Add a FIFO class.

src/fifo.cr

@@ -0,0 +1,37 @@
+# This class enables to keep track of used data.
+#
+# Each time a value is pushed, it is put on top of a FIFO stack.
+# Stack size is configurable, so in case the stack size exceeds what is allowed,
+# the least recently used value is removed.
+#
+# ```
+# fifo = FIFO(Int32).new 3 # Only 3 allowed entries.
+#
+# pp! fifo << 1 # -> nil (there is still room in the stack)
+# pp! fifo << 2 # -> nil (there is still room in the stack)
+# pp! fifo << 3 # -> nil (last entry without exceeding the allowed size)
+# pp! fifo << 4 # -> 1 (least recently used data)
+# pp! fifo << 4 # -> nil (already in the stack)
+# pp! fifo << 2 # -> nil (already in the stack)
+# pp! fifo << 5 # -> 3 (least recently used data)
+# ```
+class FIFO(V)
+	# This array is used as a *stack*.
+	property data : Array(V)
+
+	# Maximum allowed entries in the stack.
+	property max_entries : UInt32
+
+	def initialize(@max_entries : UInt32)
+		@data = Array(V).new
+	end
+
+	# Pushes a value in the FIFO and gets the oldest value whether it exceeds the allowed number of entries.
+	# NOTE: `#<<(v : V)` is the only function since it's enough for the intended use, feel free to improve this.
+	# WARNING: implementation is extremely simple (3 lines) and not designed to be highly efficient.
+	def <<(v : V) : V?
+		@data.unshift v                         # push on top of the stack
+		@data.uniq!                             # remove dups
+		@data.pop if @data.size > @max_entries  # remove least recently used entry if `@data` is too big
+	end
+end