#!/usr/bin/perl

use strict;
use warnings;

use Time::Local qw(timelocal_nocheck);

# calculate Age, days from last birthday and days to next birthday for
# a given birth date.
# birthdate is a string of the format 'YYYY-MM-DD'
#  - good for accepting dates from databases
# If the birthdate is today, then days from = days to = 0
# returns age, days from, days to

# NOTE: this script does no date validation
# NOTE: this script uses the current year to calculate age.
# NOTE: if you modify this script to use anything other than current year,
#       there may be leap year implications

sub birth_date_age_today {
    my $birthdate = shift || return 0;
    $birthdate =~ m/^(\d\d\d\d)-(\d\d)-(\d\d)$/ or return 0;

    my ($byyyy,$bmm,$bdd) = ($1,$2,$3);
        --$bmm;
    my ($yyyy) = (localtime)[5];
        $yyyy += 1900;

    my $cur_day_of_year = (localtime)[7];
        # nocheck or feb 29 birthdays will have problems
    my $birth_day_of_year = (localtime timelocal_nocheck(0, 0, 0, $bdd, $bmm, $yyyy - 1900))[7];

    # calculate age
    my $age = $yyyy - $byyyy;
    my $daysto;
    my $daysfrom;
    if ($cur_day_of_year < $birth_day_of_year) {
        # haven't hit birthday yet this year
        $age--;
        $daysto = $birth_day_of_year - $cur_day_of_year;
        # last year's birthday
        $yyyy--;
        $birth_day_of_year = (localtime timelocal_nocheck(0, 0, 0, $bdd, $bmm, $yyyy - 1900))[7];
        # correct for days (2100 isn't a leap year)
        $daysfrom = (($yyyy % 4 or $yyyy == 2100) ? 365 : 366) - $birth_day_of_year + $cur_day_of_year;
    }
    elsif ($cur_day_of_year > $birth_day_of_year) {
        # passed birthday this year
        $daysfrom = $cur_day_of_year - $birth_day_of_year;
        # next year's birthday
        # Note: we get the number of days to the birthday in the next year ($yyyy - 1899)
        # but we use the current year for checking leap year because we need to know how
        # many days are left in this year
        $birth_day_of_year = (localtime timelocal_nocheck(0, 0, 0, $bdd, $bmm, $yyyy - 1899))[7];
                # correct for leap days (2100 isn't a leap year)
        $daysto = (($yyyy % 4 or $yyyy == 2100) ? 365 : 366) - $cur_day_of_year + $birth_day_of_year;
    }
    else { $daysfrom = $daysto = 0 }

    return $age, $daysfrom, $daysto;
}

# a simple test script
# run through every date (birthdate) of a given year
# print the results (relative to current date)
my %days = (
	1 => 31,
	2 => 28,
	3 => 31,
	4 => 30,
	5 => 31,
	6 => 30,
	7 => 31,
	8 => 31,
	9 => 30,
	10 => 31,
	11 => 30,
	12 => 31,
	);

my $yyyy = 1965;
# This is not totally correct!  Just simplified for this test
$days{2} = ($yyyy % 4 or $yyyy == 1900 or $yyyy == 2100) ? 28 : 29;

for (my $mm = 1; $mm <= 12; ++$mm) {
	for (my $dd = 1; $dd <= $days{$mm}; ++$dd) {
		my $testdate = sprintf("$yyyy-%02d-%02d", $mm, $dd);
		my ($age, $daysfrom, $daysto) = birth_date_age_today($testdate);
		print "birthdate $testdate  age $age  from last $daysfrom  days to $daysto\n";
	}
}