add chapter 12 and RateLimiter

Author: rkuhn

build.sbt

@@ -10,3 +10,4 @@ lazy val chapter04 = project dependsOn (common)
 
 lazy val chapter07 = project dependsOn (common)
 
+lazy val chapter12 = project dependsOn (common)

chapter12/src/main/scala/com/reactivedesignpatterns/chapter12/RateLimiter.scala

@@ -0,0 +1,29 @@
+package com.reactivedesignpatterns.chapter12
+
+import scala.concurrent.duration.FiniteDuration
+import scala.concurrent.duration.Deadline
+import scala.concurrent.Future
+
+case object RateLimitExceeded extends RuntimeException
+
+class RateLimiter(requests: Int, period: FiniteDuration) {
+  private val startTimes = {
+    val onePeriodAgo = Deadline.now - period
+    Array.fill(requests)(onePeriodAgo)
+  }
+  private var position = 0
+  private def lastTime = startTimes(position)
+  private def enqueue(time: Deadline) = {
+    startTimes(position) = time
+    position += 1
+    if (position == requests) position = 0
+  }
+  def call[T](block: => Future[T]): Future[T] = {
+    val now = Deadline.now
+    if ((now - lastTime) < period) Future.failed(RateLimitExceeded)
+    else {
+      enqueue(now)
+      block
+    }
+  }
+}