optimize upside_down reader & 64-bit struct alignments

The UpsideDownCouchTermFieldReader.Next() only needs the doc ID from
the key, so this change provides a specialized parseKDoc() method for
that optimization.

Additionally, fields in various structs are more 64-bit aligned, in an
attempt to reduce the invocations of runtime.typedmemmove() and
runtime.heapBitsBulkBarrier(), which the go compiler seems to
automatically insert to transparently handle misaligned data.

index/upside_down/reader.go

@@ -17,12 +17,12 @@ import (
 )
 
 type UpsideDownCouchTermFieldReader struct {
+	count       uint64
 	indexReader *IndexReader
 	iterator    store.KVIterator
-	count       uint64
 	term        []byte
+	tfrNext     *TermFrequencyRow
 	field       uint16
-	tfrNext     TermFrequencyRow
 }
 
 func newUpsideDownCouchTermFieldReader(indexReader *IndexReader, term []byte, field uint16) (*UpsideDownCouchTermFieldReader, error) {
@@ -34,9 +34,10 @@ func newUpsideDownCouchTermFieldReader(indexReader *IndexReader, term []byte, fi
 	if val == nil {
 		atomic.AddUint64(&indexReader.index.stats.termSearchersStarted, uint64(1))
 		return &UpsideDownCouchTermFieldReader{
-			count: 0,
-			term:  term,
-			field: field,
+			count:   0,
+			term:    term,
+			tfrNext: &TermFrequencyRow{},
+			field:   field,
 		}, nil
 	}
 
@@ -54,6 +55,7 @@ func newUpsideDownCouchTermFieldReader(indexReader *IndexReader, term []byte, fi
 		iterator:    it,
 		count:       dictionaryRow.count,
 		term:        term,
+		tfrNext:     &TermFrequencyRow{},
 		field:       field,
 	}, nil
 }
@@ -66,8 +68,8 @@ func (r *UpsideDownCouchTermFieldReader) Next(preAlloced *index.TermFieldDoc) (*
 	if r.iterator != nil {
 		key, val, valid := r.iterator.Current()
 		if valid {
-			tfr := &r.tfrNext
-			err := tfr.parseK(key)
+			tfr := r.tfrNext
+			err := tfr.parseKDoc(key)
 			if err != nil {
 				return nil, err
 			}
@@ -82,7 +84,9 @@ func (r *UpsideDownCouchTermFieldReader) Next(preAlloced *index.TermFieldDoc) (*
 			rv.ID = string(tfr.doc)
 			rv.Freq = tfr.freq
 			rv.Norm = float64(tfr.norm)
-			rv.Vectors = r.indexReader.index.termFieldVectorsFromTermVectors(tfr.vectors)
+			if tfr.vectors != nil {
+				rv.Vectors = r.indexReader.index.termFieldVectorsFromTermVectors(tfr.vectors)
+			}
 			r.iterator.Next()
 			return rv, nil
 		}

index/upside_down/row.go

@@ -350,11 +350,11 @@ func (tv *TermVector) String() string {
 
 type TermFrequencyRow struct {
 	term    []byte
-	field   uint16
 	doc     []byte
 	freq    uint64
-	norm    float32
 	vectors []*TermVector
+	norm    float32
+	field   uint16
 }
 
 func (tfr *TermFrequencyRow) Term() []byte {
@@ -504,7 +504,7 @@ func (tfr *TermFrequencyRow) parseK(key []byte) error {
 	}
 	tfr.term = key[3 : 3+termEndPos]
 
-	docLen := len(key) - (3 + termEndPos + 1)
+	docLen := keyLen - (3 + termEndPos + 1)
 	if docLen < 1 {
 		return fmt.Errorf("invalid term frequency key, empty docid")
 	}
@@ -513,14 +513,27 @@ func (tfr *TermFrequencyRow) parseK(key []byte) error {
 	return nil
 }
 
+func (tfr *TermFrequencyRow) parseKDoc(key []byte) error {
+	termEndPos := bytes.IndexByte(key[3:], ByteSeparator)
+	if termEndPos < 0 {
+		return fmt.Errorf("invalid term frequency key, no byte separator terminating term")
+	}
+
+	tfr.doc = key[3+termEndPos+1:]
+	if len(tfr.doc) <= 0 {
+		return fmt.Errorf("invalid term frequency key, empty docid")
+	}
+
+	return nil
+}
+
 func (tfr *TermFrequencyRow) parseV(value []byte) error {
-	currOffset := 0
-	bytesRead := 0
-	tfr.freq, bytesRead = binary.Uvarint(value[currOffset:])
+	var bytesRead int
+	tfr.freq, bytesRead = binary.Uvarint(value)
 	if bytesRead <= 0 {
 		return fmt.Errorf("invalid term frequency value, invalid frequency")
 	}
-	currOffset += bytesRead
+	currOffset := bytesRead
 
 	var norm uint64
 	norm, bytesRead = binary.Uvarint(value[currOffset:])

search/searchers/search_term.go

@@ -19,10 +19,10 @@ type TermSearcher struct {
 	indexReader index.IndexReader
 	term        string
 	field       string
-	explain     bool
 	reader      index.TermFieldReader
 	scorer      *scorers.TermQueryScorer
 	tfd         index.TermFieldDoc
+	explain     bool
 }
 
 func NewTermSearcher(indexReader index.IndexReader, term string, field string, boost float64, explain bool) (*TermSearcher, error) {