decode: Fix DecodeTypeError offsets

1 files changed, 87 insertions, 0 deletions

diff --git a/methods_test.go b/methods_test.go
index 46e2601..f5d5a9a 100644
--- a/methods_test.go
+++ b/methods_test.go
@@ -232,3 +232,90 @@ func TestMethodsEncode(t *testing.T) {
 		}
 	})
 }
+
+type tstDecoder struct {
+	n   int
+	err string
+}
+
+func (d *tstDecoder) DecodeJSON(r io.RuneScanner) error {
+	for i := 0; i < d.n; i++ {
+		if _, _, err := r.ReadRune(); err != nil {
+			if err == io.EOF {
+				break
+			}
+			return err
+		}
+	}
+	if len(d.err) > 0 {
+		return errors.New(d.err)
+	}
+	return nil
+}
+
+type strUnmarshaler struct {
+	err string
+}
+
+func (u *strUnmarshaler) UnmarshalJSON([]byte) error {
+	if u.err == "" {
+		return nil
+	}
+	return errors.New(u.err)
+}
+
+type textUnmarshaler struct {
+	err string
+}
+
+func (u *textUnmarshaler) UnmarshalText([]byte) error {
+	if u.err == "" {
+		return nil
+	}
+	return errors.New(u.err)
+}
+
+type errTextUnmarshaler struct {
+	S string
+}
+
+func (u *errTextUnmarshaler) UnmarshalText(dat []byte) error {
+	u.S = string(dat)
+	return errors.New("eee")
+}
+
+func TestMethodsDecode(t *testing.T) {
+	t.Parallel()
+	type testcase struct {
+		In          string
+		Obj         any
+		ExpectedErr string
+	}
+	testcases := map[string]testcase{
+		"decode-basic":           {In: `{}`, Obj: &tstDecoder{n: 2}},
+		"decode-basic-eof":       {In: `{}`, Obj: &tstDecoder{n: 5}},
+		"decode-syntax-error":    {In: `{x}`, Obj: &tstDecoder{n: 5}, ExpectedErr: `json: v: syntax error at input byte 1: object: unexpected character: 'x'`},
+		"unmarshal-syntax-error": {In: `{x}`, Obj: &strUnmarshaler{}, ExpectedErr: `json: v: syntax error at input byte 1: object: unexpected character: 'x'`},
+		"decode-short":           {In: `{}`, Obj: &tstDecoder{n: 1}, ExpectedErr: `json: v: cannot decode JSON object at input byte 0 into Go *lowmemjson_test.tstDecoder: did not consume entire object`},
+		"decode-err":             {In: `{}`, Obj: &tstDecoder{err: "xxx"}, ExpectedErr: `json: v: cannot decode JSON object at input byte 0 into Go *lowmemjson_test.tstDecoder: xxx`},
+		"decode-err2":            {In: `{}`, Obj: &tstDecoder{n: 1, err: "yyy"}, ExpectedErr: `json: v: cannot decode JSON object at input byte 0 into Go *lowmemjson_test.tstDecoder: yyy`},
+		"unmarshal-err":          {In: `{}`, Obj: &strUnmarshaler{err: "zzz"}, ExpectedErr: `json: v: cannot decode JSON object at input byte 0 into Go *lowmemjson_test.strUnmarshaler: zzz`},
+		"unmarshaltext":          {In: `""`, Obj: &textUnmarshaler{}},
+		"unmarshaltext-nonstr":   {In: `{}`, Obj: &textUnmarshaler{}, ExpectedErr: `json: v: cannot decode JSON object at input byte 0 into Go *lowmemjson_test.textUnmarshaler`},
+		"unmarshaltext-err":      {In: `""`, Obj: &textUnmarshaler{err: "zzz"}, ExpectedErr: `json: v: cannot decode JSON string at input byte 0 into Go *lowmemjson_test.textUnmarshaler: zzz`},
+		"unmarshaltext-mapkey":   {In: `{"a":1}`, Obj: new(map[errTextUnmarshaler]int), ExpectedErr: `json: v: cannot decode JSON string at input byte 1 into Go *lowmemjson_test.errTextUnmarshaler: eee`},
+	}
+	for tcName, tc := range testcases {
+		tc := tc
+		t.Run(tcName, func(t *testing.T) {
+			t.Parallel()
+			obj := tc.Obj
+			err := lowmemjson.NewDecoder(strings.NewReader(tc.In)).Decode(&obj)
+			if tc.ExpectedErr == "" {
+				assert.NoError(t, err)
+			} else {
+				assert.EqualError(t, err, tc.ExpectedErr)
+			}
+		})
+	}
+}