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+// Copyright 2018 The Abseil Authors.
+//
+// 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.
+
+#include "absl/strings/internal/charconv_parse.h"
+#include "absl/strings/charconv.h"
+
+#include <cassert>
+#include <cstdint>
+#include <limits>
+
+#include "absl/strings/internal/memutil.h"
+
+namespace absl {
+inline namespace lts_2018_06_20 {
+namespace {
+
+// ParseFloat<10> will read the first 19 significant digits of the mantissa.
+// This number was chosen for multiple reasons.
+//
+// (a) First, for whatever integer type we choose to represent the mantissa, we
+// want to choose the largest possible number of decimal digits for that integer
+// type. We are using uint64_t, which can express any 19-digit unsigned
+// integer.
+//
+// (b) Second, we need to parse enough digits that the binary value of any
+// mantissa we capture has more bits of resolution than the mantissa
+// representation in the target float. Our algorithm requires at least 3 bits
+// of headway, but 19 decimal digits give a little more than that.
+//
+// The following static assertions verify the above comments:
+constexpr int kDecimalMantissaDigitsMax = 19;
+
+static_assert(std::numeric_limits<uint64_t>::digits10 ==
+ kDecimalMantissaDigitsMax,
+ "(a) above");
+
+// IEEE doubles, which we assume in Abseil, have 53 binary bits of mantissa.
+static_assert(std::numeric_limits<double>::is_iec559, "IEEE double assumed");
+static_assert(std::numeric_limits<double>::radix == 2, "IEEE double fact");
+static_assert(std::numeric_limits<double>::digits == 53, "IEEE double fact");
+
+// The lowest valued 19-digit decimal mantissa we can read still contains
+// sufficient information to reconstruct a binary mantissa.
+static_assert(1000000000000000000u > (uint64_t(1) << (53 + 3)), "(b) above");
+
+// ParseFloat<16> will read the first 15 significant digits of the mantissa.
+//
+// Because a base-16-to-base-2 conversion can be done exactly, we do not need
+// to maximize the number of scanned hex digits to improve our conversion. What
+// is required is to scan two more bits than the mantissa can represent, so that
+// we always round correctly.
+//
+// (One extra bit does not suffice to perform correct rounding, since a number
+// exactly halfway between two representable floats has unique rounding rules,
+// so we need to differentiate between a "halfway between" number and a "closer
+// to the larger value" number.)
+constexpr int kHexadecimalMantissaDigitsMax = 15;
+
+// The minimum number of significant bits that will be read from
+// kHexadecimalMantissaDigitsMax hex digits. We must subtract by three, since
+// the most significant digit can be a "1", which only contributes a single
+// significant bit.
+constexpr int kGuaranteedHexadecimalMantissaBitPrecision =
+ 4 * kHexadecimalMantissaDigitsMax - 3;
+
+static_assert(kGuaranteedHexadecimalMantissaBitPrecision >
+ std::numeric_limits<double>::digits + 2,
+ "kHexadecimalMantissaDigitsMax too small");
+
+// We also impose a limit on the number of significant digits we will read from
+// an exponent, to avoid having to deal with integer overflow. We use 9 for
+// this purpose.
+//
+// If we read a 9 digit exponent, the end result of the conversion will
+// necessarily be infinity or zero, depending on the sign of the exponent.
+// Therefore we can just drop extra digits on the floor without any extra
+// logic.
+constexpr int kDecimalExponentDigitsMax = 9;
+static_assert(std::numeric_limits<int>::digits10 >= kDecimalExponentDigitsMax,
+ "int type too small");
+
+// To avoid incredibly large inputs causing integer overflow for our exponent,
+// we impose an arbitrary but very large limit on the number of significant
+// digits we will accept. The implementation refuses to match a std::string with
+// more consecutive significant mantissa digits than this.
+constexpr int kDecimalDigitLimit = 50000000;
+
+// Corresponding limit for hexadecimal digit inputs. This is one fourth the
+// amount of kDecimalDigitLimit, since each dropped hexadecimal digit requires
+// a binary exponent adjustment of 4.
+constexpr int kHexadecimalDigitLimit = kDecimalDigitLimit / 4;
+
+// The largest exponent we can read is 999999999 (per
+// kDecimalExponentDigitsMax), and the largest exponent adjustment we can get
+// from dropped mantissa digits is 2 * kDecimalDigitLimit, and the sum of these
+// comfortably fits in an integer.
+//
+// We count kDecimalDigitLimit twice because there are independent limits for
+// numbers before and after the decimal point. (In the case where there are no
+// significant digits before the decimal point, there are independent limits for
+// post-decimal-point leading zeroes and for significant digits.)
+static_assert(999999999 + 2 * kDecimalDigitLimit <
+ std::numeric_limits<int>::max(),
+ "int type too small");
+static_assert(999999999 + 2 * (4 * kHexadecimalDigitLimit) <
+ std::numeric_limits<int>::max(),
+ "int type too small");
+
+// Returns true if the provided bitfield allows parsing an exponent value
+// (e.g., "1.5e100").
+bool AllowExponent(chars_format flags) {
+ bool fixed = (flags & chars_format::fixed) == chars_format::fixed;
+ bool scientific =
+ (flags & chars_format::scientific) == chars_format::scientific;
+ return scientific || !fixed;
+}
+
+// Returns true if the provided bitfield requires an exponent value be present.
+bool RequireExponent(chars_format flags) {
+ bool fixed = (flags & chars_format::fixed) == chars_format::fixed;
+ bool scientific =
+ (flags & chars_format::scientific) == chars_format::scientific;
+ return scientific && !fixed;
+}
+
+const int8_t kAsciiToInt[256] = {
+ -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+ -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+ -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8,
+ 9, -1, -1, -1, -1, -1, -1, -1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1,
+ -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+ -1, -1, 10, 11, 12, 13, 14, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+ -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+ -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+ -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+ -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+ -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+ -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+ -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+ -1, -1, -1, -1, -1, -1, -1, -1, -1};
+
+// Returns true if `ch` is a digit in the given base
+template <int base>
+bool IsDigit(char ch);
+
+// Converts a valid `ch` to its digit value in the given base.
+template <int base>
+unsigned ToDigit(char ch);
+
+// Returns true if `ch` is the exponent delimiter for the given base.
+template <int base>
+bool IsExponentCharacter(char ch);
+
+// Returns the maximum number of significant digits we will read for a float
+// in the given base.
+template <int base>
+constexpr int MantissaDigitsMax();
+
+// Returns the largest consecutive run of digits we will accept when parsing a
+// number in the given base.
+template <int base>
+constexpr int DigitLimit();
+
+// Returns the amount the exponent must be adjusted by for each dropped digit.
+// (For decimal this is 1, since the digits are in base 10 and the exponent base
+// is also 10, but for hexadecimal this is 4, since the digits are base 16 but
+// the exponent base is 2.)
+template <int base>
+constexpr int DigitMagnitude();
+
+template <>
+bool IsDigit<10>(char ch) {
+ return ch >= '0' && ch <= '9';
+}
+template <>
+bool IsDigit<16>(char ch) {
+ return kAsciiToInt[static_cast<unsigned char>(ch)] >= 0;
+}
+
+template <>
+unsigned ToDigit<10>(char ch) {
+ return ch - '0';
+}
+template <>
+unsigned ToDigit<16>(char ch) {
+ return kAsciiToInt[static_cast<unsigned char>(ch)];
+}
+
+template <>
+bool IsExponentCharacter<10>(char ch) {
+ return ch == 'e' || ch == 'E';
+}
+
+template <>
+bool IsExponentCharacter<16>(char ch) {
+ return ch == 'p' || ch == 'P';
+}
+
+template <>
+constexpr int MantissaDigitsMax<10>() {
+ return kDecimalMantissaDigitsMax;
+}
+template <>
+constexpr int MantissaDigitsMax<16>() {
+ return kHexadecimalMantissaDigitsMax;
+}
+
+template <>
+constexpr int DigitLimit<10>() {
+ return kDecimalDigitLimit;
+}
+template <>
+constexpr int DigitLimit<16>() {
+ return kHexadecimalDigitLimit;
+}
+
+template <>
+constexpr int DigitMagnitude<10>() {
+ return 1;
+}
+template <>
+constexpr int DigitMagnitude<16>() {
+ return 4;
+}
+
+// Reads decimal digits from [begin, end) into *out. Returns the number of
+// digits consumed.
+//
+// After max_digits has been read, keeps consuming characters, but no longer
+// adjusts *out. If a nonzero digit is dropped this way, *dropped_nonzero_digit
+// is set; otherwise, it is left unmodified.
+//
+// If no digits are matched, returns 0 and leaves *out unchanged.
+//
+// ConsumeDigits does not protect against overflow on *out; max_digits must
+// be chosen with respect to type T to avoid the possibility of overflow.
+template <int base, typename T>
+std::size_t ConsumeDigits(const char* begin, const char* end, int max_digits,
+ T* out, bool* dropped_nonzero_digit) {
+ if (base == 10) {
+ assert(max_digits <= std::numeric_limits<T>::digits10);
+ } else if (base == 16) {
+ assert(max_digits * 4 <= std::numeric_limits<T>::digits);
+ }
+ const char* const original_begin = begin;
+ T accumulator = *out;
+ const char* significant_digits_end =
+ (end - begin > max_digits) ? begin + max_digits : end;
+ while (begin < significant_digits_end && IsDigit<base>(*begin)) {
+ // Do not guard against *out overflow; max_digits was chosen to avoid this.
+ // Do assert against it, to detect problems in debug builds.
+ auto digit = static_cast<T>(ToDigit<base>(*begin));
+ assert(accumulator * base >= accumulator);
+ accumulator *= base;
+ assert(accumulator + digit >= accumulator);
+ accumulator += digit;
+ ++begin;
+ }
+ bool dropped_nonzero = false;
+ while (begin < end && IsDigit<base>(*begin)) {
+ dropped_nonzero = dropped_nonzero || (*begin != '0');
+ ++begin;
+ }
+ if (dropped_nonzero && dropped_nonzero_digit != nullptr) {
+ *dropped_nonzero_digit = true;
+ }
+ *out = accumulator;
+ return begin - original_begin;
+}
+
+// Returns true if `v` is one of the chars allowed inside parentheses following
+// a NaN.
+bool IsNanChar(char v) {
+ return (v == '_') || (v >= '0' && v <= '9') || (v >= 'a' && v <= 'z') ||
+ (v >= 'A' && v <= 'Z');
+}
+
+// Checks the range [begin, end) for a strtod()-formatted infinity or NaN. If
+// one is found, sets `out` appropriately and returns true.
+bool ParseInfinityOrNan(const char* begin, const char* end,
+ strings_internal::ParsedFloat* out) {
+ if (end - begin < 3) {
+ return false;
+ }
+ switch (*begin) {
+ case 'i':
+ case 'I': {
+ // An infinity std::string consists of the characters "inf" or "infinity",
+ // case insensitive.
+ if (strings_internal::memcasecmp(begin + 1, "nf", 2) != 0) {
+ return false;
+ }
+ out->type = strings_internal::FloatType::kInfinity;
+ if (end - begin >= 8 &&
+ strings_internal::memcasecmp(begin + 3, "inity", 5) == 0) {
+ out->end = begin + 8;
+ } else {
+ out->end = begin + 3;
+ }
+ return true;
+ }
+ case 'n':
+ case 'N': {
+ // A NaN consists of the characters "nan", case insensitive, optionally
+ // followed by a parenthesized sequence of zero or more alphanumeric
+ // characters and/or underscores.
+ if (strings_internal::memcasecmp(begin + 1, "an", 2) != 0) {
+ return false;
+ }
+ out->type = strings_internal::FloatType::kNan;
+ out->end = begin + 3;
+ // NaN is allowed to be followed by a parenthesized std::string, consisting of
+ // only the characters [a-zA-Z0-9_]. Match that if it's present.
+ begin += 3;
+ if (begin < end && *begin == '(') {
+ const char* nan_begin = begin + 1;
+ while (nan_begin < end && IsNanChar(*nan_begin)) {
+ ++nan_begin;
+ }
+ if (nan_begin < end && *nan_begin == ')') {
+ // We found an extra NaN specifier range
+ out->subrange_begin = begin + 1;
+ out->subrange_end = nan_begin;
+ out->end = nan_begin + 1;
+ }
+ }
+ return true;
+ }
+ default:
+ return false;
+ }
+}
+} // namespace
+
+namespace strings_internal {
+
+template <int base>
+strings_internal::ParsedFloat ParseFloat(const char* begin, const char* end,
+ chars_format format_flags) {
+ strings_internal::ParsedFloat result;
+
+ // Exit early if we're given an empty range.
+ if (begin == end) return result;
+
+ // Handle the infinity and NaN cases.
+ if (ParseInfinityOrNan(begin, end, &result)) {
+ return result;
+ }
+
+ const char* const mantissa_begin = begin;
+ while (begin < end && *begin == '0') {
+ ++begin; // skip leading zeros
+ }
+ uint64_t mantissa = 0;
+
+ int exponent_adjustment = 0;
+ bool mantissa_is_inexact = false;
+ std::size_t pre_decimal_digits = ConsumeDigits<base>(
+ begin, end, MantissaDigitsMax<base>(), &mantissa, &mantissa_is_inexact);
+ begin += pre_decimal_digits;
+ int digits_left;
+ if (pre_decimal_digits >= DigitLimit<base>()) {
+ // refuse to parse pathological inputs
+ return result;
+ } else if (pre_decimal_digits > MantissaDigitsMax<base>()) {
+ // We dropped some non-fraction digits on the floor. Adjust our exponent
+ // to compensate.
+ exponent_adjustment =
+ static_cast<int>(pre_decimal_digits - MantissaDigitsMax<base>());
+ digits_left = 0;
+ } else {
+ digits_left =
+ static_cast<int>(MantissaDigitsMax<base>() - pre_decimal_digits);
+ }
+ if (begin < end && *begin == '.') {
+ ++begin;
+ if (mantissa == 0) {
+ // If we haven't seen any nonzero digits yet, keep skipping zeros. We
+ // have to adjust the exponent to reflect the changed place value.
+ const char* begin_zeros = begin;
+ while (begin < end && *begin == '0') {
+ ++begin;
+ }
+ std::size_t zeros_skipped = begin - begin_zeros;
+ if (zeros_skipped >= DigitLimit<base>()) {
+ // refuse to parse pathological inputs
+ return result;
+ }
+ exponent_adjustment -= static_cast<int>(zeros_skipped);
+ }
+ std::size_t post_decimal_digits = ConsumeDigits<base>(
+ begin, end, digits_left, &mantissa, &mantissa_is_inexact);
+ begin += post_decimal_digits;
+
+ // Since `mantissa` is an integer, each significant digit we read after
+ // the decimal point requires an adjustment to the exponent. "1.23e0" will
+ // be stored as `mantissa` == 123 and `exponent` == -2 (that is,
+ // "123e-2").
+ if (post_decimal_digits >= DigitLimit<base>()) {
+ // refuse to parse pathological inputs
+ return result;
+ } else if (post_decimal_digits > digits_left) {
+ exponent_adjustment -= digits_left;
+ } else {
+ exponent_adjustment -= post_decimal_digits;
+ }
+ }
+ // If we've found no mantissa whatsoever, this isn't a number.
+ if (mantissa_begin == begin) {
+ return result;
+ }
+ // A bare "." doesn't count as a mantissa either.
+ if (begin - mantissa_begin == 1 && *mantissa_begin == '.') {
+ return result;
+ }
+
+ if (mantissa_is_inexact) {
+ // We dropped significant digits on the floor. Handle this appropriately.
+ if (base == 10) {
+ // If we truncated significant decimal digits, store the full range of the
+ // mantissa for future big integer math for exact rounding.
+ result.subrange_begin = mantissa_begin;
+ result.subrange_end = begin;
+ } else if (base == 16) {
+ // If we truncated hex digits, reflect this fact by setting the low
+ // ("sticky") bit. This allows for correct rounding in all cases.
+ mantissa |= 1;
+ }
+ }
+ result.mantissa = mantissa;
+
+ const char* const exponent_begin = begin;
+ result.literal_exponent = 0;
+ bool found_exponent = false;
+ if (AllowExponent(format_flags) && begin < end &&
+ IsExponentCharacter<base>(*begin)) {
+ bool negative_exponent = false;
+ ++begin;
+ if (begin < end && *begin == '-') {
+ negative_exponent = true;
+ ++begin;
+ } else if (begin < end && *begin == '+') {
+ ++begin;
+ }
+ const char* const exponent_digits_begin = begin;
+ // Exponent is always expressed in decimal, even for hexadecimal floats.
+ begin += ConsumeDigits<10>(begin, end, kDecimalExponentDigitsMax,
+ &result.literal_exponent, nullptr);
+ if (begin == exponent_digits_begin) {
+ // there were no digits where we expected an exponent. We failed to read
+ // an exponent and should not consume the 'e' after all. Rewind 'begin'.
+ found_exponent = false;
+ begin = exponent_begin;
+ } else {
+ found_exponent = true;
+ if (negative_exponent) {
+ result.literal_exponent = -result.literal_exponent;
+ }
+ }
+ }
+
+ if (!found_exponent && RequireExponent(format_flags)) {
+ // Provided flags required an exponent, but none was found. This results
+ // in a failure to scan.
+ return result;
+ }
+
+ // Success!
+ result.type = strings_internal::FloatType::kNumber;
+ if (result.mantissa > 0) {
+ result.exponent = result.literal_exponent +
+ (DigitMagnitude<base>() * exponent_adjustment);
+ } else {
+ result.exponent = 0;
+ }
+ result.end = begin;
+ return result;
+}
+
+template ParsedFloat ParseFloat<10>(const char* begin, const char* end,
+ chars_format format_flags);
+template ParsedFloat ParseFloat<16>(const char* begin, const char* end,
+ chars_format format_flags);
+
+} // namespace strings_internal
+} // inline namespace lts_2018_06_20
+} // namespace absl
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