ProbCNFGrammar.java

package aima.core.nlp.parsing.grammars;

import java.util.ArrayList;
import java.util.List;

/**
 * The CYK algorithm exploits the structure of grammars in 
 * Chomsky Normal Form in order to parse efficiently. 
 * 
 * A context-free grammar is in Chomsky Normal Form (CNF) if each
 * rule has one of the following forms:
 * 
 * 1. A -> BC,
 * 2. A -> a,
 * 3. S -> null, where B,C are non-start variables (V - {S}), 
 * and a is a terminal.
 * 
 * @author Jonathon
 *
 */
public class ProbCNFGrammar extends ProbContextFreeGrammar {
	
	// TODO: Implement conversion from ContextFree to ChomskyNormalForm
	
	// default constructor
	public ProbCNFGrammar() {
		type = 4; 
		rules = new ArrayList<Rule>();
	}
	
	public ProbCNFGrammar( ProbCNFGrammar grammar ) {
		type = 4;
		rules = grammar.rules;
	}
	
	/**
	 * Add a ruleList as the grammar's rule list if all rules in it pass
	 * both the restrictions of the Context-free grammar, and all rules 
	 * or in Chomsky-Normal-Form
	 */
	public boolean addRules( List<Rule> ruleList ) {
		for( int i=0; i < ruleList.size(); i++ ) {
			if( !validRule(ruleList.get(i)) ) {
				return false;
			}
		}
		if( !validateRuleProbabilities(ruleList)) {
			return false;
		}
		this.rules = ruleList;
		updateVarsAndTerminals();
		return true;
	}
	
	/**
	 * Add a rule to the grammar's rule list if it passes
	 * both the restrictions of the Context-free grammar, and the rule is
	 * in Chomsky Normal Form.
	 */
	public boolean addRule( Rule r ) {
		if( !validRule(r) ) {
			return false;
		}
		rules.add(r);
		updateVarsAndTerminals(r);
		return true;
	}
	
	/**
	 * For a grammar rule to be valid in a context-free grammar, 
	 * all the restrictions of the context-free grammar must hold,
	 * and the rule must be in one of three forms.
	 * 
	 * 1. A -> BC,
	 * 2. A -> a,
	 * 3. S -> null, where B,C are non-start variables (V - {S}), 
	 * and a is a terminal.
	 * 
	 * @param r,  a rule
	 * @return true, if rule is in CNF. false, otherwise
	 */
	public boolean validRule( Rule r ){
		if( !super.validRule(r) ){
			return false;
		}
		// 3. rhs is null
		if( r.rhs == null || r.rhs.size() == 0) { return true; }
		// 2. rhs is a terminal (a)
		else if( r.rhs.size() == 1 && isTerminal(r.rhs.get(0))) {
			return true;
		}
		// 1. rhs is 2 variables (BC)
		else if( r.rhs.size() == 2 && isVariable(r.rhs.get(0))
				  && isVariable(r.rhs.get(1))) { 
			return true;
		}
		// rule is not in one of the 3 CNF forms
		return false;
	}
	
} // end of CNFGrammar()