http://www.perlmonks.org?node_id=79569

in reply to Re: Finding gaps in date ranges
in thread Finding gaps in date ranges

boohoo, the above article is mine, but I posted it anonymously by mistake. oh higher powers, if it please you, transfer the XPs of the article to this humble scribe.

I'm a big fan of peanos axioms. So I started with the idea: if it weren't dates we were dealing with, but natural numbers, then I would just sort the numbers, and compute the gaps with one simple loop. I decided to convert you date-format to something that can be sorted numerically (YYYYMM, with month in two digits, like "02"), and I made up a function "nextmonth" for incrementing the date.

```       sub nextmonth {
my(\$year, \$month);
\$year = substr(\$_[0], 0, 4);
\$month = substr(\$_[0], 4, 2);

\$month++;

while (\$month > 12) {
\$year++;
\$month-=12;
}

return \$year . sprintf("%02d", \$month);

}

sub find_gaps {
my \$dates = shift;
my(@missing, @keys, \$last);

# convert your date-format to mine
@keys = map {  s/-(.)\$/0\$1/; s/-//; \$_ } keys %\$dates;

# my format can be sorted numerically
@keys = sort @keys;

# in the loop we need two variables:
# \$last for the date before the current one,
# and \$date for the current date

\$last = shift(@keys);
foreach my \$date ( @keys ) {
while (1) {
\$last = nextmonth(\$last);
last if \$last eq \$date;
push(@missing, \$last);
}
\$last = \$date;
}

# convert my date-format back to yours
@missing =
map {  s/0(.)\$/\$1/; s/^(....)/\$1-/; \$_ }  @missing;
return \@missing;
}
```--