package dict;

import java.lang.*;
import lrs.*;
import schemeFactory.*;

/**
 * An implementation of IDictionary using an LRStruct to hold the
 * DictionaryPairs. 
 */
public class DictLRS implements IDictionary {
    /*
     * A list of DictionaryPairs ordered by key
     */
    private LRStruct _lrs = new LRStruct();

    /**
     * Clears the contents of the dictionary leaving it empty.
     *
     * Implemented by replacing the existing LRStruct with a new,
     * empty one.
     */
    public void clear() {
	_lrs = new LRStruct();
    }

    /**
     * Returns true if the dictionary is empty and false otherwise.
     *
     * Implemented as a visitor to LRStruct.
     */
    public boolean isEmpty() {
	return ((Boolean)_lrs.execute(new IAlgo() {
	    public Object emptyCase(LRStruct host, Object input) {
		return Boolean.TRUE;
	    }

	    public Object nonEmptyCase(LRStruct host, Object input) {
		return Boolean.FALSE;
	    }
	}, null)).booleanValue();
    }

    /**
     * Returns an AList of DictionaryPairs corresponding to the entire
     * contents of the dictionary.
     *
     * Implemented as a visitor to LRStruct.
     */
    public AList elements() {
	return (AList)_lrs.execute(new IAlgo() {
	    IListFactory lf = new CompositeListFactory();

	    public Object emptyCase(LRStruct host, Object input) {
		return lf.makeEmptyList();
	    }

	    public Object nonEmptyCase(LRStruct host, Object input) {
		return lf.makeNEList(host.getFirst(),
				     (AList)host.getRest().execute(this,
								   input));
	    }
	}, null);
    }

    /**
     * Returns the DictionaryPair with the given key.  If there is not
     * a DictionaryPair with the given key, returns null. 
     *
     * Returns a DictionaryPair rather than the value alone so that
     * the user can distinguish between not finding the key and
     * finding the pair (key, null). 
     *
     * Implemented as a visitor to LRStruct.
     */
    public DictionaryPair lookup(Comparable key) {
	return (DictionaryPair)_lrs.execute(new IAlgo() {
	    public Object emptyCase(LRStruct host, Object input) {
		return null;
	    }

	    public Object nonEmptyCase(LRStruct host, Object input) {
		DictionaryPair first = (DictionaryPair)host.getFirst();
		int result = first.getKey().compareTo(input);

		if (result > 0)		// host > input
		    return null;
		else if (result == 0)	// host == input
		    return first;
		else			// host < input
		    return host.getRest().execute(this, input);
	    }
	}, key);
    }

    /**
     * Inserts the given key and value.  If the given key is already
     * in the dictionary, the given value replaces the key's old
     * value. 
     *
     * Implemented as a visitor to LRStruct that inserts the key and
     * value in order.
     */
    public void insert(Comparable key, Object value) {
	_lrs.execute(new IAlgo() {
	    public Object emptyCase(LRStruct host, Object input) {
		return host.insertFront(input);
	    }

	    public Object nonEmptyCase(LRStruct host, Object input) {
		DictionaryPair first = (DictionaryPair)host.getFirst();
		int result = first.compareTo(input);

		if (result > 0)		// host > input
		    return host.insertFront(input);
		else if (result == 0)	// host == input
		    return host.setFirst(input);
		else			// host < input
		    return host.getRest().execute(this, input);
	    }
	}, new DictionaryPair(key, value));
    }

    /**
     * Removes the DictionaryPair with the given key and returns it.
     * If there is not a DictionaryPair with the given key, returns
     * null.
     *
     * Implemented as a visitor to LRStruct.
     */
    public DictionaryPair remove(Comparable key) {
	return (DictionaryPair)_lrs.execute(new IAlgo() {
	    public Object emptyCase(LRStruct host, Object input) {
		return null;
	    }

	    public Object nonEmptyCase(LRStruct host, Object input) {
		DictionaryPair first = (DictionaryPair)host.getFirst();
		int result = first.getKey().compareTo(input);

		if (result > 0)		// host > input
		    return null;
		else if (result == 0) {	// host == input
		    host.removeFront();
		    return first;
		} else			// host < input
		    return host.getRest().execute(this, input);
	    }
	}, key);
    }

    /**
     * Delegates the conversion to the LRStruct toString().
     */
    public String toString() {
	return _lrs.toString();
    }
}