ascii_number.h

#ifndef FASTFLOAT_ASCII_NUMBER_H
#define FASTFLOAT_ASCII_NUMBER_H

#include <cstdio>
#include <cctype>
#include <cstdint>
#include <cstring>
#include <iterator>

#include "float_common.h"

namespace fast_float {

// Next function can be micro-optimized, but compilers are entirely
// able to optimize it well.
fastfloat_really_inline bool is_integer(char c)  noexcept  { return c >= '0' && c <= '9'; }

fastfloat_really_inline uint64_t byteswap(uint64_t val) {
  return (val & 0xFF00000000000000) >> 56
    | (val & 0x00FF000000000000) >> 40
    | (val & 0x0000FF0000000000) >> 24
    | (val & 0x000000FF00000000) >> 8
    | (val & 0x00000000FF000000) << 8
    | (val & 0x0000000000FF0000) << 24
    | (val & 0x000000000000FF00) << 40
    | (val & 0x00000000000000FF) << 56;
}

fastfloat_really_inline uint64_t read_u64(const char *chars) {
  uint64_t val;
  ::memcpy(&val, chars, sizeof(uint64_t));
#if FASTFLOAT_IS_BIG_ENDIAN == 1
  // Need to read as-if the number was in little-endian order.
  val = byteswap(val);
#endif
  return val;
}

fastfloat_really_inline void write_u64(uint8_t *chars, uint64_t val) {
#if FASTFLOAT_IS_BIG_ENDIAN == 1
  // Need to read as-if the number was in little-endian order.
  val = byteswap(val);
#endif
  ::memcpy(chars, &val, sizeof(uint64_t));
}

// credit  @aqrit
fastfloat_really_inline uint32_t  parse_eight_digits_unrolled(uint64_t val) {
  const uint64_t mask = 0x000000FF000000FF;
  const uint64_t mul1 = 0x000F424000000064; // 100 + (1000000ULL << 32)
  const uint64_t mul2 = 0x0000271000000001; // 1 + (10000ULL << 32)
  val -= 0x3030303030303030;
  val = (val * 10) + (val >> 8); // val = (val * 2561) >> 8;
  val = (((val & mask) * mul1) + (((val >> 16) & mask) * mul2)) >> 32;
  return uint32_t(val);
}

fastfloat_really_inline uint32_t parse_eight_digits_unrolled(const char *chars)  noexcept  {
  return parse_eight_digits_unrolled(read_u64(chars));
}

// credit @aqrit
fastfloat_really_inline bool is_made_of_eight_digits_fast(uint64_t val)  noexcept  {
  return !((((val + 0x4646464646464646) | (val - 0x3030303030303030)) &
     0x8080808080808080));
}

fastfloat_really_inline bool is_made_of_eight_digits_fast(const char *chars)  noexcept  {
  return is_made_of_eight_digits_fast(read_u64(chars));
}

struct parsed_number_string {
  int64_t exponent;
  uint64_t mantissa;
  const char *lastmatch;
  bool negative;
  bool valid;
  bool too_many_digits;
};


// Assuming that you use no more than 19 digits, this will
// parse an ASCII string.
fastfloat_really_inline
parsed_number_string parse_number_string(const char *p, const char *pend, parse_options options) noexcept {
  const chars_format fmt = options.format;
  const char decimal_point = options.decimal_point;

  parsed_number_string answer;
  answer.valid = false;
  answer.too_many_digits = false;
  answer.negative = (*p == '-');
  if (*p == '-') { // C++17 20.19.3.(7.1) explicitly forbids '+' sign here
    ++p;
    if (p == pend) {
      return answer;
    }
    if (!is_integer(*p) && (*p != decimal_point)) { // a sign must be followed by an integer or the dot
      return answer;
    }
  }
  const char *const start_digits = p;

  uint64_t i = 0; // an unsigned int avoids signed overflows (which are bad)

  while ((p != pend) && is_integer(*p)) {
    // a multiplication by 10 is cheaper than an arbitrary integer
    // multiplication
    i = 10 * i +
        uint64_t(*p - '0'); // might overflow, we will handle the overflow later
    ++p;
  }
  const char *const end_of_integer_part = p;
  int64_t digit_count = int64_t(end_of_integer_part - start_digits);
  int64_t exponent = 0;
  if ((p != pend) && (*p == decimal_point)) {
    ++p;
  // Fast approach only tested under little endian systems
  if ((std::distance(p, pend) >= 8) && is_made_of_eight_digits_fast(p)) {
    i = i * 100000000 + parse_eight_digits_unrolled(p); // in rare cases, this will overflow, but that's ok
    p += 8;
    if ((std::distance(p, pend) >= 8) && is_made_of_eight_digits_fast(p)) {
      i = i * 100000000 + parse_eight_digits_unrolled(p); // in rare cases, this will overflow, but that's ok
      p += 8;
    }
  }
    while ((p != pend) && is_integer(*p)) {
      uint8_t digit = uint8_t(*p - '0');
      ++p;
      i = i * 10 + digit; // in rare cases, this will overflow, but that's ok
    }
    exponent = end_of_integer_part + 1 - p;
    digit_count -= exponent;
  }
  // we must have encountered at least one integer!
  if (digit_count == 0) {
    return answer;
  }
  int64_t exp_number = 0;            // explicit exponential part
  if ((fmt & chars_format::scientific) && (p != pend) && (('e' == *p) || ('E' == *p))) {
    const char * location_of_e = p;
    ++p;
    bool neg_exp = false;
    if ((p != pend) && ('-' == *p)) {
      neg_exp = true;
      ++p;
    } else if ((p != pend) && ('+' == *p)) { // '+' on exponent is allowed by C++17 20.19.3.(7.1)
      ++p;
    }
    if ((p == pend) || !is_integer(*p)) {
      if(!(fmt & chars_format::fixed)) {
        // We are in error.
        return answer;
      }
      // Otherwise, we will be ignoring the 'e'.
      p = location_of_e;
    } else {
      while ((p != pend) && is_integer(*p)) {
        uint8_t digit = uint8_t(*p - '0');
        if (exp_number < 0x10000) {
          exp_number = 10 * exp_number + digit;
        }
        ++p;
      }
      if(neg_exp) { exp_number = - exp_number; }
      exponent += exp_number;
    }
  } else {
    // If it scientific and not fixed, we have to bail out.
    if((fmt & chars_format::scientific) && !(fmt & chars_format::fixed)) { return answer; }
  }
  answer.lastmatch = p;
  answer.valid = true;

  // If we frequently had to deal with long strings of digits,
  // we could extend our code by using a 128-bit integer instead
  // of a 64-bit integer. However, this is uncommon.
  //
  // We can deal with up to 19 digits.
  if (digit_count > 19) { // this is uncommon
    // It is possible that the integer had an overflow.
    // We have to handle the case where we have 0.0000somenumber.
    // We need to be mindful of the case where we only have zeroes...
    // E.g., 0.000000000...000.
    const char *start = start_digits;
    while ((start != pend) && (*start == '0' || *start == decimal_point)) {
      if(*start == '0') { digit_count --; }
      start++;
    }
    if (digit_count > 19) {
      answer.too_many_digits = true;
      // Let us start again, this time, avoiding overflows.
      i = 0;
      p = start_digits;
      const uint64_t minimal_nineteen_digit_integer{1000000000000000000};
      while((i < minimal_nineteen_digit_integer) && (p != pend) && is_integer(*p)) {
        i = i * 10 + uint64_t(*p - '0');
        ++p;
      }
      if (i >= minimal_nineteen_digit_integer) { // We have a big integers
        exponent = end_of_integer_part - p + exp_number;
      } else { // We have a value with a fractional component.
          p++; // skip the dot
          const char *first_after_period = p;
          while((i < minimal_nineteen_digit_integer) && (p != pend) && is_integer(*p)) {
            i = i * 10 + uint64_t(*p - '0');
            ++p;
          }
          exponent = first_after_period - p + exp_number;
      }
      // We have now corrected both exponent and i, to a truncated value
    }
  }
  answer.exponent = exponent;
  answer.mantissa = i;
  return answer;
}


// This should always succeed since it follows a call to parse_number_string
// This function could be optimized. In particular, we could stop after 19 digits
// and try to bail out. Furthermore, we should be able to recover the computed
// exponent from the pass in parse_number_string.
fastfloat_really_inline decimal parse_decimal(const char *p, const char *pend, parse_options options) noexcept {
  const char decimal_point = options.decimal_point;

  decimal answer;
  answer.num_digits = 0;
  answer.decimal_point = 0;
  answer.truncated = false;
  answer.negative = (*p == '-');
  if (*p == '-') { // C++17 20.19.3.(7.1) explicitly forbids '+' sign here
    ++p;
  }
  // skip leading zeroes
  while ((p != pend) && (*p == '0')) {
    ++p;
  }
  while ((p != pend) && is_integer(*p)) {
    if (answer.num_digits < max_digits) {
      answer.digits[answer.num_digits] = uint8_t(*p - '0');
    }
    answer.num_digits++;
    ++p;
  }
  if ((p != pend) && (*p == decimal_point)) {
    ++p;
    const char *first_after_period = p;
    // if we have not yet encountered a zero, we have to skip it as well
    if(answer.num_digits == 0) {
      // skip zeros
      while ((p != pend) && (*p == '0')) {
       ++p;
      }
    }
    // We expect that this loop will often take the bulk of the running time
    // because when a value has lots of digits, these digits often
    while ((std::distance(p, pend) >= 8) && (answer.num_digits + 8 < max_digits)) {
      uint64_t val = read_u64(p);
      if(! is_made_of_eight_digits_fast(val)) { break; }
      // We have eight digits, process them in one go!
      val -= 0x3030303030303030;
      write_u64(answer.digits + answer.num_digits, val);
      answer.num_digits += 8;
      p += 8;
    }
    while ((p != pend) && is_integer(*p)) {
      if (answer.num_digits < max_digits) {
        answer.digits[answer.num_digits] = uint8_t(*p - '0');
      }
      answer.num_digits++;
      ++p;
    }
    answer.decimal_point = int32_t(first_after_period - p);
  }
  // We want num_digits to be the number of significant digits, excluding
  // leading *and* trailing zeros! Otherwise the truncated flag later is
  // going to be misleading.
  if(answer.num_digits > 0) {
    // We potentially need the answer.num_digits > 0 guard because we
    // prune leading zeros. So with answer.num_digits > 0, we know that
    // we have at least one non-zero digit.
    const char *preverse = p - 1;
    int32_t trailing_zeros = 0;
    while ((*preverse == '0') || (*preverse == decimal_point)) {
      if(*preverse == '0') { trailing_zeros++; };
      --preverse;
    }
    answer.decimal_point += int32_t(answer.num_digits);
    answer.num_digits -= uint32_t(trailing_zeros);
  }
  if(answer.num_digits > max_digits) {
    answer.truncated = true;
    answer.num_digits = max_digits;
  }
  if ((p != pend) && (('e' == *p) || ('E' == *p))) {
    ++p;
    bool neg_exp = false;
    if ((p != pend) && ('-' == *p)) {
      neg_exp = true;
      ++p;
    } else if ((p != pend) && ('+' == *p)) { // '+' on exponent is allowed by C++17 20.19.3.(7.1)
      ++p;
    }
    int32_t exp_number = 0; // exponential part
    while ((p != pend) && is_integer(*p)) {
      uint8_t digit = uint8_t(*p - '0');
      if (exp_number < 0x10000) {
        exp_number = 10 * exp_number + digit;
      }
      ++p;
    }
    answer.decimal_point += (neg_exp ? -exp_number : exp_number);
  }
  // In very rare cases, we may have fewer than 19 digits, we want to be able to reliably
  // assume that all digits up to max_digit_without_overflow have been initialized.
  for(uint32_t i = answer.num_digits; i < max_digit_without_overflow; i++) { answer.digits[i] = 0; }

  return answer;
}
} // namespace fast_float

#endif