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Antonio Cunei
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/* __ *\
** ________ ___ / / ___ Scala API **
** / __/ __// _ | / / / _ | (c) 2006-2011, LAMP/EPFL **
** __\ \/ /__/ __ |/ /__/ __ | http://scala-lang.org/ **
** /____/\___/_/ |_/____/_/ | | **
** |/ **
\* */
package scala.util.parsing.combinator
import scala.util.parsing.combinator._
import scala.util.parsing.input.{ Reader, Position }
import scala.collection.mutable
/**
* <p>
* <code>PackratParsers</code> is a component that extends the parser combinators
* provided by <a href="Parsers.html"><code>Parsers</code></a> with a memoization facility
* (``Packrat Parsing'').
* </p>
* <p>
* Packrat Parsing is a technique for implementing backtracking, recursive-descent parsers, with the
* advantage that it guarantees unlimited lookahead and a linear parse time. Using this technique,
* left recursive grammars can also be accepted.
* </p>
* <p>
* Using <code>PackratParsers</code> is very similar to using <code>Parsers</code>:
* <ul>
* <li> any class/trait that extends <code>Parsers</code> (directly or through a subclass) can
* mix in <code>PackratParsers</code>. Example:
* <code>object MyGrammar extends StandardTokenParsers with PackratParsers </code>
* <li> each grammar production previously declared as a <code>def</code> without formal parameters
* becomes a <code>lazy val</code>, and its type is changed from <code>Parser[Elem]</code>
* to <code>PackratParser[Elem]</code>. So, for example, <code>def production: Parser[Int] = {...}</code>
* becomes <code>lazy val production: PackratParser[Int] = {...}</code>
* <li> Important: using <code>PackratParser</code>s is not an ``all or nothing'' decision. They
* can be free mixed with regular <code>Parser</code>s in a single grammar.
* </ul>
* </p>
* <p>
* Cached parse results are attached to the <i>input</i>, not the grammar.
* Therefore, <code>PackratsParser</code>s require a <code>PackratReader</code> as input, which
* adds memoization to an underlying <code>Reader</code>. Programmers can create <code>PackratReader</code>
* objects either manually, as in <code>production(new PackratReader(new lexical.Scanner("input")))</code>,
* but the common way should be to rely on the combinator <code>phrase</code> to wrap a given
* input with a <code>PackratReader</code> if the input is not one itself.
* </p>
*
* @see Bryan Ford: "Packrat Parsing: Simple, Powerful, Lazy, Linear Time." ICFP'02
* @see Alessandro Warth, James R. Douglass, Todd Millstein: "Packrat Parsers Can Support Left Recursion." PEPM'08
*
* @since 2.8
* @author Manohar Jonnalagedda, Tiark Rompf
*/
trait PackratParsers extends Parsers {
//type Input = PackratReader[Elem]
/**
* A specialized <code>Reader</code> class that wraps an underlying <code>Reader</code>
* and provides memoization of parse results.
*/
class PackratReader[+T](underlying: Reader[T]) extends Reader[T] { outer =>
/*
* caching of intermediate parse results and information about recursion
*/
private[PackratParsers] val cache = mutable.HashMap.empty[(Parser[_], Position), MemoEntry[_]]
private[PackratParsers] def getFromCache[T](p: Parser[T]): Option[MemoEntry[T]] = {
cache.get((p, pos)).asInstanceOf[Option[MemoEntry[T]]]
}
private[PackratParsers] def updateCacheAndGet[T](p: Parser[T], w: MemoEntry[T]): MemoEntry[T] = {
cache.put((p, pos),w)
w
}
/* a cache for storing parser heads: allows to know which parser is involved
in a recursion*/
private[PackratParsers] val recursionHeads: mutable.HashMap[Position, Head] = mutable.HashMap.empty
//a stack that keeps a list of all involved rules
private[PackratParsers] var lrStack: List[LR] = Nil
override def source: java.lang.CharSequence = underlying.source
override def offset: Int = underlying.offset
def first: T = underlying.first
def rest: Reader[T] = new PackratReader(underlying.rest) {
override private[PackratParsers] val cache = outer.cache
override private[PackratParsers] val recursionHeads = outer.recursionHeads
lrStack = outer.lrStack
}
def pos: Position = underlying.pos
def atEnd: Boolean = underlying.atEnd
}
/**
* <p>
* A parser generator delimiting whole phrases (i.e. programs).
* </p>
* <p>
* Overridden to make sure any input passed to the argument parser
* is wrapped in a <code>PackratReader</code>.
* </p>
*/
override def phrase[T](p: Parser[T]) = {
val q = super.phrase(p)
new PackratParser[T] {
def apply(in: Input) = in match {
case in: PackratReader[_] => q(in)
case in => q(new PackratReader(in))
}
}
}
private def getPosFromResult(r: ParseResult[_]): Position = r.next.pos
// auxiliary data structures
private case class MemoEntry[+T](var r: Either[LR,ParseResult[_]]){
def getResult: ParseResult[T] = r match {
case Left(LR(res,_,_)) => res.asInstanceOf[ParseResult[T]]
case Right(res) => res.asInstanceOf[ParseResult[T]]
}
}
private case class LR(var seed: ParseResult[_], var rule: Parser[_], var head: Option[Head]){
def getPos: Position = getPosFromResult(seed)
}
private case class Head(var headParser: Parser[_], var involvedSet: List[Parser[_]], var evalSet: List[Parser[_]]){
def getHead = headParser
}
/**
* The root class of packrat parsers.
*/
abstract class PackratParser[+T] extends super.Parser[T]
/**
* Implicitly convert a parser to a packrat parser.
* The conversion is triggered by giving the appropriate target type:
* val myParser: PackratParser[MyResult] = aParser
*/
implicit def parser2packrat[T](p: => super.Parser[T]): PackratParser[T] = {
lazy val q = p
memo(super.Parser {in => q(in)})
}
/*
* An unspecified function that is called when a packrat reader is applied.
* It verifies whether we are in the process of growing a parse or not.
* In the former case, it makes sure that rules involved in the recursion are evaluated.
* It also prevents non-involved rules from getting evaluated further
*/
private def recall(p: super.Parser[_], in: PackratReader[Elem]): Option[MemoEntry[_]] = {
val cached = in.getFromCache(p)
val head = in.recursionHeads.get(in.pos)
head match {
case None => /*no heads*/ cached
case Some(h@Head(hp, involved, evalSet)) => {
//heads found
if(cached == None && !(hp::involved contains p)) {
//Nothing in the cache, and p is not involved
return Some(MemoEntry(Right(Failure("dummy ",in))))
}
if(evalSet contains p){
//something in cache, and p is in the evalSet
//remove the rule from the evalSet of the Head
h.evalSet = h.evalSet.filterNot(_==p)
val tempRes = p(in)
//we know that cached has an entry here
val tempEntry: MemoEntry[_] = cached.get // match {case Some(x: MemoEntry[_]) => x}
//cache is modified
tempEntry.r = Right(tempRes)
}
cached
}
}
}
/*
* setting up the left-recursion. We have the LR for the rule head
* we modify the involvedSets of all LRs in the stack, till we see
* the current parser again
*/
private def setupLR(p: Parser[_], in: PackratReader[_], recDetect: LR): Unit = {
if(recDetect.head == None) recDetect.head = Some(Head(p, Nil, Nil))
in.lrStack.takeWhile(_.rule != p).foreach {x =>
x.head = recDetect.head
recDetect.head.map(h => h.involvedSet = x.rule::h.involvedSet)
}
}
/*
* growing, if needed the recursion
* check whether the parser we are growing is the head of the rule.
* Not => no grow
*/
/*
* Once the result of the recall function is known, if it is nil, then we need to store a dummy
failure into the cache (much like in the previous listings) and compute the future parse. If it
is not, however, this means we have detected a recursion, and we use the setupLR function
to update each parser involved in the recursion.
*/
private def lrAnswer[T](p: Parser[T], in: PackratReader[Elem], growable: LR): ParseResult[T] = growable match {
//growable will always be having a head, we can't enter lrAnswer otherwise
case LR(seed ,rule, Some(head)) =>
if(head.getHead != p) /*not head rule, so not growing*/ seed.asInstanceOf[ParseResult[T]]
else {
in.updateCacheAndGet(p, MemoEntry(Right[LR, ParseResult[T]](seed.asInstanceOf[ParseResult[T]])))
seed match {
case f@Failure(_,_) => f
case e@Error(_,_) => e
case s@Success(_,_) => /*growing*/ grow(p, in, head)
}
}
case _=> throw new Exception("lrAnswer with no head !!")
}
//p here should be strict (cannot be non-strict) !!
//failing left-recursive grammars: This is done by simply storing a failure if nothing is found
/**
* Explicitly convert a given parser to a memoizing packrat parser.
* In most cases, client code should avoid calling <code>memo</code> directly
* and rely on implicit conversion instead.
*/
def memo[T](p: super.Parser[T]): PackratParser[T] = {
new PackratParser[T] {
def apply(in: Input) = {
/*
* transformed reader
*/
val inMem = in.asInstanceOf[PackratReader[Elem]]
//look in the global cache if in a recursion
val m = recall(p, inMem)
m match {
//nothing has been done due to recall
case None =>
val base = LR(Failure("Base Failure",in), p, None)
inMem.lrStack = base::inMem.lrStack
//cache base result
inMem.updateCacheAndGet(p,MemoEntry(Left(base)))
//parse the input
val tempRes = p(in)
//the base variable has passed equality tests with the cache
inMem.lrStack = inMem.lrStack.tail
//check whether base has changed, if yes, we will have a head
base.head match {
case None =>
/*simple result*/
inMem.updateCacheAndGet(p,MemoEntry(Right(tempRes)))
tempRes
case s@Some(_) =>
/*non simple result*/
base.seed = tempRes
//the base variable has passed equality tests with the cache
val res = lrAnswer(p, inMem, base)
res
}
case Some(mEntry) => {
//entry found in cache
mEntry match {
case MemoEntry(Left(recDetect)) => {
setupLR(p, inMem, recDetect)
//all setupLR does is change the heads of the recursions, so the seed will stay the same
recDetect match {case LR(seed, _, _) => seed.asInstanceOf[ParseResult[T]]}
}
case MemoEntry(Right(res: ParseResult[_])) => res.asInstanceOf[ParseResult[T]]
}
}
}
}
}
}
private def grow[T](p: super.Parser[T], rest: PackratReader[Elem], head: Head): ParseResult[T] = {
//store the head into the recursionHeads
rest.recursionHeads.put(rest.pos, head /*match {case Head(hp,involved,_) => Head(hp,involved,involved)}*/)
val oldRes: ParseResult[T] = rest.getFromCache(p).get match {
case MemoEntry(Right(x)) => x.asInstanceOf[ParseResult[T]]
case _ => throw new Exception("impossible match")
}
//resetting the evalSet of the head of the recursion at each beginning of growth
head.evalSet = head.involvedSet
val tempRes = p(rest); tempRes match {
case s@Success(_,_) =>
if(getPosFromResult(oldRes) < getPosFromResult(tempRes)) {
rest.updateCacheAndGet(p, MemoEntry(Right(s)))
grow(p, rest, head)
} else {
//we're done with growing, we can remove data from recursion head
rest.recursionHeads -= rest.pos
rest.getFromCache(p).get match {
case MemoEntry(Right(x: ParseResult[_])) => x.asInstanceOf[ParseResult[T]]
case _ => throw new Exception("impossible match")
}
}
case f =>
rest.recursionHeads -= rest.pos
/*rest.updateCacheAndGet(p, MemoEntry(Right(f)));*/oldRes
}
}
}
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