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)
+		}
+	}
+}