Add TraversalsByName for zero allocation field traversal

reflectx/reflect.go

@@ -166,20 +166,39 @@ func (m *Mapper) FieldsByName(v reflect.Value, names []string) []reflect.Value {
 // traversals for each mapped name.  Panics if t is not a struct or Indirectable
 // to a struct.  Returns empty int slice for each name not found.
 func (m *Mapper) TraversalsByName(t reflect.Type, names []string) [][]int {
+	r := make([][]int, 0, len(names))
+	m.TraversalsByNameFunc(t, names, func(_ int, i []int) error {
+		if i == nil {
+			r = append(r, []int{})
+		} else {
+			r = append(r, i)
+		}
+
+		return nil
+	})
+	return r
+}
+
+// TraversalsByNameFunc traverses the mapped names and calls fn with the index of
+// each name and the struct traversal represented by that name. Panics if t is not
+// a struct or Indirectable to a struct. Returns the first error returned by fn or nil.
+func (m *Mapper) TraversalsByNameFunc(t reflect.Type, names []string, fn func(int, []int) error) error {
 	t = Deref(t)
 	mustBe(t, reflect.Struct)
 	tm := m.TypeMap(t)
-
-	r := make([][]int, 0, len(names))
-	for _, name := range names {
+	for i, name := range names {
 		fi, ok := tm.Names[name]
 		if !ok {
-			r = append(r, []int{})
+			if err := fn(i, nil); err != nil {
+				return err
+			}
 		} else {
-			r = append(r, fi.Index)
+			if err := fn(i, fi.Index); err != nil {
+				return err
+			}
 		}
 	}
-	return r
+	return nil
 }
 
 // FieldByIndexes returns a value for the field given by the struct traversal

reflectx/reflect_test.go

@@ -903,3 +903,72 @@ func BenchmarkFieldByIndexL4(b *testing.B) {
 		}
 	}
 }
+
+func BenchmarkTraversalsByName(b *testing.B) {
+	type A struct {
+		Value int
+	}
+
+	type B struct {
+		A A
+	}
+
+	type C struct {
+		B B
+	}
+
+	type D struct {
+		C C
+	}
+
+	m := NewMapper("")
+	t := reflect.TypeOf(D{})
+	names := []string{"C", "B", "A", "Value"}
+
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		if l := len(m.TraversalsByName(t, names)); l != len(names) {
+			b.Errorf("expected %d values, got %d", len(names), l)
+		}
+	}
+}
+
+func BenchmarkTraversalsByNameFunc(b *testing.B) {
+	type A struct {
+		Z int
+	}
+
+	type B struct {
+		A A
+	}
+
+	type C struct {
+		B B
+	}
+
+	type D struct {
+		C C
+	}
+
+	m := NewMapper("")
+	t := reflect.TypeOf(D{})
+	names := []string{"C", "B", "A", "Z", "Y"}
+
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		var l int
+
+		if err := m.TraversalsByNameFunc(t, names, func(_ int, _ []int) error {
+			l++
+			return nil
+		}); err != nil {
+			b.Errorf("unexpected error %s", err)
+		}
+
+		if l != len(names) {
+			b.Errorf("expected %d values, got %d", len(names), l)
+		}
+	}
+}