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>

diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index 3bf24d2250bc54d4df522dc0372204a3a3c33200..1f2c7527846cf04f64881855820b97d9e14643e3 100644 (file)
--- a/src/reflect/all_test.go
+++ b/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)
+               }
+       }
+}

diff --git a/src/reflect/swapper.go b/src/reflect/swapper.go
new file mode 100644 (file)
index 0000000..5441cb0
--- /dev/null
+++ b/src/reflect/swapper.go
@@ -0,0 +1,74 @@
+// 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)
+       }
+}