DefaultErrorStrategy.h

/* Copyright (c) 2012-2017 The ANTLR Project. All rights reserved.
 * Use of this file is governed by the BSD 3-clause license that
 * can be found in the LICENSE.txt file in the project root.
 */

#pragma once

#include "ANTLRErrorStrategy.h"
#include "misc/IntervalSet.h"

namespace antlr4 {

  /**
   * This is the default implementation of {@link ANTLRErrorStrategy} used for
   * error reporting and recovery in ANTLR parsers.
   */
  class ANTLR4CPP_PUBLIC DefaultErrorStrategy : public ANTLRErrorStrategy {
  public:
    DefaultErrorStrategy();
    DefaultErrorStrategy(DefaultErrorStrategy const& other) = delete;
    virtual ~DefaultErrorStrategy();

    DefaultErrorStrategy& operator = (DefaultErrorStrategy const& other) = delete;

  protected:
    /**
     * Indicates whether the error strategy is currently "recovering from an
     * error". This is used to suppress reporting multiple error messages while
     * attempting to recover from a detected syntax error.
     *
     * @see #inErrorRecoveryMode
     */
    bool errorRecoveryMode;

    /** 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 nauseum.  This is a failsafe mechanism to guarantee that at least
     *  one token/tree node is consumed for two errors.
     */
    int lastErrorIndex;

    misc::IntervalSet lastErrorStates;

    /// <summary>
    /// {@inheritDoc}
    /// <p/>
    /// The default implementation simply calls <seealso cref="#endErrorCondition"/> to
    /// ensure that the handler is not in error recovery mode.
    /// </summary>
  public:
    virtual void reset(Parser *recognizer) override;

    /// <summary>
    /// This method is called to enter error recovery mode when a recognition
    /// exception is reported.
    /// </summary>
    /// <param name="recognizer"> the parser instance </param>
  protected:
    virtual void beginErrorCondition(Parser *recognizer);

    /// <summary>
    /// {@inheritDoc}
    /// </summary>
  public:
    virtual bool inErrorRecoveryMode(Parser *recognizer) override;

    /// <summary>
    /// This method is called to leave error recovery mode after recovering from
    /// a recognition exception.
    /// </summary>
    /// <param name="recognizer"> </param>
  protected:
    virtual void endErrorCondition(Parser *recognizer);

    /// <summary>
    /// {@inheritDoc}
    /// <p/>
    /// The default implementation simply calls <seealso cref="#endErrorCondition"/>.
    /// </summary>
  public:
    virtual void reportMatch(Parser *recognizer) override;

    /// {@inheritDoc}
    /// <p/>
    /// The default implementation returns immediately if the handler is already
    /// in error recovery mode. Otherwise, it calls <seealso cref="#beginErrorCondition"/>
    /// and dispatches the reporting task based on the runtime type of {@code e}
    /// according to the following table.
    ///
    /// <ul>
    /// <li><seealso cref="NoViableAltException"/>: Dispatches the call to
    /// <seealso cref="#reportNoViableAlternative"/></li>
    /// <li><seealso cref="InputMismatchException"/>: Dispatches the call to
    /// <seealso cref="#reportInputMismatch"/></li>
    /// <li><seealso cref="FailedPredicateException"/>: Dispatches the call to
    /// <seealso cref="#reportFailedPredicate"/></li>
    /// <li>All other types: calls <seealso cref="Parser#notifyErrorListeners"/> to report
    /// the exception</li>
    /// </ul>
    virtual void reportError(Parser *recognizer, const RecognitionException &e) override;

    /// <summary>
    /// {@inheritDoc}
    /// <p/>
    /// The default implementation resynchronizes the parser by consuming tokens
    /// until we find one in the resynchronization set--loosely the set of tokens
    /// that can follow the current rule.
    /// </summary>
    virtual void recover(Parser *recognizer, std::exception_ptr e) override;

    /**
     * The default implementation of {@link ANTLRErrorStrategy#sync} makes sure
     * that the current lookahead symbol is consistent with what were expecting
     * at this point in the ATN. You can call this anytime but ANTLR only
     * generates code to check before subrules/loops and each iteration.
     *
     * <p>Implements Jim Idle's magic sync mechanism in closures and optional
     * subrules. E.g.,</p>
     *
     * <pre>
     * a : sync ( stuff sync )* ;
     * sync : {consume to what can follow sync} ;
     * </pre>
     *
     * At the start of a sub rule upon error, {@link #sync} performs single
     * token deletion, if possible. If it can't do that, it bails on the current
     * rule and uses the default error recovery, which consumes until the
     * resynchronization set of the current rule.
     *
     * <p>If the sub rule is optional ({@code (...)?}, {@code (...)*}, or block
     * with an empty alternative), then the expected set includes what follows
     * the subrule.</p>
     *
     * <p>During loop iteration, it consumes until it sees a token that can start a
     * sub rule or what follows loop. Yes, that is pretty aggressive. We opt to
     * stay in the loop as long as possible.</p>
     *
     * <p><strong>ORIGINS</strong></p>
     *
     * <p>Previous versions of ANTLR did a poor job of their recovery within loops.
     * A single mismatch token or missing token would force the parser to bail
     * out of the entire rules surrounding the loop. So, for rule</p>
     *
     * <pre>
     * classDef : 'class' ID '{' member* '}'
     * </pre>
     *
     * input with an extra token between members would force the parser to
     * consume until it found the next class definition rather than the next
     * member definition of the current class.
     *
     * <p>This functionality cost a little bit of effort because the parser has to
     * compare token set at the start of the loop and at each iteration. If for
     * some reason speed is suffering for you, you can turn off this
     * functionality by simply overriding this method as a blank { }.</p>
     */
    virtual void sync(Parser *recognizer) override;

    /// <summary>
    /// This is called by <seealso cref="#reportError"/> when the exception is a
    /// <seealso cref="NoViableAltException"/>.
    /// </summary>
    /// <seealso cref= #reportError
    /// </seealso>
    /// <param name="recognizer"> the parser instance </param>
    /// <param name="e"> the recognition exception </param>
  protected:
    virtual void reportNoViableAlternative(Parser *recognizer, const NoViableAltException &e);

    /// <summary>
    /// This is called by <seealso cref="#reportError"/> when the exception is an
    /// <seealso cref="InputMismatchException"/>.
    /// </summary>
    /// <seealso cref= #reportError
    /// </seealso>
    /// <param name="recognizer"> the parser instance </param>
    /// <param name="e"> the recognition exception </param>
    virtual void reportInputMismatch(Parser *recognizer, const InputMismatchException &e);

    /// <summary>
    /// This is called by <seealso cref="#reportError"/> when the exception is a
    /// <seealso cref="FailedPredicateException"/>.
    /// </summary>
    /// <seealso cref= #reportError
    /// </seealso>
    /// <param name="recognizer"> the parser instance </param>
    /// <param name="e"> the recognition exception </param>
    virtual void reportFailedPredicate(Parser *recognizer, const FailedPredicateException &e);

    /**
     * This method is called to report a syntax error which requires the removal
     * of a token from the input stream. At the time this method is called, the
     * erroneous symbol is current {@code LT(1)} symbol and has not yet been
     * removed from the input stream. When this method returns,
     * {@code recognizer} is in error recovery mode.
     *
     * <p>This method is called when {@link #singleTokenDeletion} identifies
     * single-token deletion as a viable recovery strategy for a mismatched
     * input error.</p>
     *
     * <p>The default implementation simply returns if the handler is already in
     * error recovery mode. Otherwise, it calls {@link #beginErrorCondition} to
     * enter error recovery mode, followed by calling
     * {@link Parser#notifyErrorListeners}.</p>
     *
     * @param recognizer the parser instance
     */
    virtual void reportUnwantedToken(Parser *recognizer);

    /**
     * This method is called to report a syntax error which requires the
     * insertion of a missing token into the input stream. At the time this
     * method is called, the missing token has not yet been inserted. When this
     * method returns, {@code recognizer} is in error recovery mode.
     *
     * <p>This method is called when {@link #singleTokenInsertion} identifies
     * single-token insertion as a viable recovery strategy for a mismatched
     * input error.</p>
     *
     * <p>The default implementation simply returns if the handler is already in
     * error recovery mode. Otherwise, it calls {@link #beginErrorCondition} to
     * enter error recovery mode, followed by calling
     * {@link Parser#notifyErrorListeners}.</p>
     *
     * @param recognizer the parser instance
     */
    virtual void reportMissingToken(Parser *recognizer);

  public:
    /**
     * {@inheritDoc}
     *
     * <p>The default implementation attempts to recover from the mismatched input
     * by using single token insertion and deletion as described below. If the
     * recovery attempt fails, this method throws an
     * {@link InputMismatchException}.</p>
     *
     * <p><strong>EXTRA TOKEN</strong> (single token deletion)</p>
     *
     * <p>{@code LA(1)} is not what we are looking for. If {@code LA(2)} has the
     * right token, however, then assume {@code LA(1)} is some extra spurious
     * token and delete it. Then consume and return the next token (which was
     * the {@code LA(2)} token) as the successful result of the match operation.</p>
     *
     * <p>This recovery strategy is implemented by {@link #singleTokenDeletion}.</p>
     *
     * <p><strong>MISSING TOKEN</strong> (single token insertion)</p>
     *
     * <p>If current token (at {@code LA(1)}) is consistent with what could come
     * after the expected {@code LA(1)} token, then assume the token is missing
     * and use the parser's {@link TokenFactory} to create it on the fly. The
     * "insertion" is performed by returning the created token as the successful
     * result of the match operation.</p>
     *
     * <p>This recovery strategy is implemented by {@link #singleTokenInsertion}.</p>
     *
     * <p><strong>EXAMPLE</strong></p>
     *
     * <p>For example, Input {@code i=(3;} is clearly missing the {@code ')'}. When
     * the parser returns from the nested call to {@code expr}, it will have
     * call chain:</p>
     *
     * <pre>
     * stat &rarr; expr &rarr; atom
     * </pre>
     *
     * and it will be trying to match the {@code ')'} at this point in the
     * derivation:
     *
     * <pre>
     * =&gt; ID '=' '(' INT ')' ('+' atom)* ';'
     *                    ^
     * </pre>
     *
     * The attempt to match {@code ')'} will fail when it sees {@code ';'} and
     * call {@link #recoverInline}. To recover, it sees that {@code LA(1)==';'}
     * is in the set of tokens that can follow the {@code ')'} token reference
     * in rule {@code atom}. It can assume that you forgot the {@code ')'}.
     */
    virtual Token* recoverInline(Parser *recognizer) override;

    /// <summary>
    /// This method implements the single-token insertion inline error recovery
    /// strategy. It is called by <seealso cref="#recoverInline"/> if the single-token
    /// deletion strategy fails to recover from the mismatched input. If this
    /// method returns {@code true}, {@code recognizer} will be in error recovery
    /// mode.
    /// <p/>
    /// This method determines whether or not single-token insertion is viable by
    /// checking if the {@code LA(1)} input symbol could be successfully matched
    /// if it were instead the {@code LA(2)} symbol. If this method returns
    /// {@code true}, the caller is responsible for creating and inserting a
    /// token with the correct type to produce this behavior.
    /// </summary>
    /// <param name="recognizer"> the parser instance </param>
    /// <returns> {@code true} if single-token insertion is a viable recovery
    /// strategy for the current mismatched input, otherwise {@code false} </returns>
  protected:
    virtual bool singleTokenInsertion(Parser *recognizer);

    /// <summary>
    /// This method implements the single-token deletion inline error recovery
    /// strategy. It is called by <seealso cref="#recoverInline"/> to attempt to recover
    /// from mismatched input. If this method returns null, the parser and error
    /// handler state will not have changed. If this method returns non-null,
    /// {@code recognizer} will <em>not</em> be in error recovery mode since the
    /// returned token was a successful match.
    /// <p/>
    /// If the single-token deletion is successful, this method calls
    /// <seealso cref="#reportUnwantedToken"/> to report the error, followed by
    /// <seealso cref="Parser#consume"/> to actually "delete" the extraneous token. Then,
    /// before returning <seealso cref="#reportMatch"/> is called to signal a successful
    /// match.
    /// </summary>
    /// <param name="recognizer"> the parser instance </param>
    /// <returns> the successfully matched <seealso cref="Token"/> instance if single-token
    /// deletion successfully recovers from the mismatched input, otherwise
    /// {@code null} </returns>
    virtual Token* singleTokenDeletion(Parser *recognizer);

    /// <summary>
    /// Conjure up a missing token during error recovery.
    ///
    ///  The recognizer attempts to recover from single missing
    ///  symbols. But, actions might refer to that missing symbol.
    ///  For example, x=ID {f($x);}. The action clearly assumes
    ///  that there has been an identifier matched previously and that
    ///  $x points at that token. If that token is missing, but
    ///  the next token in the stream is what we want we assume that
    ///  this token is missing and we keep going. Because we
    ///  have to return some token to replace the missing token,
    ///  we have to conjure one up. This method gives the user control
    ///  over the tokens returned for missing tokens. Mostly,
    ///  you will want to create something special for identifier
    ///  tokens. For literals such as '{' and ',', the default
    ///  action in the parser or tree parser works. It simply creates
    ///  a CommonToken of the appropriate type. The text will be the token.
    ///  If you change what tokens must be created by the lexer,
    ///  override this method to create the appropriate tokens.
    /// </summary>
    virtual Token* getMissingSymbol(Parser *recognizer);

    virtual misc::IntervalSet getExpectedTokens(Parser *recognizer);

    /// <summary>
    /// How should a token be displayed in an error message? The default
    ///  is to display just the text, but during development you might
    ///  want to have a lot of information spit out.  Override in that case
    ///  to use t.toString() (which, for CommonToken, dumps everything about
    ///  the token). This is better than forcing you to override a method in
    ///  your token objects because you don't have to go modify your lexer
    ///  so that it creates a new class.
    /// </summary>
    virtual std::string getTokenErrorDisplay(Token *t);

    virtual std::string getSymbolText(Token *symbol);

    virtual size_t getSymbolType(Token *symbol);

    virtual std::string escapeWSAndQuote(const std::string &s) const;

    /*  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. See LinearApproximator.FIRST().
     *  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
     *  context-sensitive 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     follow set       start of rule execution
     *    0         <EOF>                    a (from main())
     *    1          ']'                     b
     *    2          '^'                     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 ']' and in the set, so we would
     *  not consume anything. After printing an error, rule c would
     *  return normally.  Rule b 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 can see that the parser walks up the 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 implement context-sensitive FOLLOW sets that are combined
     *  at run-time upon error to avoid overhead during parsing.
     */
    virtual misc::IntervalSet getErrorRecoverySet(Parser *recognizer);

    /// <summary>
    /// Consume tokens until one matches the given token set. </summary>
    virtual void consumeUntil(Parser *recognizer, const misc::IntervalSet &set);

  private:
    std::vector<std::unique_ptr<Token>> _errorSymbols; // Temporarily created token.
    void InitializeInstanceFields();
  };

} // namespace antlr4