#!/bin/perl

use v5.10;
use strict;
use warnings;

use JSON::XS qw/decode_json/;
use feature qw/say/;

sub main {
    my $result = qx{ i3-msg -t get_workspaces }; # returns [{},{},{}]
    my $decoded = decode_json($result);
    my $current = 0;

    for my $workspace (@$decoded) {

        my $num = $workspace->{'num'};

        if ($workspace->{'focused'}) {
            $current = $num;
            next;
        }

        # Ignore workspaces below the currently focused one.
        if ($current == 0) { next; }

        # Found the gap!
        if ($current + 1 != $num) { last; }

        # Keep increasing the soon-to-be current workspace.
        $current = $num;

    }

    say $current + 1;
}

main();