ensure that the sort is stable

filesort.go

@@ -241,7 +241,7 @@ func newMergeReader(less func(a, b interface{}) bool, rs []reader) (reader, erro
 			}
 			return res, nil
 		}
-		if less(n0, n1) {
+		if !less(n1, n0) {
 			res := n0
 			if n0, err = rs0.Next(); err != nil {
 				return nil, err
@@ -265,16 +265,16 @@ func newMergeReader(less func(a, b interface{}) bool, rs []reader) (reader, erro
 func (ps *FileSort) merge() error {
 	defer close(ps.out)
 	var readers []reader
-	if len(ps.buffer) > 0 {
-		readers = append(readers, &sliceReader{slice: ps.buffer})
-	}
 	for _, file := range ps.files {
 		fr, err := ps.makeFileReader(file)
 		if err != nil {
 			panic(err)
 		}
 		readers = append(readers, fr)
 	}
+	if len(ps.buffer) > 0 {
+		readers = append(readers, &sliceReader{slice: ps.buffer})
+	}
 	mr, err := newMergeReader(ps.less, readers)
 	if err != nil {
 		return err

filesort_test.go

@@ -2,6 +2,7 @@ package filesort
 
 import (
 	"bufio"
+	"fmt"
 	"io"
 	"strings"
 	"testing"
@@ -88,3 +89,29 @@ func TestSort(t *testing.T) {
 		t.Errorf("expected to read %d values, but got %d", len(lines), n)
 	}
 }
+
+func TestSortStable(t *testing.T) {
+	sort, err := New(
+		WithLess(func(a, b interface{}) bool { return false }),
+		WithEncoderNew(newTestLineEncoder),
+		WithDecoderNew(newTestLineDecoder),
+		WithMaxMemoryBuffer(3),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i := 0; i < 100; i++ {
+		sort.Write(fmt.Sprintf("%d", i))
+	}
+	sort.Close()
+	for i := 0; i < 100; i++ {
+		exp := fmt.Sprintf("%d", i)
+		s, err := sort.Read()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if s.(string) != exp {
+			t.Fatalf("expected %s but got %s", exp, s.(string))
+		}
+	}
+}