type error interface {
    Error() string
}

// PathError records an error and the operation and
// file path that caused it.
type PathError struct {
    Op string    // "open", "unlink", etc.
    Path string  // The associated file.
    Err error    // Returned by the system call.
}

func (e *PathError) Error() string {
    return e.Op + " " + e.Path + ": " + e.Err.Error()
}

// A toy implementation of cube root using Newton's method.
func CubeRoot(x float64) float64 {
    z := x/3   // Arbitrary initial value
    for i := 0; i < 1e6; i++ {
        prevz := z
        z -= (z*z*z-x) / (3*z*z)
        if veryClose(z, prevz) {
            return z
        }
    }
    // A million iterations has not converged; something is wrong.
    panic(fmt.Sprintf("CubeRoot(%g) did not converge", x))
}

func fullName(firstName *string, lastName *string) {  
    defer fmt.Println("deferred call in fullName")
    if firstName == nil {
        panic("runtime error: first name cannot be nil")
    }
    if lastName == nil {
        panic("runtime error: last name cannot be nil")
    }
    fmt.Printf("%s %s\n", *firstName, *lastName)
}

func main() {  
    defer fmt.Println("deferred call in main")
    firstName := "Elon"
    fullName(&firstName, nil)
    fmt.Println("returned normally from main")
}

func recoverName() {  
    if r := recover(); r!= nil {
        fmt.Println("recovered from ", r)
    }
}

func fullName(firstName *string, lastName *string) {  
    defer recoverName()
    if firstName == nil {
        panic("runtime error: first name cannot be nil")
    }
    if lastName == nil {
        panic("runtime error: last name cannot be nil")
    }
    fmt.Printf("%s %s\n", *firstName, *lastName)
}

func main() {  
    defer fmt.Println("deferred call in main")
    firstName := "Elon"
    fullName(&firstName, nil)
    fmt.Println("returned normally from main")
}

type stringer interface{ 
   string()string
} 
  
type tester interface{ 
   stringer               // 嵌入其他接口 
   test() 
}

type Sequence []int
// Method for printing - sorts the elements before printing.
func (s Sequence) String() string {
    s = s.Copy() // Make a copy; don't overwrite argument.
    sort.Sort(s)
    str := "["
    for i, elem := range s { // Loop is O(N²); will fix that in next example.
        if i > 0 {
            str += " "
        }
        str += fmt.Sprint(elem)
    }
    return str + "]"
}

func (s Sequence) String() string {
    s = s.Copy()
    sort.Sort(s)
    return fmt.Sprint([]int(s))
}

type Stringer interface {
    String() string
}

var value interface{} // Value provided by caller.
switch str := value.(type) {
case string:
    return str
case Stringer:
    return str.String()
}

str, ok := value.(string)
if ok {
    fmt.Printf("string value is: %q\n", str)
} else {
    fmt.Printf("value is not a string\n")
}

type Block interface {
    BlockSize() int
    Encrypt(src, dst []byte)
    Decrypt(src, dst []byte)
}

type Stream interface {
    XORKeyStream(dst, src []byte)
}

// NewCTR returns a Stream that encrypts/decrypts using the given Block in
// counter mode. The length of iv must be the same as the Block's block size.
func NewCTR(block Block, iv []byte) Stream

type Reader interface {
   Read(p []byte) (n int, err error)
}

type Writer interface {
   Write(p []byte) (n int, err error)
}

type Closer interface {
   Close() error
}

type Seeker interface {
   Seek(offset int64, whence int) (int64, error)
}

type File struct { // ...
}
func (f *File) Read(buf []byte) (n int, err error)
func (f *File) Write(buf []byte) (n int, err error)
func (f *File) Seek(off int64, whence int) (pos int64, err error) 
func (f *File) Close() error

var file1 Reader = new(File)
var file2 Writer = new(File)
var file3 Closer = new(File)
var file4 Seeker = new(File)

package main

import "fmt"

type Retriever interface {
   Get(url string) string
}

func f(r Retriever) {
    s := r.Get('\args')
  	fmt.Println(s)
}

type RealRetriever struct {
   Contents string
}

// 指针接收者
func (r *RealRetriever) Get(url string) string {
   return r.Contents
}

func main(){
    retriever := RealRetriever{}
    f(&retriever) //pointer
    f(retriever)  //value
}

type MockRetriever struct {
   Contents string
}

// 值接收者
func (r MockRetriever) Get(url string) string {
   return r.Contents
}

t := []int{1, 2, 3, 4}
var s []interface{} = t

func printAll(vals []interface{}) { //1
    for _, val := range vals {
        fmt.Println(val)
    }
}

func main(){
    names := []string{"stanley", "david", "oscar"}
    printAll(names)
}

t := []int{1, 2, 3, 4}
s := make([]interface{}, len(t))

func NewFile(fd int, name string) *File {
    if fd < 0 {
        return nil
    }
    f := File{fd, name, nil, 0}
    return &f
}

return &File{fd: fd, name: name}

a := [...]string   {Enone: "no error", Eio: "Eio", Einval: "invalid argument"}
s := []string      {Enone: "no error", Eio: "Eio", Einval: "invalid argument"}
m := map[int]string{Enone: "no error", Eio: "Eio", Einval: "invalid argument"}

// Unnecessarily complex:
var p *[]int = new([]int)
*p = make([]int, 100, 100)

v := make([]int, 100)

func Contents(filename string) (string, error) {
    f, err := os.Open(filename)
    if err != nil {
        return "", err
    }
    defer f.Close()  // f.Close will run when we're finished.
  	return nil 
}

for i := 0; i < 5; i++ {
    defer fmt.Printf("%d ", i)
}

go func() { for { dst <- <-src } }()

if i < f() {
    g()
}

if i < f()  // wrong!
{           // wrong!
    g()
}

// A Context carries a deadline, cancelation signal, and request-scoped values
// across API boundaries. Its methods are safe for simultaneous use by multiple
// goroutines.
type Context interface {
    // Done returns a channel that is closed when this Context is canceled
    // or times out.
    Done() <-chan struct{}

    // Err indicates why this context was canceled, after the Done channel
    // is closed.
    Err() error

    // Deadline returns the time when this Context will be canceled, if any.
    Deadline() (deadline time.Time, ok bool)

    // Value returns the value associated with key or nil if none.
    Value(key interface{}) interface{}
}

type emptyCtx int

func (*emptyCtx) Deadline() (deadline time.Time, ok bool) {
    return
}

func (*emptyCtx) Done() <-chan struct{} {
    return nil
}

func (*emptyCtx) Err() error {
    return nil
}

func (*emptyCtx) Value(key interface{}) interface{} {
    return nil
}

func Background() Context

func WithCancel(parent Context) (ctx Context, cancel CancelFunc)

func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc)

func slowOperationWithTimeout(ctx context.Context) (Result, error) {
	ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
	defer cancel()  // releases resources if slowOperation completes before timeout elapses
	return slowOperation(ctx)
}

func WithValue(parent Context, key interface{}, val interface{}) Context

type canceler interface {
    cancel(removeFromParent bool, err error)
    Done() <-chan struct{}
}

type cancelCtx struct {
    Context

    // 保护之后的字段
    mu       sync.Mutex
    done     chan struct{}
    children map[canceler]struct{}
    err      error
}

func (c *cancelCtx) Done() <-chan struct{} {
    c.mu.Lock()
    if c.done == nil {
        c.done = make(chan struct{})
    }
    d := c.done
    c.mu.Unlock()
    return d
}

func (c *cancelCtx) cancel(removeFromParent bool, err error) {
    // 必须要传 err
    if err == nil {
        panic("context: internal error: missing cancel error")
    }
    c.mu.Lock()
    if c.err != nil {
        c.mu.Unlock()
        return // 已经被其他协程取消
    }
    // 给 err 字段赋值
    c.err = err
    // 关闭 channel，通知其他协程
    if c.done == nil {
        c.done = closedchan
    } else {
        close(c.done)
    }
    
    // 遍历它的所有子节点
    for child := range c.children {
        // 递归地取消所有子节点
        child.cancel(false, err)
    }
    // 将子节点置空
    c.children = nil
    c.mu.Unlock()

    if removeFromParent {
        // 从父节点中移除自己 
        removeChild(c.Context, c)
    }
}

type valueCtx struct {
    Context
    key, val interface{}
}

func (c *valueCtx) String() string {
    return fmt.Sprintf("%v.WithValue(%#v, %#v)", c.Context, c.key, c.val)
}

func (c *valueCtx) Value(key interface{}) interface{} {
    if c.key == key {
        return c.val
    }
    return c.Context.Value(key)
}

func main() {
	// Creates a gin router with default middleware:
	// logger and recovery (crash-free) middleware
	router := gin.Default()

	router.GET("/someGet", getting)
	router.POST("/somePost", posting)
	router.PUT("/somePut", putting)
	router.DELETE("/someDelete", deleting)
	router.PATCH("/somePatch", patching)
	router.HEAD("/someHead", head)
	router.OPTIONS("/someOptions", options)

	// By default it serves on :8080 unless a
	// PORT environment variable was defined.
	router.Run()
	// router.Run(":3000") for a hard coded port
}

func main() {
	router := gin.Default()

	// This handler will match /user/john but will not match /user/ or /user
	router.GET("/user/:name", func(c *gin.Context) {
		name := c.Param("name")
		c.String(http.StatusOK, "Hello %s", name)
	})

	// However, this one will match /user/john/ and also /user/john/send
	// If no other routers match /user/john, it will redirect to /user/john/
	router.GET("/user/:name/*action", func(c *gin.Context) {
		name := c.Param("name")
		action := c.Param("action")
		message := name + " is " + action
		c.String(http.StatusOK, message)
	})

	// For each matched request Context will hold the route definition
	router.POST("/user/:name/*action", func(c *gin.Context) {
		c.FullPath() == "/user/:name/*action" // true
	})

	router.Run(":8080")
}

router.GET("/welcome", func(c *gin.Context) {
		firstname := c.DefaultQuery("firstname", "Guest")
		lastname := c.Query("lastname") // shortcut for c.Request.URL.Query().Get("lastname")

		c.String(http.StatusOK, "Hello %s %s", firstname, lastname)
	})

router.POST("/form_post", func(c *gin.Context) {
		message := c.PostForm("message")
		nick := c.DefaultPostForm("nick", "anonymous")

		c.JSON(200, gin.H{
			"status":  "posted",
			"message": message,
			"nick":    nick,
		})
	})

func main() {
	router := gin.Default()

	router.POST("/post", func(c *gin.Context) {

		ids := c.QueryMap("ids")
		names := c.PostFormMap("names")

		fmt.Printf("ids: %v; names: %v", ids, names)
	})
	router.Run(":8080")
}

func main() {
	router := gin.Default()
	// Set a lower memory limit for multipart forms (default is 32 MiB)
	// router.MaxMultipartMemory = 8 << 20  // 8 MiB
	router.POST("/upload", func(c *gin.Context) {
		// single file
		file, _ := c.FormFile("file")
		log.Println(file.Filename)

		// Upload the file to specific dst.
		// c.SaveUploadedFile(file, dst)

		c.String(http.StatusOK, fmt.Sprintf("'%s' uploaded!", file.Filename))
	})
	router.Run(":8080")
}

// Binding from JSON
type Login struct {
	User     string `form:"user" json:"user" xml:"user"  binding:"required"`
	Password string `form:"password" json:"password" xml:"password" binding:"required"`
}
// 如果输入user没输入password，在这种情况下会报错。
// 如果把上面password的binging改成：binding:"-"就不会报错了
router.POST("/loginJSON", func(c *gin.Context) {
		var json Login
		if err := c.ShouldBindJSON(&json); err != nil {
			c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
			return
		}
		
		if json.User != "manu" || json.Password != "123" {
			c.JSON(http.StatusUnauthorized, gin.H{"status": "unauthorized"})
			return
		} 
		
		c.JSON(http.StatusOK, gin.H{"status": "you are logged in"})
	})

func main() {
	router := gin.Default()
	router.LoadHTMLGlob("templates/*")
	//router.LoadHTMLFiles("templates/template1.html", "templates/template2.html")
	router.GET("/index", func(c *gin.Context) {
		c.HTML(http.StatusOK, "index.tmpl", gin.H{
			"title": "Main website",
		})
	})
	router.Run(":8080")
}

package main

import (
	"log"
	"net/http"
	"time"

	"github.com/gin-gonic/gin"
	"golang.org/x/sync/errgroup"
)

var (
	g errgroup.Group
)

func router01() http.Handler {
	e := gin.New()
	e.Use(gin.Recovery())
	e.GET("/", func(c *gin.Context) {
		c.JSON(
			http.StatusOK,
			gin.H{
				"code":  http.StatusOK,
				"error": "Welcome server 01",
			},
		)
	})

	return e
}

func router02() http.Handler {
	e := gin.New()
	e.Use(gin.Recovery())
	e.GET("/", func(c *gin.Context) {
		c.JSON(
			http.StatusOK,
			gin.H{
				"code":  http.StatusOK,
				"error": "Welcome server 02",
			},
		)
	})

	return e
}

func main() {
	server01 := &http.Server{
		Addr:         ":8080",
		Handler:      router01(),
		ReadTimeout:  5 * time.Second,
		WriteTimeout: 10 * time.Second,
	}

	server02 := &http.Server{
		Addr:         ":8081",
		Handler:      router02(),
		ReadTimeout:  5 * time.Second,
		WriteTimeout: 10 * time.Second,
	}

	g.Go(func() error {
		err := server01.ListenAndServe()
		if err != nil && err != http.ErrServerClosed {
			log.Fatal(err)
		}
		return err
	})

	g.Go(func() error {
		err := server02.ListenAndServe()
		if err != nil && err != http.ErrServerClosed {
			log.Fatal(err)
		}
		return err
	})

	if err := g.Wait(); err != nil {
		log.Fatal(err)
	}
}

ch := make(chan int)

ch := make(chan int)

close(ch)

var sem = make(chan int, MaxOutstanding)

func handle(r *Request) {
    sem <- 1    // Wait for active queue to drain.
    process(r)  // May take a long time.
    <-sem       // Done; enable next request to run.
}
func Serve(queue chan *Request) {
    for {
        req := <-queue
        go handle(req)  // Don't wait for handle to finish.
    }
}

func handle(queue chan *Request) {
    for r := range queue {
        process(r)
    }
}

func Serve(clientRequests chan *Request, quit chan bool) {
    // Start handlers
    for i := 0; i < MaxOutstanding; i++ {
        go handle(clientRequests)
    }
    <-quit  // Wait to be told to exit.
}

type Request struct {
    args        []int
    f           func([]int) int
    resultChan  chan int
}

func sum(a []int) (s int) {
    for _, v := range a {
        s += v
    }
    return
}

request := &Request{[]int{3, 4, 5}, sum, make(chan int)}
// Send request
clientRequests <- request
// Wait for response.
fmt.Printf("answer: %d\n", <-request.resultChan)

func handle(queue chan *Request) {
    for req := range queue {
        req.resultChan <- req.f(req.args)
    }
}

package leakybuf

type LeakyBuf struct {
	bufSize  int // size of each buffer
	freeList chan []byte
}

// NewLeakyBuf creates a leaky buffer which can hold at most n buffer, each
// with bufSize bytes.
func NewLeakyBuf(n, bufSize int) *LeakyBuf {
	return &LeakyBuf{
		bufSize:  bufSize,
		freeList: make(chan []byte, n),
	}
}

// Get returns a buffer from the leaky buffer or create a new buffer.
func (lb *LeakyBuf) Get() (b []byte) {
	select {
	case b = <-lb.freeList:
	default:
		b = make([]byte, lb.bufSize)
	}
	return
}

// Put add the buffer into the free buffer pool for reuse. Panic if the buffer
// size is not the same with the leaky buffer's. This is intended to expose
// error usage of leaky buffer.
func (lb *LeakyBuf) Put(b []byte) {
	if len(b) != lb.bufSize {
		panic("invalid buffer size that's put into leaky buffer")
	}
	select {
	case lb.freeList <- b:
	default:
	}
	return
}

var sem = make(chan int, MaxOutstanding)

func handle(r *Request) {
    sem <- 1    // Wait for active queue to drain.
    process(r)  // May take a long time.
    <-sem       // Done; enable next request to run.
}

func Serve(queue chan *Request) {
    for {
        req := <-queue
        go handle(req)  // Don't wait for handle to finish.
    }
}

func Serve(queue chan *Request) {
    for req := range queue {
        sem <- 1
        go func() {
            process(req) // Buggy; see explanation below.
            <-sem
        }()
    }
}

func Serve(queue chan *Request) {
    for req := range queue {
        sem <- 1
        go func(req *Request) {
            process(req)
            <-sem
        }(req)
    }
}

var identifier type

var identifier1, identifier2 type

var v_name = value

v_name := value

var a int = 0

import (
	"fmt"
)
 
type Sample struct {
	a   int
	str string
}
 
func main() {
	s := Sample{a: 1, str: "hello"}
 
	fmt.Printf("%v\n", s)   //{1, hello}
	fmt.Printf("%+v\n", s)  //{a:1, str:hello}
	fmt.Printf("%#v\n", s)  //main.Sample{a:1, str:"hello"}
	fmt.Printf("%T\n", s)   // main.Sample
	fmt.Printf("%%\n", s.a) // %  %!(EXTRA int=1)
}

fmt.Printf("%10d\n", 353)  // will print "       353"
//将宽度指定为 * 来将宽度当作 Printf 的参数:
fmt.Printf("%*d\n", 10, 353)  // will print "       353"

// 如果你在一个格式化的字符串中多次引用一个变量，你可以使用 %[n]，其中 n 是你的参数索引（位置，从 1 开始）
fmt.Printf("The price of %[1]s was $%[2]d. $%[2]d! imagine that.\n", "carrot", 23)

s := fmt.Sprintf("是字符串 %s ","string")

fmt.Fprintf(os.Stdout, "%s\n", "hello world!")

buf := bufio.NewWriter(os.Stdout)
	// and now so does buf.
fmt.Fprintf(buf, "%s\n", "hello world! - buffered")
buf.Flush()

// helloworld.go
package main

import (
"fmt"
"./myMath"
)

func main(){
    fmt.Println("Hello World!")
    fmt.Println(mathClass.Add(1,1))
    fmt.Println(mathClass.Sub(1,1))
}
// myMath1.go
package mathClass
func Add(x,y int) int {
    return x + y
}
// myMath2.go
package mathClass
func Sub(x,y int) int {
    return x - y
}

const (
    a = iota
    b
    c
)

package main

import "fmt"

func main() {
    const (
            a = iota   //0
            b          //1
            c          //2
            d = "ha"   //独立值，iota += 1
            e          //"ha"   iota += 1
            f = 100    //iota +=1
            g          //100  iota +=1
            h = iota   //7,恢复计数
            i          //8
    )
    fmt.Println(a,b,c,d,e,f,g,h,i)
}

if err := process(); err != nil {
  return err
}

f, err := os.Open(name)
if err != nil {
    return err
}
d, err := f.Stat()
if err != nil {
    f.Close()
    return err
}
codeUsing(f, d)

for condition { }

func function_name( [parameter list] ) [return_types] {
   函数体
}
// example
/* 函数返回两个数的最大值 */
func max(num1, num2 int) int {
   /* 声明局部变量 */
   var result int

   if (num1 > num2) {
      result = num1
   } else {
      result = num2
   }
   return result
}

func ReadFull(r Reader, buf []byte) (n int, err error) {
    for len(buf) > 0 && err == nil {
        var nr int
        nr, err = r.Read(buf)
        n += nr
        buf = buf[nr:]
    }
    return
}

var 数组变量名 [元素数量]Type
 //  例子
var a [3]int             // 定义三个整数的数组
// “...”省略号，则表示数组的长度是根据初始化值的个数来计算
q := [...]int{1, 2, 3}

var identifier []type

var slice1 []type = make([]type, len)

也可以简写为

slice1 := make([]type, len)

type struct_variable_type struct {
   member definition
   member definition
   ...
   member definition
}

type Books struct {
   title string
   author string
   subject string
   book_id int
}
func main() {
		var Book1 Books 
  var Book2 *Books = &Book{}
  var Book3 *Books = new(Books)
  var struct_pointer *Books = &Book1 //指向book1结构体的指针
}

type Student struct {
    Name   string
    Age    int
    Source int
    Next   *Student
}

func creat() {
    var stu_head = Student{Name: "li", Age: 20, Source: 100, Next: nil}
    var stu_head1 = new(Student)
    var stu_head3 = &Student{
        Name: "wang",
        Age:  12,
        Next: stu_head1,
    }
    fmt.Println(stu_head)
    fmt.Println(*stu_head1)
    fmt.Println(stu_head3.Next)
}
func main() {
    creat()
}

/* 实现接口方法 */
func (struct_name_variable struct_name) method_name1() [return_type] {
   /* 方法实现 */
}

func (c Circle) getArea() float64 {
  //c.radius 即为 Circle 类型对象中的属性
  return 3.14 * c.radius * c.radius
}

package main

import (
    "fmt"
)

type Phone interface {
    call()
}

type NokiaPhone struct {
}

func (nokiaPhone NokiaPhone) call() {
    fmt.Println("I am Nokia, I can call you!")
}

type IPhone struct {
}

func (iPhone IPhone) call() {
    fmt.Println("I am iPhone, I can call you!")
}

func main() {
    var phone Phone //!!!,在具体struct iphone和nokia里是万万不能这么写的，因为new返回的是一个地址，需要赋值给一个指针来指向这个地址。

    phone = new(NokiaPhone)
    phone.call()

    phone = new(IPhone)
    phone.call()

}