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#!/usr/bin/perl -s
use v5.16;
use Test2::V0;
use DateTime::Format::Strptime;
use experimental 'signatures';
our ($examples, $today);
run_tests() if $examples; # does not return
die <<EOS unless @ARGV == 1;
usage: $0 [-examples] [-today=yyyy/mm/dd] [yyyy/mm/dd]
-examples
run the examples from the challenge
-today=yyyy/mm/dd
use given date as today
yyyy/mm/dd
day of birth
EOS
### Input and Output
{
local $, = ', ';
say mirror_date(pop, $today);
}
### Implementation
# How should we calculate the age of a person and how should "mirroring"
# be performed? A year is not a well defined time span due to leap
# days. Neither is a month. The best we can do is take the age as the
# number of days ignoring the time part. Using DateTime math in the
# 'floating' time zone and based on day deltas seems to be
# (double-)safe. At least this approach is consistent with the
# examples. For meaningful results the day of birth must not lie ahead.
sub mirror_date ($dob, $today) {
state $fmt = DateTime::Format::Strptime->new(
pattern => '%Y/%m/%d', time_zone => 'floating');
my $dob_dt = $fmt->parse_datetime($dob);
my $today_dt = $today ? $fmt->parse_datetime($today) :
DateTime->today(time_zone => 'floating');
my $age = $today_dt->delta_days($dob_dt);
($fmt->format_datetime($dob_dt - $age),
$fmt->format_datetime($today_dt + $age));
}
### Examples and tests
sub run_tests {
my $today = '2021/09/22';
is [mirror_date('2021/09/18', $today)],
[qw(2021/09/14 2021/09/26)], 'example 1';
is [mirror_date('1975/10/10', $today)],
[qw(1929/10/27 2067/09/05)], 'example 2';
is [mirror_date('1967/02/14', $today)],
[qw(1912/07/08 2076/04/30)], 'example 3';
done_testing;
exit;
}
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