/*
Copyright 2012 Google 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,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package groupcache

import (
	"bytes"
	"errors"
	"io"
	"strings"
)

// A ByteView holds an immutable view of bytes.
// Internally it wraps either a []byte or a string,
// but that detail is invisible to callers.
//
// A ByteView is meant to be used as a value type, not
// a pointer (like a time.Time).
// 封装了一个string与byte[]的统一接口，也就是说用byteview提供的接口，可以屏蔽掉string与byte[]的不同，使用时可以不用考虑是string还是byte[]
type ByteView struct {
	// If b is non-nil, b is used, else s is used.
	// 当b是nil 的时候，s作为存储内容的容器
	b []byte
	s string
}

// Len returns the view's length.
func (v ByteView) Len() int {
	if v.b != nil {
		return len(v.b)
	}
	return len(v.s)
}

// ByteSlice returns a copy of the data as a byte slice.
// 按[]byte返回一个拷贝
func (v ByteView) ByteSlice() []byte {
	if v.b != nil {
		return cloneBytes(v.b)
	}
	return []byte(v.s)
}

// String returns the data as a string, making a copy if necessary.
// 按string返回一个拷贝
func (v ByteView) String() string {
	if v.b != nil {
		return string(v.b)
	}
	return v.s
}

// At returns the byte at index i.
// 返回第i个byte
func (v ByteView) At(i int) byte {
	if v.b != nil {
		return v.b[i]
	}
	return v.s[i]
}

// Slice slices the view between the provided from and to indices.
// 返回ByteView的某个片断，不拷贝
func (v ByteView) Slice(from, to int) ByteView {
	if v.b != nil {
		return ByteView{b: v.b[from:to]}
	}
	return ByteView{s: v.s[from:to]}
}

// SliceFrom slices the view from the provided index until the end.
// 返回ByteView的从某个位置开始的片断，不拷贝
func (v ByteView) SliceFrom(from int) ByteView {
	if v.b != nil {
		return ByteView{b: v.b[from:]}
	}
	return ByteView{s: v.s[from:]}
}

// Copy copies b into dest and returns the number of bytes copied.
// 将ByteView按[]byte拷贝出来
func (v ByteView) Copy(dest []byte) int {
	if v.b != nil {
		return copy(dest, v.b)
	}
	return copy(dest, v.s)
}

// Equal returns whether the bytes in b are the same as the bytes in b2.
// 判断2个ByteView是否相等
func (v ByteView) Equal(b2 ByteView) bool {
	if b2.b == nil {
		return v.EqualString(b2.s)
	}
	return v.EqualBytes(b2.b)
}

// EqualString returns whether the bytes in b are the same as the bytes in s.
// 判断ByteView是否和string相等
func (v ByteView) EqualString(s string) bool {
	if v.b == nil {
		return v.s == s
	}
	l := v.Len()
	if len(s) != l {
		return false
	}
	for i, bi := range v.b {
		if bi != s[i] {
			return false
		}
	}
	return true
}

// EqualBytes returns whether the bytes in b are the same as the bytes
// in b2.
// 判断ByteView是否和[]byte相等
func (v ByteView) EqualBytes(b2 []byte) bool {
	if v.b != nil {
		return bytes.Equal(v.b, b2)
	}
	l := v.Len()
	if len(b2) != l {
		return false
	}
	for i, bi := range b2 {
		if bi != v.s[i] {
			return false
		}
	}
	return true
}

// Reader returns an io.ReadSeeker for the bytes in v.
// 对ByteView创建一个io.ReadSeeker
func (v ByteView) Reader() io.ReadSeeker {
	if v.b != nil {
		return bytes.NewReader(v.b)
	}
	return strings.NewReader(v.s)
}

// ReadAt implements io.ReaderAt on the bytes in v.
// 读取从off开始的后面的数据，其实下面调用的SliceFrom，这是封装成了io.Reader的一个ReadAt方法的形式
func (v ByteView) ReadAt(p []byte, off int64) (n int, err error) {
	if off < 0 {
		return 0, errors.New("view: invalid offset")
	}
	if off >= int64(v.Len()) {
		return 0, io.EOF
	}
	n = v.SliceFrom(int(off)).Copy(p)
	if n < len(p) {
		err = io.EOF
	}
	return
}

// WriteTo implements io.WriterTo on the bytes in v.
// 向w流中写入v
func (v ByteView) WriteTo(w io.Writer) (n int64, err error) {
	var m int
	if v.b != nil {
		m, err = w.Write(v.b)
	} else {
		m, err = io.WriteString(w, v.s)
	}
	if err == nil && m < v.Len() {
		err = io.ErrShortWrite
	}
	n = int64(m)
	return
}