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// Copyright 2018 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 fmtsort provides a general stable ordering mechanism
// for maps, on behalf of the fmt and text/template packages.
// It is not guaranteed to be efficient and works only for types
// that are valid map keys.
package fmtsort
import (
"reflect"
"sort"
)
// Note: Throughout this package we avoid calling reflect.Value.Interface as
// it is not always legal to do so and it's easier to avoid the issue than to face it.
// SortedMap represents a map's keys and values. The keys and values are
// aligned in index order: Value[i] is the value in the map corresponding to Key[i].
type SortedMap struct {
Key []reflect.Value
Value []reflect.Value
}
func (o *SortedMap) Len() int { return len(o.Key) }
func (o *SortedMap) Less(i, j int) bool { return compare(o.Key[i], o.Key[j]) < 0 }
func (o *SortedMap) Swap(i, j int) {
o.Key[i], o.Key[j] = o.Key[j], o.Key[i]
o.Value[i], o.Value[j] = o.Value[j], o.Value[i]
}
// Sort accepts a map and returns a SortedMap that has the same keys and
// values but in a stable sorted order according to the keys, modulo issues
// raised by unorderable key values such as NaNs.
//
// The ordering rules are more general than with Go's < operator:
//
// - when applicable, nil compares low
// - ints, floats, and strings order by <
// - NaN compares less than non-NaN floats
// - bool compares false before true
// - complex compares real, then imag
// - pointers compare by machine address
// - channel values compare by machine address
// - structs compare each field in turn
// - arrays compare each element in turn.
// Otherwise identical arrays compare by length.
// - interface values compare first by reflect.Type describing the concrete type
// and then by concrete value as described in the previous rules.
func Sort(mapValue reflect.Value) *SortedMap {
if mapValue.Type().Kind() != reflect.Map {
return nil
}
// Note: this code is arranged to not panic even in the presence
// of a concurrent map update. The runtime is responsible for
// yelling loudly if that happens. See issue 33275.
n := mapValue.Len()
key := make([]reflect.Value, 0, n)
value := make([]reflect.Value, 0, n)
iter := mapValue.MapRange()
for iter.Next() {
key = append(key, iter.Key())
value = append(value, iter.Value())
}
sorted := &SortedMap{
Key: key,
Value: value,
}
sort.Stable(sorted)
return sorted
}
// compare compares two values of the same type. It returns -1, 0, 1
// according to whether a > b (1), a == b (0), or a < b (-1).
// If the types differ, it returns -1.
// See the comment on Sort for the comparison rules.
func compare(aVal, bVal reflect.Value) int {
aType, bType := aVal.Type(), bVal.Type()
if aType != bType {
return -1 // No good answer possible, but don't return 0: they're not equal.
}
switch aVal.Kind() {
case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
a, b := aVal.Int(), bVal.Int()
switch {
case a < b:
return -1
case a > b:
return 1
default:
return 0
}
case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
a, b := aVal.Uint(), bVal.Uint()
switch {
case a < b:
return -1
case a > b:
return 1
default:
return 0
}
case reflect.String:
a, b := aVal.String(), bVal.String()
switch {
case a < b:
return -1
case a > b:
return 1
default:
return 0
}
case reflect.Float32, reflect.Float64:
return floatCompare(aVal.Float(), bVal.Float())
case reflect.Complex64, reflect.Complex128:
a, b := aVal.Complex(), bVal.Complex()
if c := floatCompare(real(a), real(b)); c != 0 {
return c
}
return floatCompare(imag(a), imag(b))
case reflect.Bool:
a, b := aVal.Bool(), bVal.Bool()
switch {
case a == b:
return 0
case a:
return 1
default:
return -1
}
case reflect.Pointer, reflect.UnsafePointer:
a, b := aVal.Pointer(), bVal.Pointer()
switch {
case a < b:
return -1
case a > b:
return 1
default:
return 0
}
case reflect.Chan:
if c, ok := nilCompare(aVal, bVal); ok {
return c
}
ap, bp := aVal.Pointer(), bVal.Pointer()
switch {
case ap < bp:
return -1
case ap > bp:
return 1
default:
return 0
}
case reflect.Struct:
for i := 0; i < aVal.NumField(); i++ {
if c := compare(aVal.Field(i), bVal.Field(i)); c != 0 {
return c
}
}
return 0
case reflect.Array:
for i := 0; i < aVal.Len(); i++ {
if c := compare(aVal.Index(i), bVal.Index(i)); c != 0 {
return c
}
}
return 0
case reflect.Interface:
if c, ok := nilCompare(aVal, bVal); ok {
return c
}
c := compare(reflect.ValueOf(aVal.Elem().Type()), reflect.ValueOf(bVal.Elem().Type()))
if c != 0 {
return c
}
return compare(aVal.Elem(), bVal.Elem())
default:
// Certain types cannot appear as keys (maps, funcs, slices), but be explicit.
panic("bad type in compare: " + aType.String())
}
}
// nilCompare checks whether either value is nil. If not, the boolean is false.
// If either value is nil, the boolean is true and the integer is the comparison
// value. The comparison is defined to be 0 if both are nil, otherwise the one
// nil value compares low. Both arguments must represent a chan, func,
// interface, map, pointer, or slice.
func nilCompare(aVal, bVal reflect.Value) (int, bool) {
if aVal.IsNil() {
if bVal.IsNil() {
return 0, true
}
return -1, true
}
if bVal.IsNil() {
return 1, true
}
return 0, false
}
// floatCompare compares two floating-point values. NaNs compare low.
func floatCompare(a, b float64) int {
switch {
case isNaN(a):
return -1 // No good answer if b is a NaN so don't bother checking.
case isNaN(b):
return 1
case a < b:
return -1
case a > b:
return 1
}
return 0
}
func isNaN(a float64) bool {
return a != a
}