xref: /external/antlr/antlr-3.4/tool/src/main/java/org/antlr/analysis/NFAContext.java

/*
 * [The "BSD license"]
 *  Copyright (c) 2010 Terence Parr
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package org.antlr.analysis;

/** A tree node for tracking the call chains for NFAs that invoke
 *  other NFAs.  These trees only have to point upwards to their parents
 *  so we can walk back up the tree (i.e., pop stuff off the stack).  We
 *  never walk from stack down down through the children.
 *
 *  Each alt predicted in a decision has its own context tree,
 *  representing all possible return nodes.  The initial stack has
 *  EOF ("$") in it.  So, for m alternative productions, the lookahead
 *  DFA will have m NFAContext trees.
 *
 *  To "push" a new context, just do "new NFAContext(context-parent, state)"
 *  which will add itself to the parent.  The root is NFAContext(null, null).
 *
 *  The complete context for an NFA configuration is the set of invoking states
 *  on the path from this node thru the parent pointers to the root.
 */
public class NFAContext {
	/** This is similar to Bermudez's m constant in his LAR(m) where
	 *  you bound the stack so your states don't explode.  The main difference
	 *  is that I bound only recursion on the stack, not the simple stack size.
	 *  This looser constraint will let the conversion roam further to find
	 *  lookahead to resolve a decision.
	 *
	 *  Bermudez's m operates differently as it is his LR stack depth
	 *  I'm pretty sure it therefore includes all stack symbols.  Here I
	 *  restrict the size of an NFA configuration to be finite because a
	 *  stack component may mention the same NFA invocation state at
	 *  most m times.  Hence, the number of DFA states will not grow forever.
	 *  With recursive rules like
	 *
	 *    e : '(' e ')' | INT ;
	 *
	 *  you could chase your tail forever if somebody said "s : e '.' | e ';' ;"
	 *  This constant prevents new states from being created after a stack gets
	 *  "too big".  Actually (12/14/2007) I realize that this example is
	 *  trapped by the non-LL(*) detector for recursion in > 1 alt.  Here is
	 *  an example that trips stack overflow:
	 *
	 *	  s : a Y | A A A A A X ; // force recursion past m=4
	 *	  a : A a | Q;
	 *
	 *  If that were:
	 *
	 *	  s : a Y | A+ X ;
	 *
	 *  it could loop forever.
	 *
	 *  Imagine doing a depth-first search on the e DFA...as you chase an input
	 *  sequence you can recurse to same rule such as e above.  You'd have a
	 *  chain of ((((.  When you get do some point, you have to give up.  The
	 *  states in the chain will have longer and longer NFA config stacks.
	 *  Must limit size.
	 *
	 *  max=0 implies you cannot ever jump to another rule during closure.
	 *  max=1 implies you can make as many calls as you want--you just
	 *        can't ever visit a state that is on your rule invocation stack.
	 * 		  I.e., you cannot ever recurse.
	 *  max=2 implies you are able to recurse once (i.e., call a rule twice
	 *  	  from the same place).
	 *
	 *  This tracks recursion to a rule specific to an invocation site!
	 *  It does not detect multiple calls to a rule from different rule
	 *  invocation states.  We are guaranteed to terminate because the
	 *  stack can only grow as big as the number of NFA states * max.
	 *
	 *  I noticed that the Java grammar didn't work with max=1, but did with
	 *  max=4.  Let's set to 4. Recursion is sometimes needed to resolve some
	 *  fixed lookahead decisions.
	 */
	public static int MAX_SAME_RULE_INVOCATIONS_PER_NFA_CONFIG_STACK = 4;

    public NFAContext parent;

    /** The NFA state that invoked another rule's start state is recorded
     *  on the rule invocation context stack.
     */
    public NFAState invokingState;

    /** Computing the hashCode is very expensive and closureBusy()
     *  uses it to track when it's seen a state|ctx before to avoid
     *  infinite loops.  As we add new contexts, record the hash code
     *  as this.invokingState + parent.cachedHashCode.  Avoids walking
     *  up the tree for every hashCode().  Note that this caching works
     *  because a context is a monotonically growing tree of context nodes
     *  and nothing on the stack is ever modified...ctx just grows
     *  or shrinks.
     */
    protected int cachedHashCode;

    public NFAContext(NFAContext parent, NFAState invokingState) {
        this.parent = parent;
        this.invokingState = invokingState;
        if ( invokingState!=null ) {
            this.cachedHashCode = invokingState.stateNumber;
        }
        if ( parent!=null ) {
            this.cachedHashCode += parent.cachedHashCode;
        }
    }

	/** Two contexts are equals() if both have
	 *  same call stack; walk upwards to the root.
	 *  Recall that the root sentinel node has no invokingStates and no parent.
	 *  Note that you may be comparing contexts in different alt trees.
	 *
	 *  The hashCode is now cheap as it's computed once upon each context
	 *  push on the stack.  Use it to make equals() more efficient.
	 */
	public boolean equals(Object o) {
		NFAContext other = ((NFAContext)o);
		if ( this.cachedHashCode != other.cachedHashCode ) {
			return false; // can't be same if hash is different
		}
		if ( this==other ) {
			return true;
		}
		// System.out.println("comparing "+this+" with "+other);
		NFAContext sp = this;
		while ( sp.parent!=null && other.parent!=null ) {
			if ( sp.invokingState != other.invokingState ) {
				return false;
			}
			sp = sp.parent;
			other = other.parent;
		}
		if ( !(sp.parent==null && other.parent==null) ) {
			return false; // both pointers must be at their roots after walk
		}
		return true;
	}

	/** Two contexts conflict() if they are equals() or one is a stack suffix
	 *  of the other.  For example, contexts [21 12 $] and [21 9 $] do not
	 *  conflict, but [21 $] and [21 12 $] do conflict.  Note that I should
	 *  probably not show the $ in this case.  There is a dummy node for each
	 *  stack that just means empty; $ is a marker that's all.
	 *
	 *  This is used in relation to checking conflicts associated with a
	 *  single NFA state's configurations within a single DFA state.
	 *  If there are configurations s and t within a DFA state such that
	 *  s.state=t.state && s.alt != t.alt && s.ctx conflicts t.ctx then
	 *  the DFA state predicts more than a single alt--it's nondeterministic.
	 *  Two contexts conflict if they are the same or if one is a suffix
	 *  of the other.
	 *
	 *  When comparing contexts, if one context has a stack and the other
	 *  does not then they should be considered the same context.  The only
	 *  way for an NFA state p to have an empty context and a nonempty context
	 *  is the case when closure falls off end of rule without a call stack
	 *  and re-enters the rule with a context.  This resolves the issue I
	 *  discussed with Sriram Srinivasan Feb 28, 2005 about not terminating
	 *  fast enough upon nondeterminism.
	 */
	public boolean conflictsWith(NFAContext other) {
		return this.suffix(other); // || this.equals(other);
	}

	/** [$] suffix any context
	 *  [21 $] suffix [21 12 $]
	 *  [21 12 $] suffix [21 $]
	 *  [21 18 $] suffix [21 18 12 9 $]
	 *  [21 18 12 9 $] suffix [21 18 $]
	 *  [21 12 $] not suffix [21 9 $]
	 *
	 *  Example "[21 $] suffix [21 12 $]" means: rule r invoked current rule
	 *  from state 21.  Rule s invoked rule r from state 12 which then invoked
	 *  current rule also via state 21.  While the context prior to state 21
	 *  is different, the fact that both contexts emanate from state 21 implies
	 *  that they are now going to track perfectly together.  Once they
	 *  converged on state 21, there is no way they can separate.  In other
	 *  words, the prior stack state is not consulted when computing where to
	 *  go in the closure operation.  ?$ and ??$ are considered the same stack.
	 *  If ? is popped off then $ and ?$ remain; they are now an empty and
	 *  nonempty context comparison.  So, if one stack is a suffix of
	 *  another, then it will still degenerate to the simple empty stack
	 *  comparison case.
	 */
	protected boolean suffix(NFAContext other) {
		NFAContext sp = this;
		// if one of the contexts is empty, it never enters loop and returns true
		while ( sp.parent!=null && other.parent!=null ) {
			if ( sp.invokingState != other.invokingState ) {
				return false;
			}
			sp = sp.parent;
			other = other.parent;
		}
		//System.out.println("suffix");
		return true;
	}

    /** Walk upwards to the root of the call stack context looking
     *  for a particular invoking state.
	public boolean contains(int state) {
        NFAContext sp = this;
		int n = 0; // track recursive invocations of state
		System.out.println("this.context is "+sp);
		while ( sp.parent!=null ) {
            if ( sp.invokingState.stateNumber == state ) {
				return true;
            }
            sp = sp.parent;
        }
        return false;
    }
	 */

	/** Given an NFA state number, how many times has the NFA-to-DFA
	 *  conversion pushed that state on the stack?  In other words,
	 *  the NFA state must be a rule invocation state and this method
	 *  tells you how many times you've been to this state.  If none,
	 *  then you have not called the target rule from this state before
	 *  (though another NFA state could have called that target rule).
	 *  If n=1, then you've been to this state before during this
	 *  DFA construction and are going to invoke that rule again.
	 *
	 *  Note that many NFA states can invoke rule r, but we ignore recursion
	 *  unless you hit the same rule invocation state again.
	 */
	public int recursionDepthEmanatingFromState(int state) {
		NFAContext sp = this;
		int n = 0; // track recursive invocations of target from this state
		//System.out.println("this.context is "+sp);
		while ( sp.parent!=null ) {
			if ( sp.invokingState.stateNumber == state ) {
				n++;
			}
			sp = sp.parent;
		}
		return n;
	}

    public int hashCode() {
        return cachedHashCode;
        /*
        int h = 0;
        NFAContext sp = this;
        while ( sp.parent!=null ) {
            h += sp.invokingState.getStateNumber();
            sp = sp.parent;
        }
        return h;
        */
    }

	/** A context is empty if there is no parent; meaning nobody pushed
	 *  anything on the call stack.
	 */
	public boolean isEmpty() {
		return parent==null;
	}

    public String toString() {
        StringBuffer buf = new StringBuffer();
        NFAContext sp = this;
        buf.append("[");
        while ( sp.parent!=null ) {
            buf.append(sp.invokingState.stateNumber);
            buf.append(" ");
            sp = sp.parent;
        }
        buf.append("$]");
        return buf.toString();
    }
}