local Observable = require "rx.observable"
local Subscription = require "rx.subscription"
local util = require "rx.util"

--- Returns a new Observable that produces its most recent value every time the specified observable
-- produces a value.
-- @arg {Observable} sampler - The Observable that is used to sample values from this Observable.
-- @returns {Observable}
function Observable:sample(sampler)
    if not sampler then
        error("Expected an Observable")
    end

    return Observable.create(function(observer)
        local latest = {}

        local function setLatest(...)
            latest = util.pack(...)
        end

        local function onNext()
            if #latest > 0 then
                return observer:onNext(util.unpack(latest))
            end
        end

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

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

        local sourceSubscription = self:subscribe(setLatest, onError)
        local sampleSubscription = sampler:subscribe(onNext, onError, onCompleted)

        return Subscription.create(function()
            if sourceSubscription then
                sourceSubscription:unsubscribe()
            end
            if sampleSubscription then
                sampleSubscription:unsubscribe()
            end
        end)
    end)
end
