local Observable = require "rx.observable"

--- Returns an Observable that produces the nth element produced by the source Observable.
-- @arg {number} index - The index of the item, with an index of 1 representing the first.
-- @returns {Observable}
function Observable:elementAt(index)
    if not index or type(index) ~= "number" then
        error("Expected a number")
    end

    return Observable.create(function(observer)
        local subscription
        local i = 1

        local function onNext(...)
            if i == index then
                observer:onNext(...)
                observer:onCompleted()
                if subscription then
                    subscription:unsubscribe()
                end
            else
                i = i + 1
            end
        end

        local function onError(e)
            return observer:onError(e)
        end

        local function onCompleted()
            return observer:onCompleted()
        end

        subscription = self:subscribe(onNext, onError, onCompleted)
        return subscription
    end)
end