/* NSC -- new Scala compiler
 * Copyright 2005-2011 LAMP/EPFL
 * @author  Martin Odersky
 */

package scala.tools.nsc
package typechecker

import scala.collection.mutable.ListBuffer
import symtab.Flags._

/** This trait ...
 *
 *  @author  Martin Odersky
 *  @version 1.0
 */
trait EtaExpansion { self: Analyzer =>

  import global._

  object etaExpansion {
    private def isMatch(vparam: ValDef, arg: Tree) = arg match {
      case Ident(name)  => vparam.name == name
      case _            => false
    }
      
    def unapply(tree: Tree): Option[(List[ValDef], Tree, List[Tree])] = tree match {
      case Function(vparams, Apply(fn, args)) if (vparams corresponds args)(isMatch) => 
        Some((vparams, fn, args))
      case _ =>
        None
    }
  }

  /** <p>
   *    Expand partial function applications of type <code>type</code>.
   *  </p><pre>
   *  p.f(es_1)...(es_n)
   *     ==>  {
   *            <b>private synthetic val</b> eta$f   = p.f   // if p is not stable
   *            ...
   *            <b>private synthetic val</b> eta$e_i = e_i    // if e_i is not stable
   *            ...
   *            (ps_1 => ... => ps_m => eta$f([es_1])...([es_m])(ps_1)...(ps_m))
   *          }</pre>
   *  <p>
   *    tree is already attributed
   *  </p>
   */
  def etaExpand(unit : CompilationUnit, tree: Tree): Tree = {
    val tpe = tree.tpe
    var cnt = 0 // for NoPosition
    def freshName() = {
      cnt += 1
      unit.freshTermName("eta$" + (cnt - 1) + "$")
    }
    val defs = new ListBuffer[Tree]

    /** Append to <code>defs</code> value definitions for all non-stable
     *  subexpressions of the function application <code>tree</code>.
     *
     *  @param tree ...
     *  @return     ...
     */
    def liftoutPrefix(tree: Tree): Tree = {
      def liftout(tree: Tree): Tree =
        if (treeInfo.isPureExpr(tree)) tree
        else {
          val vname: Name = freshName()
          // Problem with ticket #2351 here 
          defs += atPos(tree.pos) {
            ValDef(Modifiers(SYNTHETIC), vname.toTermName, TypeTree(), tree)
          }
          Ident(vname) setPos tree.pos.focus
        }
      val tree1 = tree match {
        // a partial application using named arguments has the following form:
        // { val qual$1 = qual
        //   val x$1 = arg1
        //   [...]
        //   val x$n = argn
        //   qual$1.fun(x$1, ..)..(.., x$n) }
        // Eta-expansion has to be performed on `fun'
        case Block(stats, fun) =>
          defs ++= stats
          liftoutPrefix(fun)
        case Apply(fn, args) =>
          treeCopy.Apply(tree, liftoutPrefix(fn), args mapConserve (liftout)) setType null
        case TypeApply(fn, args) =>
          treeCopy.TypeApply(tree, liftoutPrefix(fn), args) setType null
        case Select(qual, name) =>
          treeCopy.Select(tree, liftout(qual), name) setSymbol NoSymbol setType null
        case Ident(name) =>
          tree
      }
      if (tree1 ne tree) tree1 setPos tree1.pos.makeTransparent
      tree1 
    }

    /** Eta-expand lifted tree.
     */
    def expand(tree: Tree, tpe: Type): Tree = tpe match {
      case mt @ MethodType(paramSyms, restpe) if !mt.isImplicit =>
        val params = paramSyms map (sym =>
          ValDef(Modifiers(SYNTHETIC | PARAM), 
                 sym.name.toTermName, TypeTree(sym.tpe) , EmptyTree))
        atPos(tree.pos.makeTransparent) {
          Function(params, expand(Apply(tree, params map gen.paramToArg), restpe))
        }
      case _ =>
        tree
    }

    val tree1 = liftoutPrefix(tree)
    atPos(tree.pos)(Block(defs.toList, expand(tree1, tpe)))
  }
}