validate time, C++ (vc++)

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//Microsoft (R) C/C++ Optimizing Compiler Version 19.00.23506 for x64

#include <iostream>
#include <string>
#include <regex>
#include <cctype>
#include <ctime>

// convert three-character string "sun", "mon" etc. to week day (sunday==0)
int str_to_week_day( std::string str )
{
    static constexpr std::size_t NWDAYS = 7 ;
    static const std::string wdays[NWDAYS] { "sun", "mon", "tue", "wednes", "thurs", "fri", "satur" } ;

for( char& c : str ) c = char( std::tolower(c) ) ; // convert str to all lower case

// http://en.cppreference.com/w/cpp/algorithm/find (1)
    return int( std::find( wdays, wdays+NWDAYS, str ) - wdays ) ; // return the position in the array
}

// http://en.cppreference.com/w/cpp/chrono/c/tm
// return true if tm holds valid values for day, month, year and day of week
bool valid_tm( const std::tm& tm )
{
    auto cpy = tm ;

// this two step process would correct any out of range values which may be present in cpy

// http://en.cppreference.com/w/cpp/chrono/c/mktime
    //    note: "The values in time are permitted to be outside their normal ranges."
    const auto as_time_t = std::mktime( std::addressof(cpy) ) ;

// http://en.cppreference.com/w/cpp/chrono/c/localtime
    cpy = *std::localtime( std::addressof(as_time_t) ) ;

return tm.tm_mday == cpy.tm_mday && // valid day
           tm.tm_mon == cpy.tm_mon && // valid month
           tm.tm_year == cpy.tm_year && // valid year
           tm.tm_wday == cpy.tm_wday ; // valid day of week
}

// fill up tm with the parsed values and return true if dt contains a valid date
// in the form "dd/mm/yyyy, day_of_week" eg. "23/10/2017, Monday"
bool valid_date( const std::string& dt, std::tm& tm )
{
    tm = {} ; // set all members of tm to zeroes

// http://en.cppreference.com/w/cpp/regex/basic_regex

// ^       - beginning of string
    // \s*     - zero or more white space
    // (\d\d)  - two decimal digits (day), captured
    // /       - literal /
    // (\d\d)  - two decimal digits (month), captured
    // /       - literal /
    // (\d{4}) - four decimal digits (year), captured
    // \,      - literal ,
    // \s+     - one or more white space
    // (Sun|Mon|Tue|Wednes|Thurs|Fri|Satur) - any one of these seven alternatives, captured
    // day     - literal "day"
    // \s*     - zero or more white space
    // $       - end of string
    static const std::regex date_re( R"(^\s*(\d\d)/(\d\d)/(\d{4})\,\s+(Sun|Mon|Tue|Wednes|Thurs|Fri|Satur)day\s*$)",
                                     std::regex::icase ) ; // ignore case (for names of week days)

// http://en.cppreference.com/w/cpp/regex/match_results
    std::smatch match ;

// http://en.cppreference.com/w/cpp/regex/regex_match (4)
    if( std::regex_match( dt, match, date_re ) ) // if the pattern in the regex is matched
    {
        // fill up tm with dd, mm, yy and day of week
        // http://en.cppreference.com/w/cpp/chrono/c/tm
        // http://en.cppreference.com/w/cpp/string/basic_string/stol
        tm.tm_mday = std::stoi( match[1] ) ;
        tm.tm_mon = std::stoi( match[2] ) - 1 ; // -1 because in struct tm, January == 0
        tm.tm_year = std::stoi( match[3] ) - 1900 ; // years since 1900
        tm.tm_wday = str_to_week_day( match[4] ) ; // Sunday == 0
        tm.tm_hour = 12 ; // noon

return valid_tm(tm) ; // return true if tm holds values within the valid range
    }

return false ;
}

int main()
{
    const std::string str = " 23/10/2017,   MondaY" ;

std::tm tm ;
    if( valid_date( str, tm ) )
    {
       std::cout << "valid date '" << str << "'\n\n"
                 << "            day of the month: " << tm.tm_mday << '\n'
                 << "          month (january==1): " << tm.tm_mon + 1 << '\n'
                 << "                        year: " << tm.tm_year + 1900 << '\n'
                 << " day of the week (Sunday==0): " << tm.tm_wday << '\n' ;
    }
}

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Compilation time: 10,91 sec, absolute running time: 0,34 sec, absolute service time: 11,28 sec