field matching

env.go

@@ -49,6 +49,11 @@ func (e *env) load(dest Test) error {
 	destV := reflect.ValueOf(dest).Elem()
 	destT := destV.Type()
 
+	e = e.match(dest)
+	if e == nil {
+		return fmt.Errorf("failed to find a matching environment")
+	}
+
 	for i := 0; i < destT.NumField(); i++ {
 		f := destT.Field(i)
 		if !isLoadField(f) {
@@ -77,3 +82,70 @@ func (e *env) load(dest Test) error {
 	}
 	return nil
 }
+
+func (e *env) match(dest Test) *env {
+	destV := reflect.ValueOf(dest).Elem()
+	destT := destV.Type()
+
+	required := getMatchFields(destT)
+	if len(required) == 0 {
+		return e
+	}
+
+	var last *env
+	var leaf *env
+
+	for e := e; e != nil; e = e.parent {
+		present := make([]reflect.StructField, 0, len(required))
+
+		for _, f := range required {
+			ev, ok := e.data[f.Name]
+			if !ok {
+				break
+			}
+			if reflect.TypeOf(ev).AssignableTo(f.Type) {
+				present = append(present, f)
+			}
+		}
+
+		// all required fields are present in this layer
+		if len(present) == len(required) {
+			// check that the values in the env match the values that were
+			// asked for.
+			matched := make(map[string]interface{})
+			for _, f := range required {
+				fv := destV.FieldByName(f.Name)
+				if fv.Interface() == e.data[f.Name] {
+					matched[f.Name] = e.data[f.Name]
+				}
+			}
+
+			// all required match conditions are met
+			if len(matched) == len(required) {
+				if leaf == nil {
+					// if this is the first matched layer, it is the leaf of the
+					// resultant env.
+					leaf = e
+					last = leaf
+				} else {
+					// otherwise we keep this layer, since it matched our match
+					// requirements. Another layer already did, but there may
+					// be other things in the layer we want to keep.
+					last.parent = e
+					last = e
+				}
+			}
+		} else {
+			// the required fields do not exist in the layer, so this layer
+			// does not conflict with the match requirement.
+			if leaf != nil {
+				// since we have a leaf node, we have found a matching layer,
+				// and since this layer does not conflict, we keep it.
+				last.parent = e
+				last = e
+			}
+		}
+	}
+
+	return leaf
+}

env_test.go

@@ -5,18 +5,6 @@ import (
 )
 
 func TestSave(t *testing.T) {
-	type saveFoo struct {
-		empty
-		Foo int `tea:"save"`
-		Bar string
-	}
-
-	type loadFoo struct {
-		empty
-		Foo int `tea:"load"`
-		Bar string
-	}
-
 	t.Run("empty begets nil", func(t *testing.T) {
 		e := mkenv(new(empty))
 		if e != nil {
@@ -35,8 +23,15 @@ func TestSave(t *testing.T) {
 		}
 	})
 
-	t.Run("save an int", func(t *testing.T) {
-		e := mkenv(&saveFoo{Foo: 5})
+	t.Run("create an env from a test", func(t *testing.T) {
+		test := struct {
+			empty
+			Foo int `tea:"save"`
+		}{
+			Foo: 5,
+		}
+
+		e := mkenv(&test)
 		if e == nil {
 			t.Fatalf("saw nil env when expecting a valid env")
 		}
@@ -51,21 +46,169 @@ func TestSave(t *testing.T) {
 		}
 	})
 
+	t.Run("update an existing env", func(t *testing.T) {
+		test := struct {
+			empty
+			Foo int `tea:"save"`
+		}{
+			Foo: 5,
+		}
+
+		e := mkenv(&test)
+		if e == nil {
+			t.Fatalf("saw nil env when expecting a valid env")
+		}
+
+		foo, ok := e.data["Foo"]
+		if !ok {
+			t.Errorf("expected field Foo to be saved but was not saved")
+		}
+
+		if foo != 5 {
+			t.Errorf("expected value %v but saw %v instead", 5, foo)
+		}
+	})
+}
+
+func TestLoad(t *testing.T) {
 	t.Run("load an int", func(t *testing.T) {
-		e := mkenv(&saveFoo{Foo: 5})
-		test := new(loadFoo)
+		e := &env{
+			data: map[string]interface{}{"Foo": 5},
+		}
 
-		e.load(test)
+		var test struct {
+			empty
+			Foo int `tea:"load"`
+		}
+
+		e.load(&test)
 		if test.Foo != 5 {
 			t.Errorf("expected value %v but saw %v instead", 5, test.Foo)
 		}
 	})
 
 	t.Run("loads can fail", func(t *testing.T) {
-		e := mkenv(new(empty))
-		test := new(loadFoo)
-		if err := e.load(test); err == nil {
+		e := &env{
+			data: map[string]interface{}{"NotFoo": 5},
+		}
+
+		var test struct {
+			empty
+			Foo int `tea:"load"`
+		}
+
+		if err := e.load(&test); err == nil {
 			t.Errorf("expected a load error but did not see one")
 		}
 	})
 }
+
+func TestMatch(t *testing.T) {
+	t.Run("required match field not present", func(t *testing.T) {
+		e := &env{
+			data: map[string]interface{}{"Foo": 5},
+		}
+
+		var test struct {
+			empty
+			Name string `tea:"match"`
+			Foo  int    `tea:"load"`
+		}
+
+		if err := e.load(&test); err == nil {
+			t.Errorf("expected a load error but did not see one")
+		}
+	})
+
+	t.Run("required match field has wrong value", func(t *testing.T) {
+		e := &env{
+			data: map[string]interface{}{
+				"Foo":  5,
+				"Name": "alice",
+			},
+		}
+
+		var test struct {
+			empty
+			Name string `tea:"match"`
+			Foo  int    `tea:"load"`
+		}
+		test.Name = "bob"
+
+		if err := e.load(&test); err == nil {
+			t.Errorf("expected a load error but did not see one")
+		}
+	})
+
+	t.Run("simple match", func(t *testing.T) {
+		e := &env{
+			data: map[string]interface{}{
+				"Foo":  5,
+				"Name": "alice",
+			},
+		}
+
+		var test struct {
+			empty
+			Name string `tea:"match"`
+			Foo  int    `tea:"load"`
+		}
+		test.Name = "alice"
+
+		if err := e.load(&test); err != nil {
+			t.Errorf("unexpected load error: %v", err)
+		}
+		if test.Foo != 5 {
+			t.Errorf("expected Foo to load 5 but is %d instead", test.Foo)
+		}
+	})
+
+	t.Run("ancestor match", func(t *testing.T) {
+		e := &env{
+			data: map[string]interface{}{
+				"Foo":  3,
+				"Name": "bob",
+			},
+			parent: &env{
+				data: map[string]interface{}{
+					"Foo":  5,
+					"Name": "alice",
+				},
+			},
+		}
+
+		var test struct {
+			empty
+			Name string `tea:"match"`
+			Foo  int    `tea:"load"`
+		}
+		test.Name = "alice"
+
+		if err := e.load(&test); err != nil {
+			t.Errorf("unexpected load error: %v", err)
+		}
+		if test.Foo != 5 {
+			t.Errorf("expected Foo to load 5 but is %d instead", test.Foo)
+		}
+	})
+}
+
+// A.Optional(B).Child(C)
+//
+//       A           A
+//      /|          / \
+//     / |         /   \
+//    B  | ---->  B     C'
+//     \ |        |
+//      \|        |
+//       C        C
+
+// what to call this thing?
+//
+//       A               A
+//      / \             / \
+//     /   \           /   \
+//    B     C  ---->  B     C
+//     \   /          |     |
+//      \ /           |     |
+//       D            D     D'

tree.go

@@ -130,3 +130,28 @@ func isLoadField(f reflect.StructField) bool {
 	}
 	return false
 }
+
+func isMatchField(f reflect.StructField) bool {
+	// PkgPath is empty string when the identifier is unexported.
+	if f.PkgPath != "" {
+		return false
+	}
+	parts := strings.Split(f.Tag.Get("tea"), ",")
+	for _, part := range parts {
+		if part == "match" {
+			return true
+		}
+	}
+	return false
+}
+
+func getMatchFields(t reflect.Type) []reflect.StructField {
+	var fields []reflect.StructField
+	for i := 0; i < t.NumField(); i++ {
+		f := t.Field(i)
+		if isMatchField(f) {
+			fields = append(fields, f)
+		}
+	}
+	return fields
+}