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

--- Returns an Observable that produces a single value representing the number of values produced
-- by the source value that satisfy an optional predicate.
-- @arg {function=} predicate - The predicate used to match values.
function Observable:count(predicate)
    predicate = predicate or util.constant(true)

    return Observable.create(function(observer)
        local count = 0

        local function onNext(...)
            util.tryWithObserver(observer, function(...)
                if predicate(...) then
                    count = count + 1
                end
            end, ...)
        end

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

        local function onCompleted()
            observer:onNext(count)
            observer:onCompleted()
        end

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