Doomsday rule: Difference between revisions

=={{header|Nim}}==

We use the “times” standard module to parse the dates, check for leap years and compare results with those obtained with the doomsday rule (useful only to trap bugs in our implementation).

We use also the “WeekDay” type of this module which is an enumeration starting with “dMon” for Monday and ending with “dSun” for Sunday.

<lang Nim>import strformat, times

const

NormDoom = [1: 3, 7, 7, 4, 2, 6, 4, 1, 5, 3, 7, 5]

LeapDoom = [1: 4, 1, 7, 2, 4, 6, 4, 1, 5, 3, 7, 5]

proc weekday(year, month, day: int): WeekDay =

let doom = (2 + 5 * (year mod 4) + 4 * (year mod 100) + 6 * (year mod 400)) mod 7

let anchor = if year.isLeapYear: LeapDoom[month] else: NormDoom[month]

let wd = (doom + day - anchor + 7) mod 7

result = if wd == 0: dSun else: WeekDay(wd - 1)

const Dates = ["1800-01-06", "1875-03-29", "1915-12-07",

"1970-12-23", "2043-05-14", "2077-02-12", "2101-04-02"]

for date in Dates:

let dt = date.parse("yyyy-MM-dd")

let wday = weekday(dt.year, ord(dt.month), dt.monthday)

if wday != dt.weekday:

echo &"For {date}, expected {dt.weekday}, found {wday}."

else:

echo date, " → ", wday</lang>

{{out}}

<pre>1800-01-06 → Monday

1875-03-29 → Monday

1915-12-07 → Tuesday

1970-12-23 → Wednesday

2043-05-14 → Thursday

2077-02-12 → Friday

2101-04-02 → Saturday</pre>