From f41734a94c092dd850bbac5efcbbcc95d48dc902 Mon Sep 17 00:00:00 2001
From: Jörg Sommrey <28217714+jo-37@users.noreply.github.com>
Date: Thu, 23 Jun 2022 23:44:42 +0200
Subject: Solution to task 2

---
 challenge-170/jo-37/perl/ch-2.pl | 70 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)
 create mode 100755 challenge-170/jo-37/perl/ch-2.pl

diff --git a/challenge-170/jo-37/perl/ch-2.pl b/challenge-170/jo-37/perl/ch-2.pl
new file mode 100755
index 0000000000..9c4e1b135f
--- /dev/null
+++ b/challenge-170/jo-37/perl/ch-2.pl
@@ -0,0 +1,70 @@
+#!/usr/bin/perl -s
+
+use v5.16;
+use warnings;
+use PDL;
+use experimental 'signatures';
+
+our ($examples, $verbose);
+
+run_tests() if $examples;	# does not return
+
+die <<EOS unless @ARGV;
+usage: $0 [-examples] [-verbose] [--] [A B]
+
+-examples
+    run some examples
+ 
+-verbose
+    Print intermediate piddles.
+
+A B
+    Two matrices given in any format accepted by the PDL constructor, e.g.
+    '[1 2] [3 4]' or '1,2;3,4'
+
+EOS
+
+
+### Input and Output
+
+say kronecker(pdl($ARGV[0]), pdl($ARGV[1]), $verbose);
+
+
+### Implementation
+
+# PDL::Matrix has 'kroneckerproduct' but its usage would not qualify as
+# 'implementing Kronecker product'.
+# Therefore taking a different approach:
+# - replicate every element of A in the shape of B
+# - replicate the whole matrix B in the shape of A.
+# - the element-wise product of both replicated matrices results in a
+#   "true" 4-dimensional Kronecker product.
+# - "flattening" two dimensions gives the usual matrix result.
+# The potential one-liner is split into four steps to be able to show
+# the intermediate piddles.
+
+sub kronecker ($a, $b, $verbose) {
+    say "a: $a" if $verbose;
+	my $ar = $a->dummy(0, $b->dim(0))->dummy(1, $b->dim(1));
+    say "ar: $ar" if $verbose;
+
+    say "b: $b" if $verbose;
+    my $br = $b->dummy(2, $a->dim(0))->dummy(3, $a->dim(1));
+    say "br: $br" if $verbose;
+
+    my $k = $ar * $br;
+    say "k: $k" if $verbose;
+
+    $k->clump(0, 2)->clump(1, 2);
+}
+
+
+### Examples and tests
+
+sub run_tests {
+
+    say kronecker(1 + sequence(long, 2, 2), 5 + sequence(long, 2, 2), 1); 
+    say kronecker(1 + sequence(long, 2, 3), 7 + sequence(long, 3, 5), 1);
+
+    exit;
+}
-- 
cgit