improve and explain wk-082 ch-2

1 files changed, 69 insertions, 34 deletions

diff --git a/challenge-082/alexander-pankoff/perl/ch-2.pl b/challenge-082/alexander-pankoff/perl/ch-2.pl
index a5c586d085..c8587624a5 100755
--- a/challenge-082/alexander-pankoff/perl/ch-2.pl
+++ b/challenge-082/alexander-pankoff/perl/ch-2.pl
@@ -9,51 +9,86 @@ no warnings 'experimental::signatures';
 
 use Pod::Usage;
 
-use List::Util qw(min all any);
+use List::Util qw(min all any pairs);
 use Scalar::Util qw(looks_like_number);
 
-=pod
+pod2usage(
+    -message => "$0: Need exactly three arguments",
+    -exitval => 1,
+) if @ARGV != 3;
 
-=head1 SYNOPSIS
+my ( $A, $B, $C ) = @ARGV;
 
-Given three strings <A>, <B> and <C> this script will return whether <C> can be
-created by interleaving <A> and <B>
+say is_creatable_by_interleaving( $C, $A, $B ) ? 1 : 0;
 
-=head1 USAGE
+sub is_creatable_by_interleaving ( $target, $a, $b ) {
 
-ch-2.pl <A> <B> <C>
+    # first check whether the total lenght of $a and $b match with the target
+    # length
+    return 0 if length($target) != length($a) + length($b);
 
-=cut
+    # we now check wether any of $a or $b starts with the same char as $target
+    # if so, we recurse with the rest of $target and the matching item to
+    # check the remaining input.
+    # otherwise we can't find a way to interleave $a and $b to make $target
+    #
+    # to prevent checking the length condition above in every recursive case we
+    # define a helper without that check. since we consume the input charwise
+    # and pairwise, either from $target and $a or from $target and $b that
+    # condition won't change
+    my $go;
+    $go = sub ( $target, $a, $b ) {
+        # base case. we consumed all inputs - $target is $a and $b interleaved
+        # since we already made sure that the total lengths match up we only
+        # need to check wether $target became empty here.
+        return 1 if !length($target);
 
-pod2usage(
-    -message  => "$0: Need exactly three arguments",
-    -exitval  => 1,
-    -verbose  => 99,
-    -sections => "USAGE|SYNOPSIS",
-) if @ARGV != 3;
+        my $head = substr( $target, 0, 1 );
+        my $rest = substr( $target, 1 );
 
-my ( $A, $B, $C ) = @ARGV;
-say is_creatable_by_interleaving( $C, $A, $B );
+        # the order of $a and $b in the recursice call doesn't matter
+        # so we can just run the same routine on (a,b) and (b,a) instead of
+        # using two routines with the arguments flipped
+        return any(
+            sub {
+                starts_with( $_->[0], $head )
+                  && $go->( $rest, substr( $_->[0], 1 ), $_->[1] );
+            },
+            pairs( $a, $b, $b, $a )
+        );
 
-sub is_creatable_by_interleaving ( $target, $a, $b ) {
-    return 0 if length($target) != length($a) + length($b);
-    return 1 if !length($target);
-
-    my $head = substr( $target, 0, 1 );
-    my $rest = substr( $target, 1 );
-
-    return (
-          starts_with( $head, $a )
-        ? is_creatable_by_interleaving( $rest, substr( $a, 1 ), $b )
-        : 0
-      )
-      || (
-          starts_with( $head, $b )
-        ? is_creatable_by_interleaving( $rest, $a, substr( $b, 1 ) )
-        : 0
-      );
+    };
+
+    $go->( $target, $a, $b );
 }
 
-sub starts_with ( $char, $str ) {
+sub starts_with ( $str, $char ) {
     return $str =~ m/^$char/;
 }
+
+=pod
+
+=head1 NAME
+
+wk-082 ch-2 - Interleave String
+
+=head1 SYNOPSIS
+
+Given three strings <A>, <B> and <C> this script prints whether <C> can be
+created by interleaving <A> and <B>
+
+ch-2.pl <A> <B> <C>
+
+=head1 ARGUMENTS
+
+=over 8
+
+=item B<A> The first input string
+
+=item B<B> The first input string
+
+=item B<C> The target string
+
+=back
+
+=cut