2016 Update

Task/Huffman-coding/Scala/huffman-coding-2.scala

@@ -0,0 +1,43 @@
+// this version uses immutable data only, recursive functions and pattern matching
+object Huffman {
+  sealed trait Tree[+A]
+  case class Leaf[A](value: A) extends Tree[A]
+  case class Branch[A](left: Tree[A], right: Tree[A]) extends Tree[A]
+
+  // recursively build the binary tree needed to Huffman encode the text
+  def merge(xs: List[(Tree[Char], Int)]): List[(Tree[Char], Int)] = {
+    if (xs.length == 1) xs else {
+      val l = xs.head
+      val r = xs.tail.head
+      val merged = (Branch(l._1, r._1), l._2 + r._2)
+      merge((merged :: xs.drop(2)).sortBy(_._2))
+    }
+  }
+
+  // recursively search the branches of the tree for the required character
+  def contains(tree: Tree[Char], char: Char): Boolean = tree match {
+    case Leaf(c) => if (c == char) true else false
+    case Branch(l, r) => contains(l, char) || contains(r, char)
+  }
+
+  // recursively build the path string required to traverse the tree to the required character
+  def encodeChar(tree: Tree[Char], char: Char): String = {
+    def go(tree: Tree[Char], char: Char, code: String): String = tree match {
+      case Leaf(_) => code
+      case Branch(l, r) => if (contains(l, char)) go(l, char, code + '0') else go(r, char, code + '1')
+    }
+    go(tree, char, "")
+  }
+
+  def main(args: Array[String]) {
+    val text = "this is an example for huffman encoding"
+    // transform the text into a list of tuples.
+    // each tuple contains a Leaf node containing a unique character and an Int representing that character's weight
+    val frequencies = text.groupBy(chars => chars).mapValues(group => group.length).toList.map(x => (Leaf(x._1), x._2)).sortBy(_._2)
+    // build the Huffman Tree for this text
+    val huffmanTree = merge(frequencies).head._1
+    // output the resulting character codes
+    println("Char\tWeight\tCode")
+    frequencies.foreach(x => println(x._1.value + "\t" + x._2 + s"/${text.length}" + s"\t${encodeChar(huffmanTree, x._1.value)}"))
+  }
+}