From 2ad2f11326680522a8cab68c7703912c4d5e5937 Mon Sep 17 00:00:00 2001
From: IncSW <dev@incsw.in>
Date: Fri, 30 Apr 2021 19:09:15 +0300
Subject: [PATCH] when cap is enough, reuse slice data for compatibility with
 encoding/json

---
 decode_slice.go | 36 ++++++++++++++++++++----------------
 decode_test.go  | 10 ++++++++--
 2 files changed, 28 insertions(+), 18 deletions(-)

diff --git a/decode_slice.go b/decode_slice.go
index ea09f88..a80307e 100644
--- a/decode_slice.go
+++ b/decode_slice.go
@@ -96,10 +96,11 @@ 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
@@ -130,11 +131,12 @@ func (d *sliceDecoder) decodeStream(s *stream, depth int64, p unsafe.Pointer) er
 					slice.cap = capacity
 					slice.len = idx + 1
 					slice.data = data
+					dst := *(*sliceHeader)(p)
+					dst.len = idx + 1
 					dstCap := idx + 1
-					dst := sliceHeader{
-						data: newArray(d.elemType, dstCap),
-						len:  idx + 1,
-						cap:  dstCap,
+					if dstCap > dst.cap {
+						dst.data = newArray(d.elemType, dstCap)
+						dst.cap = dstCap
 					}
 					copySlice(d.elemType, dst, sliceHeader{
 						data: slice.data,
@@ -210,10 +212,11 @@ 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
@@ -245,11 +248,12 @@ func (d *sliceDecoder) decode(buf []byte, cursor, depth int64, p unsafe.Pointer)
 					slice.cap = capacity
 					slice.len = idx + 1
 					slice.data = data
+					dst := *(*sliceHeader)(p)
+					dst.len = idx + 1
 					dstCap := idx + 1
-					dst := sliceHeader{
-						data: newArray(d.elemType, dstCap),
-						len:  idx + 1,
-						cap:  dstCap,
+					if dstCap > dst.cap {
+						dst.data = newArray(d.elemType, dstCap)
+						dst.cap = dstCap
 					}
 					copySlice(d.elemType, dst, sliceHeader{
 						data: slice.data,
diff --git a/decode_test.go b/decode_test.go
index 574341f..2eb01d9 100644
--- a/decode_test.go
+++ b/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"
   }