Quickselect algorithm

func quickselect(a, k) {
    var pivot = a.pick
    var left  = a.grep{|i| i < pivot}
    var right = a.grep{|i| i > pivot}

    given(left.len) { |l|
        when(k)     { pivot }
        case(k < l) { __FUNC__(left, k) }
        default     { __FUNC__(right, k - l - 1) }
    }
}

var v = [9, 8, 7, 6, 5, 0, 1, 2, 3, 4]
say v.range.map{|i| quickselect(v, i)}

Output:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]