add perl solution for wk-099 ch-2

reading through @jacoby 's blog post helped me a lot:

https://jacoby.github.io/2021/02/11/london-patterns-perl-weekly-challenge-99.html

1 files changed, 63 insertions, 0 deletions

diff --git a/challenge-099/alexander-pankoff/perl/ch-2.pl b/challenge-099/alexander-pankoff/perl/ch-2.pl
new file mode 100755
index 0000000000..aeeea3c9ef
--- /dev/null
+++ b/challenge-099/alexander-pankoff/perl/ch-2.pl
@@ -0,0 +1,63 @@
+#!/usr/bin/env perl
+use v5.20;
+use utf8;
+use strict;
+use warnings;
+use feature qw(say signatures);
+no warnings 'experimental::signatures';
+
+{
+    if ( $ENV{TESTING} ) {
+        test_count_subsequences();
+    }
+    else {
+        my ( $S, $T ) = @ARGV;
+        die "usage: $0 STRING STRING\n" unless @ARGV == 2;
+
+        say count_subsequences( $S, $T );
+    }
+}
+
+sub test_count_subsequences() {
+
+    my @test_cases = ( [ "littleit", "lit", 5 ], [ "london", "lon", 3 ], );
+
+    require Test::More;
+    Test::More->import( tests => scalar @test_cases );
+
+    for my $test (@test_cases) {
+        my ( $string, $target, $count ) = @{$test};
+        ok(
+            count_subsequences( $string, $target ) == $count,
+            "$string contains $count subsequences matching $target"
+        );
+    }
+}
+use List::Util qw(first);
+
+sub count_subsequences ( $string, $target, $string_position = 0,
+    $target_position = 0 )
+{
+    return 0 if ( $string_position >= length($string) );
+
+    my $count = count_subsequences( $string, $target, $string_position + 1,
+        $target_position );
+
+    my $string_char = substr( $string, $string_position, 1 );
+    my $target_char = substr( $target, $target_position, 1 );
+    if ( $string_char eq $target_char ) {
+        if ( $target_position == length($target) - 1 ) {
+            $count += 1;
+        }
+        else {
+            $count += count_subsequences(
+                $string, $target,
+                $string_position + 1,
+                $target_position + 1
+            );
+        }
+    }
+
+    return $count;
+}
+