json-validator/include/jvalidate/detail/number.h

/**
 * Utility functions for managing numeric types - such as converting between
 * floating-point and integer types.
 *
 * None of these are particularly complex functions, but storing them in a
 * single header with descriptive names helps the reader quickly recognize what
 * is being done.
 */
#pragma once

#include <charconv>
#include <cmath>
#include <concepts>
#include <limits>
#include <string_view>
#include <system_error>

#include <jvalidate/compat/expected.h>
#include <jvalidate/detail/out.h>

namespace jvalidate::detail {
/**
 * @brief Determine if a floating point number is actually an integer (in the
 * mathematical sense).
 */
inline bool is_json_integer(double number) { return std::floor(number) == number; }

/**
 * @brief Determine if a floating point number is actually an integer, and
 * actually fits in the 64-bit integer type that we use for JSON Integer.
 */
inline bool fits_in_integer(double number) {
  static constexpr double g_int_max = static_cast<double>(std::numeric_limits<int64_t>::max());
  static constexpr double g_int_min = static_cast<double>(std::numeric_limits<int64_t>::min());
  return is_json_integer(number) && number <= g_int_max && number >= g_int_min;
}

/**
 * @brief Determine if an unsigned integer fits into a signed integer
 */
inline bool fits_in_integer(uint64_t number) { return (number & 0x8000'0000'0000'0000) == 0; }

struct parse_integer_args {
  int base = 10;
  out<size_t> read = discard_out;
};

template <std::integral T>
static expected<T, std::errc> parse_integer(std::string_view in, parse_integer_args args = {}) {
  T rval = 0;
  auto [ptr, ec] = std::from_chars(in.begin(), in.end(), rval, args.base);
  args.read = ptr - in.begin();
  if (ec != std::errc{}) {
    return unexpected(ec);
  }

  if (ptr != in.end()) {
    return unexpected(std::errc::result_out_of_range);
  }

  return rval;
}
}