added example package

vendor/github.com/akrylysov/pogreb/iterator.go

@@ -0,0 +1,75 @@
+package pogreb
+
+import (
+	"errors"
+	"sync"
+)
+
+// ErrIterationDone is returned by ItemIterator.Next calls when there are no more items to return.
+var ErrIterationDone = errors.New("no more items in iterator")
+
+type item struct {
+	key   []byte
+	value []byte
+}
+
+// ItemIterator is an iterator over DB key-value pairs. It iterates the items in an unspecified order.
+type ItemIterator struct {
+	db            *DB
+	nextBucketIdx uint32
+	queue         []item
+	mu            sync.Mutex
+}
+
+// fetchItems adds items to the iterator queue from a bucket located at nextBucketIdx.
+func (it *ItemIterator) fetchItems(nextBucketIdx uint32) error {
+	bit := it.db.index.newBucketIterator(nextBucketIdx)
+	for {
+		b, err := bit.next()
+		if err == ErrIterationDone {
+			return nil
+		}
+		if err != nil {
+			return err
+		}
+		for i := 0; i < slotsPerBucket; i++ {
+			sl := b.slots[i]
+			if sl.offset == 0 {
+				// No more items in the bucket.
+				break
+			}
+			key, value, err := it.db.datalog.readKeyValue(sl)
+			if err != nil {
+				return err
+			}
+			key = cloneBytes(key)
+			value = cloneBytes(value)
+			it.queue = append(it.queue, item{key: key, value: value})
+		}
+	}
+}
+
+// Next returns the next key-value pair if available, otherwise it returns ErrIterationDone error.
+func (it *ItemIterator) Next() ([]byte, []byte, error) {
+	it.mu.Lock()
+	defer it.mu.Unlock()
+
+	it.db.mu.RLock()
+	defer it.db.mu.RUnlock()
+
+	// The iterator queue is empty and we have more buckets to check.
+	for len(it.queue) == 0 && it.nextBucketIdx < it.db.index.numBuckets {
+		if err := it.fetchItems(it.nextBucketIdx); err != nil {
+			return nil, nil, err
+		}
+		it.nextBucketIdx++
+	}
+
+	if len(it.queue) > 0 {
+		item := it.queue[0]
+		it.queue = it.queue[1:]
+		return item.key, item.value, nil
+	}
+
+	return nil, nil, ErrIterationDone
+}