USL.scala

/**
 * USL - Useless Stack Language
 */

import scala.collection.mutable

object USL {
  /********************
    * Handling USL Data
    ********************/
  abstract class Dat
  case class Number(n: Float) extends Dat {
    override def toString(): String = {
      return s"$n"
    }
  }
  case class Ident(s: String) extends Dat {
    override def toString(): String = {
      return s
    }
  }
  case class Obj(contents: List[Dat]) extends Dat

  val DatStack = mutable.Stack[Dat]()
  type IStream = List[Dat]
  val Definitions = mutable.Map[Ident, Dat]()

  /**
   * Not functional (as in FP) at ALL.
   * This is an Imperative Function; there's probably a better way to do this.
   * @param strings The strings to iterate over, starting with the string '{'
   * @return The location of the first matching closing brace for the top level brace at index 0 in strings
   */
  def matchbrace(strings: Array[String]): Int = {
    var depth: Int = 0
    var i: Int = 0
    for(s <- strings) {
      i += 1
      if(s == "{") depth += 1
      if(s == "}") depth -= 1
      if(depth == 0) return i
    }
    return -1 //whoops?
  }

  //also the worst approach
  def mkStream(s: String): List[Dat] = {
    if(s.length() < 1) return List[Dat]()
    val q: Array[String] = s.split("\\s+")
    if(q(0) == "{") {
      val d: Int = matchbrace(q)
      return Obj(q.slice(1,d-1).map(Ident(_)).toList) :: mkStream(q.slice(d,q.length).mkString(" ")) ::: Nil
    } else if(q(0).matches("\\d+\\.?\\d*")) {
      return Number(java.lang.Float.parseFloat(q(0))) :: mkStream(q.slice(1,q.length).mkString(" ")) ::: Nil
    } else {
      return Ident(q(0)) :: mkStream(q.slice(1,q.length).mkString(" ")) ::: Nil
    }
  }

  def evalOne(): Boolean = {
    if(DatStack.isEmpty) return false
    DatStack.top match {
      case Ident("+") => {
        DatStack.pop()
        val a = DatStack.pop().asInstanceOf[Number]
        val b = DatStack.pop().asInstanceOf[Number]
        DatStack.push(Number(a.n + b.n))
        true
      }
      case Ident("-") => {
        DatStack.pop()
        val b = DatStack.pop().asInstanceOf[Number]
        val a = DatStack.pop().asInstanceOf[Number]
        DatStack.push(Number(a.n - b.n))
        true
      }
      case Ident("fl") => {
        DatStack.pop()
        val a = DatStack.pop()
        val b = DatStack.pop()
        DatStack.push(a)
        DatStack.push(b)
        true
      }
      case Ident("lup") => {
        DatStack.pop()
        if(DatStack.top.isInstanceOf[Ident]) {
          val i = DatStack.pop().asInstanceOf[Ident]
          Definitions.get(i) match {
            case Some(g) => DatStack.push(g)
            case None => DatStack.push(i)
          }
          true
        } else if(DatStack.top.isInstanceOf[Obj]) {
          val o = DatStack.pop().asInstanceOf[Obj]
          o.contents.reverse.foreach(DatStack.push(_))
          true
        } else {
          true
        }
      }
      case Ident("dup") => {
        DatStack.pop()
        DatStack.push(DatStack.top)
        true
      }
      case Ident("ident?") => {
        DatStack.pop()
        if(DatStack.pop().isInstanceOf[Ident]) {
          DatStack.push(Number(1))
        } else {
          DatStack.push(Number(0))
        }
        true
      }
      case Ident("obj?") => {
        DatStack.pop()
        if(DatStack.pop().isInstanceOf[Obj]) {
          DatStack.push(Number(1))
        } else {
          DatStack.push(Number(0))
        }
        true
      }
      case Ident("if") => {
        DatStack.pop()
        val test = DatStack.pop()
        val falsey = DatStack.pop().asInstanceOf[Obj]
        val truthy = DatStack.pop().asInstanceOf[Obj]
        val put = test match {
          case Number(0) => falsey
          case _ => truthy
        }
        put.contents.reverse.foreach({
          DatStack.push(_)
        })
        true
      }
      case Ident("undef") => {
        DatStack.pop()
        val l = DatStack.pop().asInstanceOf[Obj]
        l.contents.reverse.foreach({
          case s: Ident => Definitions.remove(s)
          case _ =>
        })
        true
      }
      case Ident("print") => {
        DatStack.pop()
        println(DatStack.top)
        true
      }
      case Ident("drop") => {
        DatStack.pop()
        DatStack.pop()
        true
      }
      case Ident("clear") => {
        DatStack.clear()
        false
      }
      case Ident("def") => {
        DatStack.pop()
        val bind = DatStack.pop()
        val name = DatStack.pop().asInstanceOf[Ident]
        Definitions.put(name, bind)
        true
      }
      case _ => false
    }
  }

  def run(s: IStream): Unit = {
    if(s.length == 0) {
      println("> " + DatStack)
      return
    }
    val top = s.head
    if(top.isInstanceOf[Ident] && Definitions.contains(top.asInstanceOf[Ident])) {
      val pack = Definitions.get(top.asInstanceOf[Ident]).get
      pack match {
        case Obj(l) => run(l ::: s.tail)
        case _ => run(Definitions.get(top.asInstanceOf[Ident]).get :: s.tail)
      }
    } else {
      DatStack.push(top)
      while (evalOne()) {}
      if(!DatStack.isEmpty && DatStack.top.isInstanceOf[Ident] && Definitions.contains(DatStack.top.asInstanceOf[Ident])) {
        run(Definitions.get(DatStack.pop().asInstanceOf[Ident]).get :: s.tail)
      } else {
        run(s.tail)
      }
    }
  }

  def main(args: Array[String]): Unit = {
    var v = ""
    while(v != "exit") {
      v = Console.in.readLine()
      try {
        run(mkStream(v))
      } catch {
        case e: Throwable => println(e)
      }
    }
  }
}