Added Queue problem and empty setup/solution.

menu.json

@@ -11,4 +11,5 @@
   , "Short Scrabble Words"
   , "Sublevel"
   , "Multilevel"
-]
+  , "Queue"
+]

problems/queue/problem.txt

@@ -0,0 +1,61 @@
+Write a module (not a program) that provides a FIFO queue on top
+of a `{bold}level{/bold}` database. Your module should expose a function
+that build a queue and accept a `{bold}levelup{/bold}` instance.
+This function should return an object that has the following methods:
+ * `push`: add an element to the end of the queue.
+ * `shift`: shifts an element from the front of the queue.
+ * `clear`: remove all value from the queue.
+
+`{bold}push{\bold}` accept two arguments, the value to push and
+optionally a callback to know when it is finished.
+
+`{bold}shift{\bold}` accept one argument, an optional callback to
+retrieve the value at the front of the queue.
+
+`{bold}clear{\bold}` accept one argument, an optional callback to
+to know when it is finished.
+
+Remember, all callbacks follow the style of node.js:
+
+  function cb(err, value1, value2, value3, ...) {
+    if (err) {
+      // do something
+      return;
+    }
+  }
+
+----------------------------------------------------------------------
+HINTS:
+
+Use this boilerplate to get started with your module file:
+
+  function Queue(db) {
+    if (!(this instanceof Queue)) return new Queue(db)
+
+    // ...
+  }
+
+  module.exports = Queue
+
+  Queue.prototype.push = function(element, callback) {
+    // ...
+  }
+
+  Queue.prototype.shift = function(callback) {
+    // ...
+    // at some point call callback(null, element)
+  }
+
+  Queue.prototype.clear = function(callback) {
+    // ...
+  }
+
+Before coding, think how the queue data structure can be implemented
+on top of LevelUp. In particular, you want to "translate" a `push` into a
+level `put`, and `shift` into a limited ranged query.
+
+All pushed element must be stored in separate keys, by using a growing
+monotonic identifier. What problems will you encounter if you use a counter?
+Instead, try using `Date.now()`. Is this the perfect solution?
+
+You can also use sublevel to handle the naming of the queues!

problems/queue/setup.js

@@ -0,0 +1,7 @@
+
+function setup(run, callback) {
+  console.log(arguments)
+}
+
+module.exports       = setup
+module.exports.async = true

problems/queue/solution.js

@@ -0,0 +1,46 @@
+
+function Queue(db) {
+  if (!(this instanceof Queue)) return new Queue(db)
+
+  this._db = db
+}
+
+Queue.prototype.push = function(element, callback) {
+  var key = Date.now() + '!' + Math.random().toString(16).slice(2);
+
+  this._db.put(key, element, callback)
+
+  return this
+}
+
+Queue.prototype.shift = function(callback) {
+  var stream = this._db.createReadStream({
+                 limit: 1
+               })
+
+    , db = this._db
+
+  stream.once('data', function(data) {
+    db.del(data.key, function(err) {
+      callback(err, data.value)
+    })
+  })
+
+  stream.once('error', callback)
+
+  return this
+}
+
+Queue.prototype.clear = function(callback) {
+  this._db.createReadStream()
+      .pipe(this._db.createWriteStream({
+              type: 'del'
+            }))
+      .on('close', callback)
+
+  return this
+}
+
+module.exports = Queue
+// you can find an alternative implementation
+// with more features in https://github.com/mcollina/level-queue-type