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AIMA-Java Core Algorithms from the book Artificial Intelligence a Modern Approach 3rd Ed.

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package aima.core.search.csp;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Hashtable;
import java.util.List;

/**
 * Artificial Intelligence A Modern Approach (3rd Ed.): Section 6.1, Page 202.

 * 

 * A constraint satisfaction problem or CSP consists of three components, X, D,
 * and C:
 * 
 * X is a set of variables, {X1, ... ,Xn}.
 * D is a set of domains, {D1, ... ,Dn}, one for each variable.
 * C is a set of constraints that specify allowable combinations of values.
 * 
 * 
 * @author Ruediger Lunde
 */
public class CSP {

	private List variables;
	private List domains;
	private List constraints;

	/** Lookup, which maps a variable to its index in the list of variables. */
	private Hashtable varIndexHash;
	/**
	 * Constraint network. Maps variables to those constraints in which they
	 * participate.
	 */
	private Hashtable> cnet;

	/** Creates a new CSP. */
	public CSP() {
		variables = new ArrayList();
		domains = new ArrayList();
		constraints = new ArrayList();
		varIndexHash = new Hashtable();
		cnet = new Hashtable>();
	}
	
	/** Creates a new CSP. */
	public CSP(List vars) {
		this();
		for (Variable v : vars)
			addVariable(v);
	}

	protected void addVariable(Variable var) {
		if (!varIndexHash.containsKey(var)) {
			Domain emptyDomain = new Domain(Collections.emptyList());
			variables.add(var);
			domains.add(emptyDomain);
			varIndexHash.put(var, variables.size()-1);
			cnet.put(var, new ArrayList());
		} else {
			throw new IllegalArgumentException("Variable with same name already exists.");
		}
	}
	
	public List getVariables() {
		return Collections.unmodifiableList(variables);
	}

	public int indexOf(Variable var) {
		return varIndexHash.get(var);
	}

	public Domain getDomain(Variable var) {
		return domains.get(varIndexHash.get(var));
	}

	public void setDomain(Variable var, Domain domain) {
		domains.set(indexOf(var), domain);
	}

	/**
	 * Replaces the domain of the specified variable by new domain, which
	 * contains all values of the old domain except the specified value.
	 */
	public void removeValueFromDomain(Variable var, Object value) {
		Domain currDomain = getDomain(var);
		List