kb/wiki/tech/go.md

---
title: Go
created: 2020-04-05
updated: 2020-04-05
type: summary
tags: [tech, reference]
external: https://github.com/wahyd4/knowledge/blob/master/categories/go.md
---

## pass pointer or value
- value: Variable must not be modified
- Variable is a large struct  then prefer pointer
- Variable is a map or slice then prefer value
- Passing by value often is cheaper

## struct

- make
```go
make(T, args) -> T
```
- new
```go
new(T) -> *T
```
- a := T{}

## array
```go
var a [1]int
```
Array has a exactly length, can't be modified.
## slice

```go
var a []string
[]string{"a", "b"}
[...]string{"a","b"}
```
1. Auto increment length
```go
new([]int)
make([]int, 2, 5)
```
2. nil is a valid slice which length is `0`

## Go routine

- you can run more goroutine vs thread
- go routine have a faster start up time than thread
- go routine come with built-in primitives to communicate safely by using channels

## Closure

## Channel

- Normal channel
```go
messages := make(chan string)
// _Send_ a value into a channel using the `channel <-`
go func() { messages <- "ping" }()
// channel. Here we'll receive the `"ping"` message
msg := <-messages
```

- buffered channel
```go
ch := make(chan Task, 3)
```

## Big Numbers

Package big implements arbitrary-precision arithmetic (big numbers).
For example the int numbers larger than `int64` or the float greater than `float64`.

```
int16: (-32,768 to +32,767)

int32: (-2,147,483,648 to +2,147,483,647)

int64: (-9,223,372,036,854,775,808 to +9,223,372,036,854,775,807)
```
The following numeric types are supported:
```
 Int    signed integers
 Rat    rational numbers
 Float  floating-point numbers
```

Some exmaple code
```go
import "math/big"

//string to Int
n1 := new(big.Int)
n1, ok = n1.SetString(str1, 10)

// Int1 + Int2
result := new(big.Int)
result.Add(n1, n2)

// Int to string
result.String()
```

## Sync

###  atomic

### Mutex

  - sync.Mutex()
  - mutex.Lock()
  - mutx.Unlock()

```go
package main

import (
	"fmt"
	"sync"
	"time"
)

// SafeCounter is safe to use concurrently.
type SafeCounter struct {
	v   map[string]int
	mux sync.Mutex
}

// Inc increments the counter for the given key.
func (c *SafeCounter) Inc(key string) {
	c.mux.Lock()
	// Lock so only one goroutine at a time can access the map c.v.
	c.v[key]++
	c.mux.Unlock()
}

// Value returns the current value of the counter for the given key.
func (c *SafeCounter) Value(key string) int {
	c.mux.Lock()
	// Lock so only one goroutine at a time can access the map c.v.
	defer c.mux.Unlock()
	return c.v[key]
}

func main() {
	c := SafeCounter{v: make(map[string]int)}
	for i := 0; i < 1000; i++ {
		go c.Inc("somekey")
	}

	time.Sleep(time.Second)
	fmt.Println(c.Value("somekey"))
}
```


### RWMutex
!> A RWMutex is a reader/writer mutual exclusion lock. The lock can be held by an arbitrary number of readers or a single writer. The zero value for a RWMutex is an unlocked mutex.
In other words, readers don't have to wait for each other. They only have to wait for writers holding the lock.

### string literals

```go
    `aaa bbb ccc`
```
### panic / recover

###  `sync.Map` is concurrent/ thread safe `map`, normal `map` isn't

```go
m := new(sync.Map)
m.Store("a", "b")
value, ok := m.Load("a")
```

### Defer to Clean Up

Use defer to clean up resources such as files and locks.
```go
p.Lock()
defer p.Unlock()

if p.count < 10 {
  return p.count
}

p.count++
return p.count

// more readable
```

## Monotonic Clocks

Operating systems provide both a `wall clock`, which is subject to changes for clock synchronization, and a `monotonic clock`, which is not. The general rule is that the wall clock is for telling time and the monotonic clock is for measuring time. Rather than split the API, in this package the Time returned by time.Now contains both a wall clock reading and a monotonic clock reading; later time-telling operations use the wall clock reading, but later time-measuring operations, specifically comparisons and subtractions, use the monotonic clock reading.

For example, this code always computes a positive elapsed time of approximately 20 milliseconds, even if the wall clock is changed during the operation being timed:

```go
start := time.Now()
... operation that takes 20 milliseconds ...
t := time.Now()
elapsed := t.Sub(start)
```

### wall clock

This clock is subject to potential variations. For example, if it is synchronized with NTP (Network Time Protocol). In this case after synchronization, the local clock of our server can jump backward or forward in time. So measuring a duration from the wall-clock can be biased.

### Monotonic clock

we have a guarantee that the time always moves forward and will not be impacted by variations leading to jumps in time.

Therefore, if we have to measure durations, we must use the monotonic-clock. This rule of thumb is only valid for local duration measurements though. Indeed, the monotonic-clocks of two different servers are by definition not synchronized. So, measuring a distributed execution based on these clocks will not be accurate.

## Frameworks

### db

- xorm
- gorm

### web

- beego
- mux
- gin
- go kit
    - full stack micro service framework like spring boot

## Tools

### profiler
    - go-wrk(wrk)
        - a http benchmark tool
    - go-torch
        - Stochastic flame graph profiler
### test
    - Testify <http://github.com/stretchr/testify>
    - Ginkgo <http://onsi.github.io/ginkgo/>
### Linter
    - Golangci-lint https://github.com/golangci/golangci-lint

# Tips

## Sort slice

```go
// sort users by user age ASC
sort.Slice(users, func(i, j int) bool {
  return users[i].age < planets[j].age
})
```

## slice

- byte* array //actual data
- uintgo len
- uintgo cap

## map

* implement by hash table
* slice can't be the key of a map, but sized array could. e.g. `var a map[[2]int]string`

## Go has no generics

- performance
- complexity
- If C++ and Java are about type hierarchies and the taxonomy of types, Go is about composition.
- How to solve
    - use interface
    - use type assertions
    - use reflection

## Modify item in range

- use the array index instead of the value
```go
    for _, e := range array {
        e.field = "foo"
    }

    for idx, _ := range array {
        array[idx].field = "foo"
    }
```
## merge two array

- a = append(a, b…)
    - must add … to b, otherwise you can only add one item

## error handling

### check error type

```go
ErrorSample := errors.New("some error")
if errors.Is(err, ErrorSample) {
    // something wasn't found
}
```

### error type assertion
```go
    if serr, ok := err.(*json.SyntaxError); ok {}
//or

// var e *QueryError
if errors.As(err, &e) {
    // err is a *QueryError, and e is set to the error's value
}

```

### better error handling

#### custom error type

```go
type appError struct {
    Error   error
    Message string
    Code    int
}
```
####  concat error check

```go
if err1() != nil || err2() != nil {}
```
#### some error constants

```go
errNotFound = errors.New("Item not found")
switch err {
    case errNotFound:
}
```

## date format
```go
t := time.Now()
fmt.Println(t.String())
fmt.Println(t.Format("2006-01-02 15:04:05"))
```

- var _ InterfaceX = &InterfaceXImplementation{}
	- make sure InterfaceX’s implementation works

## reference types in go
* map
* channel
* slice

## value types
* Array

## Read file line by line
```go
package main

import (
    "bufio"
    "fmt"
    "log"
    "os"
)

func main() {
    file, err := os.Open("/path/to/file.txt")
    if err != nil {
        log.Fatal(err)
    }
    defer file.Close()

    scanner := bufio.NewScanner(file)
    for scanner.Scan() {
        fmt.Println(scanner.Text())
    }

    if err := scanner.Err(); err != nil {
        log.Fatal(err)
    }
}
```

## commands

### test

- go test ./...
    -  run all tests in current directory and all of its subdirectories
- go test foo/...
    -  run all tests with import path prefixed with foo/:
- go test foo...
    - run all tests import path prefixed with foo:
- go test ...
    - run all tests in your $GOPATH:

## REPL

REPL stands for read eval print loop, basically it just like the irb in Ruby.

Wiki: <https://en.wikipedia.org/wiki/Read%E2%80%93eval%E2%80%93print_loop>

* gore <https://github.com/motemen/gore>

# Some code snippets

## defer would still run after panic

```go
package main
func main() {
	for {
		defer func() {
			for {
			}
		}()
		panic("yolo")
	}
}
```
## Some interesting mistakes

### Do not reuse HTTP request when retrying

See more information: https://stackoverflow.com/questions/55385894/http-contentlength-99-with-body-length-0



# Useful links
* [Go best practice](https://github.com/golang/go/wiki/CodeReviewComments)
* [Code Review Comments](https://github.com/golang/go/wiki/CodeReviewComments)
* [Uber Go code style](https://github.com/uber-go/guide/blob/master/style.md#pointers-to-interfaces)