Data commit

Task/LZW-compression/Scala/lzw-compression.scala

@@ -0,0 +1,45 @@
+def compress(tc:String) = {
+    //initial dictionary
+    val startDict = (1 to 255).map(a=>(""+a.toChar,a)).toMap
+    val (fullDict, result, remain) = tc.foldLeft ((startDict, List[Int](), "")) {
+      case ((dict,res,leftOver),nextChar) =>
+        if (dict.contains(leftOver + nextChar)) // current substring already in dict
+          (dict, res, leftOver+nextChar)
+        else if (dict.size < 4096) // add to dictionary
+          (dict + ((leftOver+nextChar, dict.size+1)), dict(leftOver) :: res, ""+nextChar)
+        else // dictionary is full
+          (dict, dict(leftOver) :: res, ""+nextChar)
+    }
+    if (remain.isEmpty) result.reverse else (fullDict(remain) :: result).reverse
+}
+
+def decompress(ns: List[Int]): String = {
+  val startDict = (1 to 255).map(a=>(a,""+a.toChar)).toMap
+  val (_, result, _) =
+    ns.foldLeft[(Map[Int, String], List[String], Option[(Int, String)])]((startDict, Nil, None)) {
+    case ((dict, result, conjecture), n) => {
+      dict.get(n) match {
+        case Some(output) => {
+          val (newDict, newCode) = conjecture match {
+            case Some((code, prefix)) => ((dict + (code -> (prefix + output.head))), code + 1)
+            case None => (dict, dict.size + 1)
+          }
+          (newDict, output :: result, Some(newCode -> output))
+        }
+        case None => {
+          // conjecture being None would be an encoding error
+          val (code, prefix) = conjecture.get
+          val output = prefix + prefix.head
+          (dict + (code -> output), output :: result, Some(code + 1 -> output))
+        }
+      }
+    }
+  }
+  result.reverse.mkString("")
+}
+// test
+val text = "TOBEORNOTTOBEORTOBEORNOT"
+val compressed = compress(text)
+println(compressed)
+val result = decompress(compressed)
+println(result)