implement reflect.Swapper

Signed-off-by: mathetake <takeshi@tetrate.io>

src/reflect/swapper.go

@@ -1,5 +1,40 @@
 package reflect
 
+import "unsafe"
+
+// Some of code here has been copied from the Go sources:
+//   https://github.com/golang/go/blob/go1.15.2/src/reflect/swapper.go
+// It has the following copyright note:
+//
+// 		Copyright 2016 The Go Authors. All rights reserved.
+// 		Use of this source code is governed by a BSD-style
+// 		license that can be found in the LICENSE file.
+
 func Swapper(slice interface{}) func(i, j int) {
-	panic("unimplemented: reflect.Swapper")
+	v := ValueOf(slice)
+	if v.Kind() != Slice {
+		panic(&ValueError{Method: "Swapper"})
+	}
+
+	// Just return Nop func if nothing to swap.
+	if v.Len() < 2 {
+		return func(i, j int) {}
+	}
+
+	typ := v.Type().Elem()
+	size := typ.Size()
+
+	header := (*SliceHeader)(v.value)
+	tmp := unsafe.Pointer(&make([]byte, size)[0])
+
+	return func(i, j int) {
+		if uint(i) >= uint(header.Len) || uint(j) >= uint(header.Len) {
+			panic("reflect: slice index out of range")
+		}
+		val1 := unsafe.Pointer(header.Data + uintptr(i)*size)
+		val2 := unsafe.Pointer(header.Data + uintptr(j)*size)
+		memcpy(tmp, val1, size)
+		memcpy(val1, val2, size)
+		memcpy(val2, tmp, size)
+	}
 }