Solution to task 2

1 files changed, 58 insertions, 0 deletions

diff --git a/challenge-149/jo-37/perl/ch-2.pl b/challenge-149/jo-37/perl/ch-2.pl
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+++ b/challenge-149/jo-37/perl/ch-2.pl
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+#!/usr/bin/perl -s
+
+use v5.16;
+use Test2::V0;
+use List::AllUtils qw(frequency max pairvalues);
+use Math::Prime::Util qw(fromdigits todigits todigitstring);
+use experimental 'signatures';
+
+our $examples;
+
+run_tests() if $examples;	# does not return
+
+die <<EOS unless @ARGV;
+usage: $0 [-examples] [base]
+
+-examples
+    run the examples from the challenge
+
+base
+    Find the largest perfect square having no repeated digits in the
+    given base. 
+
+EOS
+
+
+### Input and Output
+
+# Print the largest square in the requested base.
+say todigitstring(max_square($ARGV[0]), $ARGV[0]);
+
+
+### Implementation
+
+# The maximum number having no repeated digits in a given base is the
+# number built from all digits in descending order.  Trying all perfect
+# squares down from this maximum.
+#
+# Returns an integer, not the requested string.
+sub max_square ($base) {
+    for (my $r = int sqrt fromdigits [reverse 0 .. $base - 1], $base;; $r--) {
+        my $k = $r ** 2;
+        return $k if 1 == max pairvalues frequency todigits $k, $base;
+    }
+}
+
+
+
+### Examples and tests
+
+sub run_tests {
+    is todigitstring(max_square(2), 2), '1', 'example 1';
+    is todigitstring(max_square(4), 4), '3201', 'example 2';
+    is max_square(10), 9814072356, 'example 3';
+    like todigitstring(max_square(12), 12), qr/^B8750A649321$/i, 'example 4';
+
+    done_testing;
+    exit;
+}