/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

//
// Docs: https://fburl.com/fbcref_conv
//

/**
 * Conv provides the ubiquitous method `to<TargetType>(source)`, along with
 * a few other generic interfaces for converting objects to and from
 * string-like types (std::string, fbstring, StringPiece), as well as
 * range-checked conversions between numeric and enum types. The mechanisms are
 * extensible, so that user-specified types can add folly::to support.
 *
 *     folly::to<std::string>(123)
 *     // "123"
 *
 *******************************************************************************
 * ## TYPE -> STRING CONVERSIONS
 *******************************************************************************
 * You can call the `to<std::string>` or `to<fbstring>`. These are variadic
 * functions that convert their arguments to strings, and concatenate them to
 * form a result. So, for example,
 *
 *     auto str = to<std::string>(123, "456", 789);
 *
 * Sets str to `"123456789"`.
 *
 * In addition to just concatenating the arguments, related functions can
 * delimit them with some string: `toDelim<std::string>(",", "123", 456, "789")`
 * will return the string `"123,456,789"`.
 *
 * toAppend does not return a string; instead, it takes a pointer to a string as
 * its last argument, and appends the result of the concatenation into it:
 *     std::string str = "123";
 *     toAppend(456, "789", &str); // Now str is "123456789".
 *
 * The toAppendFit function acts like toAppend, but it precalculates the size
 * required to perform the append operation, and reserves that space in the
 * output string before actually inserting its arguments. This can sometimes
 * save on string expansion, but beware: appending to the same string many times
 * with toAppendFit is likely a pessimization, since it will resize the string
 * once per append.
 *
 * The combination of the append and delim variants also exist: toAppendDelim
 * and toAppendDelimFit are defined, with the obvious semantics.
 *
 *******************************************************************************
 * ## STRING -> TYPE CONVERSIONS
 *******************************************************************************
 * Going in the other direction, and parsing a string into a C++ type, is also
 * supported:
 *     to<int>("123"); // Returns 123.
 *
 * Out of range (e.g. `to<std::uint8_t>("1000")`), or invalidly formatted (e.g.
 * `to<int>("four")`) inputs will throw. If throw-on-error is undesirable (for
 * instance: you're dealing with untrusted input, and want to protect yourself
 * from users sending you down a very slow exception-throwing path), you can use
 * `tryTo<T>`, which will return an `Expected<T, ConversionCode>`.
 *
 * There are overloads of to() and tryTo() that take a `StringPiece*`. These
 * parse out a type from the beginning of a string, and modify the passed-in
 * StringPiece to indicate the portion of the string not consumed.
 *
 *******************************************************************************
 * ## NUMERIC / ENUM CONVERSIONS
 *******************************************************************************
 * Conv also supports a `to<T>(S)` overload, where T and S are numeric or enum
 * types, that checks to see that the target type can represent its argument,
 * and will throw if it cannot. This includes cases where a floating point to
 * integral conversion is attempted on a value with a non-zero fractional
 * component, and integral to floating point conversions that would lose
 * precision. Enum conversions are range-checked for the underlying type of the
 * enum, but there is no check that the input value is a valid choice of enum
 * value.
 *
 *******************************************************************************
 * ## CUSTOM TYPE CONVERSIONS
 *******************************************************************************
 * Users may customize the string conversion functionality for their own data
 * types. The key functions you should implement are:
 *     // Two functions to allow conversion to your type from a string.
 *     Expected<StringPiece, ConversionCode> parseTo(folly::StringPiece in,
 *         YourType& out);
 *     YourErrorType makeConversionError(YourErrorType in, StringPiece in);
 *     // Two functions to allow conversion from your type to a string.
 *     template <class String>
 *   void toAppend(const YourType& in, String* out);
 *       size_t estimateSpaceNeeded(const YourType& in);
 *
 * These are documented below, inline.
 *
 * @file Conv.h
 */

#pragma once

#include <algorithm>
#include <cassert>
#include <cctype>
#include <climits>
#include <cmath>
#include <cstddef>
#include <limits>
#include <optional>
#include <stdexcept>
#include <string>
#include <system_error>
#include <tuple>
#include <type_traits>
#include <utility>

#if __has_include(<charconv>)
#include <charconv>
#endif

#include <double-conversion/double-conversion.h> // V8 JavaScript implementation

#include <folly/CPortability.h>

#include <folly/Demangle.h>
#include <folly/Expected.h>
#include <folly/FBString.h>
#include <folly/Likely.h>
#include <folly/Portability.h>
#include <folly/Range.h>
#include <folly/Traits.h>
#include <folly/Unit.h>
#include <folly/Utility.h>
#include <folly/lang/Exception.h>
#include <folly/lang/Pretty.h>
#include <folly/lang/ToAscii.h>
#include <folly/portability/Math.h>

// FOLLY_CONV_AVALIABILITY_TO_CHARS_FLOATING_POINT indicates that
// std::to_chars for floating point is available
#if (defined(__cpp_lib_to_chars) && __cpp_lib_to_chars >= 201611L)
#define FOLLY_CONV_AVALIABILITY_TO_CHARS_FLOATING_POINT 1
#elif defined(_LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS) && \
    defined(_LIBCPP_AVAILABILITY_TO_CHARS_FLOATING_POINT)
#define FOLLY_CONV_AVALIABILITY_TO_CHARS_FLOATING_POINT 1
#elif defined(__APPLE__) &&                                        \
    ((defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) &&    \
      __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ >= 130300) ||  \
     (defined(__ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__) &&   \
      __ENVIRONMENT_IPHONE_OS_VERSION_MIN_REQUIRED__ >= 160300) || \
     (defined(__ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__) &&       \
      __ENVIRONMENT_TV_OS_VERSION_MIN_REQUIRED__ >= 160300) ||     \
     (defined(__ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__) &&    \
      __ENVIRONMENT_WATCH_OS_VERSION_MIN_REQUIRED__ >= 90300))
// Apple introduces std::to_chars & std::from_chars support for floating
// point types for: macOS 13.3, iOS 16.3, tvOS 16.3, watchOS 9.3.
// https://developer.apple.com/xcode/cpp/#c++17
// __builtin_available(macOS 13.3, iOS 16.3, tvOS 16.3, watchOS 9.3, *)) {
// The avaliability attributes are marked as strict, so preprocessor
// conditionals must be used to check if it's available.
#define FOLLY_CONV_AVALIABILITY_TO_CHARS_FLOATING_POINT 1
#else
#define FOLLY_CONV_AVALIABILITY_TO_CHARS_FLOATING_POINT 0
#endif

// FOLLY_CONV_USE_TO_CHARS set to 1 indicates that std::to_chars will be used
// because it's available and it was requested.
#if defined(FOLLY_CONV_DTOA_TO_CHARS) && FOLLY_CONV_DTOA_TO_CHARS == 1 && \
    defined(FOLLY_CONV_AVALIABILITY_TO_CHARS_FLOATING_POINT) &&           \
    FOLLY_CONV_AVALIABILITY_TO_CHARS_FLOATING_POINT == 1
#define FOLLY_CONV_USE_TO_CHARS 1
#else
#define FOLLY_CONV_USE_TO_CHARS 0
#endif

namespace folly {

// Keep this in sync with kErrorStrings in Conv.cpp
enum class ConversionCode : unsigned char {
  SUCCESS,
  EMPTY_INPUT_STRING,
  NO_DIGITS,
  BOOL_OVERFLOW,
  BOOL_INVALID_VALUE,
  NON_DIGIT_CHAR,
  INVALID_LEADING_CHAR,
  POSITIVE_OVERFLOW,
  NEGATIVE_OVERFLOW,
  STRING_TO_FLOAT_ERROR,
  NON_WHITESPACE_AFTER_END,
  ARITH_POSITIVE_OVERFLOW,
  ARITH_NEGATIVE_OVERFLOW,
  ARITH_LOSS_OF_PRECISION,
  NUM_ERROR_CODES, // has to be the last entry
};

struct FOLLY_EXPORT ConversionErrorBase : std::range_error {
  using std::range_error::range_error;
};

class FOLLY_EXPORT ConversionError : public ConversionErrorBase {
 public:
  ConversionError(const std::string& str, ConversionCode code)
      : ConversionErrorBase(str), code_(code) {}

  ConversionError(const char* str, ConversionCode code)
      : ConversionErrorBase(str), code_(code) {}

  ConversionCode errorCode() const { return code_; }

 private:
  ConversionCode code_;
};

/**
 * Custom Error Translation
 *
 * Your overloaded parseTo() function can return a custom error code on failure.
 * ::folly::to() will call makeConversionError to translate that error code into
 * an object to throw. makeConversionError is found by argument-dependent
 * lookup. It should have this signature:
 *
 * namespace other_namespace {
 * enum YourErrorCode { BAD_ERROR, WORSE_ERROR };
 *
 * struct YourConversionError : ConversionErrorBase {
 *   YourConversionError(const char* what) : ConversionErrorBase(what) {}
 * };
 *
 * YourConversionError
 * makeConversionError(YourErrorCode code, ::folly::StringPiece sp) {
 *   ...
 *   return YourConversionError(messageString);
 * }
 */
ConversionError makeConversionError(ConversionCode code, StringPiece input);

namespace detail {
/**
 * Enforce that the suffix following a number is made up only of whitespace.
 */
inline ConversionCode enforceWhitespaceErr(StringPiece sp) {
  for (auto c : sp) {
    if (FOLLY_UNLIKELY(!std::isspace(c))) {
      return ConversionCode::NON_WHITESPACE_AFTER_END;
    }
  }
  return ConversionCode::SUCCESS;
}

/**
 * Keep this implementation around for prettyToDouble().
 */
inline void enforceWhitespace(StringPiece sp) {
  auto err = enforceWhitespaceErr(sp);
  if (err != ConversionCode::SUCCESS) {
    throw_exception(makeConversionError(err, sp));
  }
}
} // namespace detail

/**
 * @overloadbrief to, but return an Expected
 *
 * The identity conversion function.
 * tryTo<T>(T) returns itself for all types T.
 */
template <class Tgt, class Src>
typename std::enable_if<
    std::is_same<Tgt, typename std::decay<Src>::type>::value,
    Expected<Tgt, ConversionCode>>::type
tryTo(Src&& value) noexcept {
  return static_cast<Src&&>(value);
}

/**
 * @overloadbrief Convert from one type to another.
 */
template <class Tgt, class Src>
typename std::enable_if<
    std::is_same<Tgt, typename std::decay<Src>::type>::value,
    Tgt>::type
to(Src&& value) {
  return static_cast<Src&&>(value);
}

/**
 * Arithmetic to boolean
 */

/**
 * Unchecked conversion from arithmetic to boolean. This is different from the
 * other arithmetic conversions because we use the C convention of treating any
 * non-zero value as true, instead of range checking.
 */
template <class Tgt, class Src>
typename std::enable_if<
    is_arithmetic_v<Src> && !std::is_same<Tgt, Src>::value &&
        std::is_same<Tgt, bool>::value,
    Expected<Tgt, ConversionCode>>::type
tryTo(const Src& value) noexcept {
  return value != Src();
}

template <class Tgt, class Src>
typename std::enable_if<
    is_arithmetic_v<Src> && !std::is_same<Tgt, Src>::value &&
        std::is_same<Tgt, bool>::value,
    Tgt>::type
to(const Src& value) {
  return value != Src();
}

/**
 * Anything to string
 */

namespace detail {

template <class... T>
using LastElement = type_pack_element_t<sizeof...(T) - 1, T...>;

#ifdef _MSC_VER
// MSVC can't quite figure out the LastElementImpl::call() stuff
// in the base implementation, so we have to use tuples instead,
// which result in significantly more templates being compiled,
// though the runtime performance is the same.

template <typename... Ts, typename R = LastElement<Ts...>>
const R& getLastElement(const Ts&... ts) {
  return std::get<sizeof...(Ts) - 1>(std::forward_as_tuple(ts...));
}

inline void getLastElement() {}
#else
template <typename...>
struct LastElementImpl;
template <>
struct LastElementImpl<> {
  static void call() {}
};
template <typename Ign, typename... Igns>
struct LastElementImpl<Ign, Igns...> {
  template <typename Last>
  static const Last& call(Igns..., const Last& last) {
    return last;
  }
};

template <typename... Ts, typename R = LastElement<Ts...>>
const R& getLastElement(const Ts&... ts) {
  return LastElementImpl<Ignored<Ts>...>::call(ts...);
}
#endif

} // namespace detail

/**
 * Conversions from integral types to string types.
 */

#if FOLLY_HAVE_INT128_T
namespace detail {

template <typename IntegerType>
constexpr unsigned int digitsEnough() {
  // digits10 returns the number of decimal digits that this type can represent,
  // not the number of characters required for the max value, so we need to add
  // one. ex: char digits10 returns 2, because 256-999 cannot be represented,
  // but we need 3.
  auto const digits10 = std::numeric_limits<IntegerType>::digits10;
  return static_cast<unsigned int>(digits10) + 1;
}

inline size_t unsafeTelescope128(char* outb, char* oute, unsigned __int128 x) {
  using Usrc = unsigned __int128;

  // Decompose the input into at most 3 components using the largest power-of-10
  // base that fits in a 64-bit unsigned integer, and then convert the
  // components using 64-bit arithmetic and concatenate them.
  constexpr static auto kBase = UINT64_C(10'000'000'000'000'000'000);
  constexpr static size_t kBaseDigits = 19;

  size_t p = 0;
  const auto leading = [&](Usrc v) {
    assert(v >> 64 == 0);
    p = detail::to_ascii_with_route<10, to_ascii_alphabet_lower>(
        outb, oute, static_cast<uint64_t>(v));
  };
  const auto append = [&](uint64_t v) {
    assert(v < kBase);
    assert(outb + p + kBaseDigits <= oute);
    auto v64 = static_cast<uint64_t>(v);
    detail::to_ascii_with_route<10, to_ascii_alphabet_lower>(
        outb + p, kBaseDigits, v64);
    p += kBaseDigits;
  };

  if (x >> 64 > 0) {
    const auto rem = static_cast<uint64_t>(x % kBase);
    x /= kBase;

    if (x >> 64 > 0) {
      const auto rem2 = static_cast<uint64_t>(x % kBase);
      x /= kBase;

      leading(x);
      append(rem2);
      append(rem);
      return p;
    }

    leading(x);
    append(rem);
    return p;
  }

  leading(x);
  return p;
}

} // namespace detail
#endif

/**
 * @overloadbrief Appends conversion to string.
 *
 * A single char gets appended.
 */
template <class Tgt>
void toAppend(char value, Tgt* result) {
  *result += value;
}

/**
 * @overloadbrief Estimates the number of characters in a value's string
 * representation.
 */
template <class T>
constexpr typename std::enable_if<std::is_same<T, char>::value, size_t>::type
estimateSpaceNeeded(T) {
  return 1;
}

template <size_t N>
constexpr size_t estimateSpaceNeeded(const char (&)[N]) {
  return N;
}

/**
 * Everything implicitly convertible to const char* gets appended.
 */
template <class Tgt, class Src>
typename std::enable_if<
    std::is_convertible<Src, const char*>::value &&
    IsSomeString<Tgt>::value>::type
toAppend(Src value, Tgt* result) {
  // Treat null pointers like an empty string, as in:
  // operator<<(std::ostream&, const char*).
  const char* c = value;
  if (c) {
    result->append(value);
  }
}

template <class Src>
typename std::enable_if<std::is_convertible<Src, const char*>::value, size_t>::
    type
    estimateSpaceNeeded(Src value) {
  const char* c = value;
  return c ? std::strlen(c) : 0;
}

template <class Src>
typename std::enable_if<IsSomeString<Src>::value, size_t>::type
estimateSpaceNeeded(Src const& value) {
  return value.size();
}

template <class Src>
typename std::enable_if<
    std::is_convertible<Src, folly::StringPiece>::value &&
        !IsSomeString<Src>::value &&
        !std::is_convertible<Src, const char*>::value,
    size_t>::type
estimateSpaceNeeded(Src value) {
  return folly::StringPiece(value).size();
}

template <>
inline size_t estimateSpaceNeeded(std::nullptr_t /* value */) {
  return 0;
}

template <class Src>
typename std::enable_if<
    std::is_pointer<Src>::value &&
        IsSomeString<std::remove_pointer<Src>>::value,
    size_t>::type
estimateSpaceNeeded(Src value) {
  return value->size();
}

/**
 * Strings get appended, too.
 */
template <class Tgt, class Src>
typename std::enable_if<
    IsSomeString<Src>::value && IsSomeString<Tgt>::value>::type
toAppend(const Src& value, Tgt* result) {
  result->append(value);
}

/**
 * and StringPiece objects too
 */
template <class Tgt>
typename std::enable_if<IsSomeString<Tgt>::value>::type toAppend(
    StringPiece value, Tgt* result) {
  result->append(value.data(), value.size());
}

/**
 * There's no implicit conversion from fbstring to other string types,
 * so make a specialization.
 */
template <class Tgt>
typename std::enable_if<IsSomeString<Tgt>::value>::type toAppend(
    const fbstring& value, Tgt* result) {
  result->append(value.data(), value.size());
}

#if FOLLY_HAVE_INT128_T
/**
 * Special handling for 128 bit integers.
 */

template <class Tgt>
void toAppend(__int128 value, Tgt* result) {
  typedef unsigned __int128 Usrc;
  char buffer[detail::digitsEnough<unsigned __int128>() + 1];
  const auto oute = buffer + sizeof(buffer);
  size_t p;

  if (value < 0) {
    buffer[0] = '-';
    p = 1 + detail::unsafeTelescope128(buffer + 1, oute, -Usrc(value));
  } else {
    p = detail::unsafeTelescope128(buffer, oute, value);
  }

  result->append(buffer, p);
}

template <class Tgt>
void toAppend(unsigned __int128 value, Tgt* result) {
  char buffer[detail::digitsEnough<unsigned __int128>()];
  size_t p = detail::unsafeTelescope128(buffer, buffer + sizeof(buffer), value);
  result->append(buffer, p);
}

template <class T>
constexpr
    typename std::enable_if<std::is_same<T, __int128>::value, size_t>::type
    estimateSpaceNeeded(T) {
  return detail::digitsEnough<__int128>();
}

template <class T>
constexpr typename std::
    enable_if<std::is_same<T, unsigned __int128>::value, size_t>::type
    estimateSpaceNeeded(T) {
  return detail::digitsEnough<unsigned __int128>();
}

#endif

/**
 * int32_t and int64_t to string (by appending) go through here. The
 * result is APPENDED to a preexisting string passed as the second
 * parameter. This should be efficient with fbstring because fbstring
 * incurs no dynamic allocation below 23 bytes and no number has more
 * than 22 bytes in its textual representation (20 for digits, one for
 * sign, one for the terminating 0).
 */
template <class Tgt, class Src>
typename std::enable_if<
    is_integral_v<Src> && is_signed_v<Src> && IsSomeString<Tgt>::value &&
    sizeof(Src) >= 4>::type
toAppend(Src value, Tgt* result) {
  char buffer[to_ascii_size_max_decimal<uint64_t>];
  auto uvalue = value < 0 ? ~static_cast<uint64_t>(value) + 1
                          : static_cast<uint64_t>(value);
  if (value < 0) {
    result->push_back('-');
  }
  result->append(buffer, to_ascii_decimal(buffer, uvalue));
}

template <class Src>
typename std::enable_if<
    is_integral_v<Src> && is_signed_v<Src> && sizeof(Src) >= 4 &&
        sizeof(Src) < 16,
    size_t>::type
estimateSpaceNeeded(Src value) {
  auto uvalue = value < 0 ? ~static_cast<uint64_t>(value) + 1
                          : static_cast<uint64_t>(value);
  return size_t(value < 0) + to_ascii_size_decimal(uvalue);
}

/**
 * As above, but for uint32_t and uint64_t.
 */
template <class Tgt, class Src>
typename std::enable_if<
    is_integral_v<Src> && !is_signed_v<Src> && IsSomeString<Tgt>::value &&
    sizeof(Src) >= 4>::type
toAppend(Src value, Tgt* result) {
  char buffer[to_ascii_size_max_decimal<uint64_t>];
  result->append(buffer, to_ascii_decimal(buffer, value));
}

template <class Src>
typename std::enable_if<
    is_integral_v<Src> && !is_signed_v<Src> && sizeof(Src) >= 4 &&
        sizeof(Src) < 16,
    size_t>::type
estimateSpaceNeeded(Src value) {
  return to_ascii_size_decimal(value);
}

/**
 * All small signed and unsigned integers to string go through 32-bit
 * types int32_t and uint32_t, respectively.
 */
template <class Tgt, class Src>
typename std::enable_if<
    is_integral_v<Src> && IsSomeString<Tgt>::value && sizeof(Src) < 4>::type
toAppend(Src value, Tgt* result) {
  typedef typename std::conditional<is_signed_v<Src>, int64_t, uint64_t>::type
      Intermediate;
  toAppend<Tgt>(static_cast<Intermediate>(value), result);
}

template <class Src>
typename std::enable_if<
    is_integral_v<Src> && sizeof(Src) < 4 && !std::is_same<Src, char>::value,
    size_t>::type
estimateSpaceNeeded(Src value) {
  typedef typename std::conditional<is_signed_v<Src>, int64_t, uint64_t>::type
      Intermediate;
  return estimateSpaceNeeded(static_cast<Intermediate>(value));
}

/**
 * Enumerated values get appended as integers.
 */
template <class Tgt, class Src>
typename std::enable_if<
    std::is_enum<Src>::value && IsSomeString<Tgt>::value>::type
toAppend(Src value, Tgt* result) {
  toAppend(to_underlying(value), result);
}

template <class Src>
typename std::enable_if<std::is_enum<Src>::value, size_t>::type
estimateSpaceNeeded(Src value) {
  return estimateSpaceNeeded(to_underlying(value));
}

/**
 * Conversions from floating-point types to string types.
 */

/// Operating mode for the floating point type version of
/// `folly::ToAppend`. This is modeled after
/// `double_conversion::DoubleToStringConverter::DtoaMode`.
/// Dtoa is an acryonym for Double to ASCII.
enum class DtoaMode {
  /// Outputs the shortest representation of a `double`.
  /// The output is either in decimal or exponential notation; which ever is
  /// shortest.
  SHORTEST,
  /// Outputs the shortest representation of a `float`.
  /// This outputs in either decimal or exponential notation, which ever is
  /// shortest.
  SHORTEST_SINGLE,
  /// Outputs fixed precision after the decimal point. Similar to
  /// `printf`'s %f.
  /// The output is in decimal notation.
  /// Use the `numDigits` parameter to specify the precision.
  FIXED,
  /// Outputs with a precision that is independent of the decimal point.
  /// The outputs is either decimal or exponential notation, depending on the
  /// value and the precision.
  /// Similar to `printf`'s %g formating.
  /// Use the `numDigits` parameter to specify the precision.
  PRECISION,
};

/// Flags for the floating point type version of `folly::ToAppend`.
/// This is modeled after `double_conversion::DoubleToStringConverter::Flags`.
/// Dtoa is an acryonym for Double to ASCII.
/// This enum is used to store bit wise flags, so a variable of this type may be
/// a bitwise combination of these definitions.
enum class DtoaFlags {
  NO_FLAGS = 0,
  /// Emits a plus sign for positive exponents. e.g., 1.2e+3
  EMIT_POSITIVE_EXPONENT_SIGN = 1,
  /// Emits a trailing decimal point. e.g., 123.
  EMIT_TRAILING_DECIMAL_POINT = 2,
  /// Emits a trailing decimal point. e.g., 123.0
  /// Requires `EMIT_TRAILING_DECIMAL_POINT` to be set.
  EMIT_TRAILING_ZERO_AFTER_POINT = 4,
  /// -0.0 outputs as 0.0
  UNIQUE_ZERO = 8,
  /// Trailing zeros are removed from the fractional portion
  /// of the result in precision mode. Matches `printf`'s %g.
  /// When `EMIT_TRAILING_ZERO_AFTER_POINT` is also given, one trailing zero is
  /// preserved.
  NO_TRAILING_ZERO = 16,
};

constexpr DtoaFlags operator|(DtoaFlags a, DtoaFlags b) {
  return static_cast<DtoaFlags>(to_underlying(a) | to_underlying(b));
}

constexpr DtoaFlags operator&(DtoaFlags a, DtoaFlags b) {
  return static_cast<DtoaFlags>(to_underlying(a) & to_underlying(b));
}

namespace detail {
constexpr int kConvMaxDecimalInShortestLow = -6;
/// 10^kConvMaxDecimalInShortestLow. Replace with constexpr std::pow in C++26.
constexpr double kConvMaxDecimalInShortestLowValue = 0.000001;
constexpr int kConvMaxDecimalInShortestHigh = 21;
/// 10^kConvMaxDecimalInShortestHigh. Replace with constexpr std::pow in C++26.
constexpr double kConvMaxDecimalInShortestHighValue =
    1'000'000'000'000'000'000'000.0;
constexpr int kBase10MaximalLength = 17;

enum class FloatToStringImpl {
  LibDoubleConversion,
  StdToChars,
};

#if defined(FOLLY_CONV_USE_TO_CHARS) && FOLLY_CONV_USE_TO_CHARS == 1
constexpr FloatToStringImpl kConvFloatToStringImpl =
    FloatToStringImpl::StdToChars;
constexpr int kConvMaxFixedDigitsAfterPoint = 100;
constexpr int kConvMaxPrecisionDigits = 120;
#else
constexpr FloatToStringImpl kConvFloatToStringImpl =
    FloatToStringImpl::LibDoubleConversion;
constexpr int kConvMaxFixedDigitsAfterPoint =
    double_conversion::DoubleToStringConverter::kMaxFixedDigitsAfterPoint;
constexpr int kConvMaxPrecisionDigits =
    double_conversion::DoubleToStringConverter::kMaxPrecisionDigits;

/// Converts `DtoaMode` to
/// `double_conversion::DoubleToStringConverter::DtoaMode`.
/// This is temporary until
/// `double_conversion::DoubleToStringConverter::DtoaMode` is removed.
constexpr double_conversion::DoubleToStringConverter::DtoaMode convert(
    DtoaMode mode) {
  switch (mode) {
    case DtoaMode::SHORTEST:
      return double_conversion::DoubleToStringConverter::SHORTEST;
    case DtoaMode::SHORTEST_SINGLE:
      return double_conversion::DoubleToStringConverter::SHORTEST_SINGLE;
    case DtoaMode::FIXED:
      return double_conversion::DoubleToStringConverter::FIXED;
    case DtoaMode::PRECISION:
      return double_conversion::DoubleToStringConverter::PRECISION;
  }

  assert(false);
  // Default to PRECISION per exising behavior.
  return double_conversion::DoubleToStringConverter::PRECISION;
}

/// Converts `DtoaFlags` to
/// `double_conversion::DoubleToStringConverter::DtoaFlags`.
/// This is temporary until
/// `double_conversion::DoubleToStringConverter::DtoaFlags` is removed.
constexpr double_conversion::DoubleToStringConverter::Flags convert(
    DtoaFlags flags) {
  return static_cast<double_conversion::DoubleToStringConverter::Flags>(flags);
}

/**
 * Wrapper around `double_conversion::DoubleToStringConverter`.
 */
template <class Tgt, class Src>
typename std::enable_if<
    std::is_floating_point<Src>::value && IsSomeString<Tgt>::value>::type
toAppendDoubleConversion(
    Src value,
    Tgt* result,
    DtoaMode mode,
    unsigned int numDigits,
    DtoaFlags flags = DtoaFlags::NO_FLAGS) {
  using namespace double_conversion;
  DoubleToStringConverter::Flags dcFlags = detail::convert(flags);
  DoubleToStringConverter conv(
      dcFlags,
      "Infinity",
      "NaN",
      'E',
      detail::kConvMaxDecimalInShortestLow,
      detail::kConvMaxDecimalInShortestHigh,
      6, // max leading padding zeros
      1); // max trailing padding zeros
  char buffer[256];
  StringBuilder builder(buffer, sizeof(buffer));
  DoubleToStringConverter::DtoaMode dcMode = detail::convert(mode);
  FOLLY_PUSH_WARNING
  FOLLY_CLANG_DISABLE_WARNING("-Wcovered-switch-default")
  switch (dcMode) {
    case DoubleToStringConverter::SHORTEST:
      conv.ToShortest(value, &builder);
      break;
    case DoubleToStringConverter::SHORTEST_SINGLE:
      conv.ToShortestSingle(static_cast<float>(value), &builder);
      break;
    case DoubleToStringConverter::FIXED:
      conv.ToFixed(value, int(numDigits), &builder);
      break;
    case DoubleToStringConverter::PRECISION:
    default:
      assert(dcMode == DoubleToStringConverter::PRECISION);
      conv.ToPrecision(value, int(numDigits), &builder);
      break;
  }
  FOLLY_POP_WARNING
  const size_t length = size_t(builder.position());
  builder.Finalize();
  result->append(buffer, length);
}
#endif // FOLLY_CONV_USE_TO_CHARS

#if defined(FOLLY_CONV_AVALIABILITY_TO_CHARS_FLOATING_POINT) && \
    FOLLY_CONV_AVALIABILITY_TO_CHARS_FLOATING_POINT == 1
/// Holds a set of `DtoaFlags` as a bitwise OR of the flags.
/// It has convience member functions to check if a flag is set.
struct DtoaFlagsSet {
  explicit DtoaFlagsSet(DtoaFlags flags);

  bool isSet(DtoaFlags flag) const;

  bool emitPositiveExponentSign() const;
  bool emitTrailingDecimalPoint() const;
  bool emitTrailingZeroAfterPoint() const;
  bool uniqueZero() const;
  bool noTrailingZero() const;

 private:
  DtoaFlags flags_;
};
/// This parses a decimal string into a structured format.
/// For example, given "123.456e+7", this will create pointers to the integer,
/// fractional, exponentional parts.
///
/// The decimal string is passed in as a `char` buffer with begin and end
/// pointers. The parsing will create pointers to parts of the decimal string.
///
/// e.g.,
/// -123.456e+78
/// ABCDEFGHIJK
/// negativeSign points to address A
/// integerBegin points to address B
/// integerEnd points to address E
/// and so on...
///
/// The is used to format the output of `std::to_chars` so that it is consistent
/// with `double_conversion::DoubleToStringConverter`'s format.
///
/// This also has helper member functions to identify parts needed to apply
/// `DtoaMode::PRECISION` formating.
class ParsedDecimal {
 public:
  char* negativeSign{};
  char* integerBegin{};
  char* integerEnd{};
  char* decimalPoint{};
  char* fractionalBegin{};
  char* fractionalEnd{};
  char* exponentSymbol{};
  char* exponentSign{};
  char* exponentBegin{};
  char* exponentEnd{};

  ParsedDecimal(char* begin, char* end);

  /// Returns the number of figures that count in PRECISION/general mode.
  /// This is needed to know how many more figures to add when NO_TRAILING_ZERO
  /// is unset.
  int numPrecisionFigures() const;

  /// first is the begin pointer of the fractional suffix.
  /// second is the end pointer of the fractional suffix.
  using FractionalSuffix = std::pair<char*, char*>;

  /// Returns pointers to the suffix after the fraction.
  /// e.g., for "12.34-56" this returns pointers to "e-56".
  /// Returns nothing if there is no suffix (e.g., "7.89").
  std::optional<FractionalSuffix> fractionalSuffix() const;

  /// Shifts the pointers of the fractional suffix by the given amount.
  /// This is used when inserting additional figures for `DtoaMode::PRECISION`.
  /// The pointers need to be updated after the fractional suffix is `memmove`'d
  /// to accommodate the additional figures.
  void shiftFractionalSuffixPtrs(size_t amount);
};

/// Formats the output from `std::to_chars` as if it came from
/// `double_conversion::DoubleToStringConverter`.
///
/// Specifically it adds support for:
/// - EMIT_POSITIVE_EXPONENT_SIGN
/// - EMIT_TRAILING_DECIMAL_POINT
/// - EMIT_TRAILING_ZERO_AFTER_POINT
/// - UNIQUE_ZERO
/// - NO_TRAILING_ZERO
/// - Captial E exponent sign (e.g., 1.23e4 -> 1.23E4)
/// - Removes leading zeros in exponent (e.g., 1.23e04 -> 1.23e4)
///
/// This modifies the result buffer in place to match the output format of
/// `double_conversion::DoubleToStringConverter`.
/// `resultBegin` is the begin pointer of the result from `std::to_chars`.
/// `resultEnd` is the end pointer of the result from `std::to_chars`.
/// `bufferEnd` is the end pointer of the buffer space given to `std::to_chars`.
/// The extra buffer space is used to expand the result.
/// `resultBegin`, `resultEnd`, and `bufferEnd` must point to the same buffer.
///
/// The first char* of the return type is the begin pointer of the newly
/// formatted output. The second char* of the return type is the begin pointer
/// of the newly formatted output.
std::pair<char*, char*> formatAsDoubleConversion(
    bool valueIsZero,
    DtoaMode mode,
    unsigned int numDigits,
    DtoaFlags flags,
    char* resultBegin,
    char* resultEnd,
    char* bufferEnd);

template <class Tgt, class Src>
typename std::enable_if<
    std::is_floating_point<Src>::value && IsSomeString<Tgt>::value>::type
toAppendStdToChars(
    Src value,
    Tgt* result,
    DtoaMode mode,
    unsigned int numDigits,
    DtoaFlags flags = DtoaFlags::NO_FLAGS) {
  if (std::isnan(value)) {
    // no signbit check because -nan outputs as NaN
    result->append("NaN", 3);
    return;
  }

  if (std::isinf(value)) {
    if (std::signbit(value)) {
      result->append("-", 1);
    }
    // std::to_chars returns "inf", this needs "Infinity"
    result->append("Infinity", 8);
    return;
  }

  if (mode == DtoaMode::PRECISION &&
      (numDigits == 0 || numDigits > detail::kConvMaxPrecisionDigits)) {
    // double_conversion outputs the empty string in this scenario
    return;
  }

  if (mode == DtoaMode::FIXED &&
      numDigits > detail::kConvMaxFixedDigitsAfterPoint) {
    // double_conversion outputs the empty string in this scenario
    return;
  }

  bool useShortestFixed{false};
  if (mode == DtoaMode::SHORTEST || mode == DtoaMode::SHORTEST_SINGLE) {
    Src absValue = std::abs(value);
    // use fixed decimal notation (i.e., not exponential notation)
    // for values in this range to match double-conversion formatting.
    useShortestFixed = kConvMaxDecimalInShortestLowValue <= absValue &&
        absValue < kConvMaxDecimalInShortestHighValue;
  }

  std::to_chars_result conv_result;
  char buffer[256];
  char* const bufferEnd = buffer + sizeof(buffer);
  FOLLY_PUSH_WARNING
  FOLLY_CLANG_DISABLE_WARNING("-Wcovered-switch-default")
  switch (mode) {
    case DtoaMode::SHORTEST: {
      if (useShortestFixed) {
        conv_result =
            std::to_chars(buffer, bufferEnd, value, std::chars_format::fixed);
      } else {
        conv_result = std::to_chars(buffer, bufferEnd, value);
      }
      break;
    }
    case DtoaMode::SHORTEST_SINGLE:
      if (useShortestFixed) {
        conv_result = std::to_chars(
            buffer,
            bufferEnd,
            static_cast<float>(value),
            std::chars_format::fixed);
      } else {
        conv_result =
            std::to_chars(buffer, bufferEnd, static_cast<float>(value));
      }
      break;
    case DtoaMode::FIXED:
      conv_result = std::to_chars(
          buffer, bufferEnd, value, std::chars_format::fixed, numDigits);
      break;
    case DtoaMode::PRECISION:
    default:
      assert(mode == DtoaMode::PRECISION);
      conv_result = std::to_chars(
          buffer, bufferEnd, value, std::chars_format::general, numDigits);
      break;
  }
  FOLLY_POP_WARNING

  auto [resultEnd, ec] = conv_result;
  if (ec != std::errc()) {
    folly::throw_exception<std::system_error>(std::make_error_code(ec));
  }

  char* resultBegin = buffer;
  bool valueIsZero = value == 0.0;
  auto [formattedBegin, formattedEnd] = detail::formatAsDoubleConversion(
      valueIsZero, mode, numDigits, flags, resultBegin, resultEnd, bufferEnd);

  result->append(formattedBegin, formattedEnd - formattedBegin);
}
#endif // FOLLY_CONV_AVALIABILITY_TO_CHARS_FLOATING_POINT
} // namespace detail

/**
 * `numDigits` is only used with `FIXED` && `PRECISION`.
 */
template <class Tgt, class Src>
typename std::enable_if<
    std::is_floating_point<Src>::value && IsSomeString<Tgt>::value>::type
toAppend(
    Src value,
    Tgt* result,
    DtoaMode mode,
    unsigned int numDigits,
    DtoaFlags flags = DtoaFlags::NO_FLAGS) {
#if defined(FOLLY_CONV_USE_TO_CHARS) && FOLLY_CONV_USE_TO_CHARS == 1
  detail::toAppendStdToChars(value, result, mode, numDigits, flags);
#else
  detail::toAppendDoubleConversion(value, result, mode, numDigits, flags);
#endif
}

/**
 * As above, but for floating point
 */
template <class Tgt, class Src>
typename std::enable_if<
    std::is_floating_point<Src>::value && IsSomeString<Tgt>::value>::type
toAppend(Src value, Tgt* result) {
  toAppend(value, result, DtoaMode::SHORTEST, 0);
}

/**
 * Upper bound of the length of the output from
 * DoubleToStringConverter::ToShortest(double, StringBuilder*),
 * as used in toAppend(double, string*).
 */
template <class Src>
typename std::enable_if<std::is_floating_point<Src>::value, size_t>::type
estimateSpaceNeeded(Src value) {
  // kBase10MaximalLength is 17. We add 1 for decimal point,
  // e.g. 10.0/9 is 17 digits and 18 characters, including the decimal point.
  constexpr int kMaxMantissaSpace = detail::kBase10MaximalLength + 1;
  // strlen("E-") + digits10(numeric_limits<double>::max_exponent10)
  constexpr int kMaxExponentSpace = 2 + 3;
  static const int kMaxPositiveSpace = std::max({
      // E.g. 1.1111111111111111E-100.
      kMaxMantissaSpace + kMaxExponentSpace,
      // E.g. 0.000001.1111111111111111, if kConvMaxDecimalInShortestLow is -6.
      kMaxMantissaSpace - detail::kConvMaxDecimalInShortestLow,
      // If kConvMaxDecimalInShortestHigh is 21, then 1e21 is the smallest
      // number > 1 which ToShortest outputs in exponential notation,
      // so 21 is the longest non-exponential number > 1.
      detail::kConvMaxDecimalInShortestHigh,
  });
  return size_t(
      kMaxPositiveSpace +
      (value < 0 ? 1 : 0)); // +1 for minus sign, if negative
}

template <class Src>
constexpr typename std::enable_if<
    !std::is_fundamental<Src>::value &&
#if FOLLY_HAVE_INT128_T
        // On OSX 10.10, is_fundamental<__int128> is false :-O
        !std::is_same<__int128, Src>::value &&
        !std::is_same<unsigned __int128, Src>::value &&
#endif
        !IsSomeString<Src>::value &&
        !std::is_convertible<Src, const char*>::value &&
        !std::is_convertible<Src, StringPiece>::value &&
        !std::is_enum<Src>::value,
    size_t>::type
estimateSpaceNeeded(const Src&) {
  return sizeof(Src) + 1; // dumbest best effort ever?
}

#ifndef DOXYGEN_SHOULD_SKIP_THIS
namespace detail {

FOLLY_ERASE constexpr size_t estimateSpaceToReserveOne(std::false_type, void*) {
  return 0;
}
template <typename T>
FOLLY_ERASE constexpr size_t estimateSpaceToReserveOne(
    std::true_type, const T& v) {
  return estimateSpaceNeeded(v);
}

template <typename>
struct EstimateSpaceToReserveAll;
template <size_t... I>
struct EstimateSpaceToReserveAll<std::index_sequence<I...>> {
  template <size_t J, size_t N = sizeof...(I)>
  using tag = std::bool_constant<J + 1 < N>;
  template <class... T>
  static size_t call(const T&... v) {
    const size_t sizes[] = {estimateSpaceToReserveOne(tag<I>{}, v)...};
    size_t size = 0;
    for (const auto s : sizes) {
      size += s;
    }
    return size;
  }
};

template <class O>
void reserveInTarget(const O& o) {
  (void)o;
}
template <class T, class O>
void reserveInTarget(const T& v, const O& o) {
  o->reserve(estimateSpaceNeeded(v));
}
template <class T0, class T1, class... Ts>
void reserveInTarget(const T0& v0, const T1& v1, const Ts&... vs) {
  using seq = std::index_sequence_for<T0, T1, Ts...>;
  getLastElement(vs...)->reserve(
      EstimateSpaceToReserveAll<seq>::call(v0, v1, vs...));
}

template <class Delimiter, class... Ts>
void reserveInTargetDelim(const Delimiter& d, const Ts&... vs) {
  static_assert(sizeof...(vs) >= 2, "Needs at least 2 args");
  using seq = std::index_sequence_for<Ts...>;
  size_t fordelim = (sizeof...(vs) - 2) * estimateSpaceNeeded(d);
  getLastElement(vs...)->reserve(
      fordelim + EstimateSpaceToReserveAll<seq>::call(vs...));
}

template <class T>
FOLLY_ERASE constexpr int toAppendStrImplOne(
    std::false_type, const T& v, void*) {
  (void)v;
  return 0;
}
template <class T, class Tgt>
FOLLY_ERASE int toAppendStrImplOne(std::true_type, const T& v, Tgt result) {
  return toAppend(v, result), 0;
}
template <typename>
struct ToAppendStrImplAll;
template <size_t... I>
struct ToAppendStrImplAll<std::index_sequence<I...>> {
  template <class... T>
  static void call(const T&... v) {
    using _ = int[];
    auto r = getLastElement(v...);
    void(_{toAppendStrImplOne(
        std::bool_constant<I + 1 < sizeof...(T)>{}, v, r)...});
  }
};

template <class Delimiter, class T>
FOLLY_ERASE constexpr int toAppendDelimStrImplOne(
    index_constant<0>, const Delimiter& d, const T& v, void*) {
  (void)d;
  (void)v;
  return 0;
}
template <class Delimiter, class T, class Tgt>
FOLLY_ERASE int toAppendDelimStrImplOne(
    index_constant<1>, const Delimiter& d, const T& v, Tgt result) {
  (void)d;
  toAppend(v, result);
  return 0;
}
template <class Delimiter, class T, class Tgt>
FOLLY_ERASE int toAppendDelimStrImplOne(
    index_constant<2>, const Delimiter& d, const T& v, Tgt result) {
  toAppend(v, result);
  toAppend(d, result);
  return 0;
}
template <typename>
struct ToAppendDelimStrImplAll;
template <size_t... I>
struct ToAppendDelimStrImplAll<std::index_sequence<I...>> {
  template <size_t J, size_t N = sizeof...(I), size_t K = N - J - 1>
  using tag = index_constant<(K < 2 ? K : 2)>;
  template <class Delimiter, class... T>
  static void call(const Delimiter& d, const T&... v) {
    using _ = int[];
    auto r = detail::getLastElement(v...);
    void(_{toAppendDelimStrImplOne(tag<I>{}, d, v, r)...});
  }
};
template <
    class Delimiter,
    class T,
    class... Ts,
    std::enable_if_t<
        sizeof...(Ts) >= 2 &&
            IsSomeString<typename std::remove_pointer<
                detail::LastElement<Ts...>>::type>::value,
        int> = 0>
void toAppendDelimStrImpl(const Delimiter& delim, const T& v, const Ts&... vs) {
  using seq = std::index_sequence_for<T, Ts...>;
  ToAppendDelimStrImplAll<seq>::call(delim, v, vs...);
}
} // namespace detail
#endif

/**
 * Variadic conversion to string. Appends each element in turn.
 * If we have two or more things to append, we will not reserve
 * the space for them and will depend on strings exponential growth.
 * If you just append once consider using toAppendFit which reserves
 * the space needed (but does not have exponential as a result).
 *
 * Custom implementations of toAppend() can be provided in the same namespace as
 * the type to customize printing. estimateSpaceNeed() may also be provided to
 * avoid reallocations in toAppendFit():
 *
 * namespace other_namespace {
 *
 * template <class String>
 * void toAppend(const OtherType&, String* out);
 *
 * // optional
 * size_t estimateSpaceNeeded(const OtherType&);
 *
 * }
 */
template <
    class... Ts,
    std::enable_if_t<
        sizeof...(Ts) >= 3 &&
            IsSomeString<typename std::remove_pointer<
                detail::LastElement<Ts...>>::type>::value,
        int> = 0>
void toAppend(const Ts&... vs) {
  using seq = std::index_sequence_for<Ts...>;
  detail::ToAppendStrImplAll<seq>::call(vs...);
}

/**
 * @overloadbrief toAppend, but pre-allocate the exact amount of space required.
 *
 * Special version of the call that preallocates exactly as much memory
 * as need for arguments to be stored in target. This means we are
 * not doing exponential growth when we append. If you are using it
 * in a loop you are aiming at your foot with a big perf-destroying
 * bazooka.
 * On the other hand if you are appending to a string once, this
 * will probably save a few calls to malloc.
 */
template <
    class... Ts,
    std::enable_if_t<
        IsSomeString<typename std::remove_pointer<
            detail::LastElement<Ts...>>::type>::value,
        int> = 0>
void toAppendFit(const Ts&... vs) {
  ::folly::detail::reserveInTarget(vs...);
  toAppend(vs...);
}

template <class Ts>
void toAppendFit(const Ts&) {}

/**
 * Variadic base case: do nothing.
 */
template <class Tgt>
typename std::enable_if<IsSomeString<Tgt>::value>::type toAppend(
    Tgt* /* result */) {}

/**
 * @overloadbrief Use a specified delimiter between appendees.
 *
 * Variadic base case: do nothing.
 */
template <class Delimiter, class Tgt>
typename std::enable_if<IsSomeString<Tgt>::value>::type toAppendDelim(
    const Delimiter& /* delim */, Tgt* /* result */) {}

/**
 * 1 element: same as toAppend.
 */
template <class Delimiter, class T, class Tgt>
typename std::enable_if<IsSomeString<Tgt>::value>::type toAppendDelim(
    const Delimiter& /* delim */, const T& v, Tgt* tgt) {
  toAppend(v, tgt);
}

/**
 * Append to string with a delimiter in between elements. Check out
 * comments for toAppend for details about memory allocation.
 */
template <
    class Delimiter,
    class... Ts,
    std::enable_if_t<
        sizeof...(Ts) >= 3 &&
            IsSomeString<typename std::remove_pointer<
                detail::LastElement<Ts...>>::type>::value,
        int> = 0>
void toAppendDelim(const Delimiter& delim, const Ts&... vs) {
  detail::toAppendDelimStrImpl(delim, vs...);
}

/**
 * @overloadbrief toAppend with custom delimiter and exact pre-allocation.
 *
 * Detail in comment for toAppendFit
 */
template <
    class Delimiter,
    class... Ts,
    std::enable_if_t<
        IsSomeString<typename std::remove_pointer<
            detail::LastElement<Ts...>>::type>::value,
        int> = 0>
void toAppendDelimFit(const Delimiter& delim, const Ts&... vs) {
  detail::reserveInTargetDelim(delim, vs...);
  toAppendDelim(delim, vs...);
}

template <class De, class Ts>
void toAppendDelimFit(const De&, const Ts&) {}

/**
 * to<SomeString>(v1, v2, ...) uses toAppend() (see below) as back-end
 * for all types.
 */
template <
    class Tgt,
    class... Ts,
    std::enable_if_t<
        IsSomeString<Tgt>::value &&
            (sizeof...(Ts) != 1 ||
             !std::is_same<Tgt, detail::LastElement<void, Ts...>>::value),
        int> = 0>
Tgt to(const Ts&... vs) {
  Tgt result;
  toAppendFit(vs..., &result);
  return result;
}

/**
 * Special version of to<SomeString> for floating point. When calling
 * folly::to<SomeString>(double), generic implementation above will
 * firstly reserve 24 (or 25 when negative value) bytes. This will
 * introduce a malloc call for most mainstream string implementations.
 *
 * But for most cases, a floating point doesn't need 24 (or 25) bytes to
 * be converted as a string.
 *
 * This special version will not do string reserve.
 */
template <class Tgt, class Src>
typename std::enable_if<
    IsSomeString<Tgt>::value && std::is_floating_point<Src>::value,
    Tgt>::type
to(Src value) {
  Tgt result;
  toAppend(value, &result);
  return result;
}

/**
 * @overloadbrief Like `to`, but uses a custom delimiter.
 *
 * toDelim<SomeString>(SomeString str) returns itself.
 */
template <class Tgt, class Delim, class Src>
typename std::enable_if<
    IsSomeString<Tgt>::value &&
        std::is_same<Tgt, typename std::decay<Src>::type>::value,
    Tgt>::type
toDelim(const Delim& /* delim */, Src&& value) {
  return static_cast<Src&&>(value);
}

/**
 * toDelim<SomeString>(delim, v1, v2, ...) uses toAppendDelim() as
 * back-end for all types.
 */
template <
    class Tgt,
    class Delim,
    class... Ts,
    std::enable_if_t<
        IsSomeString<Tgt>::value &&
            (sizeof...(Ts) != 1 ||
             !std::is_same<Tgt, detail::LastElement<void, Ts...>>::value),
        int> = 0>
Tgt toDelim(const Delim& delim, const Ts&... vs) {
  Tgt result;
  toAppendDelimFit(delim, vs..., &result);
  return result;
}

/**
 * Conversions from string types to integral types.
 */

namespace detail {

Expected<bool, ConversionCode> str_to_bool(StringPiece* src) noexcept;

template <typename T>
Expected<T, ConversionCode> str_to_floating(StringPiece* src) noexcept;

extern template Expected<float, ConversionCode> str_to_floating<float>(
    StringPiece* src) noexcept;
extern template Expected<double, ConversionCode> str_to_floating<double>(
    StringPiece* src) noexcept;

template <typename T>
Expected<T, ConversionCode> str_to_floating_fast_float_from_chars(
    StringPiece* src) noexcept;

extern template Expected<float, ConversionCode>
str_to_floating_fast_float_from_chars<float>(StringPiece* src) noexcept;
extern template Expected<double, ConversionCode>
str_to_floating_fast_float_from_chars<double>(StringPiece* src) noexcept;

template <class Tgt>
Expected<Tgt, ConversionCode> digits_to(const char* b, const char* e) noexcept;

extern template Expected<char, ConversionCode> digits_to<char>(
    const char*, const char*) noexcept;
extern template Expected<signed char, ConversionCode> digits_to<signed char>(
    const char*, const char*) noexcept;
extern template Expected<unsigned char, ConversionCode>
digits_to<unsigned char>(const char*, const char*) noexcept;

extern template Expected<short, ConversionCode> digits_to<short>(
    const char*, const char*) noexcept;
extern template Expected<unsigned short, ConversionCode>
digits_to<unsigned short>(const char*, const char*) noexcept;

extern template Expected<int, ConversionCode> digits_to<int>(
    const char*, const char*) noexcept;
extern template Expected<unsigned int, ConversionCode> digits_to<unsigned int>(
    const char*, const char*) noexcept;

extern template Expected<long, ConversionCode> digits_to<long>(
    const char*, const char*) noexcept;
extern template Expected<unsigned long, ConversionCode>
digits_to<unsigned long>(const char*, const char*) noexcept;

extern template Expected<long long, ConversionCode> digits_to<long long>(
    const char*, const char*) noexcept;
extern template Expected<unsigned long long, ConversionCode>
digits_to<unsigned long long>(const char*, const char*) noexcept;

#if FOLLY_HAVE_INT128_T
extern template Expected<__int128, ConversionCode> digits_to<__int128>(
    const char*, const char*) noexcept;
extern template Expected<unsigned __int128, ConversionCode>
digits_to<unsigned __int128>(const char*, const char*) noexcept;
#endif

template <class T>
Expected<T, ConversionCode> str_to_integral(StringPiece* src) noexcept;

extern template Expected<char, ConversionCode> str_to_integral<char>(
    StringPiece* src) noexcept;
extern template Expected<signed char, ConversionCode>
str_to_integral<signed char>(StringPiece* src) noexcept;
extern template Expected<unsigned char, ConversionCode>
str_to_integral<unsigned char>(StringPiece* src) noexcept;

extern template Expected<short, ConversionCode> str_to_integral<short>(
    StringPiece* src) noexcept;
extern template Expected<unsigned short, ConversionCode>
str_to_integral<unsigned short>(StringPiece* src) noexcept;

extern template Expected<int, ConversionCode> str_to_integral<int>(
    StringPiece* src) noexcept;
extern template Expected<unsigned int, ConversionCode>
str_to_integral<unsigned int>(StringPiece* src) noexcept;

extern template Expected<long, ConversionCode> str_to_integral<long>(
    StringPiece* src) noexcept;
extern template Expected<unsigned long, ConversionCode>
str_to_integral<unsigned long>(StringPiece* src) noexcept;

extern template Expected<long long, ConversionCode> str_to_integral<long long>(
    StringPiece* src) noexcept;
extern template Expected<unsigned long long, ConversionCode>
str_to_integral<unsigned long long>(StringPiece* src) noexcept;

#if FOLLY_HAVE_INT128_T
extern template Expected<__int128, ConversionCode> str_to_integral<__int128>(
    StringPiece* src) noexcept;
extern template Expected<unsigned __int128, ConversionCode>
str_to_integral<unsigned __int128>(StringPiece* src) noexcept;
#endif

template <typename T>
typename std::
    enable_if<std::is_same<T, bool>::value, Expected<T, ConversionCode>>::type
    convertTo(StringPiece* src) noexcept {
  return str_to_bool(src);
}

template <typename T>
typename std::enable_if<
    std::is_floating_point<T>::value,
    Expected<T, ConversionCode>>::type
convertTo(StringPiece* src) noexcept {
  return str_to_floating<T>(src);
}

template <typename T>
typename std::enable_if<
    is_integral_v<T> && !std::is_same<T, bool>::value,
    Expected<T, ConversionCode>>::type
convertTo(StringPiece* src) noexcept {
  return str_to_integral<T>(src);
}

} // namespace detail

/**
 * String represented as a pair of pointers to char to unsigned
 * integrals. Assumes NO whitespace before or after.
 */
template <typename Tgt>
typename std::enable_if<
    is_integral_v<Tgt> && !std::is_same<Tgt, bool>::value,
    Expected<Tgt, ConversionCode>>::type
tryTo(const char* b, const char* e) noexcept {
  return detail::digits_to<Tgt>(b, e);
}

template <typename Tgt>
typename std::enable_if< //
    is_integral_v<Tgt> && !std::is_same<Tgt, bool>::value,
    Tgt>::type
to(const char* b, const char* e) {
  return tryTo<Tgt>(b, e).thenOrThrow(identity, [=](ConversionCode code) {
    return makeConversionError(code, StringPiece(b, e));
  });
}

/**
 * Conversions from string types to arithmetic types.
 */

/**
 * Parsing strings to numeric types.
 */
template <typename Tgt>
FOLLY_NODISCARD inline typename std::enable_if< //
    is_arithmetic_v<Tgt>,
    Expected<StringPiece, ConversionCode>>::type
parseTo(StringPiece src, Tgt& out) {
  return detail::convertTo<Tgt>(&src).then(
      [&](Tgt res) { return void(out = res), src; });
}

/**
 * Integral / Floating Point to integral / Floating Point
 */

namespace detail {

/**
 * Bool to integral/float doesn't need any special checks, and this
 * overload means we aren't trying to see if a bool is less than
 * an integer.
 */
template <class Tgt>
typename std::enable_if<
    !std::is_same<Tgt, bool>::value &&
        (is_integral_v<Tgt> || std::is_floating_point<Tgt>::value),
    Expected<Tgt, ConversionCode>>::type
convertTo(const bool& value) noexcept {
  return static_cast<Tgt>(value ? 1 : 0);
}

/**
 * Checked conversion from integral to integral. The checks are only
 * performed when meaningful, e.g. conversion from int to long goes
 * unchecked.
 */
template <class Tgt, class Src>
typename std::enable_if<
    is_integral_v<Src> && !std::is_same<Tgt, Src>::value &&
        !std::is_same<Tgt, bool>::value && is_integral_v<Tgt>,
    Expected<Tgt, ConversionCode>>::type
convertTo(const Src& value) noexcept {
  if /* constexpr */ (
      make_unsigned_t<Tgt>(std::numeric_limits<Tgt>::max()) <
      make_unsigned_t<Src>(std::numeric_limits<Src>::max())) {
    if (greater_than<Tgt, std::numeric_limits<Tgt>::max()>(value)) {
      return makeUnexpected(ConversionCode::ARITH_POSITIVE_OVERFLOW);
    }
  }
  if /* constexpr */ (
      is_signed_v<Src> && (!is_signed_v<Tgt> || sizeof(Src) > sizeof(Tgt))) {
    if (less_than<Tgt, std::numeric_limits<Tgt>::min()>(value)) {
      return makeUnexpected(ConversionCode::ARITH_NEGATIVE_OVERFLOW);
    }
  }
  return static_cast<Tgt>(value);
}

/**
 * Checked conversion from floating to floating. The checks are only
 * performed when meaningful, e.g. conversion from float to double goes
 * unchecked.
 */
template <class Tgt, class Src>
typename std::enable_if<
    std::is_floating_point<Tgt>::value && std::is_floating_point<Src>::value &&
        !std::is_same<Tgt, Src>::value,
    Expected<Tgt, ConversionCode>>::type
convertTo(const Src& value) noexcept {
  if (FOLLY_UNLIKELY(std::isinf(value))) {
    return static_cast<Tgt>(value);
  }
  if /* constexpr */ (
      std::numeric_limits<Tgt>::max() < std::numeric_limits<Src>::max()) {
    if (value > std::numeric_limits<Tgt>::max()) {
      return makeUnexpected(ConversionCode::ARITH_POSITIVE_OVERFLOW);
    }
    if (value < std::numeric_limits<Tgt>::lowest()) {
      return makeUnexpected(ConversionCode::ARITH_NEGATIVE_OVERFLOW);
    }
  }
  return static_cast<Tgt>(value);
}

/**
 * Check if a floating point value can safely be converted to an
 * integer value without triggering undefined behaviour.
 */
template <typename Tgt, typename Src>
inline typename std::enable_if<
    std::is_floating_point<Src>::value && is_integral_v<Tgt> &&
        !std::is_same<Tgt, bool>::value,
    bool>::type
checkConversion(const Src& value) {
  constexpr Src tgtMaxAsSrc = static_cast<Src>(std::numeric_limits<Tgt>::max());
  constexpr Src tgtMinAsSrc = static_cast<Src>(std::numeric_limits<Tgt>::min());
  // NOTE: The following two comparisons also handle the case where value is
  // NaN, as all comparisons with NaN are false.
  if (!(value < tgtMaxAsSrc)) {
    if (!(value <= tgtMaxAsSrc)) {
      return false;
    }
    const Src mmax = folly::nextafter(tgtMaxAsSrc, Src());
    if (static_cast<Tgt>(value - mmax) >
        std::numeric_limits<Tgt>::max() - static_cast<Tgt>(mmax)) {
      return false;
    }
  } else if (value <= tgtMinAsSrc) {
    if (value < tgtMinAsSrc) {
      return false;
    }
    const Src mmin = folly::nextafter(tgtMinAsSrc, Src());
    if (static_cast<Tgt>(value - mmin) <
        std::numeric_limits<Tgt>::min() - static_cast<Tgt>(mmin)) {
      return false;
    }
  }
  return true;
}

// Integers can always safely be converted to floating point values
template <typename Tgt, typename Src>
constexpr typename std::enable_if<
    is_integral_v<Src> && std::is_floating_point<Tgt>::value,
    bool>::type
checkConversion(const Src&) {
  return true;
}

// Also, floating point values can always be safely converted to bool
// Per the standard, any floating point value that is not zero will yield true
template <typename Tgt, typename Src>
constexpr typename std::enable_if<
    std::is_floating_point<Src>::value && std::is_same<Tgt, bool>::value,
    bool>::type
checkConversion(const Src&) {
  return true;
}

/**
 * Checked conversion from integral to floating point and back. The
 * result must be convertible back to the source type without loss of
 * precision. This seems Draconian but sometimes is what's needed, and
 * complements existing routines nicely. For various rounding
 * routines, see <math>.
 */
template <typename Tgt, typename Src>
typename std::enable_if<
    (is_integral_v<Src> && std::is_floating_point<Tgt>::value) ||
        (std::is_floating_point<Src>::value && is_integral_v<Tgt>),
    Expected<Tgt, ConversionCode>>::type
convertTo(const Src& value) noexcept {
  if (FOLLY_LIKELY(checkConversion<Tgt>(value))) {
    Tgt result = static_cast<Tgt>(value);
    if (FOLLY_LIKELY(checkConversion<Src>(result))) {
      Src witness = static_cast<Src>(result);
      if (FOLLY_LIKELY(value == witness)) {
        return result;
      }
    }
  }
  return makeUnexpected(ConversionCode::ARITH_LOSS_OF_PRECISION);
}

template <typename Tgt, typename Src>
inline std::string errorValue(const Src& value) {
  return to<std::string>("(", pretty_name<Tgt>(), ") ", value);
}

template <typename Tgt, typename Src>
using IsArithToArith = std::bool_constant<
    !std::is_same<Tgt, Src>::value && !std::is_same<Tgt, bool>::value &&
    is_arithmetic_v<Src> && is_arithmetic_v<Tgt>>;

} // namespace detail

template <typename Tgt, typename Src>
typename std::enable_if<
    detail::IsArithToArith<Tgt, Src>::value,
    Expected<Tgt, ConversionCode>>::type
tryTo(const Src& value) noexcept {
  return detail::convertTo<Tgt>(value);
}

template <typename Tgt, typename Src>
typename std::enable_if<detail::IsArithToArith<Tgt, Src>::value, Tgt>::type to(
    const Src& value) {
  return tryTo<Tgt>(value).thenOrThrow(identity, [&](ConversionCode e) {
    return makeConversionError(e, detail::errorValue<Tgt>(value));
  });
}

/**
 * Custom Conversions
 *
 * Any type can be used with folly::to by implementing parseTo. The
 * implementation should be provided in the namespace of the type to facilitate
 * argument-dependent lookup:
 *
 * namespace other_namespace {
 * ::folly::Expected<::folly::StringPiece, SomeErrorCode>
 *   parseTo(::folly::StringPiece, OtherType&) noexcept;
 * }
 */
template <class T>
FOLLY_NODISCARD typename std::enable_if<
    std::is_enum<T>::value,
    Expected<StringPiece, ConversionCode>>::type
parseTo(StringPiece in, T& out) noexcept {
  typename std::underlying_type<T>::type tmp{};
  auto restOrError = parseTo(in, tmp);
  out = static_cast<T>(tmp); // Harmless if parseTo fails
  return restOrError;
}

FOLLY_NODISCARD
inline Expected<StringPiece, ConversionCode> parseTo(
    StringPiece in, StringPiece& out) noexcept {
  out = in;
  return StringPiece{in.end(), in.end()};
}

namespace detail {

template <class Str>
FOLLY_ERASE Expected<StringPiece, ConversionCode> parseToStr(
    StringPiece in, Str& out) {
  out.clear();
  out.append(in.data(), in.size()); // TODO try/catch?
  return StringPiece{in.end(), in.end()};
}

} // namespace detail

FOLLY_NODISCARD
inline Expected<StringPiece, ConversionCode> parseTo(
    StringPiece in, std::string& out) {
  return detail::parseToStr(in, out);
}

FOLLY_NODISCARD
inline Expected<StringPiece, ConversionCode> parseTo(
    StringPiece in, std::string_view& out) {
  out = std::string_view(in.data(), in.size());
  return StringPiece{in.end(), in.end()};
}

FOLLY_NODISCARD
inline Expected<StringPiece, ConversionCode> parseTo(
    StringPiece in, fbstring& out) {
  return detail::parseToStr(in, out);
}

template <class Str>
FOLLY_NODISCARD inline typename std::enable_if<
    IsSomeString<Str>::value,
    Expected<StringPiece, ConversionCode>>::type
parseTo(StringPiece in, Str& out) {
  return detail::parseToStr(in, out);
}

namespace detail {
template <typename Tgt>
using ParseToResult = decltype(parseTo(StringPiece{}, std::declval<Tgt&>()));

struct CheckTrailingSpace {
  Expected<Unit, ConversionCode> operator()(StringPiece sp) const {
    auto e = enforceWhitespaceErr(sp);
    if (FOLLY_UNLIKELY(e != ConversionCode::SUCCESS)) {
      return makeUnexpected(e);
    }
    return unit;
  }
};

template <class Error>
struct ReturnUnit {
  template <class T>
  constexpr Expected<Unit, Error> operator()(T&&) const {
    return unit;
  }
};

// Older versions of the parseTo customization point threw on error and
// returned void. Handle that.
template <class Tgt>
inline typename std::enable_if<
    std::is_void<ParseToResult<Tgt>>::value,
    Expected<StringPiece, ConversionCode>>::type
parseToWrap(StringPiece sp, Tgt& out) {
  parseTo(sp, out);
  return StringPiece(sp.end(), sp.end());
}

template <class Tgt>
inline typename std::enable_if<
    !std::is_void<ParseToResult<Tgt>>::value,
    ParseToResult<Tgt>>::type
parseToWrap(StringPiece sp, Tgt& out) {
  return parseTo(sp, out);
}

template <typename Tgt>
using ParseToError = ExpectedErrorType<decltype(detail::parseToWrap(
    StringPiece{}, std::declval<Tgt&>()))>;

} // namespace detail

/**
 * String or StringPiece to target conversion. Accepts leading and trailing
 * whitespace, but no non-space trailing characters.
 */

template <class Tgt>
inline typename std::enable_if<
    !std::is_same<StringPiece, Tgt>::value,
    Expected<Tgt, detail::ParseToError<Tgt>>>::type
tryTo(StringPiece src) noexcept {
  Tgt result{};
  using Error = detail::ParseToError<Tgt>;
  using Check = typename std::conditional<
      is_arithmetic_v<Tgt>,
      detail::CheckTrailingSpace,
      detail::ReturnUnit<Error>>::type;
  return parseTo(src, result).then(Check(), [&](Unit) {
    return std::move(result);
  });
}

template <class Tgt, class Src>
inline typename std::enable_if<
    IsSomeString<Src>::value && !std::is_same<StringPiece, Tgt>::value,
    Tgt>::type
to(Src const& src) {
  return to<Tgt>(StringPiece(src.data(), src.size()));
}

template <class Tgt>
inline
    typename std::enable_if<!std::is_same<StringPiece, Tgt>::value, Tgt>::type
    to(StringPiece src) {
  Tgt result{};
  using Error = detail::ParseToError<Tgt>;
  using Check = typename std::conditional<
      is_arithmetic_v<Tgt>,
      detail::CheckTrailingSpace,
      detail::ReturnUnit<Error>>::type;
  auto tmp = detail::parseToWrap(src, result);
  return tmp
      .thenOrThrow(
          Check(),
          [&](Error e) { throw_exception(makeConversionError(e, src)); })
      .thenOrThrow(
          [&](Unit) { return std::move(result); },
          [&](Error e) {
            throw_exception(makeConversionError(e, tmp.value()));
          });
}

/**
 * tryTo/to that take the strings by pointer so the caller gets information
 * about how much of the string was consumed by the conversion. These do not
 * check for trailing whitespace.
 */
template <class Tgt>
Expected<Tgt, detail::ParseToError<Tgt>> tryTo(StringPiece* src) noexcept {
  Tgt result;
  return parseTo(*src, result).then([&, src](StringPiece sp) -> Tgt {
    *src = sp;
    return std::move(result);
  });
}

template <class Tgt>
Tgt to(StringPiece* src) {
  Tgt result{};
  using Error = detail::ParseToError<Tgt>;
  return parseTo(*src, result)
      .thenOrThrow(
          [&, src](StringPiece sp) -> Tgt {
            *src = sp;
            return std::move(result);
          },
          [=](Error e) { return makeConversionError(e, *src); });
}

/**
 * Enum to anything and back
 */

template <class Tgt, class Src>
typename std::enable_if<
    std::is_enum<Src>::value && !std::is_same<Src, Tgt>::value &&
        !std::is_convertible<Tgt, StringPiece>::value,
    Expected<Tgt, ConversionCode>>::type
tryTo(const Src& value) noexcept {
  return tryTo<Tgt>(to_underlying(value));
}

template <class Tgt, class Src>
typename std::enable_if<
    !std::is_convertible<Src, StringPiece>::value && std::is_enum<Tgt>::value &&
        !std::is_same<Src, Tgt>::value,
    Expected<Tgt, ConversionCode>>::type
tryTo(const Src& value) noexcept {
  using I = typename std::underlying_type<Tgt>::type;
  return tryTo<I>(value).then([](I i) { return static_cast<Tgt>(i); });
}

template <class Tgt, class Src>
typename std::enable_if<
    std::is_enum<Src>::value && !std::is_same<Src, Tgt>::value &&
        !std::is_convertible<Tgt, StringPiece>::value,
    Tgt>::type
to(const Src& value) {
  return to<Tgt>(to_underlying(value));
}

template <class Tgt, class Src>
typename std::enable_if<
    !std::is_convertible<Src, StringPiece>::value && std::is_enum<Tgt>::value &&
        !std::is_same<Src, Tgt>::value,
    Tgt>::type
to(const Src& value) {
  return static_cast<Tgt>(to<typename std::underlying_type<Tgt>::type>(value));
}

} // namespace folly