Challenge manwar#1 answer

Author: kianmeng

challenge-015/kian-meng-ang/perl5/ch-1.pl

@@ -0,0 +1,60 @@
+#!/usr/bin/env perl
+# vi:et:sw=4 ts=4 ft=perl
+
+use strict;
+use warnings;
+use utf8;
+use feature qw(say);
+
+# Interestigly, both CPAN modules can be used interchangeably.
+# use Math::Prime::Util qw(nth_prime next_prime prev_prime);
+use ntheory qw(nth_prime next_prime prev_prime);
+
+# See https://en.wikipedia.org/wiki/Strong_prime
+# See https://oeis.org/A051634
+# See https://oeis.org/A051635
+sub prime_type {
+    my ($prime) = @_;
+
+    my $pprime = prev_prime($prime);
+    my $nprime = next_prime($prime);
+
+    return 'strong'  if ($prime > ($pprime + $nprime) / 2);
+    return 'weak'    if ($prime < ($pprime + $nprime) / 2);
+}
+
+sub nth_strong_primes {
+    my ($nth) = @_;
+
+    my ($pos, @primes) = (2, ());
+    while (scalar @primes < $nth) {
+        my $prime = nth_prime($pos);
+        push @primes, $prime if (prime_type($prime) eq 'strong');
+        $pos++;
+    }
+
+    return \@primes;
+}
+
+sub nth_weak_primes {
+    my ($nth) = @_;
+
+    my ($pos, @primes) = (2, ());
+    while (scalar @primes < $nth) {
+        my $prime = nth_prime($pos);
+        push @primes, $prime if (prime_type($prime) eq 'weak');
+        $pos++;
+    }
+
+    return \@primes;
+}
+
+say 'Strong primes: ' , join ', ', @{nth_strong_primes(10)};
+say 'Weak primes: ', join ', ', @{nth_weak_primes(10)};
+
+1;
+
+__END__
+$ perl ch-1.pl
+Strong primes: 11, 17, 29, 37, 41, 59, 67, 71, 79, 97
+Weak primes: 3, 7, 13, 19, 23, 31, 43, 47, 61, 73