Challenge 100

• Program: raku/ch-1.raku

One should use DateTime module to solve this but that is not fun so we solve it the wrong way!

The program will accept any string as $time. We do the format check later.

#| convert 12-hour formatted time to 24-hour format and vice-versa
unit sub MAIN (
    Str $time, #= time (format: 05:15pm or "05:15 pm" or 17:00)
);

The grammar Time will parse $time to give us meaningful information required to do the task.

• hour & minute match any digit ranging from 00 to 99
• meridiem matches either am or pm
• Note: This grammar will consider "99:99" as a valid timestamp.

grammar Time {
    token TOP { <hour> ':' <minute> ' '? <meridiem>? }
    token hour { \d ** 1..2 }
    token minute { \d ** 1..2 }
    token meridiem { ['am'|'pm'] }
}

We parse $time with Time grammar. If meridiem is set then it must be 12-hour format time, otherwise it'll be 24-hour format time.

# Match for time format.
if Time.parse($time) -> $m {
    given $m<meridiem> {
        ...
    }
} else {
    note "Wrong format!";
    exit 1;
}

For "am" we just check if the hour is 12, if so then we print it as "00" otherwise just print the hour.

when 'am' {
    printf "%02d:%02d\n",
    $m<hour> == 12 ?? "00" !! $m<hour>,
    $m<minute>;
}

If the hour is < 12 then we print hour + 12 otherwise just print the hour.

when 'pm' {
    printf "%02d:%02d\n",
    $m<hour> < 12 ?? $m<hour> + 12 !! $m<hour>,
    $m<minute>;
}

If the hour is 0 then print 12 otherwise check if it's > 12, if so then print hour - 12 otherwise just print the hour.

23 -> 11

greater than 12

00 -> 12

equal to 0

12 -> 12

neither equal to 0, nor greater than 12

If the hour is < 12 then print "am" otherwise print "pm".

default {
    printf "%02d:%02d%s\n",
    $m<hour> == 0 ?? "12" !! $m<hour> > 12 ?? $m<hour> - 12 !! $m<hour>,
    $m<minute>, $m<hour> < 12 ?? "am" !! "pm";
}