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+/* 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