Day of the week of the next Feb 29th

Question

Write a function which takes a date and returns the day of the week of the next February 29th after that date.

• The input is a string in the ISO Extended format: YYYY-MM-DD (e.g. May 27th, 2010 would be "2010-05-27").

• The output is a string which is the name of the day of the week (e.g. "Monday"). Capitalization doesn't matter, but do give the full name in English.

• If the given date is February 29th, then return the day of the week of the next Feburary 29th.

• Use the calculations for the Proleptic Gregorian Calendar (so, it uses the Gregorian leap year calculations for it's entire length). Don't worry about the Julian Calendar or when the switch from Julian to Gregorian occurred. Just assume Gregorian for everything.

• The function should work for at least the range of "0001-01-01" - "2100-01-01".

• Feel free to use whatever standard libraries your language of choice provides, but do not use 3rd party libraries unless you want to include that code as part of your solution.

• Shortest code (fewest characters) wins.

Examples:

• func("0001-01-01") -> "Sunday"
• func("1899-12-03") -> "Monday"
• func("1970-01-01") -> "Tuesday"
• func("1999-07-06") -> "Tuesday"
• func("2003-05-22") -> "Sunday"
• func("2011-02-17") -> "Wednesday"
• func("2100-01-01") -> "Friday"

(and no, you don't have to name the function func)

Hints:

• Remember that years ending in 00 which aren't divisable by 400 aren't leap years.
• January 1st, 0001 is a Monday.

Answer 1

Windows PowerShell, 65

Fairly straightforward.

filter f{for($d=date $_;($d+='1').day*$d.month-58){}$d.dayofweek}

As usual, we can shave off two bytes if we are willing to wait a long time till completion:

filter f{for($d=date $_;($d+=9).day*$d.month-58){}$d.dayofweek}

Test:

> '0001-01-01','1899-12-03','1970-01-01','1999-07-06','2003-05-22','2011-02-17','2100-01-01'|f
Sunday
Monday
Tuesday
Tuesday
Sunday
Wednesday
Friday

History:

• 2011-02-18 00:06 (65) First attempt.

Answer 2

Ruby 1.9, 85 characters

f=->a{require"date";d=Date.parse(a,0,0)+1;d+=1until d.day*d.month==58;d.strftime"%A"}

Straightforward solution. Call the function with f[args].

• Edit: (87 -> 97) Fixed the 0001-01-01 testcase.
• Edit 2: (97 -> 91) Date.parse allows specifying the date of the calendar reform as well.
• Edit 3: (91 -> 87) Use a lambda instead of a function. Thanks Dogbert!
• Edit 4: (87 -> 85) Remove unnecessary spaces. Thanks again, Dogbert!

Answer 3

T-SQL 166 185 Characters

CREATE PROCEDURE f29 (@d DATETIME) AS
DECLARE @i int
SET @i=YEAR(DATEADD(M,-2,@d)+3)
SET @i=@i+(4-@i%4)
IF @i%100=0 AND @i%400<>0 SET @i=@i+4
SELECT DATENAME(W,CAST(@i AS CHAR)+'-2-29')

I was already messing with T-SQL date functions, so I figured why not...

Original solution was incorrect...

Here's what I actually have to do to get that strategy to work:

CREATE PROCEDURE f29 (@d DATE) AS
DECLARE @i int
SET @i = YEAR(@d)
BEGIN TRY 
SET @i=YEAR(DATEADD(D, 3, DATEADD(M,-2,@d)))
END TRY
BEGIN CATCH
END CATCH
SET @i=@i+(4-@i%4)
IF @i%100=0 AND @i%400<>0 SET @i=@i+4
SELECT DATENAME(W,CAST(@i AS CHAR)+'-2-29')

Answer 4

C#, 176

Func<String,String>f=(d)=>{DateTime n;for(n=DateTime.Parse(d).AddDays(307);!(DateTime.IsLeapYear(n.Year));n=n.AddYears(1)){}return new DateTime(n.Year,2,29).ToString("dddd");};

Answer 5

Bash, 96 bytes

Thanks Peter... Highly golfed version, 96 bytes:

i=1;until [ `date -d"$1 $i days" +%m-%d` = "02-29" ];do((i++));done
date -d "${1} ${i} days" +%A

Old version, 229 bytes

#!/bin/bash
from=$1
i=1
while [ "$isFeb" = "" ] || [ "$is29" = "" ]
do
isFeb=`date -d "${from} ${i} days" | grep Feb`
is29=`date -d "${from} ${i} days" +%Y-%m-%d | grep "\-29"`
((i++))
done
((i--))
date -d "${from} ${i} days" +%A

SAMPLE I/O

:~/aman>./29Feb.sh 0001-01-01
Sunday
:~/aman>./29Feb.sh 1899-12-03
Monday
:~/aman>./29Feb.sh 1970-01-01
Tuesday

Answer 6

Perl, no date library: 160 159 155

sub f{($y,$m)=split/-/,@_[0],2;$y++if($m>'02-28');$y=($y+3)%400>>2;$y+=$y&&!($y%25);@r=(Tues,Wednes,Thurs,Fri,Satur,Sun,Mon);@r[(5*$y-($y/25&3))%7]."day";}

The real benefit of these date libraries is pushing off the length of the names of the days to someone else.

On the other hand, I think this is the only solution so far which works regardless of locale.

Answer 7

DATE and some BASHy glue (90)

The function:

f(){ while :;do $(date -d$1+1day +'set - %F %A 1%m%d');(($3==10229))&&break;done;echo $2;}

Testing:

$ for x in 0001-01-01 1899-12-03 1970-01-01 1999-07-06 2003-05-22 2011-02-17 2100-01-01 ; do f $x ; done
Sunday
Monday
Tuesday
Tuesday
Sunday
Wednesday
Friday

Answer 8

PHP, 104 bytes

function f($s){for($y=$s-(substr($s,5)<"02-29");!date(L,$t=strtotime(++$y."-2-1")););return date(l,$t);}

breakdown

for($y=$s-;                 // "-" casts the string to int; decrement year if ...
    (substr($s,5)<"02-29")  // ... date is before feb 29
    !date(L,                        // loop while incremented $y is no leap year
        $t=strtotime(++$y."-2-1")   // $t=feb 01 in that year (same weekday, but shorter)
    );
);
return date(l,$t);          // return weekday name of that timestamp

date(L): 1 for leap year, 0 else
date(l): full textual representation of the day of the week

Answer 9

D: 175 Characters

S f(S)(S s){auto d=Date.fromISOExtString(s)+days(1);while(d.month!=Month.feb||d.day!=29)d+=days(1);return["Sun","Mon","Tues","Wednes","Thurs","Fri","Sat"][d.dayOfWeek]~"day";}

More Legibly:

S f(S)(S s)
{
    auto d = Date.fromISOExtString(s) + days(1);

    while(d.month != Month.feb || d.day != 29)
        d += days(1);

    return ["Sun", "Mon", "Tues", "Wednes", "Thurs", "Fri", "Sat"][d.dayOfWeek] ~ "day";
}

It's very easy to write in D, but it's definitely not going to win any code golfing contests. Still, outside of code golfing, I'd much rather have it easy to write and understand but long than have it terse but hard to write and understand.

Answer 10

Java 8 - 252 chars

Golfed:

import java.time.*;
import java.time.format.*;
public class S{public static void main(String[] a){LocalDate d=LocalDate.parse(a[0]).plusDays(1);while(d.getMonthValue()!=2||d.getDayOfMonth()!=29){d=d.plusDays(1);}System.out.println(d.getDayOfWeek());}}

Ungolfed:

import java.time.*;
import java.time.format.*;
public class S {
    public static void main(String[] a) {
        LocalDate d = LocalDate.parse(a[0]).plusDays(1);

        while(d.getMonthValue()!=2 || d.getDayOfMonth()!=29) {
            d = d.plusDays(1);
        }
        System.out.println(d.getDayOfWeek());
    }
}

Answer 11

Rebol - 78

f: func[d][system/locale/days/(until[d: d + 1 d/day * d/month = 58]d/weekday)]

Ungolfed:

f: func [d] [
    system/locale/days/(
        until [
            d: d + 1
            d/day * d/month = 58
        ]
        d/weekday
    )
]

Example usage (in Rebol console):

>> f 0001-01-01
== "Sunday"

>> f 2100-01-01
== "Friday"

Answer 12

GNU coreutils, 71 bytes

f(){
seq -f "$1 %gday" 2921|date -f- +'%m%d%A'|sed 's/0229//;t;d;:;q'
}

Linear search of the next 8 years (worst case, given 2096-02-29). Sed finds and outputs the first line that matches 29th of February.

I was surprised to find that t;d;:;q was shorter than testing to see whether the digits remained (/[01]/d;q), despite being twice as many commands.

Tests

I added an extra line of tests for difficult corner cases:

for i in 0001-01-01.Sunday 1899-12-03.Monday \
    1970-01-01.Tuesday     1999-07-06.Tuesday \
    2003-05-22.Sunday      2011-02-17.Wednesday 2100-01-01.Friday \
    2000-02-28.Tuesday     2000-02-29.Sunday    2000-03-01.Sunday   2096-02-29.Friday
do
    printf "%s => %s (expected %s)\n" ${i%.*} $(f ${i%.*}) ${i#*.}
done