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#!/usr/bin/env perl
use strict;
use warnings;
use feature qw'say signatures';
no warnings qw'experimental::signatures';
use List::Util qw'first';
use constant DEBUG => $ENV{DEBUG} // 0;
run() unless caller();
sub run() {
my $stealthy = stealthy( $ARGV[0] );
say $stealthy ? 1 : 0;
explain($stealthy) if DEBUG;
}
sub stealthy($n) {
my @divisors = find_divisors_pairs($n);
my @pairs = pairs(@divisors);
my $stealthy = first {
my ( $a, $b, $c, $d ) = flatten($_);
$a * $b == $c * $d && $a + $b == $c + $d + 1;
}
@pairs;
return $stealthy;
}
sub explain($stealthy) {
say "Not stealthy" && return if !$stealthy;
my ( $a, $b, $c, $d ) = flatten($stealthy);
say
"Since $a (a) * $b (b) = $c (c) * $d (d) and $a (a) + $b (b) = $c (c) + $d (d) + 1";
}
sub pairs(@xs) {
my @out;
for my $i ( 0 .. $#xs - 1 ) {
push @out, map { [ $xs[$i], $xs[$_] ] } ( $i + 1 .. $#xs );
}
return @out;
}
sub flatten($xs) {
map { @$_ } @$xs;
}
sub find_divisors_pairs($x) {
my @out;
my $max = sqrt($x);
for my $i ( 1 .. $max ) {
if ( $x % $i == 0 ) {
push @out, [ $i, $x / $i ];
}
}
return @out;
}
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