package main

import (
    "fmt"
    "log"
    "net/http"
    "net/url"
    "os"
    "strings"

    "github.com/PuerkitoBio/goquery"
    "github.com/juju/persistent-cookiejar"
    "golang.org/x/term"
)

func main() {
    jar, _ := cookiejar.New(nil)
    http.DefaultClient.Jar = jar
    defer jar.Save()

    // If the file exists, check whether logged in
    _, err := os.Stat(cookiejar.DefaultCookieFile())
    if err == nil {
        doc, _ := getDocument("https://atcoder.jp/home")
        navbarRight := doc.Find("div#navbar-collapse > ul.navbar-right")
        if navbarRight.Children().Length() == 2 {
            fmt.Println("Already logged in!")
            return
        }
    }

    var username string
    fmt.Print("Username: ")
    fmt.Scan(&username)

    fmt.Print("Password: ")
    // Don't press ctrl+c while Go reads password because your terminal won't display any inputs after that.
    // To resolve it, see gist.github.com/montanaflynn/5ae3eeae7212b0ba232f46e88f1ab67f
    bypePassword, _ := term.ReadPassword(int(os.Stdin.Fd()))

    loginUrl := "https://atcoder.jp/login"
    doc, _ := getDocument(loginUrl)
    token, found := doc.Find(`form input[type="hidden"]`).Attr("value")
    if !found {
        log.Fatal("error: cannot find CSRF token")
    }

    values := url.Values{
        "username":   {username},
        "password":   {string(bypePassword)},
        "csrf_token": {token},
    }
    req, _ := http.NewRequest("POST", loginUrl, strings.NewReader(values.Encode()))
    req.Header.Set("Content-Type", "application/x-www-form-urlencoded")

    resp, err := http.DefaultClient.Do(req)
    if err != nil {
        log.Fatal(err)
    }
    defer resp.Body.Close()

    if resp.Request.URL.String() == loginUrl {
        log.Fatal("Failed to login. Check your username/password")
    }

    fmt.Println("Successfully logged in!")
}

func getDocument(url string) (*goquery.Document, error) {
    resp, err := http.Get(url)
    if err != nil {
        log.Fatal(err)
    }
    defer resp.Body.Close()

    return goquery.NewDocumentFromReader(resp.Body)
}