module.exports = function (filter) {

var value = null, listeners = [], oncers = []

function trigger (_value) {

value = _value

var length = listeners.length

for(var i = 0; i< length && value === _value; i++) {

var listener = listeners[i](value)

//if we remove a listener, must decrement i also

}

// decrement from length, incase a !immediately

// listener is added during a trigger

var l = oncers.length

var _oncers = oncers

oncers = []

while(l-- && _value === value) {

_oncers.shift()(value)

}

}

function many (ready, immediately) {

var i = listeners.push(ready) - 1

if(value !== null && immediately !== false) ready(value)

return function () { //manually remove...

//fast path, will happen if an earlier listener has not been removed.

if(listeners[i] !== ready)

i = listeners.indexOf(ready)

listeners.splice(i, 1)

}

}

many.set = function (_value) {

if(filter ? filter(value, _value) : true) trigger(many.value = _value)

return many

}

many.once = function (once, immediately) {

if(value !== null && immediately !== false) {

once(value)

return function () {}

}

else {

var i = oncers.push(once) - 1

return function () {

if(oncers[i] !== once)

i = oncers.indexOf(once)

}

}

}

return many

}