Merge pull request #200 from IncSW/slice-decoding-compatibility

when cap is enough, reuse slice data for compatibility with encoding/json

Author: goccy

decode_slice.go

@@ -101,10 +101,11 @@ func (d *sliceDecoder) decodeStream(s *stream, depth int64, p unsafe.Pointer) er
 			s.cursor++
 			s.skipWhiteSpace()
 			if s.char() == ']' {
-				*(*sliceHeader)(p) = sliceHeader{
-					data: newArray(d.elemType, 0),
-					len:  0,
-					cap:  0,
+				dst := (*sliceHeader)(p)
+				if dst.data == nil {
+					dst.data = newArray(d.elemType, 0)
+				} else {
+					dst.len = 0
 				}
 				s.cursor++
 				return nil
@@ -138,18 +139,13 @@ func (d *sliceDecoder) decodeStream(s *stream, depth int64, p unsafe.Pointer) er
 					slice.cap = capacity
 					slice.len = idx + 1
 					slice.data = data
-					dstCap := idx + 1
-					dst := sliceHeader{
-						data: newArray(d.elemType, dstCap),
-						len:  idx + 1,
-						cap:  dstCap,
+					dst := (*sliceHeader)(p)
+					dst.len = idx + 1
+					if dst.len > dst.cap {
+						dst.data = newArray(d.elemType, dst.len)
+						dst.cap = dst.len
 					}
-					copySlice(d.elemType, dst, sliceHeader{
-						data: slice.data,
-						len:  slice.len,
-						cap:  slice.cap,
-					})
-					*(*sliceHeader)(p) = dst
+					copySlice(d.elemType, *dst, *slice)
 					d.releaseSlice(slice)
 					s.cursor++
 					return nil
@@ -218,10 +214,11 @@ func (d *sliceDecoder) decode(buf []byte, cursor, depth int64, p unsafe.Pointer)
 			cursor++
 			cursor = skipWhiteSpace(buf, cursor)
 			if buf[cursor] == ']' {
-				**(**sliceHeader)(unsafe.Pointer(&p)) = sliceHeader{
-					data: newArray(d.elemType, 0),
-					len:  0,
-					cap:  0,
+				dst := (*sliceHeader)(p)
+				if dst.data == nil {
+					dst.data = newArray(d.elemType, 0)
+				} else {
+					dst.len = 0
 				}
 				cursor++
 				return cursor, nil
@@ -256,18 +253,13 @@ func (d *sliceDecoder) decode(buf []byte, cursor, depth int64, p unsafe.Pointer)
 					slice.cap = capacity
 					slice.len = idx + 1
 					slice.data = data
-					dstCap := idx + 1
-					dst := sliceHeader{
-						data: newArray(d.elemType, dstCap),
-						len:  idx + 1,
-						cap:  dstCap,
+					dst := (*sliceHeader)(p)
+					dst.len = idx + 1
+					if dst.len > dst.cap {
+						dst.data = newArray(d.elemType, dst.len)
+						dst.cap = dst.len
 					}
-					copySlice(d.elemType, dst, sliceHeader{
-						data: slice.data,
-						len:  slice.len,
-						cap:  slice.cap,
-					})
-					**(**sliceHeader)(unsafe.Pointer(&p)) = dst
+					copySlice(d.elemType, *dst, *slice)
 					d.releaseSlice(slice)
 					cursor++
 					return cursor, nil

decode_test.go

@@ -103,6 +103,12 @@ func Test_Decoder(t *testing.T) {
 		assertErr(t, json.Unmarshal([]byte(` [ 1 , 2 , 3 , 4 ] `), &v))
 		assertEq(t, "slice", fmt.Sprint([]int{1, 2, 3, 4}), fmt.Sprint(v))
 	})
+	t.Run("slice_reuse_data", func(t *testing.T) {
+		v := make([]int, 0, 10)
+		assertErr(t, json.Unmarshal([]byte(` [ 1 , 2 , 3 , 4 ] `), &v))
+		assertEq(t, "slice", fmt.Sprint([]int{1, 2, 3, 4}), fmt.Sprint(v))
+		assertEq(t, "cap", 10, cap(v))
+	})
 	t.Run("array", func(t *testing.T) {
 		var v [4]int
 		assertErr(t, json.Unmarshal([]byte(` [ 1 , 2 , 3 , 4 ] `), &v))
@@ -121,10 +127,10 @@ func Test_Decoder(t *testing.T) {
 {
   "a": {
     "nestedA": "value of nested a"
-  },  
+  },
   "b": {
     "nestedB": "value of nested b"
-  },  
+  },
   "c": {
     "nestedC": "value of nested c"
   }