digraphs: parse plan9 keyboard combinations

Import <https://9fans.github.io/plan9port/man/man7/keyboard.html>
definitions for personal comparison and evaluation.

diff --git a/tools/mkdigraphs-plan9 b/tools/mkdigraphs-plan9
new file mode 100755 (executable)
index 0000000..54b8b3c
--- /dev/null
+++ b/tools/mkdigraphs-plan9
@@ -0,0 +1,78 @@
+#!/usr/bin/env perl
+use 5.014;
+use warnings;
+use utf8;
+use open IO => ':utf8', ':std';
+use Data::Dump 'pp';
+
+our $VERSION = '1.00';
+
+# translation table for deprecated code points
+my %replace = (
+       0xF015 => '⎇',  # alt
+       0xF016 => '⇧',  # shift
+       0xF017 => '⎈',  # control
+);
+
+# expect input data source at command line
+@ARGV or die "Specify input source file or - for STDIN\n";
+
+# convert each character line to perl code
+# (assume no backslashes or curlies, so we can just q{} w/o escaping)
+say "# automatically generated by $0";
+say 'use utf8;';
+say '+{';
+while ($_ = readline) {
+       my ($chrhex, $mnems, $sample, $name) = m{\A([0-9A-F]{4})  (.{11}) (.)\t(.*)}i
+               or warn("syntax error on line $.: $_"), next;
+       my $chrnum = hex $chrhex;
+       my $chr = chr $chrnum;
+       $chr eq $sample
+               or warn("character mismatch on line $.: $_"), next;
+       $chr = $replace{$chrnum} or next if defined $replace{$chrnum};
+       my $chrstr = pp($replace{$chrnum} // $chr);
+       for my $mnem (split / /, $mnems) {
+#              next if length $mnem != 2;
+               say "q{$mnem} => $chrstr, # $name";
+       }
+}
+say '}';
+
+__END__
+
+=head1 NAME
+
+mkdigraphs-plan9 - Output digraph data from Plan9 keyboard data
+
+=head1 SYNOPSIS
+
+Extract digraphs from text specifications as a perl hash:
+
+    mkdigraphs-plan9 keyboard >digraphs-plan9.inc.pl
+
+Input can be the literal RFC (or similar) document:
+
+    curl https://9fans.github.io/usr/local/plan9/lib/keyboard | mkdigraphs-plan9 -
+
+Test by printing the character for DO (should be a dollar sign):
+
+    perl -e'$di = do "digraphs-plan9.inc.pl"; print chr $di->{DO}'
+
+=head1 DESCRIPTION
+
+Parses the official RFC-1345 document, searching the
+'character mnemonic table' for all digraph definitions.
+If successful, Perl code is output resulting in a hash
+with Unicode code points keyed by digraph.
+Obsolete values (references to private use area)
+are converted to modern alternatives.
+Any errors and warnings are given at STDERR.
+
+=head1 AUTHOR
+
+Mischa POSLAWSKY <perl@shiar.org>
+
+=head1 LICENSE
+
+Licensed under the GNU Affero General Public License version 3.
+