Reverse iterators (#224)

with passing tests

db/c_level_db.go

@@ -190,7 +190,8 @@ func (db *CLevelDB) Iterator(start, end []byte) Iterator {
 }
 
 func (db *CLevelDB) ReverseIterator(start, end []byte) Iterator {
-	panic("not implemented yet") // XXX
+	itr := db.db.NewIterator(db.ro)
+	return newCLevelDBIterator(itr, start, end, true)
 }
 
 var _ Iterator = (*cLevelDBIterator)(nil)
@@ -204,12 +205,25 @@ type cLevelDBIterator struct {
 
 func newCLevelDBIterator(source *levigo.Iterator, start, end []byte, isReverse bool) *cLevelDBIterator {
 	if isReverse {
-		panic("not implemented yet") // XXX
-	}
-	if start != nil {
-		source.Seek(start)
+		if start == nil {
+			source.SeekToLast()
+		} else {
+			source.Seek(start)
+			if source.Valid() {
+				soakey := source.Key() // start or after key
+				if bytes.Compare(start, soakey) < 0 {
+					source.Prev()
+				}
+			} else {
+				source.SeekToLast()
+			}
+		}
 	} else {
-		source.SeekToFirst()
+		if start == nil {
+			source.SeekToFirst()
+		} else {
+			source.Seek(start)
+		}
 	}
 	return &cLevelDBIterator{
 		source:    source,
@@ -243,9 +257,16 @@ func (itr cLevelDBIterator) Valid() bool {
 	// If key is end or past it, invalid.
 	var end = itr.end
 	var key = itr.source.Key()
-	if end != nil && bytes.Compare(end, key) <= 0 {
-		itr.isInvalid = true
-		return false
+	if itr.isReverse {
+		if end != nil && bytes.Compare(key, end) <= 0 {
+			itr.isInvalid = true
+			return false
+		}
+	} else {
+		if end != nil && bytes.Compare(end, key) <= 0 {
+			itr.isInvalid = true
+			return false
+		}
 	}
 
 	// It's valid.
@@ -267,7 +288,11 @@ func (itr cLevelDBIterator) Value() []byte {
 func (itr cLevelDBIterator) Next() {
 	itr.assertNoError()
 	itr.assertIsValid()
-	itr.source.Next()
+	if itr.isReverse {
+		itr.source.Prev()
+	} else {
+		itr.source.Next()
+	}
 }
 
 func (itr cLevelDBIterator) Close() {