updated attempt to reuse []byte

previous attempt was flawed (but maked by Reset() method)
new approach is to do this work in the Reset() method itself,
logically this is where it belongs.

but further we acknowledge that IndexInternalID []byte lifetime
lives beyond the TermFieldDoc, so another copy is made into
the DocumentMatch.  Although this introduces yet another copy
the theory being tested is that it allows each of these
structuress to reuse memory without additional allocation.

index/index.go

@@ -123,7 +123,9 @@ type TermFieldDoc struct {
 }
 
 func (tfd *TermFieldDoc) Reset() *TermFieldDoc {
+	id := tfd.ID
 	*tfd = TermFieldDoc{}
+	tfd.ID = id[:0]
 	return tfd
 }
 

index/upside_down/reader.go

@@ -83,7 +83,6 @@ func (r *UpsideDownCouchTermFieldReader) Next(preAlloced *index.TermFieldDoc) (*
 			if rv == nil {
 				rv = &index.TermFieldDoc{}
 			}
-			rv.ID = rv.ID[:0]
 			rv.ID = append(rv.ID, tfr.doc...)
 			rv.Freq = tfr.freq
 			rv.Norm = float64(tfr.norm)
@@ -111,7 +110,6 @@ func (r *UpsideDownCouchTermFieldReader) Advance(docID index.IndexInternalID, pr
 			if rv == nil {
 				rv = &index.TermFieldDoc{}
 			}
-			rv.ID = rv.ID[:0]
 			rv.ID = append(rv.ID, tfr.doc...)
 			rv.Freq = tfr.freq
 			rv.Norm = float64(tfr.norm)

search/scorers/scorer_term.go

@@ -132,7 +132,7 @@ func (s *TermQueryScorer) Score(termMatch *index.TermFieldDoc, preAllocated *sea
 	if rv == nil {
 		rv = &search.DocumentMatch{}
 	}
-	rv.IndexInternalID = termMatch.ID
+	rv.IndexInternalID = append(rv.IndexInternalID, termMatch.ID...)
 	rv.Score = score
 	if s.explain {
 		rv.Expl = scoreExplanation

search/search.go

@@ -89,7 +89,9 @@ func (dm *DocumentMatch) AddFieldValue(name string, value interface{}) {
 }
 
 func (dm *DocumentMatch) Reset() *DocumentMatch {
+	indexInternalId := dm.IndexInternalID
 	*dm = DocumentMatch{}
+	dm.IndexInternalID = indexInternalId[:0]
 	return dm
 }