#!/usr/bin/env perl

use strict;
use warnings;
use v5.22;

if (@ARGV !=1) {
    die "Usage: $0 [input-filename]";
}

my $input_filename = $ARGV[0];
open my $filehandle, '<', $input_filename or
    die "Could not open input file $input_filename: $!";

chomp( my @input = ( <$filehandle> ) );

# Advent of Code 2021 Day 03 Part 2 
my $len = length($input[0]) - 1; # number of bits - 1

my @ogr = @input;
foreach my $index (0 .. $len) { # first iterate through indices...
    my $count;
    foreach my $line (@ogr) { # ...and then through the numbers/lines
        if ( (split //, $line)[$index] ) { # if bit at position $index is 1
            $count++; # postive or zero count means 1
        } else {
            $count--; # negative count means 0
        }
    }
    # now see which is the most common bit at the index
    # if $count < 0, it's 0; otherwise it's 1
    my $most_freq_bit = ($count < 0) ? 0 : 1;
    my @temp_ogr = grep { (split //)[$index] == $most_freq_bit  } @ogr;
    @ogr = @temp_ogr; # replace real array with temporary array
    last if (scalar( @ogr ) == 1);
}

my @co2sr = @input;
foreach my $index (0 .. $len) { # first iterate through indices...
    my $count;
    foreach my $line (@co2sr) { # ...and then through the numbers/lines
        if ( (split //, $line)[$index] ) { # if bit at position $index is 1
            $count++; # postive or zero count means 1
        } else {
            $count--; # negative count means 0
        }
    }
    # now see which is the most common bit at the index
    # if $count < 0, it's 0; otherwise it's 1
    my $most_freq_bit = ($count < 0) ? 0 : 1;
    my @temp_co2sr = grep { (split //)[$index] != $most_freq_bit  } @co2sr;
    @co2sr = @temp_co2sr; # replace real array with temporary array
    last if (scalar( @co2sr ) == 1);
}

# From the perlfunc man page:
#        oct EXPR
#        oct Interprets EXPR as an octal string and returns the corresponding
#            value.  (If EXPR happens to start off with "0x", interprets it as a
#            hex string.  If EXPR starts off with "0b", it is interpreted as a
#            binary string.  Leading whitespace is ignored in all three cases.)
say oct("0b" . join("", @ogr)) * oct("0b" . join("", @co2sr));