Implement Postings Iterator Over Bytes

Closes fabxc/tsdb#18

index.go

@@ -698,20 +698,11 @@ func (r *indexReader) Postings(name, value string) (Postings, error) {
 		return nil, errors.Wrapf(errInvalidFlag, "section at %d", off)
 	}
 
-	// TODO(fabxc): just read into memory as an intermediate solution.
-	// Add iterator over serialized data.
-	var l []uint32
-
-	for len(b) > 0 {
-		if len(b) < 4 {
-			return nil, errors.Wrap(errInvalidSize, "plain postings entry")
-		}
-		l = append(l, binary.BigEndian.Uint32(b[:4]))
-
-		b = b[4:]
+	// Add iterator over the bytes.
+	if len(b)%4 != 0 {
+		return nil, errors.Wrap(errInvalidSize, "plain postings entry")
 	}
-
-	return &listPostings{list: l, idx: -1}, nil
+	return newBytePostings(b), nil
 }
 
 type stringTuples struct {

postings.go

@@ -1,6 +1,7 @@
 package tsdb
 
 import (
+	"encoding/binary"
 	"sort"
 	"strings"
 )
@@ -240,6 +241,40 @@ func (it *listPostings) Err() error {
 	return nil
 }
 
+type bytePostings struct {
+	list []byte
+	idx  int
+}
+
+func newBytePostings(list []byte) *bytePostings {
+	return &bytePostings{list: list, idx: -1}
+}
+
+func (it *bytePostings) At() uint32 {
+	idx := 4 * it.idx
+	return binary.BigEndian.Uint32(it.list[idx : idx+4])
+}
+
+func (it *bytePostings) Next() bool {
+	it.idx++
+	return it.idx*4 < len(it.list)
+}
+
+func (it *bytePostings) Seek(x uint32) bool {
+	num := len(it.list) / 4
+	// Do binary search between current position and end.
+	it.idx += sort.Search(num-it.idx, func(i int) bool {
+		idx := 4 * (it.idx + i)
+		val := binary.BigEndian.Uint32(it.list[idx : idx+4])
+		return val >= x
+	})
+	return it.idx*4 < len(it.list)
+}
+
+func (it *bytePostings) Err() error {
+	return nil
+}
+
 type stringset map[string]struct{}
 
 func (ss stringset) set(s string) {