// Debouncer runs a given function, debounced
type Debouncer struct {
    currDebounceID *int32
}

// NewDebouncer creates a new Debouncer
func NewDebouncer() *Debouncer {
    return &Debouncer{}
}

func (d *Debouncer) debounce(f func(), timeout time.Duration) {
    if d.currDebounceID != nil {
        // stop currently scheduled function
        v := atomic.AddInt32(d.currDebounceID, 1)
        d.currDebounceID = nil
        if v > 1 {
            // it was already executed
            return
        }
    }

    d.currDebounceID = new(int32)
    go func(f func(), timeout time.Duration, debounceID *int32) {
        for {
            select {
            case <-time.After(timeout):
                v := atomic.AddInt32(debounceID, 1)
                // if v != 1, it was cancelled
                if v == 1 {
                    f()
                }
            }
        }
    }(f, timeout, d.currDebounceID)
}

var articleLoadDebouncer *Debouncer

func reloadArticlesDelayed() {
    if articleLoadDebouncer == nil {
        articleLoadDebouncer = NewDebouncer()
    }
    articleLoadDebouncer.debounce(loadArticles, time.Second)
}

func TimeoutDialer(cTimeout time.Duration, rwTimeout time.Duration) func(net, addr string) (c net.Conn, err error) {
    return func(netw, addr string) (net.Conn, error) {
        conn, err := net.DialTimeout(netw, addr, cTimeout)
        if err != nil {
            return nil, err
        }
        conn.SetDeadline(time.Now().Add(rwTimeout))
        return conn, nil
    }
}

// can be used for http.Get() requests with better timeouts. New one must be created
// for each Get() request
func NewTimeoutClient(connectTimeout time.Duration, readWriteTimeout time.Duration) *http.Client {
    return &http.Client{
        Transport: &http.Transport{
            Dial:  TimeoutDialer(connectTimeout, readWriteTimeout),
            Proxy: http.ProxyFromEnvironment,
        },
    }
}

func DownloadUrl(url string) ([]byte, error) {
    // default timeout for http.Get() is really long, so dial it down
    // for both connection and read/write timeouts
    timeoutClient := NewTimeoutClient(time.Second*120, time.Second*120)
    resp, err := timeoutClient.Get(url)
    if err != nil {
        return nil, err
    }
    defer resp.Body.Close()
    if resp.StatusCode != 200 {
        return nil, errors.New(fmt.Sprintf("'%s': status code not 200 (%d)", url, resp.StatusCode))
    }
    return ioutil.ReadAll(resp.Body)
}

select {
case ch <- value:
default:
}

package util

import (
	"sync"
	"time"
)

// ProgressEstimatorData contains fields readable after Next()
type ProgressEstimatorData struct {
	Total             int
	Curr              int
	Left              int
	PercDone          float64 // 0...1
	Skipped           int
	TimeSoFar         time.Duration
	EstimatedTimeLeft time.Duration
}

// ProgressEstimator is for estimating progress
type ProgressEstimator struct {
	timeStart time.Time
	sync.Mutex
	ProgressEstimatorData
}

// NewProgressEstimator creates a ProgressEstimator
func NewProgressEstimator(total int) *ProgressEstimator {
	d := ProgressEstimatorData{
		Total: total,
	}
	return &ProgressEstimator{
		ProgressEstimatorData: d,
		timeStart:             time.Now(),
	}
}

// Next advances estimator
func (pe *ProgressEstimator) next(isSkipped bool) ProgressEstimatorData {
	pe.Lock()
	defer pe.Unlock()
	if isSkipped {
		pe.Skipped++
	}
	pe.Curr++
	pe.Left = pe.Total - pe.Curr
	pe.TimeSoFar = time.Since(pe.timeStart)

	realTotal := pe.Total - pe.Skipped
	realCurr := pe.Curr - pe.Skipped
	if realCurr == 0 || realTotal == 0 {
		pe.EstimatedTimeLeft = pe.TimeSoFar
	} else {
		pe.PercDone = float64(realCurr) / float64(realTotal) // 0..1 range
		realPerc := float64(realTotal) / float64(realCurr)
		estimatedTotalTime := float64(pe.TimeSoFar) * realPerc
		pe.EstimatedTimeLeft = time.Duration(estimatedTotalTime) - pe.TimeSoFar
	}
	cpy := pe.ProgressEstimatorData
	return cpy
}

// Next advances estimator
func (pe *ProgressEstimator) Next() ProgressEstimatorData {
	return pe.next(false)
}

// Skip advances estimator but allows to mark this file as taking no time,
// to allow better estimates
func (pe *ProgressEstimator) Skip() ProgressEstimatorData {
	return pe.next(true)
}

https://gist.github.com/kennwhite/f9ea1afff776049974678fb21a6f3846

package main

import (
	"crypto/tls"
	"golang.org/x/crypto/acme/autocert"
	"log"
	"net"
	"net/http"
)

var domain string = "example.com"

func redirect(w http.ResponseWriter, req *http.Request) {

	ip, _, _ := net.SplitHostPort(req.RemoteAddr)
	target := "https://" + req.Host
	agent := req.UserAgent()
	log.Printf("Redirecting [%s] - [%s] to: %s", agent, ip, target)
	http.Redirect(w, req, target, http.StatusTemporaryRedirect)

}

func main() {

	staticDir := "static"
	certCache := "tmp/certs"
	TLSAddr := ":https"

	// Use LE key & cert retrieved from tls.Config GetCertificate
	certFile, keyFile := "", ""

	certManager := autocert.Manager{
		Prompt:     autocert.AcceptTOS,
		HostPolicy: autocert.HostWhitelist(domain),
		Cache:      autocert.DirCache(certCache),
	}

	SetHSTSHeader := func(h http.Handler) http.HandlerFunc {
		return func(w http.ResponseWriter, r *http.Request) {
			ip, _, _ := net.SplitHostPort(r.RemoteAddr)

			l := "[" + r.UserAgent() + "] - [" + ip + "]"
			log.Print(l)

			headers := map[string]string{
				"Strict-Transport-Security": "max-age=31557600; includeSubDomains",
				"X-Content-Type-Options":    "nosniff",
				"X-XSS-Protection":          "1; mode=block",
				"Content-Security-Policy":   "default-src 'self'; script-src 'self'",
				"X-Frame-Options":           "DENY",
				"Referrer-Policy":           "no-referrer",
				"Public-Key-Pins": 
					"pin-sha256=\"uIgDNRW0N1ZqFBvx6qJWFqIlaR2rZH/Yr35ZNB+KdHE=\";" + // Site cert
					"pin-sha256=\"BackupBackupBackupBackupBackupBackupBackups=\";" + // Site backup cert
					"pin-sha256=\"YLh1dUR9y6Kja30RrAn7JKnbQG/uEtLMkBgFF2Fuihg=\";" + // LE X3 root CA
					"pin-sha256=\"Vjs8r4z+80wjNcr1YKepWQboSIRi63WsWXhIMN+eWys=\";" + // LE DST X3 root CA
					"includeSubdomains; max-age=2592000",
			}
			for k, v := range headers {
				//fmt.Println(k, ":", v)
				w.Header().Add(k, v)
			}
			h.ServeHTTP(w, r)
		}
	}

	HTTPSServer := &http.Server{
		Addr: TLSAddr,
		TLSConfig: &tls.Config{
			GetCertificate: certManager.GetCertificate, MinVersion: tls.VersionTLS10, // MaxVersion: tls.VersionTLS12,
			CipherSuites: []uint16{
				tls.TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305,
				tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
				tls.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
				tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, // Required by Go (and HTTP/2 RFC), even if you only present ECDSA certs
				tls.TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA,
				/*
					tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
					tls.TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA,
					tls.TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305,
					tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
					tls.TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA,
				*/
			},
		},
	}

	http.Handle("/", SetHSTSHeader(http.FileServer(http.Dir(staticDir))))

	// HTTP (redirect) Listener
	go http.ListenAndServe(":80", http.HandlerFunc(redirect))

	// HTTPS Listener
	err := HTTPSServer.ListenAndServeTLS(certFile, keyFile)

	log.Fatal(err)
}

https://gist.github.com/hyg/9c4afcd91fe24316cbf0

func openBrowser(url string) {
	var err error

	switch runtime.GOOS {
	case "linux":
		err = exec.Command("xdg-open", url).Start()
	case "windows":
		err = exec.Command("rundll32", "url.dll,FileProtocolHandler", url).Start()
	case "darwin":
		err = exec.Command("open", url).Start()
	default:
		err = fmt.Errorf("unsupported platform")
	}
	if err != nil {
		log.Fatal(err)
	}

}

func panicIfErr(err error) {
	if err != nil {
		panic(err.Error())
	}
}

type Foo struct {
	Str     string   `json:"str"`
	Str2    string   `json:"str-2" firestore:"str-2"`
	StrArr  []string `json:"strarr"`
	StrArr2 []string `json:"str-arr" firestore:"str-arr"`
}

func dbTestSetGet() {
	db, ctx := getDB()
	d := map[string]interface{}{
		"str":     "string value str",
		"str-2":   "string value str-2",
		"strarr":  []string{"string", "array", "for", "strarr"},
		"str-arr": []string{"string", "array", "for", "str-arr"},
	}
	_, err := db.Collection("foo").Doc("bar").Set(ctx, d)
	panicIfErr(err)
	doc, err := db.Collection("foo").Doc("bar").Get(ctx)
	panicIfErr(err)
	data := doc.Data()
	fmt.Printf("Data: %#v\n", data)
	var foo Foo
	err = doc.DataTo(&foo)
	panicIfErr(err)
	fmt.Printf("foo: %#v\n", foo)
}

func normalizeNewlines(d []byte) []byte {
	// replace CR LF (windows) with LF (unix)
	d = bytes.Replace(d, []byte{13, 10}, []byte{10}, -1)
	// replace CF (mac) with LF (unix)
	d = bytes.Replace(d, []byte{13}, []byte{10}, -1)
	return d
}

// return first line of d and the rest
func bytesRemoveFirstLine(d []byte) (string, []byte) {
	idx := bytes.IndexByte(d, 10)
	//u.PanicIf(-1 == idx)
	if -1 == idx {
		return string(d), nil
	}
	l := d[:idx]
	return string(l), d[idx+1:]
}

// FmtArgs formats args as a string. First argument should be format string
// and the rest are arguments to the format
func FmtArgs(args ...interface{}) string {
	if len(args) == 0 {
		return ""
	}
	format := args[0].(string)
	if len(args) == 1 {
		return format
	}
	return fmt.Sprintf(format, args[1:]...)
}

func panicWithMsg(defaultMsg string, args ...interface{}) {
	s := FmtArgs(args...)
	if s == "" {
		s = defaultMsg
	}
	fmt.Printf("%s\n", s)
	panic(s)
}

// PanicIf panics if cond is true
func PanicIf(cond bool, args ...interface{}) {
	if !cond {
		return
	}
	panicWithMsg("PanicIf: condition failed", args...)
}

// RemoveDuplicateStrings removes duplicate strings from an array of strings.
// It's optimized for the case of no duplicates. It modifes a in place.
func RemoveDuplicateStrings(a []string) []string {
    if len(a) < 2 {
        return a
    }
    sort.Strings(a)
    writeIdx := 1
    for i := 1; i < len(a); i++ {
        if a[i-1] == a[i] {
            continue
        }
        if writeIdx != i {
            a[writeIdx] = a[i]
        }
        writeIdx++
    }
    return a[:writeIdx]
}