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use strict;
use warnings;
use v5.20;
# get input array from command line
my @N = @ARGV;
die "usage: perl $0 <space separate numbers (at least 3)>" unless @N > 2;
# check that all numbers are positive, and find max value
my $max = -1;
for my $n (@N) {
die "$n is not a proper positive number" unless $n && $n =~ /^\d+$/;
$max = $n if $n > $max;
}
# we need to take care of the amount of decimal places needed
# form $max, so the histogram can look ok
my $places = length $max;
# so the format to print numbers should be:
my $dfmt = "\%${places}d";
my $acc_water = 0;
for my $r (reverse 1 .. $max) {
# construct horizontal row, starting from $max
my $line = join('', map { $_ >= $r ? '#' : ' ' } @N);
# we need to identify in this particular line how many units
# of rain water we can trap. We will replace with a "W" every
# one of those units.
#
# We search for the pattern #< n spaces>#, and as this will
# allow to trap n units, we will replace the spaces with the
# letter "W"
# Note that after replacing one group, we should start over the
# same line. We cannot continue looking because the last caracter
# matched ('#') may be necesary for the next match. So we cannot
# use a /g to repeat the search, we need to start over the
# matching, but with the spaces changed to "W" so we don't get
# the same match but the following one, if any.
# For the regex, we use substitution (s///) and we calculate
# the replacement with "W" x length(spaces captured), and leave
# the '#'s in place. /e means we are evaluating the second
# argument, instead of taking it literally
#
while ($line =~ s/#(\s+)#/'#'. 'W' x length($1) . '#'/e) { }
# units this row can trap is the amount of "W"s we have on it
# feel free to change the transliteration operator to tr/W/W/
# to keep an indication of the allocated water units
$acc_water += $line =~ tr/W/ /;
# we will add separating spaces between symbols, note we do that after any
# calculation
$line = ' ' . join(' ', split('', $line));
say sprintf($dfmt, $r) . $line;
}
# closing line
say " " x ($places - 1) . '_' . ' _' x @N;
# now print the base rows of the histogram
for my $i (0 .. $places - 1) {
my $line = join(' ', map { substr(sprintf($dfmt, $_), $i, 1) } @N);
say " " x ($places + 1) . $line;
}
say "Trapped water: $acc_water";
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