenericCase(current, end);
  }
  void ParseOneByte(const uint8_t* start) final { return ParseInternal(start); }
  void ParseTwoByte(const base::uc16* start) final {
    return ParseInternal(start);
  }
 
  double GetResult() {
    ParseInt();
    switch (state()) {
      case State::kJunk:
      case State::kEmpty:
        return JunkStringValue();
      case State::kZero:
        return SignedZero(negative());
      case State::kDone:
        return negative() ? -result_ : result_;
      case State::kError:
      case State::kRunning:
        break;
    }
    UNREACHABLE();
  }
 
 private:
  template <class Char>
  void HandleGenericCase(const Char* current, const Char* end);
 
  template <class Char>
  double HandlePowerOfTwoCase(const Char* current, const Char* end) {
    const bool allow_trailing_junk = true;
    // GetResult() will take care of the sign bit, so ignore it for now.
    const bool negative = false;
    switch (radix()) {
      case 2:
        return InternalStringToIntDouble<1>(current, end, negative,
                                            allow_trailing_junk);
      case 4:
        return InternalStringToIntDouble<2>(current, end, negative,
                                            allow_trailing_junk);
      case 8:
        return InternalStringToIntDouble<3>(current, end, negative,
                                            allow_trailing_junk);
 
      case 16:
        return InternalStringToIntDouble<4>(current, end, negative,
                                            allow_trailing_junk);
 
      case 32:
        return InternalStringToIntDouble<5>(current, end, negative,
                                            allow_trailing_junk);
      default:
        UNREACHABLE();
    }
  }
 
  template <class Char>
  void HandleBaseTenCase(const Char* current, const Char* end) {
    // Parsing with strtod.
    // Doubles are less than 1.8e308.
    constexpr size_t kMaxSignificantDigits = 309;
    // The buffer may contain up to kMaxSignificantDigits + 1 digits and a zero
    // end.
    constexpr size_t kBufferSize = kMaxSignificantDigits + 2;
    char buffer[kBufferSize];
    size_t buffer_pos = 0;
    while (*current >= '0' && *current <= '9') {
      if (buffer_pos <= kMaxSignificantDigits) {
        // If the number has more than kMaxSignificantDigits it will be parsed
        // as infinity.
        static_assert(kMaxSignificantDigits < kBufferSize);
        buffer[buffer_pos++] = static_cast<char>(*current);
      }
      ++current;
      if (current == end) break;
    }
 
    SLOW_DCHECK(buffer_pos < kBufferSize);
    buffer[buffer_pos] = '\0';
    base::Vector<const char> buffer_vector(buffer, buffer_pos);
    result_ = Strtod(buffer_vector, 0);
    set_state(State::kDone);
  }
 
  double result_ = 0;
};
 
template <class Char>
void NumberParseIntHelper::HandleGenericCase(const Char* current,
                                             const Char* end) {
  // The following code causes accumulating rounding error for numbers greater
  // than ~2^56. It's explicitly allowed in the spec: "if R is not 2, 4, 8, 10,
  // 16, or 32, then mathInt may be an implementation-dependent approximation to
  // the mathematical integer value" (15.1.2.2).
 
  int lim_0 = '0' + (radix() < 10 ? radix() : 10);
  int lim_a = 'a' + (radix() - 10);
  int lim_A = 'A' + (radix() - 10);
 
  // NOTE: The code for computing the value may seem a bit complex at
  // first glance. It is structured to use 32-bit multiply-and-add
  // loops as long as possible to avoid losing precision.
 
  bool done = false;
  do {
    // Parse the longest part of the string starting at {current}
    // possible while keeping the multiplier, and thus the part
    // itself, within 32 bits.
    uint32_t part = 0, multiplier = 1;
    while (true) {
      uint32_t d;
      if (*current >= '0' && *current < lim_0) {
        d = *current - '0';
      } else if (*current >= 'a' && *current < lim_a) {
        d = *current - 'a' + 10;
      } else if (*current >= 'A' && *current < lim_A) {
        d = *current - 'A' + 10;
      } else {
        done = true;
        break;
      }
 
      // Update the value of the part as long as the multiplier fits
      // in 32 bits. When we can't guarantee that the next iteration
      // will not overflow the multiplier, we stop parsing the part
      // by leaving the loop.
      const uint32_t kMaximumMultiplier = 0xFFFFFFFFU / 36;
      uint32_t m = multiplier * static_cast<uint32_t>(radix());
      if (m > kMaximumMultiplier) break;
      part = part * radix() + d;
      multiplier = m;
      DCHECK(multiplier > part);
 
      ++current;
      if (current == end) {
        done = true;
        break;
      }
    }
    result_ = result_ * multiplier + part;
  } while (!done);
 
  if (!allow_trailing_junk() && AdvanceToNonspace(&current, end)) {
    return set_state(State::kJunk);
  }
  return set_state(State::kDone);
}
 
// Converts a string to a double value.
template <class Char>
double InternalStringToDouble(const Char* current, const Char* end,
                              ConversionFlag flag, double empty_string_val) {
  // To make sure that iterator dereferencing is valid the following
  // convention is used:
  // 1. Each '++current' statement is followed by check for equality to 'end'.
  // 2. If AdvanceToNonspace returned false then current == end.
  // 3. If 'current' becomes be equal to 'end' the function returns or goes to
  // 'parsing_done'.
  // 4. 'current' is not dereferenced after the 'parsing_done' label.
  // 5. Code before 'parsing_done' may rely on 'current != end'.
  if (!AdvanceToNonspace(&current, end)) {
    return empty_string_val;
  }
 
  // The non-decimal prefix has to be the first thing after any whitespace,
  // so check for this first.
  if (flag == ALLOW_NON_DECIMAL_PREFIX) {
    // Copy the current iterator, so that on a failure to find the prefix, we
    // rewind to the start.
    const Char* prefixed = current;
    if (*prefixed == '0') {
      ++prefixed;
      if (prefixed == end) return 0;
 
      if (*prefixed == 'x' || *prefixed == 'X') {
        ++prefixed;
        if (prefixed == end) return JunkStringValue();  // "0x".
        return InternalStringToIntDouble<4>(prefixed, end, false, false);
      } else if (*prefixed == 'o' || *prefixed == 'O') {
        ++prefixed;
        if (prefixed == end) return JunkStringValue();  // "0o".
        return InternalStringToIntDouble<3>(prefixed, end, false, false);
      } else if (*prefixed == 'b' || *prefixed == 'B') {
        ++prefixed;
        if (prefixed == end) return JunkStringValue();  // "0b".
        return InternalStringToIntDouble<1>(prefixed, end, false, false);
      }
    }
  }
 
  // From here we are parsing a StrDecimalLiteral, as per
  // https://tc39.es/ecma262/#sec-tonumber-applied-to-the-string-type
  const bool allow_trailing_junk = flag == ALLOW_TRAILING_JUNK;
 
  double value;
  // fast_float takes a char/char16_t instead of a uint8_t/uint16_t. Cast the
  // pointers to match.
  using UC = std::conditional_t<std::is_same_v<Char, uint8_t>, char, char16_t>;
  static_assert(sizeof(UC) == sizeof(Char));
  const UC* current_uc = reinterpret_cast<const UC*>(current);
  const UC* end_uc = reinterpret_cast<const UC*>(end);
  auto ret =
      fast_float::from_chars(current_uc, end_uc, value,
                             static_cast<fast_float::chars_format>(
                                 fast_float::chars_format::general |
                                 fast_float::chars_format::no_infnan |
                                 fast_float::chars_format::allow_leading_plus));
  if (ret.ptr == end_uc) return value;
  if (ret.ptr > current_uc) {
    current = reinterpret_cast<const Char*>(ret.ptr);
    if (!allow_trailing_junk && AdvanceToNonspace(&current, end)) {
      return JunkStringValue();
    }
    return value;
  }
 
  // Failed to parse any number -- handle ±Infinity before giving up.
  DCHECK_EQ(ret.ptr, current_uc);
  DCHECK_NE(current, end);
  static constexpr char kInfinityString[] = "Infinity";
  switch (*current) {
    case '+':
      // Ignore leading plus sign.
      ++current;
      if (current == end) return JunkStringValue();
      if (*current != kInfinityString[0]) return JunkStringValue();
      [[fallthrough]];
    case kInfinityString[0]:
      if (!SubStringEquals(&current, end, kInfinityString)) {
        return JunkStringValue();
      }
      if (!allow_trailing_junk && AdvanceToNonspace(&current, end)) {
        return JunkStringValue();
      }
      return V8_INFINITY;
 
    case '-':
      ++current;
      if (current == end) return JunkStringValue();
      if (*current != kInfinityString[0]) return JunkStringValue();
      if (!SubStringEquals(&current, end, kInfinityString)) {
        return JunkStringValue();
      }
      if (!allow_trailing_junk && AdvanceToNonspace(&current, end)) {
        return JunkStringValue();
      }
      return -V8_INFINITY;
 
    default:
      return JunkStringValue();
  }
}
 
double StringToDouble(const char* str, ConversionFlag flags,
                      double empty_string_val) {
  // We use {base::OneByteVector} instead of {base::CStrVector} to avoid
  // instantiating the InternalStringToDouble() template for {const char*} as
  // well.
  return StringToDouble(base::OneByteVector(str), flags, empty_string_val);
}
 
double StringToDouble(base::Vector<const uint8_t> str, ConversionFlag flags,
                      double empty_string_val) {
  return InternalStringToDouble(str.begin(), str.end(), flags,
                                empty_string_val);
}
 
double StringToDouble(base::Vector<const base::uc16> str, ConversionFlag flags,
                      double empty_string_val) {
  return InternalStringToDouble(str.begin(), str.end(), flags,
                                empty_string_val);
}
 
double BinaryStringToDouble(base::Vector<const uint8_t> str) {
  DCHECK_EQ(str[0], '0');
  DCHECK_EQ(tolower(str[1]), 'b');
  return InternalStringToIntDouble<1>(str.begin() + 2, str.end(), false, false);
}
 
double OctalStringToDouble(base::Vector<const uint8_t> str) {
  DCHECK_EQ(str[0], '0');
  DCHECK_EQ(tolower(str[1]), 'o');
  return InternalStringToIntDouble<3>(str.begin() + 2, str.end(), false, false);
}
 
double HexStringToDouble(base::Vector<const uint8_t> str) {
  DCHECK_EQ(str[0], '0');
  DCHECK_EQ(tolower(str[1]), 'x');
  return InternalStringToIntDouble<4>(str.begin() + 2, str.end(), false, false);
}
 
double ImplicitOctalStringToDouble(base::Vector<const uint8_t> str) {
  return InternalStringToIntDouble<3>(str.begin(), str.end(), false, false);
}
 
double StringToInt(Isolate* isolate, DirectHandle<String> string, int radix) {
  NumberParseIntHelper helper(string, radix);
  return helper.GetResult();
}
 
template <typename IsolateT>
class StringToBigIntHelper : public StringToIntHelper {
 public:
  enum class Behavior { kStringToBigInt, kLiteral };
 
  // Used for StringToBigInt operation (BigInt constructor and == operator).
  StringToBigIntHelper(IsolateT* isolate, DirectHandle<String> string)
      : StringToIntHelper(string),
        isolate_(isolate),
        behavior_(Behavior::kStringToBigInt) {
    set_allow_binary_and_octal_prefixes();
    set_disallow_trailing_junk();
  }
 
  // Used for parsing BigInt literals, where the input is a buffer of
  // one-byte ASCII digits, along with an optional radix prefix.
  StringToBigIntHelper(IsolateT* isolate, const uint8_t* string, size_t length)
      : StringToIntHelper(string, length),
        isolate_(isolate),
        behavior_(Behavior::kLiteral) {
    set_allow_binary_and_octal_prefixes();
  }
 
  void ParseOneByte(const uint8_t* start) final { return ParseInternal(start); }
  void ParseTwoByte(const base::uc16* start) final {
    return ParseInternal(start);
  }
 
  MaybeHandle<BigInt> GetResult() {
    ParseInt();
    if (behavior_ == Behavior::kStringToBigInt && sign() != Sign::kNone &&
        radix() != 10) {
      return MaybeHandle<BigInt>();
    }
    if (state() == State::kEmpty) {
      if (behavior_ == Behavior::kStringToBigInt) {
        set_state(State::kZero);
      } else {
        UNREACHABLE();
      }
    }
    switch (this->state()) {
      case State::kJunk:
      case State::kError:
        return MaybeHandle<BigInt>();
      case State::kZero:
        return BigInt::Zero(isolate(), allocation_type());
      case State::kDone:
        return BigInt::Allocate(isolate(), &accumulator_, negative(),
                                allocation_type());
      case State::kEmpty:
      case State::kRunning:
        break;
    }
    UNREACHABLE();
  }
 
  // Used for converting BigInt literals. The scanner has already checked
  // that the literal is valid and not too big, so this always succeeds.
  std::unique_ptr<char[]> DecimalString(bigint::Processor* processor) {
    DCHECK_EQ(behavior_, Behavior::kLiteral);
    ParseInt();
    if (state() == State::kZero) {
      // Input may have been "0x0" or similar.
      return std::unique_ptr<char[]>(new char[2]{'0', '\0'});
    }
    DCHECK_EQ(state(), State::kDone);
    int num_digits = accumulator_.ResultLength();
    base::SmallVector<bigint::digit_t, 8> digit_storage(num_digits);
    bigint::RWDigits digits(digit_storage.data(), num_digits);
    processor->FromString(digits, &accumulator_);
    uint32_t num_chars = bigint::ToStringResultLength(digits, 10, false);
    std::unique_ptr<char[]> out(new char[num_chars + 1]);
    processor->ToString(out.get(), &num_chars, digits, 10, false);
    out[num_chars] = '\0';
    return out;
  }
  IsolateT* isolate() { return isolate_; }
 
 private:
  template <class Char>
  void ParseInternal(const Char* start) {
    using Result = bigint::FromStringAccumulator::Result;
    const Char* current = start + cursor();
    const Char* end = start + length();
    current = accumulator_.Parse(current, end, radix());
 
    Result result = accumulator_.result();
    if (result == Result::kMaxSizeExceeded) {
      return set_state(State::kError);
    }
    if (!allow_trailing_junk() && AdvanceToNonspace(&current, end)) {
      return set_state(State::kJunk);
    }
    return set_state(State::kDone);
  }
 
  AllocationType allocation_type() {
    // For literals, we pretenure the allocated BigInt, since it's about
    // to be stored in the interpreter's constants array.
    return behavior_ == Behavior::kLiteral ? AllocationType::kOld
                                           : AllocationType::kYoung;
  }
 
  IsolateT* isolate_;
  bigint::FromStringAccumulator accumulator_{BigInt::kMaxLength};
  Behavior behavior_;
};
 
MaybeHandle<BigInt> StringToBigInt(Isolate* isolate,
                                   DirectHandle<String> string) {
  string = String::Flatten(isolate, string);
  StringToBigIntHelper<Isolate> helper(isolate, string);
  return helper.GetResult();
}
 
template <typename IsolateT>
MaybeHandle<BigInt> BigIntLiteral(IsolateT* isolate, const char* string) {
  StringToBigIntHelper<IsolateT> helper(
      isolate, reinterpret_cast<const uint8_t*>(string), strlen(string));
  return helper.GetResult();
}
template EXPORT_TEMPLATE_DEFINE(V8_EXPORT_PRIVATE)
    MaybeHandle<BigInt> BigIntLiteral(Isolate* isolate, const char* string);
template EXPORT_TEMPLATE_DEFINE(V8_EXPORT_PRIVATE)
    MaybeHandle<BigInt> BigIntLiteral(LocalIsolate* isolate,
                                      const char* string);
 
std::unique_ptr<char[]> BigIntLiteralToDecimal(
    LocalIsolate* isolate, base::Vector<const uint8_t> literal) {
  StringToBigIntHelper<LocalIsolate> helper(nullptr, literal.begin(),
                                            literal.size());
  return helper.DecimalString(isolate->bigint_processor());
}
 
std::string_view DoubleToStringView(double v, base::Vector<char> buffer) {
  switch (FPCLASSIFY_NAMESPACE::fpclassify(v)) {
    case FP_NAN:
      return "NaN";
    case FP_INFINITE:
      return (v < 0.0 ? "-Infinity" : "Infinity");
    case FP_ZERO:
      return "0";
    default: {
      if (IsInt32Double(v)) {
        // This will trigger if v is -0 and -0.0 is stringified to "0".
        // (see ES section 7.1.12.1 #sec-tostring-applied-to-the-number-type)
        return IntToStringView(FastD2I(v), buffer);
      }
      SimpleStringBuilder builder(buffer.begin(), buffer.size());
      int decimal_point;
      int sign;
      constexpr int kV8DtoaBufferCapacity = base::kBase10MaximalLength + 1;
      char decimal_rep[kV8DtoaBufferCapacity];
      int length;
 
      base::DoubleToAscii(
          v, base::DTOA_SHORTEST, 0,
          base::Vector<char>(decimal_rep, kV8DtoaBufferCapacity), &sign,
          &length, &decimal_point);
 
      if (sign) builder.AddCharacter('-');
 
      if (length <= decimal_point && decimal_point <= 21) {
        // ECMA-262 section 9.8.1 step 6.
        builder.AddString(decimal_rep, length);
        builder.AddPadding('0', decimal_point - length);
 
      } else if (0 < decimal_point && decimal_point <= 21) {
        // ECMA-262 section 9.8.1 step 7.
        builder.AddSubstring(decimal_rep, decimal_point);
        builder.AddCharacter('.');
        builder.AddString(decimal_rep + decimal_point, length - decimal_point);
 
      } else if (decimal_point <= 0 && decimal_point > -6) {
        // ECMA-262 section 9.8.1 step 8.
        builder.AddStringLiteral("0.");
        builder.AddPadding('0', -decimal_point);
        builder.AddString(decimal_rep, length);
 
      } else {
        // ECMA-262 section 9.8.1 step 9 and 10 combined.
        builder.AddCharacter(decimal_rep[0]);
        if (length != 1) {
          builder.AddCharacter('.');
          builder.AddString(decimal_rep + 1, length - 1);
        }
        builder.AddCharacter('e');
        builder.AddCharacter((decimal_point >= 0) ? '+' : '-');
        int exponent = decimal_point - 1;
        if (exponent < 0) exponent = -exponent;
        builder.AddDecimalInteger(exponent);
      }
      return {buffer.begin(), builder.Finalize()};
    }
  }
}
 
std::string_view IntToStringView(int n, base::Vector<char> buffer) {
  bool negative = true;
  if (n >= 0) {
    n = -n;
    negative = false;
  }
  // Build the string backwards from the least significant digit.
  size_t i = buffer.size();
  do {
    // We ensured n <= 0, so the subtraction does the right addition.
    buffer[--i] = '0' - (n % 10);
    n /= 10;
  } while (n);
  if (negative) buffer[--i] = '-';
  return {buffer.begin() + i, buffer.end()};
}
 
std::string_view DoubleToFixedStringView(double value, int f,
                                         base::Vector<char> buffer) {
  const double kFirstNonFixed = 1e21;
  DCHECK_GE(f, 0);
  DCHECK_LE(f, kMaxFractionDigits);
 
  bool negative = false;
  double abs_value = value;
  if (value < 0) {
    abs_value = -value;
    negative = true;
  }
 
  // If abs_value has more than kDoubleToFixedMaxDigitsBeforePoint digits before
  // the point use the non-fixed conversion routine.
  if (abs_value >= kFirstNonFixed) {
    return DoubleToStringView(value, buffer);
  }
 
  // Find a sufficiently precise decimal representation of n.
  int decimal_point;
  int sign;
  // Add space for the '\0' byte.
  constexpr int kDecimalRepCapacity =
      kDoubleToFixedMaxDigitsBeforePoint + kMaxFractionDigits + 1;
  char decimal_rep[kDecimalRepCapacity];
  int decimal_rep_length;
  base::DoubleToAscii(value, base::DTOA_FIXED, f,
                      base::Vector<char>(decimal_rep, kDecimalRepCapacity),
                      &sign, &decimal_rep_length, &decimal_point);
 
  // Create a representation that is padded with zeros if needed.
  int zero_prefix_length = 0;
  int zero_postfix_length = 0;
 
  if (decimal_point <= 0) {
    zero_prefix_length = -decimal_point + 1;
    decimal_point = 1;
  }
 
  if (zero_prefix_length + decimal_rep_length < decimal_point + f) {
    zero_postfix_length =
        decimal_point + f - decimal_rep_length - zero_prefix_length;
  }
 
  unsigned rep_length =
      zero_prefix_length + decimal_rep_length + zero_postfix_length;
  // TODO(pthier): Get rid of this intermediate string builder.
  base::Vector<char> rep_buffer = base::Vector<char>::New(rep_length + 1);
  SimpleStringBuilder rep_builder(rep_buffer.begin(), rep_buffer.size());
  rep_builder.AddPadding('0', zero_prefix_length);
  rep_builder.AddString(decimal_rep, decimal_rep_length);
  rep_builder.AddPadding('0', zero_postfix_length);
  char* rep_end = rep_builder.Finalize();
  // AddSubstring requires a null-terminated string (for DCHECKs only).
  *rep_end = '\0';
 
  // Create the result string by appending a minus and putting in a
  // decimal point if needed.
  SimpleStringBuilder builder(buffer.begin(), buffer.size());
  if (negative) builder.AddCharacter('-');
  builder.AddSubstring(rep_buffer.begin(), decimal_point);
  if (f > 0) {
    builder.AddCharacter('.');
    builder.AddSubstring(rep_buffer.begin() + decimal_point, f);
  }
  DeleteArray(rep_buffer.begin());
  return {buffer.begin(), builder.Finalize()};
}
 
static std::string_view CreateExponentialRepresentation(
    char* decimal_rep, int rep_length, int exponent, bool negative,
    int significant_digits, base::Vector<char> buffer) {
  bool negative_exponent = false;
  if (exponent < 0) {
    negative_exponent = true;
    exponent = -exponent;
  }
 
  SimpleStringBuilder builder(buffer.begin(), buffer.size());
 
  if (negative) builder.AddCharacter('-');
  builder.AddCharacter(decimal_rep[0]);
  if (significant_digits != 1) {
    builder.AddCharacter('.');
    DCHECK_EQ(rep_length, strlen(decimal_rep));
    DCHECK_GE(significant_digits, rep_length);
    builder.AddString(decimal_rep + 1, rep_length - 1);
    builder.AddPadding('0', significant_digits - rep_length);
  }
 
  builder.AddCharacter('e');
  builder.AddCharacter(negative_exponent ? '-' : '+');
  builder.AddDecimalInteger(exponent);
  return {buffer.begin(), builder.Finalize()};
}
 
std::string_view DoubleToExponentialStringView(double value, int f,
                                               base::Vector<char> buffer) {
  // f might be -1 to signal that f was undefined in JavaScript.
  DCHECK(f >= -1 && f <= kMaxFractionDigits);
 
  bool negative = false;
  if (value < 0) {
    value = -value;
    negative = true;
  }
 
  // Find a sufficiently precise decimal representation of n.
  int decimal_point;
  int sign;
  // f corresponds to the digits after the point. There is always one digit
  // before the point. The number of requested_digits equals hence f + 1.
  // And we have to add one character for the null-terminator.
  constexpr int kV8DtoaBufferCapacity = kMaxFractionDigits + 1 + 1;
  // Make sure that the buffer is big enough, even if we fall back to the
  // shortest representation (which happens when f equals -1).
  DCHECK_LE(base::kBase10MaximalLength, kMaxFractionDigits + 1);
  char decimal_rep[kV8DtoaBufferCapacity];
  int decimal_rep_length;
 
  if (f == -1) {
    base::DoubleToAscii(value, base::DTOA_SHORTEST, 0,
                        base::Vector<char>(decimal_rep, kV8DtoaBufferCapacity),
                        &sign, &decimal_rep_length, &decimal_point);
    f = decimal_rep_length - 1;
  } else {
    base::DoubleToAscii(value, base::DTOA_PRECISION, f + 1,
                        base::Vector<char>(decimal_rep, kV8DtoaBufferCapacity),
                        &sign, &decimal_rep_length, &decimal_point);
  }
  DCHECK_GT(decimal_rep_length, 0);
  DCHECK(decimal_rep_length <= f + 1);
 
  int exponent = decimal_point - 1;
  return CreateExponentialRepresentation(decimal_rep, decimal_rep_length,
                                         exponent, negative, f + 1, buffer);
}
 
std::string_view DoubleToPrecisionStringView(double value, int p,
                                             base::Vector<char> buffer) {
  constexpr int kMinimalDigits = 1;
  DCHECK(p >= kMinimalDigits && p <= kMaxFractionDigits);
  USE(kMinimalDigits);
 
  bool negative = false;
  if (value < 0) {
    value = -value;
    negative = true;
  }
 
  // Find a sufficiently precise decimal representation of n.
  int decimal_point;
  int sign;
  // Add one for the terminating null character.
  constexpr int kV8DtoaBufferCapacity = kMaxFractionDigits + 1;
  char decimal_rep[kV8DtoaBufferCapacity];
  int decimal_rep_length;
 
  base::DoubleToAscii(value, base::DTOA_PRECISION, p,
                      base::Vector<char>(decimal_rep, kV8DtoaBufferCapacity),
                      &sign, &decimal_rep_length, &decimal_point);
  DCHECK(decimal_rep_length <= p);
 
  int exponent = decimal_point - 1;
 
  std::string_view result;
 
  if (exponent < -6 || exponent >= p) {
    result = CreateExponentialRepresentation(decimal_rep, decimal_rep_length,
                                             exponent, negative, p, buffer);
  } else {
    // Use fixed notation.
    SimpleStringBuilder builder(buffer.begin(), buffer.size());
    if (negative) builder.AddCharacter('-');
    if (decimal_point <= 0) {
      builder.AddStringLiteral("0.");
      builder.AddPadding('0', -decimal_point);
      builder.AddString(decimal_rep, decimal_rep_length);
      builder.AddPadding('0', p - decimal_rep_length);
    } else {
      const size_t m = std::min(decimal_rep_length, decimal_point);
      builder.AddSubstring(decimal_rep, m);
      builder.AddPadding('0', decimal_point - decimal_rep_length);
      if (decimal_point < p) {
        builder.AddCharacter('.');
        const int extra = negative ? 2 : 1;
        if (decimal_rep_length > decimal_point) {
          DCHECK_EQ(decimal_rep_length - decimal_point,
                    strlen(decimal_rep + decimal_point));
          const int len = decimal_rep_length - decimal_point;
          DCHECK_LE(builder.position(), kMaxInt);
          const size_t n =
              std::min(len, p - static_cast<int>(builder.position() - extra));
          builder.AddSubstring(decimal_rep + decimal_point, n);
        }
        builder.AddPadding('0',
                           extra + (p - static_cast<int>(builder.position())));
      }
    }
    result = {buffer.begin(), builder.Finalize()};
  }
 
  return result;
}
 
std::string_view DoubleToRadixStringView(double value, int radix,
                                         base::Vector<char> buffer) {
  // We don't expect to see zero here (callers should handle it).
  DCHECK_NE(0.0, value);
 
  // Certain invalid inputs will cause this function to corrupt memory (write
  // out-of-bounds of the given buffer), so defend against that with CHECKs.
  CHECK(radix >= 2 && radix <= 36);
  CHECK(std::isfinite(value));
 
  // Character array used for conversion.
  static const char chars[] = "0123456789abcdefghijklmnopqrstuvwxyz";
 
  size_t integer_cursor = buffer.size() / 2;
  size_t fraction_cursor = integer_cursor;
 
  bool negative = value < 0;
  if (negative) value = -value;
 
  // Split the value into an integer part and a fractional part.
  double integer = std::floor(value);
  double fraction = value - integer;
  // We only compute fractional digits up to the input double's precision.
  double delta = 0.5 * (base::Double(value).NextDouble() - value);
  bool delta_is_positive = true;
  // If the delta rounded down to zero, use the minimum (denormal) delta
  // value. Be careful around denormal flushing when doing so.
  if (delta <= 0) {
    if (base::FPU::GetFlushDenormals()) {
      // We're flushing the delta value to zero, so the loop below won't
      // make progress. Skip it instead.
      delta_is_positive = false;
    } else {
      static_assert(base::Double(0.0).NextDouble() > 0);
      delta = base::Double(0.0).NextDouble();
    }
  }
  if (delta_is_positive && fraction >= delta) {
    // Insert decimal point.
    buffer[fraction_cursor++] = '.';
    do {
      // Shift up by one digit.
      fraction *= radix;
      delta *= radix;
      // Write digit.
      int digit = static_cast<int>(fraction);
      buffer[fraction_cursor++] = chars[digit];
      // Calculate remainder.
      fraction -= digit;
      // Round to even.
      if (fraction > 0.5 || (fraction == 0.5 && (digit & 1))) {
        if (fraction + delta > 1) {
          // We need to back trace already written digits in case of carry-over.
          while (true) {
            fraction_cursor--;
            if (fraction_cursor == buffer.size() / 2) {
              CHECK_EQ('.', buffer[fraction_cursor]);
              // Carry over to the integer part.
              integer += 1;
              break;
            }
            char c = buffer[fraction_cursor];
            // Reconstruct digit.
            digit = c > '9' ? (c - 'a' + 10) : (c - '0');
            if (digit + 1 < radix) {
              buffer[fraction_cursor++] = chars[digit + 1];
              break;
            }
          }
          break;
        }
      }
    } while (fraction >= delta);
  }
 
  // Compute integer digits. Fill unrepresented digits with zero.
  while (base::Double(integer / radix).Exponent() > 0) {
    integer /= radix;
    buffer[--integer_cursor] = '0';
  }
  do {
    double remainder = Modulo(integer, radix);
    buffer[--integer_cursor] = chars[static_cast<int>(remainder)];
    integer = (integer - remainder) / radix;
  } while (integer > 0);
 
  // Add sign and terminate string.
  if (negative) buffer[--integer_cursor] = '-';
  DCHECK_LE(integer_cursor, 1u << 31);  // Didn't underflow.
  DCHECK_GT(fraction_cursor, integer_cursor);
  return {buffer.begin() + integer_cursor, fraction_cursor - integer_cursor};
}
 
// ES6 18.2.4 parseFloat(string)
double StringToDouble(Isolate* isolate, DirectHandle<String> string,
                      ConversionFlag flag, double empty_string_val) {
  DirectHandle<String> flattened = String::Flatten(isolate, string);
  return FlatStringToDouble(*flattened, flag, empty_string_val);
}
 
double FlatStringToDouble(Tagged<String> string, ConversionFlag flag,
                          double empty_string_val) {
  DisallowGarbageCollection no_gc;
  DCHECK(string->IsFlat());
  String::FlatContent flat = string->GetFlatContent(no_gc);
  DCHECK(flat.IsFlat());
  if (flat.IsOneByte()) {
    return StringToDouble(flat.ToOneByteVector(), flag, empty_string_val);
  } else {
    return StringToDouble(flat.ToUC16Vector(), flag, empty_string_val);
  }
}
 
std::optional<double> TryStringToDouble(LocalIsolate* isolate,
                                        DirectHandle<String> object,
                                        uint32_t max_length_for_conversion) {
  DisallowGarbageCollection no_gc;
  uint32_t length = object->length();
  if (length > max_length_for_conversion) {
    return std::nullopt;
  }
 
  auto buffer = std::make_unique<base::uc16[]>(max_length_for_conversion);
  SharedStringAccessGuardIfNeeded access_guard(isolate);
  String::WriteToFlat(*object, buffer.get(), 0, length, access_guard);
  base::Vector<const base::uc16> v(buffer.get(), length);
  return StringToDouble(v, ALLOW_NON_DECIMAL_PREFIX);
}
 
std::optional<double> TryStringToInt(LocalIsolate* isolate,
                                     DirectHandle<String> object, int radix) {
  DisallowGarbageCollection no_gc;
  const uint32_t kMaxLengthForConversion = 20;
  uint32_t length = object->length();
  if (length > kMaxLengthForConversion) {
    return std::nullopt;
  }
 
  if (String::IsOneByteRepresentationUnderneath(*object)) {
    uint8_t buffer[kMaxLengthForConversion];
    SharedStringAccessGuardIfNeeded access_guard(isolate);
    String::WriteToFlat(*object, buffer, 0, length, access_guard);
    NumberParseIntHelper helper(buffer, radix, length);
    return helper.GetResult();
  } else {
    base::uc16 buffer[kMaxLengthForConversion];
    SharedStringAccessGuardIfNeeded access_guard(isolate);
    String::WriteToFlat(*object, buffer, 0, length, access_guard);
    NumberParseIntHelper helper(buffer, radix, length);
    return helper.GetResult();
  }
}
 
bool IsSpecialIndex(Tagged<String> string) {
  DCHECK(!SharedStringAccessGuardIfNeeded::IsNeeded(string));
  SharedStringAccessGuardIfNeeded access_guard =
      SharedStringAccessGuardIfNeeded::NotNeeded();
  return IsSpecialIndex(string, access_guard);
}
 
bool IsSpecialIndex(Tagged<String> string,
                    SharedStringAccessGuardIfNeeded& access_guard) {
  // Max length of canonical double: -X.XXXXXXXXXXXXXXXXX-eXXX
  const uint32_t kBufferSize = 24;
  const uint32_t length = string->length();
  if (length == 0 || length > kBufferSize) return false;
  uint16_t buffer[kBufferSize];
  String::WriteToFlat(string, buffer, 0, length, access_guard);
  // If the first char is not a digit or a '-' or we can't match 'NaN' or
  // '(-)Infinity', bailout immediately.
  uint32_t offset = 0;
  if (!IsDecimalDigit(buffer[0])) {
    if (buffer[0] == '-') {
      if (length == 1) return false;  // Just '-' is bad.
      if (!IsDecimalDigit(buffer[1])) {
        if (buffer[1] == 'I' && length == 9) {
          // Allow matching of '-Infinity' below.
        } else {
          return false;
        }
      }
      offset++;
    } else if (buffer[0] == 'I' && length == 8) {
      // Allow matching of 'Infinity' below.
    } else if (buffer[0] == 'N' && length == 3) {
      // Match NaN.
      return buffer[1] == 'a' && buffer[2] == 'N';
    } else {
      return false;
    }
  }
  // Expected fast path: key is an integer.
  static const uint32_t kRepresentableIntegerLength = 15;  // (-)XXXXXXXXXXXXXXX
  if (length - offset <= kRepresentableIntegerLength) {
    const uint32_t initial_offset = offset;
    bool matches = true;
    for (; offset < length; offset++) {
      matches &= IsDecimalDigit(buffer[offset]);
    }
    if (matches) {
      // Match 0 and -0.
      if (buffer[initial_offset] == '0') return initial_offset == length - 1;
      return true;
    }
  }
  // Slow path: test DoubleToString(StringToDouble(string)) == string.
  base::Vector<const uint16_t> vector(buffer, length);
  double d = StringToDouble(vector, NO_CONVERSION_FLAG);
  if (std::isnan(d)) return false;
  // Compute reverse string.
  char reverse_buffer[kBufferSize + 1];  // Result will be /0 terminated.
  base::Vector<char> reverse_vector(reverse_buffer, arraysize(reverse_buffer));
  std::string_view reverse_string = DoubleToStringView(d, reverse_vector);
 
  if (reverse_string.length() != length) return false;
  for (uint32_t i = 0; i < length; ++i) {
    if (static_cast<uint16_t>(reverse_string[i]) != buffer[i]) return false;
  }
  return true;
}
 
float DoubleToFloat32_NoInline(double x) { return DoubleToFloat32(x); }
 
int32_t DoubleToInt32_NoInline(double x) { return DoubleToInt32(x); }
 
}  // namespace internal
}  // namespace v8
 
#undef FPCLASSIFY_NAMESPACE