Perl solution for week 108, part 2

1 files changed, 68 insertions, 0 deletions

diff --git a/challenge-108/abigail/perl/ch-2.pl b/challenge-108/abigail/perl/ch-2.pl
new file mode 100644
index 0000000000..d89e32391e
--- /dev/null
+++ b/challenge-108/abigail/perl/ch-2.pl
@@ -0,0 +1,68 @@
+#!/opt/perl/bin/perl
+
+use 5.032;
+
+use strict;
+use warnings;
+no  warnings 'syntax';
+
+use experimental 'signatures';
+use experimental 'lexical_subs';
+
+#
+# See ../README.md
+#
+
+#
+# Run as: perl ch-2.pl [plain|fetch|compute]
+#
+#
+# Three solutions, depending on the command line argument:
+#   1) We just print the first 10 Bell numbers. The simplest solution.
+#   2) We fetch the first 10 Bell numbers from the OEIS.
+#   3) We calculate the Bell numbers from the Bell triangle.
+#
+
+my $COUNT        = 10;
+
+my $TYPE_PLAIN   =  0;
+my $TYPE_FETCH   =  1;
+my $TYPE_COMPUTE =  2;
+
+my $type = $TYPE_PLAIN;   # Default
+   $type = $TYPE_FETCH   if @ARGV && $ARGV [0] eq "fetch";
+   $type = $TYPE_COMPUTE if @ARGV && $ARGV [0] eq "compute";
+
+
+if ($type == $TYPE_PLAIN) {
+    say "1, 1, 2, 5, 15, 52, 203, 877, 4140, 21147";
+}
+elsif ($type == $TYPE_FETCH) {
+    use OEIS;
+    $, = ", ";
+    say oeis (110 => 10)
+}
+elsif ($type == $TYPE_COMPUTE) {
+    my $COUNT = 10;
+
+    #
+    # Create Bell triangle (See https://en.wikipedia.org/wiki/Bell_triangle)
+    # The Bell numbers (starting from the second one), are on the
+    # right diagonal.
+    #
+    # The Bell triangle is defined as follows:
+    #    Bell (0, 0) = 1
+    #    Bell (n, 0) = Bell (n - 1, n - 1), k > 0
+    #    Bell (n, m) = Bell (n, m - 1) + Bell (n - 1, m - 1), n > 0, m > 0
+    #
+    my @bell = [1];
+    for (my $x = 1; $x < $COUNT - 1; $x ++) {
+        $bell [$x] [0] = $bell [$x - 1] [-1];
+        for (my $y = 1; $y <= $x; $y ++) {
+            $bell [$x] [$y] = $bell [$x] [$y - 1] + $bell [$x - 1] [$y - 1]
+        }
+    }
+
+    $, = ", ";
+    say 1, map {$$_ [-1]} @bell;
+}