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

--- Returns a new Observable that produces values computed by accumulating the results of running a
-- function on each value produced by the original Observable.
-- @arg {function} accumulator - Accumulates the values of the original Observable. Will be passed
--                               the return value of the last call as the first argument and the
--                               current values as the rest of the arguments.  Each value returned
--                               from this function will be emitted by the Observable.
-- @arg {*} seed - A value to pass to the accumulator the first time it is run.
-- @returns {Observable}
function Observable:scan(accumulator, seed)
    return Observable.create(function(observer)
        local result = seed
        local first = true

        local function onNext(...)
            if first and seed == nil then
                result = ...
                first = false
            else
                return util.tryWithObserver(observer, function(...)
                    result = accumulator(result, ...)
                    observer:onNext(result)
                end, ...)
            end
        end

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

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

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