remember previous evaluations

which avoids a lot of calls.  previously:

A( 3, 10 ) = 8189  ...after 44698325 calls.

becomes:

A( 3, 10 ) = 8189  ...after 24584 calls.

1 files changed, 6 insertions, 6 deletions

diff --git a/challenge-017/michael-hamlin/perl5/t1-ackermann-simple.pl b/challenge-017/michael-hamlin/perl5/t1-ackermann-simple.pl
index 02e03a644f..406fe6073e 100644
--- a/challenge-017/michael-hamlin/perl5/t1-ackermann-simple.pl
+++ b/challenge-017/michael-hamlin/perl5/t1-ackermann-simple.pl
@@ -15,13 +15,13 @@ no warnings 'recursion';
 my $count = 0;
 
 sub _ack( $m, $n ) {
+    state @known;
     $count++;
-    if ($m > 0) {
-        return _ack( $m - 1, 1 ) unless $n;
-        return _ack( $m - 1, _ack( $m, $n - 1));
-    } else {
-        return $n + 1;
-    }
+    return $known[$m][$n] ||= do {
+        !$m ? $n + 1 :
+        !$n ? _ack($m - 1, 1) :
+        _ack($m - 1, _ack($m, $n - 1));
+    };
 }
 sub ack ( $m, $n ) {
     die "the function is not defined for negative parameters" if $m < 0 || $n < 0;