package simpledb.index.query;import simpledb.record.RID;import simpledb.query.*;import simpledb.index.Index;/** * The scan class corresponding to the indexjoin relational * algebra operator. * The code is very similar to that of ProductScan,  * which makes sense because an index join is essentially * the product of each LHS record with the matching RHS index records. * @author Edward Sciore */public class IndexJoinScan implements RidScan {	private Scan s;	private Index idx;	private String joinfield;	/**	 * Creates an index join scan for the specified LHS scan and 	 * RHS index.	 * @param s the LHS scan	 * @param idx the RHS index	 * @param joinfield the LHS field used for joining	 */	public IndexJoinScan(Scan s, Index idx, String joinfield) {		this.s = s;		this.idx  = idx;		this.joinfield = joinfield;		beforeFirst();	}	/**	 * Positions the scan before the first record.	 * That is, the LHS scan will be positioned at its	 * first record, and the index will be positioned	 * before the first record for the join value.	 * @see simpledb.query.Scan#beforeFirst()	 */	public void beforeFirst() {		s.beforeFirst();		s.next();		resetIndex();	}	/**	 * Moves the scan to the next record.	 * The method moves to the next index record, if possible.	 * Otherwise, it moves to the next LHS record and the	 * first index record.	 * If there are no more LHS records, the method returns false.	 * @see simpledb.query.Scan#next()	 */	public boolean next() {		while (true) {			if (idx.next())				return true;			if (!s.next())				return false;			resetIndex();		}	}	/**	 * Closes the scan by closing its LHS scan and its RHS index.	 * @see simpledb.query.Scan#close()	 */	public void close() {		s.close();		idx.close();	}	/**	 * Returns the Constant value of the specified field.	 * Since the index contains no interesting fields,	 * the value will be obtained from the LHS scan.	 * @see simpledb.query.Scan#getVal(java.lang.String)	 */	public Constant getVal(String fldname) {		return s.getVal(fldname);	}	/**	 * Returns the integer value of the specified field.	 * Since the index contains no interesting fields,	 * the value will be obtained from the LHS scan.	 * @see simpledb.query.Scan#getVal(java.lang.String)	 */	public int getInt(String fldname) {		return s.getInt(fldname);	}	/**	 * Returns the string value of the specified field.	 * Since the index contains no interesting fields,	 * the value will be obtained from the LHS scan.	 * @see simpledb.query.Scan#getVal(java.lang.String)	 */	public String getString(String fldname) {		return s.getString(fldname);	}	/** Returns true if the field is in the schema.	 * Since the index has an empty schema, the method	 * returns the value that the LHS returns.	 * @see simpledb.query.Scan#hasField(java.lang.String)	 */	public boolean hasField(String fldname) {		return s.hasField(fldname);	}	/**	 * Returns the dataRid of the current index record.	 * @see simpledb.index.query.RidScan#getDataRid()	 */	public RID getDataRid() {		return idx.getDataRid();	}	private void resetIndex() {		Constant searchkey = s.getVal(joinfield);		idx.beforeFirst(searchkey);	}}