// Require the core node modules.
var stream = require( "stream" );
var util = require( "util" );
var crypto = require( "crypto" );
var fileSystem = require( "fs" );


// ----------------------------------------------------------------------------------- //
// ----------------------------------------------------------------------------------- //


// CAUTION: Run the code in the next tick to give the full prototype chain a chance to
// initialize. If we try to run immediately, we'll get the function hoisting for the
// ETagStream constructor, but the prototype chain will not yet be defined fully.
process.nextTick(
	function run() {

		var fileReadStream = fileSystem.createReadStream( "./gina-carano.jpg" );

		// Once the file is finished piping into the etagStream, it will emit an etag
		// event with the computed MD5 hash.
		var etagStream = new ETagStream()
			.on(
				"etag",
				function handleETag( etag ) {

					console.log( "ETag:", etag );

				}
			)
		;

		fileReadStream.pipe( etagStream );

	}
);


// ----------------------------------------------------------------------------------- //
// ----------------------------------------------------------------------------------- //


// I provide a writable stream that will emit an "etag" event once the stream is closed.
// The etag will be an MD5 hash of the content that was written to the stream.
// --
// NOTE: In this version, we'll be using the "legacy" methods of the underlying Hash
// object which allow for intuitive .update() and .digest() methods.
function ETagStream() {

	// Call the super constructor.
	stream.Writable.call( this );

	this._hasher = crypto.createHash( "md5" );

	// Listen for the "finish" event, which will indicate that we have all the data that
	// we need in order to generate the MD5 has of the stream content.
	this.once( "finish", this._handleFinish.bind( this ) );

}

util.inherits( ETagStream, stream.Writable );


// ---
// PRIVATE METHODS.
// ---


// I handle the finish event, which, in turn, emits an "etag" event.
ETagStream.prototype._handleFinish = function() {

	// When dealing with "legacy" crypto methods, all we have to do is digest all of
	// the data that has been aggregated in the _write() method.
	this.emit( "etag", this._hasher.digest( "hex" ) );

};


// I write data to the etag stream.
ETagStream.prototype._write = function( chunk, encoding, writeComplete ) {

	// When dealing with "legacy" crypto methods, we can simply pass the chunk into
	// the underlying hash without giving any concern to back-pressure.
	this._hasher.update( chunk, encoding );

	writeComplete();

};