Add Queue data structure: Array, Queue and SizedQueue

data_structures/queues/queue.rb

@@ -0,0 +1,102 @@
+# A queue is like a waiting list.
+# Imagine you’re waiting in line to buy the latest Android product
+# or getting a parking ticket. These are queues!
+
+#
+#
+# 1. An array can behave like a Queue if you use the right methods.
+#
+# These methods are:
+# - unshift (or prepend with Ruby 2.5+): when you unshift,
+# you are adding one item to the queue
+# - pop
+
+class ArrayQueue
+  def initialize(queue = [])
+    @queue = queue
+  end
+
+  attr_accessor :queue
+
+  def add(item)
+    queue.unshift(item)
+  end
+
+  def pop
+    queue.pop
+  end
+
+  def peek
+    queue[-1]
+  end
+end
+
+queue = ArrayQueue.new
+queue.add(3)
+queue.add(4)
+queue.add(5)
+
+puts queue.inspect
+# => #<ArrayQueue:0x00007fc78200f5e0 @queue=[5, 4, 3]>
+
+queue.pop
+
+puts queue.inspect
+# => #<ArrayQueue:0x00007fc78200f5e0 @queue=[5, 4]>
+
+puts(queue.peek)
+# => 4
+
+#
+#
+# 2. Ruby Concurrent Queue
+# Ruby has a proper thread-safe, blocking, Queue class.
+# You can use this queue for coordinating work in a multi-threaded program.
+#
+
+que = Queue.new
+
+que << 1
+que << 2
+que << 3
+
+que.pop
+# 1
+
+que.pop
+# 2
+
+# If the queue is empty, calling pop will put your current
+# thread to sleep & wait until something is added to the queue.
+
+#
+#
+# 3. How to Use A Sized Queue
+# A sized queue is the same as a regular queue but with a size limit.
+#
+
+que = SizedQueue.new(5)
+
+# When the queue is full, the push (same as <<) operation
+# will suspend the current thread until an item is taken off the queue.
+
+que.push(:oranges)
+que.push(:apples)
+que.push(:blue)
+que.push(:orange)
+que.push(:green)
+# At this point, the SizedQueue is full
+
+que.push(:throw_expection)
+# data_structures/queues/queue.rb:81:in `push': No live threads left. Deadlock? (fatal)
+# 1 threads, 1 sleeps current:0x00007ff54f407130 main thread:0x00007ff54f407130
+# * #<Thread:0x00007ff54f86ef38 sleep_forever>
+#    rb_thread_t:0x00007ff54f407130 native:0x000000010dd24dc0 int:0
+#    data_structures/queues/queue.rb:81:in `push'
+#    data_structures/queues/queue.rb:81:in `<main>'
+# 	from data_structures/queues/queue.rb:81:in `<main>'
+
+# You can choose to raise an exception, passing true as an argument as follows:
+que.push(:throw_expection, true)
+# data_structures/queues/queue.rb:83:in `push': queue full (ThreadError)
+# 	from data_structures/queues/queue.rb:83:in `<main>'