reflect: add Swapper func

Swapper returns a func that swaps two elements in a slice.

Updates #16721

Change-Id: I7f2287a675c10a05019e02b7d62fb870af31216f
Reviewed-on: https://go-review.googlesource.com/30088
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/reflect/all_test.go

@@ -5780,3 +5780,84 @@ func BenchmarkNew(b *testing.B) {
 		New(v)
 	}
 }
+
+func TestSwapper(t *testing.T) {
+	type I int
+	var a, b, c I
+	type pair struct {
+		x, y int
+	}
+	type pairPtr struct {
+		x, y int
+		p    *I
+	}
+	type S string
+
+	tests := []struct {
+		in   interface{}
+		i, j int
+		want interface{}
+	}{
+		{
+			in:   []int{1, 20, 300},
+			i:    0,
+			j:    2,
+			want: []int{300, 20, 1},
+		},
+		{
+			in:   []uintptr{1, 20, 300},
+			i:    0,
+			j:    2,
+			want: []uintptr{300, 20, 1},
+		},
+		{
+			in:   []int16{1, 20, 300},
+			i:    0,
+			j:    2,
+			want: []int16{300, 20, 1},
+		},
+		{
+			in:   []int8{1, 20, 100},
+			i:    0,
+			j:    2,
+			want: []int8{100, 20, 1},
+		},
+		{
+			in:   []*I{&a, &b, &c},
+			i:    0,
+			j:    2,
+			want: []*I{&c, &b, &a},
+		},
+		{
+			in:   []string{"eric", "sergey", "larry"},
+			i:    0,
+			j:    2,
+			want: []string{"larry", "sergey", "eric"},
+		},
+		{
+			in:   []S{"eric", "sergey", "larry"},
+			i:    0,
+			j:    2,
+			want: []S{"larry", "sergey", "eric"},
+		},
+		{
+			in:   []pair{{1, 2}, {3, 4}, {5, 6}},
+			i:    0,
+			j:    2,
+			want: []pair{{5, 6}, {3, 4}, {1, 2}},
+		},
+		{
+			in:   []pairPtr{{1, 2, &a}, {3, 4, &b}, {5, 6, &c}},
+			i:    0,
+			j:    2,
+			want: []pairPtr{{5, 6, &c}, {3, 4, &b}, {1, 2, &a}},
+		},
+	}
+	for i, tt := range tests {
+		inStr := fmt.Sprint(tt.in)
+		Swapper(tt.in)(tt.i, tt.j)
+		if !DeepEqual(tt.in, tt.want) {
+			t.Errorf("%d. swapping %v and %v of %v = %v; want %v", i, tt.i, tt.j, inStr, tt.in, tt.want)
+		}
+	}
+}

src/reflect/swapper.go

+// 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.
+
+package reflect
+
+import "unsafe"
+
+// Swapper returns a function that swaps the elements in the provided
+// slice.
+//
+// Swapper panics if the provided interface is not a slice.
+func Swapper(slice interface{}) func(i, j int) {
+	v := ValueOf(slice)
+	if v.Kind() != Slice {
+		panic(&ValueError{Method: "Swapper", Kind: v.Kind()})
+	}
+	// Fast path for slices of size 0 and 1. Nothing to swap.
+	switch v.Len() {
+	case 0:
+		return func(i, j int) { panic("reflect: slice index out of range") }
+	case 1:
+		return func(i, j int) {
+			if i != 0 || j != 0 {
+				panic("reflect: slice index out of range")
+			}
+		}
+	}
+
+	typ := v.Type().Elem().(*rtype)
+	size := typ.Size()
+	hasPtr := typ.kind&kindNoPointers == 0
+
+	// Some common & small cases, without using memmove:
+	if hasPtr {
+		if size == ptrSize {
+			ps := *(*[]unsafe.Pointer)(v.ptr)
+			return func(i, j int) { ps[i], ps[j] = ps[j], ps[i] }
+		}
+		if typ.Kind() == String {
+			ss := *(*[]string)(v.ptr)
+			return func(i, j int) { ss[i], ss[j] = ss[j], ss[i] }
+		}
+	} else {
+		switch size {
+		case 8:
+			is := *(*[]int64)(v.ptr)
+			return func(i, j int) { is[i], is[j] = is[j], is[i] }
+		case 4:
+			is := *(*[]int32)(v.ptr)
+			return func(i, j int) { is[i], is[j] = is[j], is[i] }
+		case 2:
+			is := *(*[]int16)(v.ptr)
+			return func(i, j int) { is[i], is[j] = is[j], is[i] }
+		case 1:
+			is := *(*[]int8)(v.ptr)
+			return func(i, j int) { is[i], is[j] = is[j], is[i] }
+		}
+	}
+
+	s := (*sliceHeader)(v.ptr)
+	tmp := unsafe_New(typ) // swap scratch space
+
+	return func(i, j int) {
+		if uint(i) >= uint(s.Len) || uint(j) >= uint(s.Len) {
+			panic("reflect: slice index out of range")
+		}
+		val1 := arrayAt(s.Data, i, size)
+		val2 := arrayAt(s.Data, j, size)
+		typedmemmove(typ, tmp, val1)
+		typedmemmove(typ, val1, val2)
+		typedmemmove(typ, val2, tmp)
+	}
+}