// Copyright 2015 PingCAP, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.

package hack

import (
	"reflect"
	"unsafe"
)

// MutableString can be used as string via string(MutableString) without performance loss.
type MutableString string

// String converts slice to MutableString without copy.
// The MutableString can be converts to string without copy.
// Use it at your own risk.
func String(b []byte) (s MutableString) {
	if len(b) == 0 {
		return ""
	}
	pbytes := (*reflect.SliceHeader)(unsafe.Pointer(&b))
	pstring := (*reflect.StringHeader)(unsafe.Pointer(&s))
	pstring.Data = pbytes.Data
	pstring.Len = pbytes.Len
	return
}

// Slice converts string to slice without copy.
// Use at your own risk.
func Slice(s string) (b []byte) {
	pbytes := (*reflect.SliceHeader)(unsafe.Pointer(&b))
	pstring := (*reflect.StringHeader)(unsafe.Pointer(&s))
	pbytes.Data = pstring.Data
	pbytes.Len = pstring.Len
	pbytes.Cap = pstring.Len
	return
}

// LoadFactor is the maximum average load of a bucket that triggers growth is 6.5 in Golang Map.
// Represent as LoadFactorNum/LoadFactorDen, to allow integer math.
// They are from the golang definition. ref: https://github.com/golang/go/blob/go1.13.15/src/runtime/map.go#L68-L71
const (
	// LoadFactorNum is the numerator of load factor
	LoadFactorNum = 13
	// LoadFactorDen is the denominator of load factor
	LoadFactorDen = 2
)
