balanced-brakets.pl

Author: Filip Sergot

best-of-rosettacode/balanced-brackets.pl

@@ -0,0 +1,97 @@
+=begin pod
+
+=head1 Problem
+
+Generate a string with N opening brackets (“[”) and N closing brackets (“]”), in some arbitrary order.
+Determine whether the generated string is balanced; that is, whether it consists entirely of pairs of opening/closing brackets (in that order), none of which mis-nest.
+
+=head1 More
+
+L<http://rosettacode.org/wiki/Balanced_brackets#Perl_6>
+
+=head1 What's interesting here?
+* idiomatic solutions
+* hyper operators
+* switch statement
+* roll
+* grammar
+
+=head2 Depth counter
+
+=end pod
+
+sub balanced($s) {
+    my $l = 0;
+    for $s.comb {
+        when "]" {
+            --$l;
+            return False if $l < 0;
+        }
+        when "[" {
+            ++$l;
+        }
+    }
+    return $l == 0;
+}
+ 
+my $n = prompt "Number of brackets";
+my $s = (<[ ]> xx $n).pick(*).join;
+say "$s {balanced($s) ?? "is" !! "is not"} well-balanced"
+
+=begin pod
+
+=head2 FP oriented
+
+=end pod
+
+sub balanced($s) {
+    .none < 0 and .[*-1] == 0
+        given [\+] '\\' «leg« $s.comb;
+}
+ 
+my $n = prompt "Number of bracket pairs: ";
+my $s = <[ ]>.roll($n*2).join;
+say "$s { balanced($s) ?? "is" !! "is not" } well-balanced"
+
+=begin pod
+
+=head2 String munging
+
+=end pod
+
+sub balanced($_ is copy) {
+    () while s:g/'[]'//;
+    $_ eq '';
+}
+ 
+my $n = prompt "Number of bracket pairs: ";
+my $s = <[ ]>.roll($n*2).join;
+say "$s is", ' not' xx not balanced($s)), " well-balanced";
+
+=begin pod
+
+=head2 Prasing with a grammar
+
+=end pod
+
+grammar BalBrack {
+    token TOP { ^ <balanced>* $ };
+    token balanced { '[]' | '[' ~ ']' <balanced> }
+}
+ 
+my $n = prompt "Number of bracket pairs: ";
+my $s = <[ ]>.roll($n*2).join;
+say "$s { BalBrack.parse($s) ?? "is" !! "is not" } well-balanced";
+
+=begin pod
+
+=head1 Features used
+
+C<roll> - L<http://perlcabal.org/syn/S32/Containers.html#roll>
+C<given> - L<http://perlcabal.org/syn/S04.html#Switch_statements>
+C<prompt> - L<http://perlcabal.org/syn/S32/IO.html#prompt>
+C<grammar> - L<http://perlcabal.org/syn/S05.html#Grammars>
+
+=end pod
+
+# vim: expandtab shiftwidth=2 ft=perl6: