func zig_zag(w, h) {

    var r = []
    var n = 0

    h.of { |e|
        w.of { |f|
            [e, f]
        }
    }.reduce('+').sort { |a, b|
           (a[0]+a[1] <=> b[0]+b[1]) ||
           (a[0]+a[1] -> is_even ? a[0]<=>b[0]
                                 : a[1]<=>b[1])
    }.each { |a|
       r[a[1]][a[0]] = n++
    }

    return r
}

zig_zag(5, 5).each { say .join('', {|i| "%4i" % i}) }