package main

import (
	"fmt"
	"sync"
	"sync/atomic"
)

func main() {
	var atomicCounter, nonAtomicCounter uint64
	wg := sync.WaitGroup{}

	for i := 0; i < 50; i++ {
		wg.Add(1)

		go func() { // run 50 goroutines, each goroutine increases the counter 1000 times
			for c := 0; c < 1000; c++ {
				atomic.AddUint64(&atomicCounter, 1) // increase atomic counter
				nonAtomicCounter++ // increase non-atomic counter
			}

			wg.Done()
		}()
	}

	wg.Wait() // wait until all goroutines finish
	
	fmt.Println("atomic counter value:", atomicCounter)
	fmt.Println("non atomic counter value:", nonAtomicCounter)
}

// go run counters.go 
// atomic counter value: 50000
// non atomic counter value: 30648

package main

import (
	"fmt"
	"time"
)

func main() {
	first := make(chan string, 1)

	go func() {
		time.Sleep(2 * time.Second) // wait 2 seconds before send value to channel
		first <- "first result"     // send value to the channel
	}()

	select {
	case result := <-first:
		fmt.Println(result)
	case <-time.After(time.Second): // timeout occurs before the value is read from the first channel
		fmt.Println("first timeout")
	}

	second := make(chan string, 1)

	go func() {
		time.Sleep(time.Second)   // wait 1 second before send value to the channel
		second <- "second result" // send value to the channel
	}()

	select {
	case result := <-second: // the value from the channel is read before the timeout expires
		fmt.Println(result)
	case <-time.After(2 * time.Second):
		fmt.Println("second timeout")
	}
}

// $ go run timeouts.go 
// first timeout
// second result

package main

import (
	"fmt"
	"time"
)

func main() {
	ticker := time.NewTicker(time.Second)
	fmt.Println("ticker is started")

	done := make(chan struct{})

	go func() {
		for {
			select {
			case <-done:
				return
			case t := <-ticker.C:
				fmt.Println("tick", t)
			}
		}
	}()

	time.Sleep(5 * time.Second)

	ticker.Stop()
	fmt.Println("ticker is stopped")

	time.Sleep(5 * time.Second)
	done <- struct{}{}
}

// go run tickers.go 
// ticker is started
// tick 2024-02-09 23:40:25.809616086 +0700 +07 m=+1.000091237
// tick 2024-02-09 23:40:26.809742343 +0700 +07 m=+2.000152938
// tick 2024-02-09 23:40:27.809804037 +0700 +07 m=+3.000214622
// tick 2024-02-09 23:40:28.809852444 +0700 +07 m=+4.000263029
// ticker is stopped
// tick 2024-02-09 23:40:29.809908932 +0700 +07 m=+5.000319557

package main

import (
	"fmt"
	"time"
)

func main() {
	firstTimer := time.NewTimer(3 * time.Second)
	value := <-firstTimer.C // value is a current time

	fmt.Printf("first timer expired: %v\n", value) // the line printed 3 seconds after running code

	secondTimer := time.NewTimer(2 * time.Second)

	go func() {
		<-secondTimer.C
		fmt.Println("second timer expired") // the line is not printed because the second timer is stopped before expiration
	}()

	stop := secondTimer.Stop() // stop second timer before it expired

	if stop {
		fmt.Println("second timer is stopped")
	}
}

// $ go run timers.go
// first timer expired: 2024-02-09 22:32:57.570649221 +0700 +07 m=+3.002054417
// second timer is stopped

package main

import (
    "fmt"
    "net/http"
)

func hello(w http.ResponseWriter, req *http.Request) {
    fmt.Fprintf(w, "Hello, World!\n")
}

func headers(w http.ResponseWriter, req *http.Request) {
    for name, headers := range req.Header {
        for _, h := range headers {
            fmt.Fprintf(w, "%v: %v\n", name, h)
        }
    }
}

func main() {
    http.HandleFunc("/hello", hello)
    http.HandleFunc("/headers", headers)

    http.ListenAndServe(":8080", nil) // run http server on port 8080
}

package main

import (
	"errors"
	"fmt"
	"os"
)

func main() {
	filePath := "file.txt"

	if _, err := os.Stat(filePath); errors.Is(err, os.ErrNotExist) {
		fmt.Println("file not exists")
	} else {
		fmt.Println("file exists")
	}
}