Write a program to convert dates between the Gregorian calendar and the French Republican calendar.

The year 1 of the Republican calendar began on 22 September 1792. There were twelve months (Vendémiaire, Brumaire, Frimaire, Nivôse, Pluviôse, Ventôse, Germinal, Floréal, Prairial, Messidor, Thermidor, and Fructidor) of 30 days each, followed by five intercalary days or Sansculottides (Fête de la vertu / Virtue Day, Fête du génie / Talent Day, Fête du travail / Labour Day, Fête de l'opinion / Opinion Day, and Fête des récompenses / Honours Day). In leap years (the years 3, 7, and 11) a sixth Sansculottide was added: Fête de la Révolution / Revolution Day.

As a minimum, your program should give correct results for dates in the range from 1 Vendémiaire 1 = 22 September 1792 to 10 Nivôse 14 = 31 December 1805 (the last day when the Republican calendar was officially in use). If you choose to accept later dates, be aware that there are several different methods (described on the Wikipedia page) about how to determine leap years after the year 14. You should indicate which method you are using. (Because of these different methods, correct programs may sometimes give different results for dates after 1805.)

Test your program by converting the following dates both from Gregorian to Republican and from Republican to Gregorian:

• 1 Vendémiaire 1 = 22 September 1792

• 1 Prairial 3 = 20 May 1795

• 27 Messidor 7 = 15 July 1799 (Rosetta Stone discovered)

• Fête de la Révolution 11 = 23 September 1803

• 10 Nivôse 14 = 31 December 1805

use v6;
constant @month_names = <
    Vendémiaire Brumaire Frimaire  Nivôse   Pluviôse  Ventôse
    Germinal    Floréal  Prairial  Messidor Thermidor Fructidor
>;
constant @intercalary =
    'Fête de la vertu',  'Fête du génie',        'Fête du travail',
    "Fête de l'opinion", 'Fête des récompenses', 'Fête de la Révolution',
;
constant %month_nums  = %( @month_names Z=> 1..12 );
constant %i_cal_nums  = %( @intercalary Z=> 1.. 6 );
constant $i_cal_month = 13;
constant $epoch       = Date.new: '1792-09-22';
 
sub is_republican_leap_year ( Int:D $year --> Bool ) {
    my $y := $year + 1;
    return ?( $y %% 4 and ($y !%% 100 or $y %% 400) );
}
 
sub Republican_to_Gregorian ( Str:D $rep_date --> Date ) {
    grammar Republican_date_text {
        token day   { \d+ }
        token year  { \d+ }
        token ic    { @intercalary }
        token month { @month_names }
        rule TOP    { ^ [ <ic> | <day> <month> ] <year> $ }
    }
 
    Republican_date_text.parse($rep_date)
        orelse die "Republican date not recognized: '$rep_date'";
 
    my $day1   := $/<ic>    ?? %i_cal_nums{~$/<ic>}    !! +$/<day>;
    my $month1 := $/<month> ?? %month_nums{~$/<month>} !! $i_cal_month;
 
    my @ymd0 := ($/<year>, $month1, $day1) »-» 1;
    my $days_since_epoch := [+] @ymd0 Z* (365, 30, 1);
 
    my $leap_days := +grep &is_republican_leap_year, 1 ..^ $/<year>;
    return $epoch + $days_since_epoch + $leap_days;
}
 
sub Gregorian_to_Republican ( Date:D $greg_date --> Str ) {
    my $days_since_epoch := $greg_date - $epoch;
    die if $days_since_epoch < 0;
 
    my ( $year, $days ) = 1, $days_since_epoch;
    loop {
        my $year_length = 365 + (1 if $year.&is_republican_leap_year);
        last if $days < $year_length;
        $days -= $year_length;
        $year += 1;
    }
 
    my ( $day0, $month0 ) = $days.polymod( 30 );
    my ( $day1, $month1 ) = ($day0, $month0) X+ 1;
 
    return $month1 == $i_cal_month
        ??       "@intercalary[$day0  ] $year"
        !! "$day1 @month_names[$month0] $year";
}
 
my @test_data =
    ( '1792-09-22', '1 Vendémiaire 1' ),
    ( '1795-05-20', '1 Prairial 3' ),
    ( '1799-07-15', '27 Messidor 7' ),
    ( '1803-09-23', 'Fête de la Révolution 11' ),
    ( '1805-12-31', '10 Nivôse 14' ),
 
    ( '1871-03-18', '27 Ventôse 79' ),
    ( '1944-08-25', '7 Fructidor 152' ),
    ( '2016-09-19', 'Fête du travail 224' ),
 
    ( '1871-05-06', '16 Floréal 79'  ), # Paris Commune begins
    ( '1871-05-23', '3 Prairial 79'  ), # Paris Commune ends
    ( '1799-11-09', '18 Brumaire 8'  ), # Revolution ends by Napoléon coup
    ( '1804-12-02', '11 Frimaire 13' ), # Republic   ends by Napoléon coronation
    ( '1794-10-30', '9 Brumaire 3'   ), # École Normale Supérieure established
    ( '1794-07-27', '9 Thermidor 2'  ), # Robespierre falls
    ( '1799-05-27', '8 Prairial 7'   ), # Fromental Halévy born
 
    ( '1792-09-22', '1 Vendémiaire 1'   ),
    ( '1793-09-22', '1 Vendémiaire 2'   ),
    ( '1794-09-22', '1 Vendémiaire 3'   ),
    ( '1795-09-23', '1 Vendémiaire 4'   ),
    ( '1796-09-22', '1 Vendémiaire 5'   ),
    ( '1797-09-22', '1 Vendémiaire 6'   ),
    ( '1798-09-22', '1 Vendémiaire 7'   ),
    ( '1799-09-23', '1 Vendémiaire 8'   ),
    ( '1800-09-23', '1 Vendémiaire 9'   ),
    ( '1801-09-23', '1 Vendémiaire 10'  ),
    ( '1802-09-23', '1 Vendémiaire 11'  ),
    ( '1803-09-24', '1 Vendémiaire 12'  ),
    ( '1804-09-23', '1 Vendémiaire 13'  ),
    ( '1805-09-23', '1 Vendémiaire 14'  ),
    ( '1806-09-23', '1 Vendémiaire 15'  ),
    ( '1807-09-24', '1 Vendémiaire 16'  ),
    ( '1808-09-23', '1 Vendémiaire 17'  ),
    ( '1809-09-23', '1 Vendémiaire 18'  ),
    ( '1810-09-23', '1 Vendémiaire 19'  ),
    ( '1811-09-24', '1 Vendémiaire 20'  ),
    ( '2015-09-23', '1 Vendémiaire 224' ),
    ( '2016-09-22', '1 Vendémiaire 225' ),
    ( '2017-09-22', '1 Vendémiaire 226' ),
;
 
for @test_data -> ( $g_text, $r ) {
    my $g = Date.new: $g_text;
 
    die if Republican_to_Gregorian($r) != $g
        or Gregorian_to_Republican($g) ne $r;
 
    die if Gregorian_to_Republican(Republican_to_Gregorian($r)) ne $r
        or Republican_to_Gregorian(Gregorian_to_Republican($g)) != $g;
}
say 'All tests successful.';