From 16cd2ef35476783142ed6eed2a8f90c755899c34 Mon Sep 17 00:00:00 2001
From: Jörg Sommrey <28217714+jo-37@users.noreply.github.com>
Date: Tue, 6 Apr 2021 11:26:29 +0200
Subject: Solution to task 1

---
 challenge-107/jo-37/perl/ch-1.pl | 96 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 96 insertions(+)
 create mode 100755 challenge-107/jo-37/perl/ch-1.pl

diff --git a/challenge-107/jo-37/perl/ch-1.pl b/challenge-107/jo-37/perl/ch-1.pl
new file mode 100755
index 0000000000..8ef8f2b098
--- /dev/null
+++ b/challenge-107/jo-37/perl/ch-1.pl
@@ -0,0 +1,96 @@
+#!/usr/bin/perl -s
+
+use v5.16;
+use Test2::V0;
+use Math::Prime::Util qw(fromdigits todigitstring forcomp);
+use List::Util 'all';
+use List::MoreUtils 'frequency';
+use experimental 'signatures';
+
+our ($tests, $examples, $verbose);
+
+run_tests() if $tests || $examples;	# does not return
+
+die <<EOS unless @ARGV;
+usage: $0 [-examples] [-tests] [-verbose] [base...]
+
+-examples
+    run the examples from the challenge
+ 
+-tests
+    run some tests
+
+-verbose
+    include the decimal representation in the output
+
+base...
+    find self-descriptive numbers for given base(s)
+    Call "$0 4 5" for the task's solution.
+
+EOS
+
+
+### Input and Output
+
+for my $base (@ARGV) {
+    say todigitstring($_, $base), " ($base) = $_ (10)" x !!$verbose
+        for @{self_descriptive($base)};
+}
+
+
+### Implementation
+
+# A self-descriptive number has (besides its self-descriptiveness) two
+# basic properties: In the given base the sum of its digits equals the
+# base and the least significant digit is zero.  These properties may be
+# used to select candidates.  There are utility subroutines that iterate
+# over compositions, i.e. all the (ordered) summands for a given sum.
+# However, the smallest value for a summand in a composition is one.  If
+# we add one to our digits, they are in the range 1 .. base and give a
+# sum of 2*base.  Furthermore, the last summand becomes one (instead of
+# zero) and so the (restricted) compositions consist of base - 1
+# remaining summands with a sum of 2*base - 1.  Such a composition
+# iteration is provided by Math::Prime::Util::forcomp.
+# A candidate is self-descriptive, if it resembles its digit
+# frequencies, which can be easily calculated using
+# List::MoreUtils::frequency.
+# Finally, assemble the digits into a number.  This extra step assures
+# the correct processing of bases larger than 10.
+#
+sub self_descriptive ($base) {
+    my @sd;
+    forcomp {
+        # Get the true digits in the range 0 .. $base - 1.
+        my @digit = map $_ - 1, @_, 1;
+        my %freq = frequency @digit;
+        push @sd, fromdigits(\@digit, $base) if all {
+            $digit[$_] ?
+                $freq{$_} && $digit[$_] == $freq{$_} :
+                !$freq{$_}
+        } 0 .. $#digit;
+    } 2 * $base - 1, {n => $base - 1, amax => $base};
+
+    \@sd;
+}
+
+
+### Examples and tests
+
+sub run_tests {
+    SKIP: {
+        skip "examples" unless $examples;
+
+        is self_descriptive(4), [100, 136], 'first two';
+        is self_descriptive(5), [1425], 'third';
+    }
+
+    SKIP: {
+        skip "tests" unless $tests;
+        is self_descriptive(6), [], 'base 6';
+        is self_descriptive(7), [389305], 'base 7';
+        is self_descriptive(8), [8946176], 'base 8';
+	}
+
+    done_testing;
+    exit;
+}
-- 
cgit