metaprogramming: parse 24-hour clock to seconds

time_parse.cpp

// Copyright (c) 2020 Julio Montes
//
// SPDX-License-Identifier: Apache-2.0
//

// convert a string in format 24h to seconds at compile time

#include <stdexcept>
#include <string>
using namespace std;

namespace time_parse {
// HH:MM:SS
// ^
constexpr const char hoursOffset = 0;

// HH:MM:SS
//    ^
constexpr const char minutesOffset = 3;

// HH:MM:SS
//       ^
constexpr const char secondsOffset = 6;

// HH:MM:SS
constexpr const char timeLength = 8;

constexpr time_t parseDigit(char d) {
    return ('0' <= d && d <= '9')
               ? d - '0'
               : throw std::domain_error{"invalid character in time"};
}

constexpr time_t parseTime(const char *t, time_t multipler) {
    return (parseDigit(t[0]) * 10 * multipler) + (parseDigit(t[1]) * multipler);
}

template <size_t N> constexpr time_t format24hToSeconds(const char (&s)[N]) {
    return (N - 1 != timeLength)
               ? throw std::
                     domain_error{"expecting string in the form hh:mm:ss"}
               : parseTime(s + hoursOffset, 60 * 60) +
                     parseTime(s + minutesOffset, 60) +
                     parseTime(s + secondsOffset, 1);
}
} // namespace time_parse

int main() {
    constexpr const char t[] = "01:01:20";
    constexpr const time_t v = time_parse::format24hToSeconds(t);
    printf("%s to seconds: %ld\n", t, v);
}