adding entity.discard() to stop data events

entity.discard() is a performance optimization for those using this library to only read specific files from the tar.
the parser looks at this property in the same manner it looks at _remaining to know when to skip calling write on the entity.

an entity which discard() has been called on will not emit any more data events.
discard() does not take any arguments and only sets the value of _discard to true.

this saves 100ms or so for me just `stating` all of the files in the npm v 3 tar

lib/entry.js

@@ -24,6 +24,7 @@ function Entry (header, extended, global) {
   this._ending = false
   this._ended = false
   this._remaining = 0
+  this._discard = false
   this._queue = []
   this._index = 0
   this._queueLen = 0
@@ -209,5 +210,11 @@ Entry.prototype._setProps = function () {
   this._remaining = props.size
 }
 
+// the parser may not call write if discard is true. 
+// useful for skipping data from some files quickly.
+Entry.prototype.discard = function(){
+  this._discard = true
+}
+
 Entry.prototype.warn = fstream.warn
 Entry.prototype.error = fstream.error

lib/parse.js

@@ -102,7 +102,11 @@ Parse.prototype._process = function (c) {
 
   if (this._entry) {
     var entry = this._entry
-    entry.write(c)
+    if(!entry._discard) entry.write(c)
+    else {
+      entry._remaining -= c.length
+      if(entry._remaining < 0) entry._remaining = 0
+    }
     if (entry._remaining === 0) {
       entry.end()
       this._entry = null

test/parse-discard.js

@@ -0,0 +1,29 @@
+var tap = require("tap")
+  , tar = require("../tar.js")
+  , fs = require("fs")
+  , path = require("path")
+  , file = path.resolve(__dirname, "fixtures/c.tar")
+
+tap.test("parser test", function (t) {
+  var parser = tar.Parse()
+  var total = 0;
+  var dataTotal = 0;
+
+  parser.on("end", function () {
+
+    t.equals(total-513,dataTotal,'should have discarded only c.txt')
+
+    t.end()
+  })
+
+  fs.createReadStream(file)
+    .pipe(parser)
+    .on('entry',function(entry){
+      if(entry.path === 'c.txt') entry.discard()
+
+      total += entry.size;
+      entry.on('data',function(data){
+        dataTotal += data.length        
+      })
+    })
+})