Just another update

Task/Probabilistic-choice/Scala/probabilistic-choice.scala

@@ -0,0 +1,64 @@
+object ProbabilisticChoice extends App {
+  import scala.collection.mutable.LinkedHashMap
+
+  def weightedProb[A](prob: LinkedHashMap[A,Double]): A = {
+    require(prob.forall{case (_, p) => p > 0 && p < 1})
+    assume(prob.values.sum == 1)
+    def weighted(todo: Iterator[(A,Double)], rand: Double, accum: Double = 0): A = todo.next match {
+      case (s, i) if rand < (accum + i) => s
+      case (_, i) => weighted(todo, rand, accum + i)
+    }
+    weighted(prob.toIterator, scala.util.Random.nextDouble)
+  }
+
+  def weightedFreq[A](freq: LinkedHashMap[A,Int]): A = {
+    require(freq.forall{case (_, f) => f >= 0})
+    require(freq.values.sum > 0)
+    def weighted(todo: Iterator[(A,Int)], rand: Int, accum: Int = 0): A = todo.next match {
+      case (s, i) if rand < (accum + i) => s
+      case (_, i) => weighted(todo, rand, accum + i)
+    }
+    weighted(freq.toIterator, scala.util.Random.nextInt(freq.values.sum))
+  }
+
+  // Tests:
+
+  val probabilities = LinkedHashMap(
+    'aleph  -> 1.0/5,
+    'beth   -> 1.0/6,
+    'gimel  -> 1.0/7,
+    'daleth -> 1.0/8,
+    'he     -> 1.0/9,
+    'waw    -> 1.0/10,
+    'zayin  -> 1.0/11,
+    'heth   -> 1759.0/27720
+  )
+
+  val frequencies = LinkedHashMap(
+    'aleph  -> 200,
+    'beth   -> 167,
+    'gimel  -> 143,
+    'daleth -> 125,
+    'he     -> 111,
+    'waw    -> 100,
+    'zayin  -> 91,
+    'heth   -> 63
+  )
+
+  def check[A](original: LinkedHashMap[A,Double], results: Seq[A]) {
+    val freq = results.groupBy(x => x).mapValues(_.size.toDouble/results.size)
+    original.foreach{case (k, v) =>
+      val a = v/original.values.sum
+      val b = freq(k)
+      val c = if (Math.abs(a - b) < 0.001) "ok" else "**"
+      println(f"$k%10s  $a%.4f  $b%.4f  $c")
+    }
+    println(" "*10 + f"  ${1}%.4f  ${freq.values.sum}%.4f")
+  }
+
+  println("Checking weighted probabilities:")
+  check(probabilities, for (i <- 1 to 1000000) yield weightedProb(probabilities))
+  println
+  println("Checking weighted frequencies:")
+  check(frequencies.map{case (a, b) => a -> b.toDouble}, for (i <- 1 to 1000000) yield weightedFreq(frequencies))
+}