#! perl -slw
use strict;
package Q;
use threads;
use threads::shared;
use constant {
    NEXT_WRITE  => -2,
    N           => -1,
};

sub new {
#    warn "new: @_\n";
    my( $class, $Qsize ) = @_;
    $Qsize //= 3;
    my @Q :shared;
    $#Q = $Qsize;
    @Q[ NEXT_WRITE, N ] = ( 0, 0 ); ## nextWrite, N
#    warn sprintf "new: size %d\n\n", scalar @Q;
    return bless \@Q, $class;
}

sub nq {
#    warn "nq: @_\n";
    my $self = shift;
    lock @$self;
    for( @_ ) {
        cond_wait @$self until $self->[ N ] < ( @$self-2 );
        $self->[ $self->[ NEXT_WRITE ]++ ] = $_;
        ++$self->[ N ];
        $self->[ NEXT_WRITE ] %= ( @$self - 2 );
        cond_signal @$self;
    }
}

sub dq {
#    warn "dq: @_\n";
    my $self = shift;
    lock @$self;
    cond_wait @$self until $self->[ N ] > 0;
    my $p = $self->[ NEXT_WRITE ] - $self->[ N ]--;
    $p += @$self -2 if $p < 0;
    my $out =  $self->[ $p ];
    cond_signal @$self;
    return $out;
}

sub n {
    my $self = shift;
#    lock @$self;
    return $self->[ N ];
}

sub _state {
    no warnings;
    my $self = shift;
    lock @$self;
    return join '|', @{ $self };
}

return 1 if caller;