# French Republican calendar

```ruby
require('DateTime')
 
var month_names = %w(
  Vendémiaire Brumaire Frimaire  Nivôse   Pluviôse  Ventôse
  Germinal    Floréal  Prairial  Messidor Thermidor Fructidor
)
 
var intercalary = [
                   'Fête de la vertu',
                   'Fête du génie',
                   'Fête du travail',
                   "Fête de l'opinion",
                   'Fête des récompenses',
                   'Fête de la Révolution',
                  ]
 
var i_cal_month = 13
var epoch = %O<DateTime>.new(year => 1792, month => 9, day => 22)
 
var month_nums = Hash(month_names.kv.map {|p| [p[1], p[0]+1] }.flat...)
var i_cal_nums = Hash(intercalary.kv.map {|p| [p[1], p[0]+1] }.flat...)
 
func is_republican_leap_year(Number year) -> Bool {
    var y = (year + 1)
    !!(4.divides(y) && (!100.divides(y) || 400.divides(y)))
}
 
func Republican_to_Gregorian(String rep_date) -> String {
    static months   = month_names.map { .escape }.join('|')
    static intercal = intercalary.map { .escape }.join('|')
    static re       = Regex("^
        \\s* (?:
                (?<ic> #{intercal})
              | (?<day> \\d+) \\s+ (?<month> #{months})
            )
        \\s+ (?<year> \\d+)
        \\s*
      \\z", 'x')
 
    var m = (rep_date =~ re)
    m || die "Republican date not recognized: '#{rep_date}'"
 
    var ncap = m.named_captures
 
    var day1   = Number(ncap{:ic}    ? i_cal_nums{ncap{:ic}}    : ncap{:day})
    var month1 = Number(ncap{:month} ? month_nums{ncap{:month}} : i_cal_month)
    var year1  = Number(ncap{:year})
 
    var days_since_epoch = (365*(year1 - 1) + 30*(month1 - 1) + (day1 - 1))
 
    var leap_days = (1 ..^ year1 -> grep { is_republican_leap_year(_) })
    epoch.clone.add(days => (days_since_epoch + leap_days)).strftime("%Y-%m-%d")
}
 
func Gregorian_to_Republican(String greg_date) -> String {
    var m = (greg_date =~ /^(\d{4})-(\d{2})-(\d{2})\z/)
    m || die "Gregorian date not recognized: '#{greg_date}'"
 
    var g = %O<DateTime>.new(year => m[0], month => m[1], day => m[2])
    var days_since_epoch = epoch.delta_days(g).in_units('days')
    days_since_epoch < 0 && die "unexpected error"
    var (year, days) = (1, days_since_epoch)
 
    loop {
        var year_length = (365 + (is_republican_leap_year(year) ? 1 : 0))
        days < year_length && break
        days -= year_length
        year += 1;
    }
 
    var day0   = (days % 30)
    var month0 = (days - day0)/30
 
    var (day1, month1) = (day0 + 1, month0 + 1)
 
    (month1 == i_cal_month
        ? "#{intercalary[day0]} #{year}"
        : "#{day1} #{month_names[month0]} #{year}")
}
 
for line in DATA {
 
    line.sub!(/\s*\#.+\R?\z/, '')
 
    var m = (line =~ /^(\d{4})-(\d{2})-(\d{2})\s+(\S.+?\S)\s*$/)
    m || die "error for: #{line.dump}"
 
    var g = "#{m[0]}-#{m[1]}-#{m[2]}"
    var r = m[3]
 
    var r2g = Republican_to_Gregorian(r)
    var g2r = Gregorian_to_Republican(g)
 
    #say "#{r} -> #{r2g}"
    #say "#{g} -> #{g2r}"
 
    if ((g2r != r) || (r2g != g)) {
        die "1-way error"
    }
 
    if ((Gregorian_to_Republican(r2g) != r) ||
        (Republican_to_Gregorian(g2r) != g)
    ) {
        die "2-way error"
    }
}
 
say 'All tests successful.'
 
__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
```

#### Output:

```
All tests successful.
```


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