Add filter support for walker

walker.go

@@ -15,6 +15,7 @@ import (
 type WalkOpt struct {
 	IncludePaths    []string // todo: remove?
 	ExcludePatterns []string
+	Map             func(*Stat) bool
 }
 
 func Walk(ctx context.Context, p string, opt *WalkOpt, fn filepath.WalkFunc) error {
@@ -138,7 +139,12 @@ func Walk(ctx context.Context, p string, opt *WalkOpt, fn filepath.WalkFunc) err
 		case <-ctx.Done():
 			return ctx.Err()
 		default:
-			if err := fn(path, &StatInfo{stat}, nil); err != nil {
+			if opt != nil && opt.Map != nil {
+				if allowed := opt.Map(stat); !allowed {
+					return nil
+				}
+			}
+			if err := fn(stat.Path, &StatInfo{stat}, nil); err != nil {
 				return err
 			}
 		}

walker_test.go

@@ -6,6 +6,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/pkg/errors"
@@ -72,6 +73,33 @@ file foo/bar2
 
 }
 
+func TestWalkerMap(t *testing.T) {
+	d, err := tmpDir(changeStream([]string{
+		"ADD bar file",
+		"ADD foo dir",
+		"ADD foo2 file",
+		"ADD foo/bar2 file",
+	}))
+	assert.NoError(t, err)
+	defer os.RemoveAll(d)
+	b := &bytes.Buffer{}
+	err = Walk(context.Background(), d, &WalkOpt{
+		Map: func(s *Stat) bool {
+			if strings.HasPrefix(s.Path, "foo") {
+				s.Path = "_" + s.Path
+				return true
+			}
+			return false
+		},
+	}, bufWalk(b))
+	assert.NoError(t, err)
+
+	assert.Equal(t, `dir _foo
+file _foo/bar2
+file _foo2
+`, string(b.Bytes()))
+}
+
 func bufWalk(buf *bytes.Buffer) filepath.WalkFunc {
 	return func(path string, fi os.FileInfo, err error) error {
 		stat, ok := fi.Sys().(*Stat)