From c2c4171a3a1432695e14c62c2f174bff78f14980 Mon Sep 17 00:00:00 2001
From: Yonah <contrib@gensokyo.uk>
Date: Wed, 17 Sep 2025 07:47:51 +0900
Subject: [PATCH] generic: emulate nullable string behaviour

This fills the gap for nullable strings returned by the API.

Signed-off-by: Yonah <contrib@gensokyo.uk>
---
 generic.go      | 23 ++++++++++++++++++++++
 generic_test.go | 51 +++++++++++++++++++++++++++++++------------------
 2 files changed, 55 insertions(+), 19 deletions(-)

diff --git a/generic.go b/generic.go
index 47a71d9..309bc15 100644
--- a/generic.go
+++ b/generic.go
@@ -13,6 +13,29 @@ type Response[T any] struct {
 	Data    T      `json:"data"`
 }
 
+// NullableString is a JSON string where its zero value behaves like null.
+type NullableString string
+
+func (s *NullableString) MarshalJSON() ([]byte, error) {
+	if *s == "" {
+		return []byte("null"), nil
+	}
+	return json.Marshal(string(*s))
+}
+
+func (s *NullableString) UnmarshalJSON(data []byte) (err error) {
+	var v *string
+	err = json.Unmarshal(data, &v)
+	if err == nil {
+		if v != nil {
+			*s = NullableString(*v)
+		} else {
+			*s = ""
+		}
+	}
+	return
+}
+
 // StringInt is a JSON string representing an integer.
 type StringInt int
 
diff --git a/generic_test.go b/generic_test.go
index bc053cf..7b93e16 100644
--- a/generic_test.go
+++ b/generic_test.go
@@ -11,13 +11,17 @@ import (
 	. "git.gensokyo.uk/yonah/monstersirenfetch"
 )
 
+func TestNullableString(t *testing.T) {
+	checkJSONRoundTripM(t, []jsonRoundTripTestCase[NullableString]{
+		{"zero", nil, `null`, ""},
+		{"string", nil, `"null"`, "null"},
+
+		{"invalid json", newSyntaxError("unexpected end of JSON input", 5), `"null`, "\x00"},
+	})
+}
+
 func TestStringInt(t *testing.T) {
-	testCases := []struct {
-		name    string
-		wantErr error
-		data    string
-		val     int
-	}{
+	checkJSONRoundTripM(t, []jsonRoundTripTestCase[StringInt]{
 		{"valid long", nil, `"3735928559"`, 0xdeadbeef},
 		{"valid pad 4", nil, `"0000"`, 0},
 		{"valid 4", nil, `"1000"`, 1e3},
@@ -26,13 +30,29 @@ func TestStringInt(t *testing.T) {
 
 		{"invalid json", newSyntaxError("unexpected end of JSON input", 11), `"3735928559`, -1},
 		{"invalid number", &strconv.NumError{Func: "Atoi", Num: ":3735928559", Err: strconv.ErrSyntax}, `":3735928559"`, -1},
-	}
+	})
+}
+
+func newSyntaxError(msg string, offset int64) *json.SyntaxError {
+	e := &json.SyntaxError{Offset: offset}
+	msgV := reflect.ValueOf(e).Elem().FieldByName("msg")
+	reflect.NewAt(msgV.Type(), unsafe.Pointer(msgV.UnsafeAddr())).Elem().SetString(msg)
+	return e
+}
+
+type jsonRoundTripTestCase[T comparable] struct {
+	name    string
+	wantErr error
+	data    string
+	val     T
+}
+
+func checkJSONRoundTripM[T comparable](t *testing.T, testCases []jsonRoundTripTestCase[T]) {
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
 			if tc.wantErr == nil {
 				t.Run("marshal", func(t *testing.T) {
-					v := StringInt(tc.val)
-					if got, err := json.Marshal(&v); err != nil {
+					if got, err := json.Marshal(&tc.val); err != nil {
 						t.Fatalf("Marshal: error = %v", err)
 					} else if string(got) != tc.data {
 						t.Errorf("Marshal: %s, want %s", string(got), tc.data)
@@ -41,7 +61,7 @@ func TestStringInt(t *testing.T) {
 			}
 
 			t.Run("unmarshal", func(t *testing.T) {
-				var got StringInt
+				var got T
 				err := json.Unmarshal([]byte(tc.data), &got)
 
 				if !reflect.DeepEqual(err, tc.wantErr) {
@@ -49,8 +69,8 @@ func TestStringInt(t *testing.T) {
 				}
 
 				if tc.wantErr == nil {
-					if int(got) != tc.val {
-						t.Errorf("Unmarshal: %d, want %d", got, tc.val)
+					if got != tc.val {
+						t.Errorf("Unmarshal: %v, want %v", got, tc.val)
 					}
 				}
 			})
@@ -58,13 +78,6 @@ func TestStringInt(t *testing.T) {
 	}
 }
 
-func newSyntaxError(msg string, offset int64) *json.SyntaxError {
-	e := &json.SyntaxError{Offset: offset}
-	msgV := reflect.ValueOf(e).Elem().FieldByName("msg")
-	reflect.NewAt(msgV.Type(), unsafe.Pointer(msgV.UnsafeAddr())).Elem().SetString(msg)
-	return e
-}
-
 func checkJSONRoundTrip[T any](t *testing.T, v T, data []byte) {
 	t.Run("marshal", func(t *testing.T) {
 		buf := new(bytes.Buffer)