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Task/Huffman-coding/Swift/huffman-coding.swift

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+enum HuffmanTree<T> {
+  case Leaf(T)
+  indirect case Node(HuffmanTree<T>, HuffmanTree<T>)
+
+  func printCodes(prefix: String) {
+    switch(self) {
+    case let .Leaf(c):
+      print("\(c)\t\(prefix)")
+    case let .Node(l, r):
+      l.printCodes(prefix + "0")
+      r.printCodes(prefix + "1")
+    }
+  }
+}
+
+func buildTree<T>(freqs: [(T, Int)]) -> HuffmanTree<T> {
+  assert(freqs.count > 0, "must contain at least one character")
+  // leaves sorted by increasing frequency
+  let leaves : [(Int, HuffmanTree<T>)] = freqs.sort { (p1, p2) in p1.1 < p2.1 }.map { (x, w) in (w, .Leaf(x)) }
+  // nodes sorted by increasing frequency
+  var nodes = [(Int, HuffmanTree<T>)]()
+  // iterate through leaves and nodes in order of increasing frequency
+  for var i = 0, j = 0; ; {
+    assert(i < leaves.count || j < nodes.count)
+    // get subtree of least frequency
+    var e1 : (Int, HuffmanTree<T>)
+    if j == nodes.count || i < leaves.count && leaves[i].0 < nodes[j].0 {
+      e1 = leaves[i]
+      i++
+    } else {
+      e1 = nodes[j]
+      j++
+    }
+
+    // if there's no subtrees left, then that one was the answer
+    if i == leaves.count && j == nodes.count {
+      return e1.1
+    }
+
+    // get next subtree of least frequency
+    var e2 : (Int, HuffmanTree<T>)
+    if j == nodes.count || i < leaves.count && leaves[i].0 < nodes[j].0 {
+      e2 = leaves[i]
+      i++
+    } else {
+      e2 = nodes[j]
+      j++
+    }
+    // create node from two subtrees
+    nodes.append((e1.0 + e2.0, .Node(e1.1, e2.1)))
+  }
+}
+
+func getFreqs<S : SequenceType where S.Generator.Element : Hashable>(seq: S) -> [(S.Generator.Element, Int)] {
+  var freqs : [S.Generator.Element : Int] = [:]
+  for c in seq {
+    freqs[c] = (freqs[c] ?? 0) + 1
+  }
+  return Array(freqs)
+}
+
+let str = "this is an example for huffman encoding"
+let charFreqs = getFreqs(str.characters)
+let tree = buildTree(charFreqs)
+print("Symbol\tHuffman code")
+tree.printCodes("")