cal.d

auto daysPerYaer(int year) {
    if (year % 4 == 0 && !(year % 100 == 0 && year % 400 != 0)) {
        return 366;
    } else {
        return 365;
    }
}
unittest {
    import std.algorithm;
    import std.range;
    assert(iota(2000, 2400).map!daysPerYaer().map!`a - 365`().sum() == 97);
    static assert(iota(2000, 2400).map!daysPerYaer().map!`a - 365`().sum() == 97);
}

enum Day {
    Saturday = 0,
    Sunday,
    Mondey,
    Tuesday,
    Wednesday,
    Thursday,
    Friday,
}
auto zeller(int y, int m, int d) {
    if (m <= 2) {
        m += 12;
        y -= 1;
    }
    auto C = y / 100;
    auto Y = y % 100;
    auto Gamma = 5 * C + C / 4;
    return cast(Day)(5 * C + Y + Y / 4 + C / 4 + 26 * (m + 1) / 10 + d) % 7;
}
unittest {
    assert(zeller(2015, 2, 1) == Day.Sunday);
    assert(zeller(2009, 2, 1) == Day.Sunday);
}
unittest {
    import std.algorithm;
    import std.range;
    import std.typecons;
    auto years = iota(1583, 2016).filter!(a => daysPerYaer(a) == 365)().filter!(a => zeller(a, 2, 1) == Day.Sunday)().array;
    std.stdio.writeln(years);
    std.stdio.writeln(years.length);
    std.stdio.writeln(cast(real)(2016 - 1583) / years.length);
}