baserecognizer.java

ANTLR(ANother Tool for Language Recognition)它是这样的一种工具

package org.antlr.runtime;

import java.util.*;

/** A generic recognizer that can handle recognizers generated from
 *  lexer, parser, and tree grammars.  This is all the parsing
 *  support code essentially; most of it is error recovery stuff and
 *  backtracking.
 */
public abstract class BaseRecognizer {
	public static final int MEMO_RULE_FAILED = -2;
	public static final int MEMO_RULE_UNKNOWN = -1;
	public static final int INITIAL_FOLLOW_STACK_SIZE = 100;

	// copies from Token object for convenience in actions
	public static final int DEFAULT_TOKEN_CHANNEL = Token.DEFAULT_CHANNEL;
	public static final int HIDDEN = Token.HIDDEN_CHANNEL;

	/** Track the set of token types that can follow any rule invocation.
	 *  Stack grows upwards.  When it hits the max, it grows 2x in size
	 *  and keeps going.
	 */
	protected BitSet[] following = new BitSet[INITIAL_FOLLOW_STACK_SIZE];
	protected int _fsp = -1;

	/** This is true when we see an error and before having successfully
	 *  matched a token.  Prevents generation of more than one error message
	 *  per error.
	 */
	protected boolean errorRecovery = false;

	/** The index into the input stream where the last error occurred.
	 * 	This is used to prevent infinite loops where an error is found
	 *  but no token is consumed during recovery...another error is found,
	 *  ad naseum.  This is a failsafe mechanism to guarantee that at least
	 *  one token/tree node is consumed for two errors.
	 */
	protected int lastErrorIndex = -1;

	/** In lieu of a return value, this indicates that a rule or token
	 *  has failed to match.  Reset to false upon valid token match.
	 */
	protected boolean failed = false;

	/** If 0, no backtracking is going on.  Safe to exec actions etc...
	 *  If >0 then it's the level of backtracking.
	 */
	protected int backtracking = 0;

	/** An array[size num rules] of Map<Integer,Integer> that tracks
	 *  the stop token index for each rule.  ruleMemo[ruleIndex] is
	 *  the memoization table for ruleIndex.  For key ruleStartIndex, you
	 *  get back the stop token for associated rule or MEMO_RULE_FAILED.
	 *
	 *  This is only used if rule memoization is on (which it is by default).
	 */
	protected Map[] ruleMemo;

	/** reset the parser's state */
	public void reset() {
		// TODO: implement this!
		//following.setSize(0);
	}

	/** Match current input symbol against ttype.  Upon error, do one token
	 *  insertion or deletion if possible.  You can override to not recover
	 *  here and bail out of the current production to the normal error
	 *  exception catch (at the end of the method) by just throwing
	 *  MismatchedTokenException upon input.LA(1)!=ttype.
	 */
	public void match(IntStream input, int ttype, BitSet follow)
		throws RecognitionException
	{
		if ( input.LA(1)==ttype ) {
			input.consume();
			errorRecovery = false;
			failed = false;
			return;
		}
		if ( backtracking>0 ) {
			failed = true;
			return;
		}
		mismatch(input, ttype, follow);
		return;
	}

	public void matchAny(IntStream input) {
		errorRecovery = false;
		failed = false;
		input.consume();
	}

	/** factor out what to do upon token mismatch so tree parsers can behave
	 *  differently.
	 */
	protected void mismatch(IntStream input, int ttype, BitSet follow)
		throws RecognitionException
	{
		MismatchedTokenException mte =
			new MismatchedTokenException(ttype, input);
		recoverFromMismatchedToken(input, mte, ttype, follow);
	}

	/** Report a recognition problem.  Java is not polymorphic on the
	 *  argument types so you have to check the type of exception yourself.
	 *  That's not very clean but it's better than generating a bunch of
	 *  catch clauses in each rule and makes it easy to extend with
	 *  more exceptions w/o breaking old code.
	 *
	 *  This method sets errorRecovery to indicate the parser is recovering
	 *  not parsing.  Once in recovery mode, no errors are generated.
	 *  To get out of recovery mode, the parser must successfully match
	 *  a token (after a resync).  So it will go:
	 *
	 * 		1. error occurs
	 * 		2. enter recovery mode, report error
	 * 		3. consume until token found in resynch set
	 * 		4. try to resume parsing
	 * 		5. next match() will reset errorRecovery mode
	 */
	public void reportError(RecognitionException e) {
		// if we've already reported an error and have not matched a token
		// yet successfully, don't report any errors.
		if ( errorRecovery ) {
			//System.err.print("[SPURIOUS] ");
			return;
		}
		errorRecovery = true;

		displayRecognitionError(this.getClass().getName(),
								this.getTokenNames(),
								e);
	}

	public static void displayRecognitionError(String name,
											   String[] tokenNames,
											   RecognitionException e)
	{
		System.err.print(getRuleInvocationStack(e, name)+
						 ": line "+e.line+":"+e.charPositionInLine+" ");
		if ( e instanceof MismatchedTokenException ) {
			MismatchedTokenException mte = (MismatchedTokenException)e;
			String tokenName="<unknown>";
			if ( mte.expecting==Token.EOF ) {
				tokenName = "EOF";
			}
			else {
				tokenName = tokenNames[mte.expecting];
			}
			System.err.println("mismatched token: "+
							   e.token+
							   "; expecting type "+
							   tokenName);
		}
		else if ( e instanceof MismatchedTreeNodeException ) {
			MismatchedTreeNodeException mtne = (MismatchedTreeNodeException)e;
			String tokenName="<unknown>";
			if ( mtne.expecting==Token.EOF ) {
				tokenName = "EOF";
			}
			else {
				tokenName = tokenNames[mtne.expecting];
			}
			System.err.println("mismatched tree node: "+
							   mtne.foundNode+
							   "; expecting type "+
							   tokenName);
		}
		else if ( e instanceof NoViableAltException ) {
			NoViableAltException nvae = (NoViableAltException)e;
			System.err.println(//"decision=<<"+nvae.grammarDecisionDescription+">>"+
							   "state "+nvae.stateNumber+
							   " (decision="+nvae.decisionNumber+
							   ") no viable alt; token="+
							   e.token);
		}
		else if ( e instanceof EarlyExitException ) {
			EarlyExitException eee = (EarlyExitException)e;
			System.err.println("required (...)+ loop (decision="+
							   eee.decisionNumber+
							   ") did not match anything; token="+
							   e.token);
		}
		else if ( e instanceof MismatchedSetException ) {
			MismatchedSetException mse = (MismatchedSetException)e;
			System.err.println("mismatched token: "+
							   e.token+
							   "; expecting set "+mse.expecting);
		}
		else if ( e instanceof MismatchedNotSetException ) {
			MismatchedNotSetException mse = (MismatchedNotSetException)e;
			System.err.println("mismatched token: "+
							   e.token+
							   "; expecting set "+mse.expecting);
		}
		else if ( e instanceof FailedPredicateException ) {
			FailedPredicateException fpe = (FailedPredicateException)e;
			System.err.println("rule "+fpe.ruleName+" failed predicate: {"+
							   fpe.predicateText+"}?");
		}
	}

	/** Recover from an error found on the input stream.  Mostly this is
	 *  NoViableAlt exceptions, but could be a mismatched token that
	 *  the match() routine could not recover from.
	 */
	public void recover(IntStream input, RecognitionException re) {
		if ( lastErrorIndex==input.index() ) {
			// uh oh, another error at same token index; must be a case
			// where LT(1) is in the recovery token set so nothing is
			// consumed; consume a single token so at least to prevent
			// an infinite loop; this is a failsafe.
			input.consume();
		}
		lastErrorIndex = input.index();
		BitSet followSet = computeErrorRecoverySet();
		beginResync();
		consumeUntil(input, followSet);
		endResync();
	}

	/** A hook to listen in on the token consumption during error recovery.
	 *  The DebugParser subclasses this to fire events to the listenter.
	 */
	public void beginResync() {
	}

	public void endResync() {
	}

	/*  Compute the error recovery set for the current rule.  During
	 *  rule invocation, the parser pushes the set of tokens that can
	 *  follow that rule reference on the stack; this amounts to
	 *  computing FIRST of what follows the rule reference in the
	 *  enclosing rule. This local follow set only includes tokens
	 *  from within the rule; i.e., the FIRST computation done by
	 *  ANTLR stops at the end of a rule.
	 *
	 *  EXAMPLE
	 *
	 *  When you find a "no viable alt exception", the input is not
	 *  consistent with any of the alternatives for rule r.  The best
	 *  thing to do is to consume tokens until you see something that
	 *  can legally follow a call to r *or* any rule that called r.
	 *  You don't want the exact set of viable next tokens because the
	 *  input might just be missing a token--you might consume the
	 *  rest of the input looking for one of the missing tokens.
	 *
	 *  Consider grammar:
	 *
	 *  a : '[' b ']'
	 *    | '(' b ')'
	 *    ;
	 *  b : c '^' INT ;
	 *  c : ID
	 *    | INT
	 *    ;
	 *
	 *  At each rule invocation, the set of tokens that could follow
	 *  that rule is pushed on a stack.  Here are the various "local"
	 *  follow sets:
	 *
	 *  FOLLOW(b1_in_a) = FIRST(']') = ']'
	 *  FOLLOW(b2_in_a) = FIRST(')') = ')'
	 *  FOLLOW(c_in_b) = FIRST('^') = '^'
	 *
	 *  Upon erroneous input "[]", the call chain is
	 *
	 *  a -> b -> c
	 *
	 *  and, hence, the follow context stack is:
	 *
	 *  depth  local follow set     after call to rule
	 *    0         <EOF>                    a (from main())
	 *    1          ']'                     b
	 *    3          '^'                     c
	 *
	 *  Notice that ')' is not included, because b would have to have
	 *  been called from a different context in rule a for ')' to be
	 *  included.
	 *
	 *  For error recovery, we cannot consider FOLLOW(c)
	 *  (context-sensitive or otherwise).  We need the combined set of
	 *  all context-sensitive FOLLOW sets--the set of all tokens that
	 *  could follow any reference in the call chain.  We need to
	 *  resync to one of those tokens.  Note that FOLLOW(c)='^' and if
	 *  we resync'd to that token, we'd consume until EOF.  We need to
	 *  sync to context-sensitive FOLLOWs for a, b, and c: {']','^'}.
	 *  In this case, for input "[]", LA(1) is in this set so we would
	 *  not consume anything and after printing an error rule c would
	 *  return normally.  It would not find the required '^' though.
	 *  At this point, it gets a mismatched token error and throws an
	 *  exception (since LA(1) is not in the viable following token
	 *  set).  The rule exception handler tries to recover, but finds
	 *  the same recovery set and doesn't consume anything.  Rule b
	 *  exits normally returning to rule a.  Now it finds the ']' (and
	 *  with the successful match exits errorRecovery mode).
	 *
	 *  So, you cna see that the parser walks up call chain looking
	 *  for the token that was a member of the recovery set.
	 *
	 *  Errors are not generated in errorRecovery mode.
	 *
	 *  ANTLR's error recovery mechanism is based upon original ideas:
	 *
	 *  "Algorithms + Data Structures = Programs" by Niklaus Wirth
	 *
	 *  and
	 *
	 *  "A note on error recovery in recursive descent parsers":
	 *  http://portal.acm.org/citation.cfm?id=947902.947905
	 *
	 *  Later, Josef Grosch had some good ideas:
	 *
	 *  "Efficient and Comfortable Error Recovery in Recursive Descent
	 *  Parsers":
	 *  ftp://www.cocolab.com/products/cocktail/doca4.ps/ell.ps.zip
	 *
	 *  Like Grosch I implemented local FOLLOW sets that are combined
	 *  at run-time upon error to avoid overhead during parsing.
	 */
	protected BitSet computeErrorRecoverySet() {
		return combineFollows(false);
	}

	/** Compute the context-sensitive FOLLOW set for current rule.
	 *  This is set of token types that can follow a specific rule
	 *  reference given a specific call chain.  You get the set of
	 *  viable tokens that can possibly come next (lookahead depth 1)
	 *  given the current call chain.  Contrast this with the
	 *  definition of plain FOLLOW for rule r:
	 *
	 *   FOLLOW(r)={x | S=>*alpha r beta in G and x in FIRST(beta)}
	 *
	 *  where x in T* and alpha, beta in V*; T is set of terminals and
	 *  V is the set of terminals and nonterminals.  In other words,
	 *  FOLLOW(r) is the set of all tokens that can possibly follow
	 *  references to r in *any* sentential form (context).  At
	 *  runtime, however, we know precisely which context applies as
	 *  we have the call chain.  We may compute the exact (rather
	 *  than covering superset) set of following tokens.
	 *
	 *  For example, consider grammar:
	 *
	 *  stat : ID '=' expr ';'      // FOLLOW(stat)=={EOF}
	 *       | "return" expr '.'
	 *       ;
	 *  expr : atom ('+' atom)* ;   // FOLLOW(expr)=={';','.',')'}
	 *  atom : INT                  // FOLLOW(atom)=={'+',')',';','.'}
	 *       | '(' expr ')'
	 *       ;
	 *
	 *  The FOLLOW sets are all inclusive whereas context-sensitive
	 *  FOLLOW sets are precisely what could follow a rule reference.
	 *  For input input "i=(3);", here is the derivation:
	 *
	 *  stat => ID '=' expr ';'
	 *       => ID '=' atom ('+' atom)* ';'