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//filter will reemit the data if cb(err,pass) pass is truthy
// reduce is more tricky
// maybe we want to group the reductions or emit progress updates occasionally
// the most basic reduce just emits one 'data' event after it has recieved 'end'
var Stream = require('stream').Stream
//create an event stream and apply function to each .write
//emitting each response as data
//unless it's an empty callback
module.exports = function (mapper, opts) {
var stream = new Stream()
, self = this
, inputs = 0
, outputs = 0
, ended = false
, paused = false
, destroyed = false
, lastWritten = 0
, inNext = false
this.opts = opts || {};
var errorEventName = this.opts.failures ? 'failure' : 'error';
// Items that are not ready to be written yet (because they would come out of
// order) get stuck in a queue for later.
var writeQueue = {}
stream.writable = true
stream.readable = true
function queueData (data, number) {
var nextToWrite = lastWritten + 1
if (number === nextToWrite) {
// If it's next, and its not undefined write it
if (data !== undefined) {
stream.emit.apply(stream, ['data', data])
}
lastWritten ++
nextToWrite ++
} else {
// Otherwise queue it for later.
writeQueue[number] = data
}
// If the next value is in the queue, write it
if (writeQueue.hasOwnProperty(nextToWrite)) {
var dataToWrite = writeQueue[nextToWrite]
delete writeQueue[nextToWrite]
return queueData(dataToWrite, nextToWrite)
}
outputs ++
if(inputs === outputs) {
if(paused) paused = false, stream.emit('drain') //written all the incoming events
if(ended) end()
}
}
function next (err, data, number) {
if(destroyed) return
inNext = true
if (!err || self.opts.failures) {
queueData(data, number)
}
if (err) {
stream.emit.apply(stream, [ errorEventName, err ]);
}
inNext = false;
}
// Wrap the mapper function by calling its callback with the order number of
// the item in the stream.
function wrappedMapper (input, number, callback) {
return mapper.call(null, input, function(err, data){
callback(err, data, number)
})
}
stream.write = function (data) {
if(ended) throw new Error('map stream is not writable')
inNext = false
inputs ++
try {
//catch sync errors and handle them like async errors
var written = wrappedMapper(data, inputs, next)
paused = (written === false)
return !paused
} catch (err) {
//if the callback has been called syncronously, and the error
//has occured in an listener, throw it again.
if(inNext)
throw err
next(err)
return !paused
}
}
function end (data) {
//if end was called with args, write it,
ended = true //write will emit 'end' if ended is true
stream.writable = false
if(data !== undefined) {
return queueData(data, inputs)
} else if (inputs == outputs) { //wait for processing
stream.readable = false, stream.emit('end'), stream.destroy()
}
}
stream.end = function (data) {
if(ended) return
end(data)
}
stream.destroy = function () {
ended = destroyed = true
stream.writable = stream.readable = paused = false
process.nextTick(function () {
stream.emit('close')
})
}
stream.pause = function () {
paused = true
}
stream.resume = function () {
paused = false
}
return stream
}
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