generic: parse string representation of integer

This makes the data easier to handle down the pipeline.

Signed-off-by: Yonah <contrib@gensokyo.uk>

generic.go

@@ -0,0 +1,23 @@
+package monstersirenfetch
+
+import (
+	"encoding/json"
+	"strconv"
+)
+
+// StringInt is a JSON string representing an integer.
+type StringInt int
+
+func (i *StringInt) MarshalJSON() ([]byte, error) { return json.Marshal(strconv.Itoa(int(*i))) }
+func (i *StringInt) UnmarshalJSON(data []byte) (err error) {
+	var v string
+	err = json.Unmarshal(data, &v)
+	if err == nil {
+		var n int
+		n, err = strconv.Atoi(v)
+		if err == nil {
+			*i = StringInt(n)
+		}
+	}
+	return
+}

generic_test.go

@@ -0,0 +1,60 @@
+package monstersirenfetch_test
+
+import (
+	"encoding/json"
+	"reflect"
+	"strconv"
+	"testing"
+	"unsafe"
+
+	. "git.gensokyo.uk/yonah/monstersirenfetch"
+)
+
+func TestStringInt(t *testing.T) {
+	testCases := []struct {
+		name    string
+		wantErr error
+		data    string
+		val     int
+	}{
+		{"valid", nil, `"3735928559"`, 0xdeadbeef},
+		{"invalid json", newSyntaxError("unexpected end of JSON input", 11), `"3735928559`, -1},
+		{"invalid number", &strconv.NumError{Func: "Atoi", Num: ":3735928559", Err: strconv.ErrSyntax}, `":3735928559"`, -1},
+	}
+	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 {
+						t.Fatalf("Marshal: error = %v", err)
+					} else if string(got) != tc.data {
+						t.Errorf("Marshal: %s, want %s", string(got), tc.data)
+					}
+				})
+			}
+
+			t.Run("unmarshal", func(t *testing.T) {
+				var got StringInt
+				err := json.Unmarshal([]byte(tc.data), &got)
+
+				if !reflect.DeepEqual(err, tc.wantErr) {
+					t.Errorf("Unmarshal: error = %v, want %v", err, tc.wantErr)
+				}
+
+				if tc.wantErr == nil {
+					if int(got) != tc.val {
+						t.Errorf("Unmarshal: %d, want %d", got, tc.val)
+					}
+				}
+			})
+		})
+	}
+}
+
+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
+}