Add solution for challenge-147 task 2

1 files changed, 48 insertions, 0 deletions

diff --git a/challenge-147/alexander-pankoff/perl/ch-2.pl b/challenge-147/alexander-pankoff/perl/ch-2.pl
new file mode 100644
index 0000000000..1d9b7bdc7f
--- /dev/null
+++ b/challenge-147/alexander-pankoff/perl/ch-2.pl
@@ -0,0 +1,48 @@
+#!/usr/bin/env perl
+use strict;
+use warnings;
+use feature qw'say signatures';
+no warnings qw'experimental::signatures';
+
+use List::Util qw(first);
+
+run() unless caller();
+
+sub run() {
+    say join( ', ', @{ find_first_pair() } );
+}
+
+sub find_first_pair() {
+    my $found;
+
+    my @pentagon_numbers;
+
+    my $i = 2;
+    for ( my $i = 2 ; !$found ; $i++ ) {
+        my $p     = pentagon_number($i);
+        my @pairs = map { [ $_, $p ] } @pentagon_numbers;
+        push @pentagon_numbers, $p;
+        my $found = first { check(@$_) } @pairs;
+        if ($found) {
+            return $found;
+        }
+    }
+
+}
+
+# Pentagon numbers can be defined as P(n) = n(3n - 1)/2.
+sub pentagon_number($n) {
+    return $n * ( 3 * $n - 1 ) / 2;
+}
+
+sub check ( $a, $b ) {
+    return is_pentagon_number( $a + $b )
+      && is_pentagon_number( abs( $a - $b ) );
+
+}
+
+# https://en.wikipedia.org/wiki/Pentagonal_number#Tests_for_pentagonal_numbers
+sub is_pentagon_number($x) {
+    my $n = ( sqrt( 24 * $x + 1 ) + 1 ) / 6;
+    return $n == int($n);
+}