1 package simpledb.log;
2
3 import static simpledb.file.Page.INT_SIZE;
4 import simpledb.file.*;
5 import java.util.Iterator;
6
7 /**
8 * A class that provides the ability to move through the
9 * records of the log file in reverse order.
10 *
11 * @author Edward Sciore
12 */
13 class LogIterator implements Iterator<BasicLogRecord> {
14 private Block blk;
15 private Page pg = new Page();
16 private int currentrec;
17
18 /**
19 * Creates an iterator for the records in the log file,
20 * positioned after the last log record.
21 * This constructor is called exclusively by
22 * {@link LogMgr#iterator()}.
23 */
24 LogIterator(Block blk) {
25 this.blk = blk;
26 pg.read(blk);
27 currentrec = pg.getInt(LogMgr.LAST_POS);
28 }
29
30 /**
31 * Determines if the current log record
32 * is the earliest record in the log file.
33 * @return true if there is an earlier record
34 */
35 public boolean hasNext() {
36 return currentrec>0 || blk.number()>0;
37 }
38
39 /**
40 * Moves to the next log record in reverse order.
41 * If the current log record is the earliest in its block,
42 * then the method moves to the next oldest block,
43 * and returns the log record from there.
44 * @return the next earliest log record
45 */
46 public BasicLogRecord next() {
47 if (currentrec == 0)
48 moveToNextBlock();
49 currentrec = pg.getInt(currentrec);
50 return new BasicLogRecord(pg, currentrec+INT_SIZE);
51 }
52
53 public void remove() {
54 throw new UnsupportedOperationException();
55 }
56
57 /**
58 * Moves to the next log block in reverse order,
59 * and positions it after the last record in that block.
60 */
61 private void moveToNextBlock() {
62 blk = new Block(blk.fileName(), blk.number()-1);
63 pg.read(blk);
64 currentrec = pg.getInt(LogMgr.LAST_POS);
65 }
66 }