Golang

Golang training

1. Development Environment

Install

# Download and Install
# https://go.dev/doc/install

# Managing Go installations
# https://go.dev/doc/manage-install

# Installing from source
# https://go.dev/doc/install/source

Goland

# active
cat ./ideaActive/ja-netfilter-all/ja-netfilter/readme.txt

# plugins
gradianto # themes
rainbow brackets # json

2. ProjectManage

go mod init go-example

go mod tidy

go mod vendor

3. Training

Basic grammar

Commentary

/*
Go programs doc.

Usage:

    gofmt [flags] [path ...]

The flags are:

    -x xxx
    -y yyy
*/
package main
import "fmt"

func main() {
    fmt.Println("hello world!") // Oneline comment

}
// go doc tmp.go

Constants

type ByteSize float64

const (
    _           = iota // ignore first value by assigning to blank identifier
    KB ByteSize = 1 << (10 * iota)
    MB
    GB
    TB
    PB
    EB
    ZB
    YB
)
func (b ByteSize) String() string {
    switch {
    case b >= YB:
        return fmt.Sprintf("%.2fYB", b/YB)
    case b >= ZB:
        return fmt.Sprintf("%.2fZB", b/ZB)
    case b >= EB:
        return fmt.Sprintf("%.2fEB", b/EB)
    case b >= PB:
        return fmt.Sprintf("%.2fPB", b/PB)
    case b >= TB:
        return fmt.Sprintf("%.2fTB", b/TB)
    case b >= GB:
        return fmt.Sprintf("%.2fGB", b/GB)
    case b >= MB:
        return fmt.Sprintf("%.2fMB", b/MB)
    case b >= KB:
        return fmt.Sprintf("%.2fKB", b/KB)
    }
    return fmt.Sprintf("%.2fB", b)
}

Variables

// option1
var (
    x string
    y int
)
x,y = "a",2
// option2
var x,y = "a",2
// option3
x,y := "a",2

// anonymous variable
_, _ = call_function()

Operator

// arithmetic
+
-
*
/
%

// relational
x == x
x != x
>
<
>=
<=

// logical
x && y
x || y
!(x < 18)

// assignment
x = 1
x += 1
x -= 1
x *= 1
x /= 1
x %= 1
x <= 1
x >= 1
x &= 1
x ^= 1
x |= 1

Printing and Formatting

/*
    %s    string
    %d    decimal integer
    %f    floating-point number
    %b    binary
    %o    octal
    %x    hexadecimal(lowercase)
    %X    hexadecimal(uppercase)
    %v    default format
    %T    typeof variable
    %p    pointer address
*/
// Print, Println
fmt.Print("xxx")
fmt.Println("yyy")

var name string = "logic"
var age int = 33
var height float64 = 1.1
var p *string = &name
// Printf, Sprintf
fmt.Printf("%s %d %f %p %b", name, age, height, p, age)
s := fmt.Sprintf("%s %d %f %p %b", name, age, height, p, age)
fmt.Println(s)

var (
    name     string
    age      int
    married  bool
)
// Scan, Scanf, Scanln
fmt.Scan(&name, &age, &married)
fmt.Scanf("1:%s 2:%d 3:%t", &name, &age, &married)
fmt.Scanln(&name, &age, &married)
//
fmt.Printf("name:%s age:%d married:%t", name, age, married)

package

// import multiple package
import (
    "fmt"
    "myproject/api"
)

// alias
import F "fmt"
F.Println()

// omit citation format
import . "myproject/api"
RestfulAPI()

// only execute init function
import _ "myproject/api"

// go mod
go mod download        // download modules to local cache: $GOPATH/pkg
go mod graph           // print module requirement graph
go mod init myproject  // initialize new module in current directory
go mod tidy            // add missing and remove unused modules
go mod verify          // add missing and remove unused modules

Data type

bool

var b1 = true
var b2 = false
fmt.Println(reflect.TypeOf(b1), reflect.TypeOf(b2))

float

var f1 float32 = 3.1234567890123456789
f1 = 2e10
fmt.Println(f1, reflect.TypeOf(f1))

var f2 float64 = 3.1234567890123456789
f2 = 2e-2
fmt.Println(f2, reflect.TypeOf(f2))

int

/*
    int8	-128~127
    uint8	0~255
    int16	-32768~32767
    uint16	0~65535
    int32	-2147483648~2147483647
    uint32	0~4294967295
    int64	-9223372036854775808~9223372036854775807
    uint64	0~18446744073709551615
    uint	32-bit operating system is uint32,64-bit operating system is uint64
    int     32-bit operating system is int32,64-bit operating system is int64
*/
var i int = 10
fmt.Printf("%d \n", i)  // 10
fmt.Printf("%b \n", i)  // 1010
fmt.Printf("%o \n", i)  // 12
fmt.Printf("%x \n", i)  // a

string

/*
    \r	回车符（返回行首）
    \n	换行符（直接跳到下一行的同列位置）
    \t	制表符
    \'	单引号
    \"	双引号
    \\	反斜杠
*/
var s string = "hello world"
s1 := s[1:3]
s2 := string(s[0]) + string(s[4])
s3 := `
multi line one
multi line two
`
fmt.Println(s1, s2, s3)


// strings method
fmt.Println(strings.ToUpper(s))
fmt.Println(strings.ToLower(s))
fmt.Println(strings.TrimLeft(s, ""))
fmt.Println(strings.Trim(s, ""))
fmt.Println(strings.Index(s, "world"))
fmt.Println(strings.HasPrefix(s, "hello"))
fmt.Println(strings.Contains(s, "llo"))
fmt.Println(strings.Join([]string{"a","b","c"}, ","))

// strconv method
// int to int
i = 100
fmt.Println(int64(i), reflect.TypeOf(int64(i)))
fmt.Println(float64(i), reflect.TypeOf(float64(i)))
// int to string
x := strconv.Itoa(98)
fmt.Println(x, reflect.TypeOf(x))
// string to int
y, _ := strconv.Atoi("97")
fmt.Println(y, reflect.TypeOf(y))
i1, _ := strconv.ParseInt("1000", 10, 8)
println(i1)
i2, _ := strconv.ParseInt("1000", 10, 64)
println(i2)
// string to float
f2, _ := strconv.ParseFloat("3.1415926", 64)
fmt.Println(f2, reflect.TypeOf(f2))
f1, _ := strconv.ParseFloat("3.1415926", 32)
fmt.Println(f1, reflect.TypeOf(f1))
// string to bool
b1, _ := strconv.ParseBool("true")
fmt.Println(b1, reflect.TypeOf(b1))
b2, _ := strconv.ParseBool("T")
fmt.Println(b2, reflect.TypeOf(b2))
b3, _ := strconv.ParseBool("1")
fmt.Println(b3, reflect.TypeOf(b3))
b4, _ := strconv.ParseBool("100")
fmt.Println(b4, reflect.TypeOf(b4))

pointer

/*
    &  address character,return address
    *  value character, return value
*/
// example
var i = 100
fmt.Println("i address:", &i)
fmt.Println("i value:", i)
var p *int
p = &i
fmt.Println("p address:", &p)
fmt.Printf("p address: %p\n", &p)
fmt.Println("p value:", p)
fmt.Println("p value's address's value:", *p)
fmt.Println(*&p)

// **p
var a = 100
var b = &a
var c = &b
**c = 200
fmt.Println(a)

// new function, allcated memory
var p *int = new(int)
fmt.Println(p)
fmt.Println(*p)

array

// example
var arr = [3]int8{1,2,3}
fmt.Printf("%p\n", &arr)
fmt.Println(&arr[0])
fmt.Println(&arr[1])
fmt.Println(&arr[2])

// infinity array
var names = [...]string{"alice","bob","candy"}
fmt.Printf("%p\n", &names)
fmt.Println(&names[0])
// get element
fmt.Println(names[0])
// modify element
names[0] = "logic"
// slice
fmt.Println(names[0:2])
// range
for k,v := range names {
    fmt.Println(k,v)
}

slice

/*
type Slice struct {
      Data uintptr   // 指针，指向底层数组中切片指定的开始位置
      Len int        // 长度，即切片的长度
      Cap int        // 最大长度（容量），也就是切片开始位置到数组的最后位置的长度
}
*/
// option1: from array
var arr = [5]int{10, 11, 12, 13, 14}
sa := arr[1:2]

// option2: declare variable, not allocated memory
var sv []string{}

// option3: make([]Type, size, cap), allocated memory
var sm = make([]string, 3, 5)


// address check
var arr = [5]int{10, 11, 12, 13, 14}  // int64 = 8Bytes
s1 := arr[0:3] // 对数组切片
s2 := arr[2:5]
s3 := s2[0:2] // 对切片切片

fmt.Println(s1) // [10, 11, 12]
fmt.Println(s2) // [12, 13, 14]
fmt.Println(s3) // [12, 13]

// 地址是连续的
fmt.Printf("%p\n", &arr)
fmt.Printf("%p\n", &arr[0]) // 相差8个字节
fmt.Printf("%p\n", &arr[1])
fmt.Printf("%p\n", &arr[2])
fmt.Printf("%p\n", &arr[3])
fmt.Printf("%p\n", &arr[4])

// 每一个切片都有一块自己的空间地址，分别存储了对于数组的引用地址，长度和容量
fmt.Printf("%p\n", &s1) // s1自己的地址
fmt.Printf("%p\n", &s1[0])
fmt.Println(len(s1), cap(s1))

fmt.Printf("%p\n", &s2) // s2自己的地址
fmt.Printf("%p\n", &s2[0])
fmt.Println(len(s2), cap(s2))

fmt.Printf("%p\n", &s3) // s3自己的地址
fmt.Printf("%p\n", &s3[0])
fmt.Println(len(s3), cap(s3))


// append, extend slice
var emps = make([]string, 3, 5)
emps[0] = "aaa"
emps[1] = "bbb"
emps[2] = "ccc"
fmt.Println(emps)
emps2 := append(emps, "ddd")
fmt.Println(emps2)
emps3 := append(emps2, "eee")
fmt.Println(emps3)
// 容量不够时默认发生二倍扩容
emps4 := append(emps3, "xxx")
fmt.Println(emps4, len(emps4), cap(emps4)) // 此时底层数组已经发生变化

// append element
append(emps, "xxx", "yyy")
// apend a slice
append(emps, []string{"A","B","C"}...)

// delete element
append(emps[:1], emps[2:]...)

// sort
si := []int{3,5,1}
sort.Ints(si)
ss := []string{"melon","banana","apple"}
sort.Strings(ss)
sf := []float64{3.14,5.25,1.12,4,78}
sort.Float64s(sf)

// copy
var s1 = []int{1, 2, 3, 4, 5}
var s2 = make([]int, len(s1))
copy(s2, s1)
fmt.Println(s2)
s3 := []int{4, 5}
s4 := []int{6, 7, 8, 9}
copy(s4, s3)
fmt.Println(s4)

map

/*
type hmap struct {
    count     int // 当前 map 中元素数量
    flags     uint8
    B         uint8  // 当前 buckets 数量，2^B 等于 buckets 个数
    noverflow uint16 // approximate number of overflow buckets; see incrnoverflow for details
    hash0     uint32 // 哈希种子

    buckets    unsafe.Pointer // buckets 数组指针
    oldbuckets unsafe.Pointer // 扩容时保存之前 buckets 数据。
    nevacuate  uintptr        // progress counter for evacuation (buckets less than this have been evacuated)

    extra *mapextra // optional fields
}
*/
// option1: declare
var m map[string]string
m["name"] = "logic"
// option2: declare
m := map[string]string{"name": "logic"}
// option3: make(map[keytype]valuetype, cap)
m := make(map[string]string, 10)
fmt.Println(m, len(m))


// select
for k,v := range m {
    fmt.Println(k,v)
}

// add and delete
m["new_key"] = "new_value"
delete(m, "name")

struct

/*  alloc stack memory
*/

// declaration and definition
type Student struct {
    dstr    string // default: ""
    dint    int   // default: 0
    dbool   bool  // default: false
    dsli    []string  // default: nil
    dmap    map[string]int  // default: nil
}
var s1,s2,s3 Stuent
// option1
s1.dstr = "xxx"
s1.dint = 11
...
// option2
s2 := Student{"xxx", 11, true, []string{"a","b"}, map[string]int{"x":12}}
// option3: composite literal
s3 := Student{dstr: "xxx", dint: 11, dbool: true, dsli: []string{"a","b"}, map[string]int{"x":12}}
// option4: new method
ps := new(Student)
fmt.Println(reflect.TypeOf(ps))

// syntactic sugar: (*instance).key
sptr := &s1
fmt.Println((*sptr).dstr))
fmt.Println(sptr.dstr))

// constructor function
func NewStudent(dstr string, dint int8, dbool bool, dsli []string, dmap map[string]int) *Student {
    return &Student{
        dstr:    dstr,
        dint:    dint,
        dbool:   dbool,
        dsli:    dsli,
        dmap:    dmap
    }
}
s4 := NewStudent("xxx", 11, true, []string{"a", "b"}, map[string]int{"x": 12})
fmt.Println(s4)

// method receiver
func (s Student) Read(book string) {
    fmt.Println("Reading book:", book)
}
func (s *Student) Attacked(book string) {
    fmt.Printf("%s is attacked.\n", s.dstr)
}
s4.Read()

// anonymous field
type Addr struct {
    country  string
    province string
    city     string
}
type Person struct {
    string
    int
    Addr
}
p := Person{
    "logic",
    35,
    Addr{"cn", "gd", "gz"},
}
fmt.Println(p.Addr.country)

// inherited struct
type Animal struct {
	name string
}
func (a *Animal) eat() {
	fmt.Printf("%s is eating.\n", a.name)
}
type Dog struct {
	Kind string
	*Animal
}
func (d *Dog) bark() {
	fmt.Printf("%s is barking.\n", d.name)
}
func main() {
	d := &Dog{
		"X",
		&Animal{"dog1"},
	}
	fmt.Println(d)
	d.bark()
	d.eat()
}

// serialize
type Addr struct {
	Province string
	City     string
}
type Stu struct {
	Name string `json:"name"` // json tag
	Age  int    `json:"-"`    // ignore json field
	Addr Addr
}
func main() {
	// serialization
	var stuStruct = Stu{Name: "myname", Age: 33, Addr: Addr{Province: "gd", City: "sz"}}
	jsonStuStruct, _ := json.Marshal(stuStruct)
	fmt.Println(string(jsonStuStruct))
	// deserialization
    var StuStruct Stu
    err := json.Unmarshal(jsonStuStruct, &StuStruct)
    if err != nil {
        return
    }
    fmt.Println(StuStruct.Age)
    fmt.Println(StuStruct.Addr.City)

}

interface

/* Polymorphism
type interfaceName interface {
    method1(argList1) returnList1
    method2(argList2) returnList2
    ...
}
*/
// example
type Sperker interface {
	speak()
}
// pointer receiver
type Chinese struct{}
func (c *Chinese) speak() {
	fmt.Println("Chinese speak")
}
// value receiver
type Japanese struct{}
func (j Japanese) speak() {
	fmt.Println("Japanese speak")
}
// public function
func PublicSpeak(s Sperker){
    s.speak()
}
func main() {
	var cv,jv,jp Sperker
	cv = &Chinese{}
	cv.speak()
	jv = Japanese{}
	jv.speak()
	jp = &Japanese{}
	jp.speak()

    // interface type variable
    c := &Chinese{}
    j1 := Japanese{}
    j2 := &Japanese{}
    PublicSpeak(c)
    PublicSpeak(j1)
    PublicSpeak(j2)
}

// type nesting
type Animal interface {
    bark()
    run()
}
type Barker struct {
    name string
}
func (b *Barker) bark(){
    fmt.Printf("%v is barking\n", b.name)
}
type Dog struct {
    name string
    Barker
}
func (d *Dog) run(){
    fmt.Printf("%v is running\n", d.name)
}
func main(){
    var a Animal
    a = &Dog{
        name: "dog gbk",
        Barker: Barker{
            name: "loud barker",
        },
    }
    a.bark()
    a.run()
}

// interface nesting
type Barker interface {
    bark()
}
type Runner interface {
    run()
}
type Animal interface {
    Barker
    Runner
}
type Dog struct {
    name string
}
func (d Dog) bark(){}
func (d Dog) run(){}

channel

/*  alloc heap memory
type hchan struct {
    qcount   uint           // total data in the queue 当前队列里还剩余元素个数
    dataqsiz uint           // size of the circular queue 环形队列长度，即缓冲区的大小，即make(chan T,N) 中的N
    buf      unsafe.Pointer // points to an array of dataqsiz elements 环形队列指针
    elemsize uint16 //每个元素的大小
    closed   uint32 //标识当前通道是否处于关闭状态，创建通道后，该字段设置0，即打开通道；通道调用close将其设置为1，通道关闭
    elemtype *_type // element type 元素类型，用于数据传递过程中的赋值
    sendx    uint   // send index 环形缓冲区的状态字段，它只是缓冲区的当前索引-支持数组，它可以从中发送数据
    recvx    uint   // receive index 环形缓冲区的状态字段，它只是缓冲区当前索引-支持数组，它可以从中接受数据
    recvq    waitq  // list of recv waiters 等待读消息的goroutine队列
    sendq    waitq  // list of send waiters 等待写消息的goroutine队列

    // lock protects all fields in hchan, as well as several
    // fields in sudogs blocked on this channel.
    //
    // Do not change another G's status while holding this lock
    // (in particular, do not ready a G), as this can deadlock
    // with stack shrinking.
    lock mutex //互斥锁，为每个读写操作锁定通道，因为发送和接受必须是互斥操作
}

// sudog 代表goroutine
type waitq struct {
    first *sudog
    last  *sudog
}
*/
// declaration and definition
var ch chan int
ch := make(chan int)
ch := make(chan int,3)

// example
package main
import (
    "fmt"
)
func main() {
    ch := make(chan int, 10)
    fmt.Println(len(ch), cap(ch))
    ch <- 1
    ch <- 2
    ch <- 3
    fmt.Println(len(ch), cap(ch))
    fmt.Println(<-ch)
    fmt.Println(<-ch)
    fmt.Println(<-ch)

    // copy by value
    ch2 := make(chan int, 3)
    x := 10
    ch2 <- x
    x = 20
    fmt.Println(<-ch2)

    // copy by reference
    ch3 := make(chan *int, 3)
    y := 20
    ch3 <- &y
    y = 30
    p := <-ch3
    fmt.Println(*p)

    // reference type
    var ch4 = make(chan int, 3)
    var ch5 = ch4
    ch5 <- 100
    ch5 <- 200
    fmt.Println(<-ch4)
    fmt.Println(<-ch4)
}

// close and loop
package main
import "fmt"
func main() {
    ch := make(chan int, 10)
    ch <- 1
    ch <- 2
    ch <- 3

    // close use goroutine
    go func(){
        time.Sleep(time.Second * 3)
        ch <-4
    }()
    i, ok := <-ch
    fmt.Println(i, ok) // 1 true
    i, ok = <-ch
    i, ok = <-ch
    i, ok = <-ch
    fmt.Println(i, ok) // 4 true

    // close use function
    close(ch)
    i, ok := <-ch
    i, ok = <-ch
    i, ok = <-ch
    fmt.Println(i, ok) // 3 true
    i, ok = <-ch
    fmt.Println(i, ok) // 0 false

    // loop
    close(ch)
    for c := range ch {
        fmt.Println(c, len(ch))
    }
}

// producer consumer model
package main
import (
    "fmt"
    "sync"
    "time"
)
func producer(ch chan int) {
    for i := 1; i < 11; i++ {
        time.Sleep(time.Second * 1)
        // time.Sleep(time.Second * 3)
        ch <- i
        fmt.Println("send key: ", i)
    }
    wg.Done()
}
func consumer(ch chan int) {
    for i := 1; i < 11; i++ {
        // time.Sleep(time.Second * 1)
        time.Sleep(time.Second * 3)
        fmt.Println("get key: ", <-ch)
    }
    wg.Done()
}
var wg sync.WaitGroup
func main() {
    // ch := make(chan int) // sync channel
    ch := make(chan int, 100)
    wg.Add(2)
    go producer(ch)
    go consumer(ch)
    wg.Wait()
    fmt.Println("done")
}

// unbuffered channel(sync channel)
package main
import "fmt"
func main() {
    ch := make(chan int)
    go func() {
            x := <- ch
            fmt.Println(x)
    }()
    ch <- 10
    fmt.Println("done")
}

// deadlock
package main

import (
    "fmt"
    "sync"
    "time"
)
var wg sync.WaitGroup
func main() {
    pipline := make(chan int)
    wg.Add(2)
    // producer
    go func() {
        defer wg.Done()
        for i := 0; i < 10; i++ {
                time.Sleep(time.Second)
                pipline <- i
                fmt.Println("send key:", i)
        }
        // resolve deadlock
        // close(pipline)
    }()
    // consumer
    go func() {
        defer wg.Done()
        for v := range pipline {
                fmt.Println("get key:", v)
        }
    }()
    wg.Wait()
}

// oneway channel
package main
import (
    "fmt"
    "sync"
    "time"
)
var wg sync.WaitGroup
func main() {
    ch := make(chan int)
    wg.Add(2)
    // producer
    go func(ch chan<- int) {
        defer wg.Done()
        for i := range 11 {
            time.Sleep(time.Second * 1)
            ch <- i
            fmt.Println("send key: ", i)
        }
    }(ch)
    // consumer
    go func(ch <-chan int) {
        defer wg.Done()
        for i:=0;i<=11;i++ {
            time.Sleep(time.Second * 3)
            fmt.Println("get key: ", <-ch)
        }
    }(ch)
    wg.Wait()
    fmt.Println("done")
}

// select
// permanent sleep state
func main(){
    select {}
}
// timeout
package main
import (
    "fmt"
    "time"
)
func main() {
    ch1 := make(chan string)
    ch2 := make(chan string)
    // channel1
    go func(c chan string) {
        time.Sleep(time.Second * 1)
        // time.Sleep(time.Second * 3)
        c <- "get key from channel1"
    }(ch1)
    // channel2
    go func(c chan string) {
        time.Sleep(time.Second * 1)
        c <- "get key from channel2"
    }(ch2)
    select {
    case v1 := <-ch1:
        fmt.Println(v1)
    case v2 := <-ch2:
        fmt.Println(v2)
    case <-time.After(time.Second * 2):
        fmt.Println("timeout branch")
    // default:
    //     fmt.Println("default branch")
    }
}

Control structures

if

// if
if true {
    // do something
}

// if, else
score := 33
if score < 10 {
    fmt.Println("less than 10")
} else {
    fmt.Println("greater than 10")
}

// if, else if, else
if score < 10 {
    fmt.Println("less than 10")
} else if score < 20 {
    fmt.Println("less than 20")
} else {
    fmt.Println("greater than 20")
}

switch

choice := "fff"
switch choice {
    case "a":
        fmt.Println("a")
    case "b":
        fmt.Println("b")
    case "x", "y":
        fmt.Println("xy")
    default:
        fmt.Println("x")
}

for

// option1
count := 0
for count < 10 {
    fmt.Println(count)
    // count ++
    count += 1
}

// option2
for i:=1; i<10; i++ {
    if i % 2 == 0{
        fmt.Println(i)
    } else if i == 6 {
        continue
    } else if i == 7 {
        break
    }
}

// option3
var names [3]int = [3]int{1,2,3}
var names = [...]string{"a","b","c"}
for k,v := range names {
    fmt.Println(k,v)
}

Functions

Example

/*
func FunctionName(p1 paramType, p2 paramType) (returnType, returnType) {
    doSomething...
    return r1, r2
}
*/

// positional parameter
func add_cal(x int, y int) {
    fmt.Pringln(x+y)
}

// indefinite length parameter
func sum(nums ...int) {
    for _, num := range nums {
        fmt.Println(num)
    }
}

// return value
func get_name_age() (string, int) {
    return "logic", 99
}
func calc(x, y int) (sum, sub int) {
    sum = x + y
    sub = x - y
    return     //  return sum sub
}

Pass-by-xxx

/*
Pass-by-value
    @int
    @string
    @struct

Pass-by-reference
    @pointer
    @slice
    @map
    @chan
*/
func func01(x int) {
    x = 100
}
func func02(s []int) {
    fmt.Printf("func02 var s address: %p\n",&s)
    s[0] = 100
    // s = append(s, 1000)
}

func func03(p *int)  {
    *p = 100
}
func main() {
    // example1
    var x = 10
    func01(x)
    fmt.Println(x)

    // example2
    var s = []int{1, 2, 3}
    fmt.Printf("main var s address: %p\n",&s)
    func02(s)
    fmt.Println(s)

    // example3
    var a = 10
    var p *int = &a
    func03(p)
    fmt.Println(a)
}

anonymous function

func main() {
    var c = (func(x,y int) int {
        return x + y
    })(1,2)
    fmt.Println(c)
}

higher-order function

// function parameter
func add(x,y int) int {
    return x+y
}
func mul(x,y int) int {
    return x*y
}
func cal(f func(x,y int) int, x,y int) int {
    return f(x,y)
}
func main() {
    ret1 := cal(add,12,3)
    fmt.Println(ret1)
    ret2 := cal(mul,12,3)
    fmt.Println(ret2)
}
// function return
func foo() func() {
    inner := func() {
        fmt.Println("inner running..")
    }
    fmt.Println("outer running..")
    return inner
}
func main() {
    foo()()
}

closure

// example1
func getCounter(f func()) func() {
    calledNum := 0
    return func() {
        f()
        calledNum += 1
        fn := runtime.FuncForPC(reflect.ValueOf(f).Pointer()).Name()
        fmt.Printf("function %s called by number %d\n", fn, calledNum)
    }
}

func foo() {
    fmt.Println("foo running")
}
func bar() {
    fmt.Println("bar running")
}

func main() {
    foo := getCounter(foo)
    foo()
    foo()
    foo()

    bar := getCounter(bar)
    bar()
    bar()
}


// example2
func GetTimer(f func(t time.Duration)) func(t time.Duration) {
    return func(t time.Duration) {
        t1 := time.Now().Unix()
        f(t)
        t2 := time.Now().Unix()
        fmt.Println("function duration: ", t2-t1)
    }
}

func foo(t time.Duration) {
    fmt.Println("foo start")
    time.Sleep(time.Second * t)
    fmt.Println("foo end")
}

func bar(t time.Duration) {
    fmt.Println("bar start")
    time.Sleep(time.Second * t)
    fmt.Println("bar end")
}

func main() {
    var foo = GetTimer(foo)
    foo(3)
    var bar = GetTimer(bar)
    bar(2)
}

defer

// example: close open file socket
fileObj, err:=os.Open("./t.txt")
if err != nil {
    fmt.Println("Open file failed, error: ",err)
}
defer fileObj.Close()


// excution order: fifo
fmt.Println("test01")
defer fmt.Println("test02")
fmt.Println("test03")
defer fmt.Println("test04")
fmt.Println("test05")


// copy mechanism
func main() {
    x := 10
    defer func(a int) {
      fmt.Println(a)
    }(x)
    x++
}


// defer return
/*
    return first step: rval=xxx
    execute defer
    return second step: ret rval
*/
func f1() *int {
    i := 5
    defer func() {
        i++
        fmt.Printf(":::%p\n", &i)
    }()
    fmt.Printf(":::%p\n", &i)
    return &i    // 5
}
func f2() (result int) {
    defer func() {
        result++
    }()
    return 5    // 6
}
func f3() (result int) {
    defer func() {
        result++
    }()
    return result  // 1
}
func f4() (r int) {
    fmt.Println(&r)
    defer func(r int) {
        r = r + 1
        fmt.Println(&r)
    }(r)
    return 5    // rval=5, defer func, ret rval
}

recursive function

/*
递归特性:
1. 调用自身函数。
2. 必须有一个明确的结束条件。
3. 在计算机中函数调用通过栈（stack）这种数据结构实现的，每当进入一个函数调用，栈就会加一层栈帧，每当函数返回，栈就会减一层栈帧。由于栈的大小不是无限的，递归调用的次数过多，会导致栈溢出。
*/

// example 1
func factorial(n int)int{
    if n == 0{
        return 1
    }
    return n * factorial(n-1)
}
func main() {
    var ret = factorial(4)
    fmt.Println(ret)
}


// example 2
func fib(n int) int {
    if n == 2 || n == 1 {
        return 1
    }
    return fib(n-1) + fib(n-2)

}
func main() {
    ret := fib(6)
    fmt.Println(ret)
}

init function

func init() {
    // init log
    // init config

    if user == "" {
        log.Fatal("$USER not set")
    }
    if home == "" {
        home = "/home/" + user
    }
    if gopath == "" {
        gopath = home + "/go"
    }
    // gopath may be overridden by --gopath flag on command line.
    flag.StringVar(&gopath, "gopath", gopath, "override default GOPATH")
}
func main(){
    fmt.Println("xxx")
}

File IO

character and byte

/*
    ASCII: 1byte = 1character
    Unicode: 2byte = 1character
    UTF8: 1~3byte = 1character


    type stringStruct struct {
        str unsafe.Pointer
        len int
    }
    stringStruct.str: string first point address
    stringStruct.len: string length
*/
// byte
var b1 byte
b1 = 'A'
fmt.Println(reflect.TypeOf(b1)) // uint8
fmt.Printf("%c\n",b1)
fmt.Printf("%d\n",b1)
fmt.Println(b1)  // ASCII

// uint8
var b2 uint8
b2 = 65
fmt.Println(reflect.TypeOf(b2)) // uint8
fmt.Printf("%c\n",b2)
fmt.Printf("%d\n",b2)
fmt.Println(b2)  // ASCII

// int32
var b3 rune
b3 = '中'
fmt.Println(reflect.TypeOf(b3)) // int32
fmt.Println(b3, string(b3))


// byte and character
s := "Hello,世界"
r1 := []byte(s)  // utf8 encode
r2 := []rune(s)  // Unicode character
fmt.Println(r1)
fmt.Println(r2)
// byte length
fmt.Println(len(r1))
// character length
fmt.Println(len(r2))
// byte to string
string(r1)

Read/Write File

import (
    "bufio"
    "fmt"
    "io"
    "os"
)
func main() {
    // open file
    file, err := os.OpenFile("/tmp/x.txt", os.O_CREATE|os.O_RDWR|os.O_APPEND, 0666)
    if err != nil {
        fmt.Println("err: ", err)
    }
    // close file
    defer file.Close()

    // (1) for bytes
    var b = make([]byte, 3)
    // read
    n, err := file.Read(b)
    if err != nil {
        fmt.Println("err:", err)
        return
    }
    fmt.Printf("read byte count: %d\n", n)
    fmt.Printf("read byte value: %v\n", b)
    fmt.Printf("read to string content: %v\n", string(b[:n]))
    // write
    str = "new content"
    file.Write([]byte(str))
    file.WriteString(str)

    // (2) for lines
    // read
    reader := bufio.NewReader(file)
    for {
        // read line with string
        strs, err := reader.ReadString('\n')
        fmt.Print(err, strs)
        // read line with bytes
        bytes, err := reader.ReadBytes('\n')
        fmt.Print(bytes)
        if err == io.EOF {
            break
        }
    }
    // write
    str = "new content"
    writer := bufio.NewWriter(file)
    writer.WriteString("str")
    writer.Flush()

    // (3) for all file content
    // read
    content, err := os.ReadFile("x.txt")
    if err != nil {
        fmt.Println("read file failed, err:", err)
        return
    }
    fmt.Print(string(content))
    // write
    if _, err = file.WriteAt([]byte(str), 0); err != nil {
        fmt.Println("Error writing data:", err)
    }

    // others
    os.Remove(fname)
    os.Mkdir(dname, os.ModelDir|os.ModePerm)
    os.Stat(fname)
}

Network IO

/* Socket
src ip
src port
dst ip
dst port
protocol
*/

// socket send io buffer data: write() / send()
// socket recv io buffer data: read() / recv()
// socket io buffer: default 8k


// TCP
// sync mode

// UDP

// HTTP

// tcp example
// server.go
package main
import (
        "bufio"
        "fmt"
        "net"
    "strings"
)
func main() {
        // listen local port
        listener, err := net.Listen("tcp", ":8080")
        if err != nil {
                fmt.Println("Error listening:", err.Error())
                return
        }
        defer listener.Close()
        fmt.Println("Server is listening on port 8080...")

        for {
                // recv client connect
                conn, err := listener.Accept()
                if err != nil {
                        fmt.Println("Error accepting connection:", err.Error())
                }
                fmt.Println("New client connected: ", conn.RemoteAddr())

                // handler request
                go handleClient(conn)
        }
}
func handleClient(conn net.Conn) {
        defer conn.Close()

        // recv client data
        reader := bufio.NewReader(conn)
        recvData, err := reader.ReadString('\n')
        if err != nil {
                fmt.Println("Error reading message:", err.Error())
                return
        }
        fmt.Printf("Received from client: %s", recvData)

    // send server response data
    sendData := strings.ToUpper(recvData)
        writer := bufio.NewWriter(conn)
        _, err = writer.WriteString(sendData)
        if err != nil {
                fmt.Println("Error sending response:", err.Error())
                return
        }
        writer.Flush()
        fmt.Printf("Sent to client: %s", sendData)
}
// client.go
package main
import (
        "bufio"
        "fmt"
        "net"
        "os"
)
func main() {
        // connect to server
        conn, err := net.Dial("tcp", "localhost:8080")
        if err != nil {
                fmt.Println("Error connecting to server:", err.Error())
                return
        }
        defer conn.Close()
        fmt.Println("Connected to server.")

        // get data from stdin
        reader := bufio.NewReader(os.Stdin)
        fmt.Print("Enter a message: ")
        data, _ := reader.ReadString('\n')

        // send data to server
        writer := bufio.NewWriter(conn)
        _, err = writer.WriteString(data)
        if err != nil {
                fmt.Println("Error sending message:", err.Error())
                return
        }
        writer.Flush()
        fmt.Printf("Sent to server: %s", data)

        // recv data from server
        responseReader := bufio.NewReader(conn)
        response, err := responseReader.ReadString('\n')
        if err != nil {
                fmt.Println("Error receiving response:", err.Error())
                return
        }
        fmt.Printf("Received from server: %s", response)
}


// http example
// net_web.go
package main
import (
    "fmt"
    "net"
)
func main()  {
    listenner, err := net.Listen("tcp", "0.0.0.0:8888")
    if err != nil {
        fmt.Println(err)
        return
    }
    defer listenner.Close()
    for {
        conn, err := listenner.Accept()
        if err != nil {
            fmt.Println(err)
            continue
        }
        buf := make([]byte, 1024)
        n, err := conn.Read(buf)
        fmt.Println("n",n)
        conn.Write([]byte("HTTP/1.1 200 OK\r\n\r\n<h1>Welcome to Web World!</h1>"))
    }
}
// http_web.go
package main
import (
    "fmt"
    "net/http"
)
func foo (w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "Hello Yuan")
}
func main() {
    http.HandleFunc("/foo", foo)
    http.ListenAndServe(":8080", nil)
}

Concurrency

// Process
// Thread
// Coroutines

// kernel space model
// user space model

// scheduling and switching

// thread model
// M:1
// multiple user space thread -> one kernel thread
//
// 1:1
// one user space thread -> one kernel thread
//
// M:N
// multiple user space thread -> multiple kernel thread

// goroutine example
package main
import (
    "fmt"
    "time"
)
func foo() {
    fmt.Println("foo")
    time.Sleep(time.Second * 1)
    fmt.Println("foo end")
}
func main() {
    go foo()
    time.Sleep(time.Second * 5)
}

// sync.WaitGroup
package main
import (
        "fmt"
        "sync"
        "time"
)
var wg sync.WaitGroup
func foo() {
        defer wg.Done()
        fmt.Println("foo")
        time.Sleep(time.Second * 1)
        fmt.Println("foo end")
}
func bar() {
        defer wg.Done()
        fmt.Println("bar")
        time.Sleep(time.Second * 3)
        fmt.Println("bar end")
}
func main() {
        start := time.Now()
        wg.Add(2)
        go foo()
        go bar()
        wg.Wait()
        fmt.Println("Run time duration: ", time.Now().Sub(start))
}

// GOMAXPROCS(default cpu core number)
package main
import (
    "fmt"
    "runtime"
    "sync"
)
var wg sync.WaitGroup
func foo() {
    for i := 1; i < 10; i++ {
        fmt.Println("A:", i)
        //time.Sleep(time.Millisecond*20)
    }
    wg.Done()
}
func bar() {
    for i := 1; i < 10; i++ {
        fmt.Println("B:", i)
        //time.Sleep(time.Millisecond*30)
    }
    wg.Done()
}
func main() {
    wg.Add(2)
    fmt.Println(runtime.NumCPU())
    runtime.GOMAXPROCS(1)
    // runtime.GOMAXPROCS(4)
    go foo()
    go bar()
    wg.Wait()
}

// mutex
package main
import (
    "fmt"
    "sync"
)
var wg sync.WaitGroup
var lock sync.Mutex
var x = 0
func add() {
    //lock.Lock()
    x++
    //lock.Unlock()
    wg.Done()
}
func main() {
    wg.Add(1000)
    for i := 0; i < 1000 ; i++  {
        go add()
    }
    wg.Wait()
    fmt.Println(x)
}

// read write mutex
package main
import (
    "fmt"
    "sync"
    "time"
)
var rwlock sync.RWMutex
// var mutex sync.Mutex
var wg sync.WaitGroup
var x int
func write() {
        //mutex.Lock()
        rwlock.Lock()
        x += 1
        fmt.Println("x",x)
        time.Sleep(10 * time.Millisecond)
        //mutex.Unlock()
        rwlock.Unlock()
        wg.Done()
}
func read(i int) {
        //mutex.Lock()
        rwlock.RLock()
        time.Sleep(time.Millisecond)
        fmt.Println(x)
        //mutex.Unlock()
        rwlock.RUnlock()
        wg.Done()
}
func main() {
    start := time.Now()
    wg.Add(1)
    go write()
    for i := 0; i < 1000; i++ {
        wg.Add(1)
        go read(i)
    }
    wg.Wait()
    fmt.Println("run time duration", time.Now().Sub(start))

}

// map mutex
package main
import (
    "fmt"
    "sync"
)
func main() {
    wg := sync.WaitGroup{}
    var m = make(map[int]int)
    //var m = sync.Map{}
    // concurrency
    for i := 0; i < 100; i++ {
        wg.Add(1)
        go func(i int) {
            defer wg.Done()
            m.Store(i,i*i)
        }(i)
    }
    wg.Wait()
    fmt.Println(m.Load(1))
    fmt.Println(m.Load(2))
}

// Atomic operation
/*
AddXxx()
CompareAndSwapXxx()
StoreXxx()
SwapXxx()
*/

Reference:

1. Official Website

2. Repository

3. Go语言中文网