package main

import (
	"context"
	"encoding/json"
	"fmt"
	"log"
	"net/http"
	"net/url"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"time"
)

// Mail is outbound only, plain text. Unsubscribe says the message respects the
// notify toggle, and puts List-Unsubscribe on it.
type Message struct {
	To          string `json:"to"`
	Subject     string `json:"subject"`
	Body        string `json:"body"`
	Unsubscribe bool   `json:"unsubscribe,omitempty"`
	Attempts    int    `json:"attempts,omitempty"`
}

type Mailer interface {
	Send(m Message) error
}

const (
	mailIdle     = 60 * time.Second // how often to look when nothing woke Drain
	mailBackoff  = 5 * time.Minute  // the ceiling on backing off after a failure
	mailAttempts = 6
)

// Mail is the spool and its drainer: a directory, where adding and removing
// are atomic.
type Mail struct {
	dir    string
	driver Mailer
	wake   chan struct{}
}

func newMail(cfg *Config, dir string) *Mail {
	var driver Mailer = dropMailer{}
	switch cfg.Mail.Driver {
	case "log":
		driver = logMailer{from: cfg.Mail.From}
	case "mailgun":
		driver = &mailgunMailer{
			from:   cfg.Mail.From,
			base:   cfg.Mail.Mailgun.BaseURL,
			domain: cfg.Mail.Mailgun.Domain,
			key:    cfg.Mail.Mailgun.APIKey,
			list:   cfg.BaseURL + "/settings",
			client: &http.Client{Timeout: 30 * time.Second},
		}
	}
	return &Mail{dir: dir, driver: driver, wake: make(chan struct{}, 1)}
}

// Enqueue never fails the request that caused it: mail is a side effect.
func (m *Mail) Enqueue(msg Message) {
	if msg.To == "" {
		return
	}
	data, err := marshalJSON(msg)
	if err != nil {
		log.Printf("mail: %v", err)
		return
	}
	// A sortable timestamp plus a random suffix, so oldest first is a
	// directory sort.
	name := time.Now().UTC().Format("20060102T150405.000000000Z") + "-" + randomToken(4) + ".json"
	if err := writeFile(filepath.Join(m.dir, name), data, 0600); err != nil {
		log.Printf("mail: %v", err)
		return
	}
	select {
	case m.wake <- struct{}{}:
	default:
	}
}

// Drain is the only background goroutine. Delivery is at-least-once: a crash
// between sending and deleting resends once.
func (m *Mail) Drain(ctx context.Context) {
	backoff := time.Second
	for {
		failed := m.drainOnce()
		wait := mailIdle
		if failed {
			wait = backoff
			if backoff *= 2; backoff > mailBackoff {
				backoff = mailBackoff
			}
		} else {
			backoff = time.Second
		}
		// After a failure a wake would only retry the head early. A nil
		// channel never fires.
		wake := m.wake
		if failed {
			wake = nil
		}
		select {
		case <-ctx.Done():
			return
		case <-wake:
		case <-time.After(wait):
		}
	}
}

// drainOnce reports whether it stopped on a failure; it stops rather than
// skipping ahead, to keep oldest first.
func (m *Mail) drainOnce() bool {
	entries, err := os.ReadDir(m.dir)
	if err != nil {
		log.Printf("mail: %v", err)
		return true
	}
	names := make([]string, 0, len(entries))
	for _, entry := range entries {
		if strings.HasSuffix(entry.Name(), ".json") {
			names = append(names, entry.Name())
		}
	}
	sort.Strings(names)

	for _, name := range names {
		path := filepath.Join(m.dir, name)
		data, err := os.ReadFile(path)
		if err != nil {
			log.Printf("mail: %s: %v", name, err)
			return true
		}
		var msg Message
		if err := json.Unmarshal(data, &msg); err != nil {
			log.Printf("mail: %s does not parse, dropping it: %v", name, err)
			os.Remove(path)
			continue
		}
		if err := m.driver.Send(msg); err != nil {
			msg.Attempts++
			if msg.Attempts >= mailAttempts {
				// Never the body: it may carry a set-password link.
				log.Printf("mail: giving up on %q to %s after %d attempts: %v",
					msg.Subject, msg.To, msg.Attempts, err)
				os.Remove(path)
				return true
			}
			log.Printf("mail: %q to %s failed, attempt %d: %v", msg.Subject, msg.To, msg.Attempts, err)
			// Unrecorded, the attempt would repeat for ever.
			again, err := marshalJSON(msg)
			if err == nil {
				err = writeFile(path, again, 0600)
			}
			if err != nil {
				log.Printf("mail: cannot record the attempt on %q to %s, dropping it: %v",
					msg.Subject, msg.To, err)
				os.Remove(path)
			}
			return true
		}
		if err := os.Remove(path); err != nil {
			// Or it is sent again on every drain, silently.
			log.Printf("mail: %q to %s was sent but is still spooled: %v", msg.Subject, msg.To, err)
			return true
		}
	}
	return false
}

// dropMailer is the none driver, and the default: nothing is sent, and the log
// names what was not, never the body.
type dropMailer struct{}

func (dropMailer) Send(m Message) error {
	log.Printf("mail: none, dropped %q to %s", m.Subject, m.To)
	return nil
}

// logMailer writes whole messages to the log, for development: the one driver
// that puts a set-password link there.
type logMailer struct{ from string }

func (l logMailer) Send(m Message) error {
	log.Printf("mail: from %s to %s\nSubject: %s\n\n%s", l.from, m.To, m.Subject, m.Body)
	return nil
}

// mailgunMailer is one form-encoded POST, the whole API surface in use.
type mailgunMailer struct {
	from, base, domain, key, list string
	client                        *http.Client
}

func (g *mailgunMailer) Send(m Message) error {
	form := url.Values{
		"from":    {g.from},
		"to":      {m.To},
		"subject": {m.Subject},
		"text":    {m.Body},
	}
	if m.Unsubscribe {
		form.Set("h:List-Unsubscribe", "<"+g.list+">")
	}
	endpoint := g.base + "/" + g.domain + "/messages"
	req, err := http.NewRequest(http.MethodPost, endpoint, strings.NewReader(form.Encode()))
	if err != nil {
		return err
	}
	req.SetBasicAuth("api", g.key)
	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")

	resp, err := g.client.Do(req)
	if err != nil {
		return err
	}
	defer resp.Body.Close()
	if resp.StatusCode/100 != 2 {
		return fmt.Errorf("mailgun: %s", resp.Status)
	}
	return nil
}

// The notification rules. Mail is sent for these events and no others.

// mailAccount tells the account holder about their own account: it ignores
// the notify toggle, and still needs a proved address.
func (f *Forge) mailAccount(u User, subject, body string) {
	if !u.EmailConfirmed {
		return
	}
	f.mail.Enqueue(Message{To: u.Email, Subject: f.subject(subject), Body: body})
}

// mailLink carries a capability to an address confirmed or not: following it
// is what confirms it.
func (f *Forge) mailLink(to, subject, body string) {
	f.mail.Enqueue(Message{To: to, Subject: f.subject(subject), Body: body})
}

// mailRepo is a message to everyone holding any grant on a repository,
// respecting the toggle and never to whoever caused the event.
func (f *Forge) mailRepo(repo Repo, except, subject, body string) {
	for name := range repo.Grants {
		if name == except {
			continue
		}
		if u, ok := f.store.User(name); ok && u.Notify && u.EmailConfirmed {
			f.mail.Enqueue(Message{To: u.Email, Subject: f.subject(subject), Body: body, Unsubscribe: true})
		}
	}
}

// mailAdmins is a message to every global admin, respecting the toggle.
func (f *Forge) mailAdmins(except, subject, body string) {
	for name, u := range f.store.Users() {
		if name == except || !u.Admin || !u.Notify || !u.EmailConfirmed {
			continue
		}
		f.mail.Enqueue(Message{To: u.Email, Subject: f.subject(subject), Body: body, Unsubscribe: true})
	}
}

func (f *Forge) subject(s string) string { return "[" + f.siteName() + "] " + s }

func (f *Forge) url(path string) string { return f.cfg.BaseURL + path }

// notifyIssue tells everyone with a grant that an issue moved.
func (f *Forge) notifyIssue(path string, repo Repo, issue Issue, actor, verb string) {
	subject := fmt.Sprintf("%s#%d %s: %s", path, issue.ID, verb, issue.Title)
	body := fmt.Sprintf("%s %s %s#%d.\n\n%s\n\n%s\n",
		actor, verb, path, issue.ID, issue.Title,
		f.url(fmt.Sprintf("/%s/issues/%d", path, issue.ID)))
	f.mailRepo(repo, actor, subject, body)
}