Add unmarshal funcs (#8)

jsn.go

@@ -1,5 +1,12 @@
 package jsn
 
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"io"
+)
+
 // A represents JSON array.
 type A = []any
 
@@ -15,3 +22,26 @@ type N string
 func (n N) MarshalJSON() ([]byte, error) {
 	return []byte(n), nil
 }
+
+// Unmarshal only 1 JSON entity from the input.
+// Disallows unknown fields if the argument is a struct.
+func Unmarshal(b []byte, v any) error {
+	return UnmarshalFrom(bytes.NewReader(b), v)
+}
+
+// UnmarshalFrom only 1 JSON entity from the input.
+// Disallows unknown fields if the argument is a struct.
+func UnmarshalFrom(r io.Reader, v any) error {
+	d := json.NewDecoder(r)
+	d.DisallowUnknownFields()
+
+	if err := d.Decode(v); err != nil {
+		return err
+	}
+	if d.More() {
+		return errMore
+	}
+	return nil
+}
+
+var errMore = errors.New("body must contain only one JSON entity")

jsn_test.go

@@ -0,0 +1,65 @@
+package jsn
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestUnmarshal(t *testing.T) {
+	testCases := []struct {
+		input  string
+		want   any
+		errStr string
+	}{
+		{
+			input: `"abc"`,
+			want:  "abc",
+		},
+		{
+			input: `123`,
+			want:  float64(123),
+		},
+		{
+			input: `{"abc": 123}`,
+			want:  map[string]any{"abc": float64(123)},
+		},
+		{
+			input: `{"abc": 123} `,
+			want:  map[string]any{"abc": float64(123)},
+		},
+		{
+			input: `        ["abc", 123] `,
+			want:  []any{"abc", float64(123)},
+		},
+		{
+			input:  `{"abc": 123}a`,
+			errStr: "body must contain only one JSON entity",
+		},
+		{
+			input:  `[123]{"a":"b"}`,
+			errStr: "body must contain only one JSON entity",
+		},
+		{
+			input:  `{"abc`,
+			errStr: "unexpected EOF",
+		},
+	}
+
+	for _, tc := range testCases {
+		var val any
+		err := Unmarshal([]byte(tc.input), &val)
+		if err != nil {
+			if tc.errStr == "" {
+				t.Fatal(err)
+			}
+			if have := err.Error(); have != tc.errStr {
+				t.Fatalf("\nhave: %+v\nwant: %+v\n", have, tc.errStr)
+			}
+			continue
+		}
+
+		if !reflect.DeepEqual(val, tc.want) {
+			t.Fatalf("\nhave: %+v\nwant: %+v\n", val, tc.want)
+		}
+	}
+}