Wanted: script to list uninstalled optdepends and packages that need t

Question

I had a thought today that a script to list package that are optdepends but not installed could be quite useful...  I have not written one, but thought it would be good for pacman-contrib if someone wanted to contribute.
Example output:
tk - optional for:
   ruby - for Ruby/TK
   python - for IDLE, pynche and modulator
fakeroot - optional for:
   pacman - for makepkg usage as normal user
So get script writing!  Preferably in bash or perl (which the current pacman-contrib scripts use).

Answer 1

I cheated and used my perl ALPM module.  I think the logic is sound, though, and you can replace the module calls with backticking pacman (preferably functions wrapping them).  I planned to but ran out of time.
#!/usr/bin/perl
use warnings;
use strict;
use ALPM ( dbpath => '/var/lib/pacman/' );
my %reverse_depends;
my $db = ALPM->register_db;
OPT_SEARCH:
for my $pkg ( @{$db->get_pkg_cache} ) {
my $pkg_optdeps = $pkg->get_optdepends;
next OPT_SEARCH unless @$pkg_optdeps;
my $depender = $pkg->get_name;
my %reason_for = map {
/\A([^:]*):\s*(.*)/ ? ($1 => $2) : ($_ => '')
} @$pkg_optdeps;
DEP_LOOP:
for my $dependee ( keys %reason_for ) {
next DEP_LOOP if $db->get_pkg($dependee); #ignore installed
$reverse_depends{$dependee}->{$depender} = $reason_for{$dependee};
for my $opt_dep ( sort keys %reverse_depends ) {
print "$opt_dep\n";
my $reason_of = $reverse_depends{$opt_dep};
for my $depender ( sort keys %$reason_of ) {
my $reason = $reason_of->{$depender};
print " $depender",
( $reason ? ": $reason\n" : "\n" );