package RJT::LocalHash {
    use parent 'Tie::Hash';
    use List::MoreUtils qw/uniq/;
    use Carp;

    sub TIEHASH {
        my ($class, $orig) = @_;
        croak 'Expected HASH ref, not `'.ref($orig)."'"
            unless 'HASH' eq ref $orig;
        bless { orig => $orig, del => { }, new => { } }, $class;
    }
    sub STORE  { delete $_[0]{del}{$_[1]};  $_[0]{new}{$_[1]} = $_[2]  }
    sub EXISTS { not exists $_[0]{del}{$_[1]} and
                 (exists $_[0]{new}{$_[1]} or exists $_[0]{orig}{$_[1]}) }
    sub FETCH  { return if exists $_[0]{del}{$_[1]};
                 exists $_[0]{new}{$_[1]} ? $_[0]{new}{$_[1]}
                                          : $_[0]{orig}{$_[1]} }
    sub FIRSTKEY {
        # Initialize the iterator as union of both hash key sets
        # minus anything that's been locally deleted
        my @each = grep { not exists $_[0]{del}{$_} } 
                   uniq keys $_[0]{orig}, keys $_[0]{new};
        $_[0]{each} = \@each;
        shift @each;
    }
    sub NEXTKEY { shift $_[0]{each} }

    sub DELETE {
        $_[0]{del}{$_[1]} = 1;
        $_[0]{new}{$_[1]} // $_[0]{orig}{$_[1]};
    }
}