go concurrency

limit.go

package main

import (
	"fmt"
	"strings"
	"time"
)

func upper(input string) string {
	fmt.Println("upper " + input)
	time.Sleep(time.Second)
	return strings.ToUpper(input)
}

func main() {
	concurrency := 5
	sem := make(chan bool, concurrency)
	urls := []string{"url1", "url2", "url3", "url4", "url5", "url6", "url7", "url8", "url9", "url2", "url10", "url11", "url12", "url13", "url14", "url15", "url16", "url17"}

	resCh := make(chan string, len(urls))

	for _, url := range urls {
		sem <- true
		go func(u string) {
			defer func() { <-sem }()
			resCh <- upper(u)
		}(url)
	}

	for i := 0; i < cap(sem); i++ {
		sem <- true
	}

	results := make([]string, 0, len(urls))

	for i := 0; i < cap(resCh); i++ {
		r := <-resCh
		results = append(results, r)
	}

	fmt.Printf("%+v", results)
}