Sort batches of indexes to reduce cursor churn (#99)

* Sort batches of indexes to reduce cursor churn

pebble/pebble.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"errors"
 	"io"
+	"sort"
 
 	"github.com/cockroachdb/pebble"
 	"github.com/ipni/dhstore"
@@ -48,9 +49,15 @@ func NewPebbleDHStore(path string, opts *pebble.Options) (*PebbleDHStore, error)
 }
 
 func (s *PebbleDHStore) MergeIndexes(indexes []dhstore.Index) error {
+	// Sort indexes to reduce cursor churn.
+	sort.Slice(indexes, func(i, j int) bool {
+		return bytes.Compare(indexes[i].Key, indexes[j].Key) == -1
+	})
+
 	keygen := s.p.leaseSimpleKeyer()
 	defer keygen.Close()
 	batch := s.db.NewBatch()
+
 	for _, index := range indexes {
 		dmh, err := multihash.Decode(index.Key)
 		if err != nil {
@@ -82,6 +89,11 @@ func (s *PebbleDHStore) MergeIndexes(indexes []dhstore.Index) error {
 // DeleteIndexes removes dh-multihash to encrypted-valueKey mappings. This is
 // the inverse of MergeIndexes.
 func (s *PebbleDHStore) DeleteIndexes(indexes []dhstore.Index) error {
+	// Sort indexes to reduce cursor churn.
+	sort.Slice(indexes, func(i, j int) bool {
+		return bytes.Compare(indexes[i].Key, indexes[j].Key) == -1
+	})
+
 	keygen := s.p.leaseSimpleKeyer()
 	defer keygen.Close()
 	batch := s.db.NewBatch()

pebble/pebble_merger.go

@@ -25,15 +25,14 @@ type valueKeysValueMerger struct {
 func (s *PebbleDHStore) newValueKeysMerger() *pebble.Merger {
 	return &pebble.Merger{
 		Merge: func(k, value []byte) (pebble.ValueMerger, error) {
-			// Fall back on default merger if the key is not of type multihash, i.e. the only key
-			// type that corresponds to value-keys.
-			switch keyPrefix(k[0]) {
-			case multihashKeyPrefix:
+			// Use specialized merger for multihash keys.
+			if keyPrefix(k[0]) == multihashKeyPrefix {
 				v := &valueKeysValueMerger{s: s}
 				return v, v.MergeNewer(value)
-			default:
-				return pebble.DefaultMerger.Merge(k, value)
 			}
+			// Use default merger for non-multihash type keys, i.e. the
+			// only key type that corresponds to value-keys.
+			return pebble.DefaultMerger.Merge(k, value)
 		},
 		Name: valueKeysMergerName,
 	}