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package main

import (
	"bytes"
	"fmt"
	"io"
	"log"
	"strings"
	"time"

	"github.com/emersion/go-imap"
	"github.com/emersion/go-imap/client"
	gomessage "github.com/emersion/go-message"
	"github.com/emersion/go-message/mail"
	"github.com/emersion/go-sasl"
	gosmtp "github.com/emersion/go-smtp"
)

// ============================================================================
// SMTP credentials
// ============================================================================

type smtpCreds struct {
	host string
	port int
	user string
	pass string
}

// ============================================================================
// Message identity
// ============================================================================

// setMessageID stamps a fresh Message-ID on an outgoing message, using the
// domain of the From address as the right-hand side.
//
// Every message mailweb sends needs one, and until this existed none of them
// had one: go-message writes no Message-ID unless asked, so every unsubscribe
// request and every spam report went out unidentified. That is not merely
// untidy. A Message-ID is what In-Reply-To and References point at, so mail
// sent without one cannot be replied to in a way any client can thread —
// including mailweb itself, which stores the field and would have nothing to
// join a reply back to. It is also what a bounce names when reporting a
// message undeliverable, and what a receiving spam filter uses to recognise a
// retransmission rather than a duplicate.
//
// The domain comes from the From address rather than from os.Hostname, which
// is what GenerateMessageID would use. A local hostname is neither unique nor
// a FQDN — this machine is "rolery", and the one message hand-sent from it
// carries <…@rolery>, which no resolver can place and which tells every
// recipient the name of the sending workstation. The From domain is by
// construction a real domain that the account demonstrably sends from.
//
// A From address with no domain leaves the message without a Message-ID rather
// than inventing one: a fabricated right-hand side is worse than a missing
// header, since it claims an origin that does not exist.
func setMessageID(h *mail.Header, from string) {
	_, domain, ok := strings.Cut(from, "@")
	if !ok || domain == "" {
		log.Printf("smtp: no domain in From %q; sending without a Message-ID", from)
		return
	}
	if err := h.GenerateMessageIDWithHostname(domain); err != nil {
		// Only the CSPRNG can fail here. The mail is still worth sending, so
		// this is reported rather than fatal.
		log.Printf("smtp: could not generate Message-ID: %v", err)
	}
}

// ============================================================================
// Build RFC 5322 message
// ============================================================================

// buildMessage constructs a minimal RFC 5322 message and returns its raw bytes.
// contentType should be "text/plain" or "text/html"; defaults to "text/plain" if empty.
// unsubscribeFor, if non-empty, is written as X-Mailweb-Unsubscribe: <addr> so the
// sent mail can be matched back to the contact's conversation.
func buildMessage(from, to, subject, body, contentType, unsubscribeFor string) ([]byte, error) {
	var buf bytes.Buffer

	if contentType == "" {
		contentType = "text/plain"
	}

	var h mail.Header
	h.SetDate(time.Now())
	setMessageID(&h, from)
	h.SetAddressList("From", []*mail.Address{{Address: from}})
	h.SetAddressList("To", []*mail.Address{{Address: to}})
	h.Set("Subject", subject)
	h.Set("Content-Type", contentType+"; charset=utf-8")
	if unsubscribeFor != "" {
		h.Set("X-Mailweb-Unsubscribe", unsubscribeFor)
	}

	wc, err := mail.CreateSingleInlineWriter(&buf, h)
	if err != nil {
		return nil, fmt.Errorf("create writer: %w", err)
	}
	if _, err := wc.Write([]byte(body)); err != nil {
		return nil, fmt.Errorf("write body: %w", err)
	}
	if err := wc.Close(); err != nil {
		return nil, fmt.Errorf("close writer: %w", err)
	}

	return buf.Bytes(), nil
}

// ============================================================================
// Reply messages
// ============================================================================

// draftHeader builds the header block of a message being sent from a draft.
//
// In-Reply-To and References are what make a reply a reply: without them it is
// a new message that happens to start with "Re:", and every threading client
// shows it as one. Both come off the draft, where they were copied from the
// parent when it was created — so a parent since expunged does not cost the
// threading. A draft that answers nothing carries neither, and the two headers
// are simply absent rather than empty.
func draftHeader(d *draft, from string, set recipientSet) mail.Header {
	var h mail.Header
	h.SetDate(time.Now())
	setMessageID(&h, from)
	h.SetAddressList("From", []*mail.Address{{Address: from}})
	h.SetAddressList("To", recipientAddressList(set.To))
	if len(set.Cc) > 0 {
		h.SetAddressList("Cc", recipientAddressList(set.Cc))
	}
	h.Set("Subject", d.Subject)
	if d.InReplyTo != "" {
		h.Set("In-Reply-To", d.InReplyTo)
	}
	if d.References != "" {
		h.Set("References", d.References)
	}
	return h
}

// recipientAddressList converts recipients into the address form go-message
// wants. Recipients carry no display name: what mailweb knows a correspondent
// as is either their own claim, which is theirs to make and not mailweb's to
// repeat, or a petname, which must never leave this machine.
func recipientAddressList(rs []Recipient) []*mail.Address {
	out := make([]*mail.Address, 0, len(rs))
	for _, r := range rs {
		out = append(out, &mail.Address{Address: r.Address()})
	}
	return out
}

// writePlainBody writes a single-part text/plain message.
//
// It takes an io.Writer rather than a buffer because it is used twice: once as
// the whole message, and once as a part inside a multipart/mixed when the draft
// carries attachments. The bytes it writes are identical either way, which is
// what keeps "what will be sent" the same sentence in both cases.
func writePlainBody(w io.Writer, h mail.Header, body string) error {
	h.Set("Content-Type", "text/plain; charset=utf-8")
	wc, err := mail.CreateSingleInlineWriter(w, h)
	if err != nil {
		return fmt.Errorf("create writer: %w", err)
	}
	if _, err := wc.Write([]byte(body)); err != nil {
		return fmt.Errorf("write body: %w", err)
	}
	if err := wc.Close(); err != nil {
		return fmt.Errorf("close writer: %w", err)
	}
	return nil
}

// writeAlternativeBody writes a multipart/alternative message carrying the same
// content as text and as HTML.
//
// The plain part is written first, which is required rather than stylistic:
// RFC 2046 orders the parts of an alternative from least to most faithful, and
// a client shows the last one it can render. Reversing them shows plain text to
// everybody.
//
// The alternative is the top level of the message rather than being wrapped in
// a multipart/mixed. go-message's mail.CreateWriter always writes mixed, which
// is what a message with attachments needs and is a layer of nesting that this
// one does not: mixed says "several things are enclosed" where there is exactly
// one thing in two renderings. Every client copes with the extra layer, but the
// structure would be describing a message other than the one being sent. So the
// content type is set here and the generic entity writer used directly.
func writeAlternativeBody(w io.Writer, h mail.Header, plain, htmlBody string) error {
	h.Set("Content-Type", "multipart/alternative")
	mw, err := gomessage.CreateWriter(w, h.Header)
	if err != nil {
		return fmt.Errorf("create writer: %w", err)
	}
	if err := writeAlternativeParts(mw, plain, htmlBody); err != nil {
		return err
	}
	if err := mw.Close(); err != nil {
		return fmt.Errorf("close message: %w", err)
	}
	return nil
}

// writeTextPart writes one text part of a multipart entity.
func writeTextPart(mw *gomessage.Writer, contentType, body string) error {
	var ph gomessage.Header
	ph.Set("Content-Type", contentType+"; charset=utf-8")
	ph.Set("Content-Transfer-Encoding", "quoted-printable")
	pw, err := mw.CreatePart(ph)
	if err != nil {
		return fmt.Errorf("create %s part: %w", contentType, err)
	}
	if _, err := pw.Write([]byte(body)); err != nil {
		return fmt.Errorf("write %s part: %w", contentType, err)
	}
	return pw.Close()
}

// writeAlternativeParts fills a multipart/alternative with the two renderings,
// plain first.
//
// Shared by the two places an alternative is written — as the whole message,
// and as the body part of a message that also carries attachments — so the
// ordering rule is stated once. RFC 2046 orders the parts least to most
// faithful and a client shows the last one it can render; reversing them shows
// plain text to everybody.
func writeAlternativeParts(mw *gomessage.Writer, plain, htmlBody string) error {
	if err := writeTextPart(mw, "text/plain", plain); err != nil {
		return err
	}
	return writeTextPart(mw, "text/html", htmlBody)
}

// writePlainInto writes a text/plain body as a part of an enclosing multipart
// entity, rather than as the whole message.
func writePlainInto(mw *gomessage.Writer, body string) error {
	return writeTextPart(mw, "text/plain", body)
}

// writeAlternativeInto writes a multipart/alternative body as a part of an
// enclosing multipart entity.
//
// The part's own header is what makes it an alternative: CreatePart reads the
// Content-Type it is given and hands back a writer that is itself multipart, so
// the two renderings become children of this part rather than of the message.
// Writing a fresh entity into an untyped part instead — the obvious shortcut —
// produces a text/plain part whose body happens to begin with a header block,
// which is what this looked like before it was tested.
func writeAlternativeInto(mw *gomessage.Writer, plain, htmlBody string) error {
	var ph gomessage.Header
	ph.Set("Content-Type", "multipart/alternative")
	aw, err := mw.CreatePart(ph)
	if err != nil {
		return fmt.Errorf("create alternative part: %w", err)
	}
	if err := writeAlternativeParts(aw, plain, htmlBody); err != nil {
		return err
	}
	if err := aw.Close(); err != nil {
		return fmt.Errorf("close alternative part: %w", err)
	}
	return nil
}

// writeMixedBody writes a message whose body is enclosed alongside attachments.
//
// This is where mixed is correct and where writeAlternativeBody's argument
// against it does not apply. There, mixed would have claimed several things
// were enclosed when there was one thing in two renderings; here several things
// really are enclosed, and mixed is the structure that says so. The two
// functions are opposite halves of one rule rather than a preference reversed.
//
// The body is written by a callback rather than passed as bytes, so the choice
// between plain and alternative is made in exactly one place — buildDraftMessage
// — and is the same choice whether or not anything is attached. A message with
// attachments must not quietly become HTML because it was assembled by
// different code.
//
// The callback is handed the multipart writer rather than an io.Writer, because
// a part's type lives in the part's own header: an alternative nested here is a
// part whose Content-Type says so, not a whole entity written into an untyped
// one. See writeAlternativeInto.
func writeMixedBody(
	w io.Writer,
	h mail.Header,
	writeBody func(*gomessage.Writer) error,
	attachments []draftAsset,
) error {
	h.Set("Content-Type", "multipart/mixed")
	mw, err := gomessage.CreateWriter(w, h.Header)
	if err != nil {
		return fmt.Errorf("create writer: %w", err)
	}

	// The body goes first: From, To, Subject and the threading headers stay on
	// the message, since they address the whole of it and not one of its parts.
	if err := writeBody(mw); err != nil {
		return err
	}

	for _, a := range attachments {
		var ah gomessage.Header
		// The type is the one derived from the bytes at upload; see
		// detectAssetType. What the uploading client called it never reaches
		// here.
		ah.Set("Content-Type", a.MimeType)
		ah.Set("Content-Disposition",
			fmt.Sprintf("attachment; filename=%q", sanitiseFilename(a.Filename)))
		// base64 because an attachment is arbitrary bytes: quoted-printable
		// would inflate a JPEG by roughly three times, and 8bit is not
		// guaranteed by every hop.
		ah.Set("Content-Transfer-Encoding", "base64")
		aw, err := mw.CreatePart(ah)
		if err != nil {
			return fmt.Errorf("create attachment %q: %w", a.Filename, err)
		}
		if _, err := aw.Write(a.Bytes); err != nil {
			return fmt.Errorf("write attachment %q: %w", a.Filename, err)
		}
		if err := aw.Close(); err != nil {
			return fmt.Errorf("close attachment %q: %w", a.Filename, err)
		}
	}

	if err := mw.Close(); err != nil {
		return fmt.Errorf("close message: %w", err)
	}
	return nil
}

// sendRaw hands already-built message bytes to the SMTP server.
//
// Separate from sendMail because a reply is built before it is sent — the draft
// page shows what will go out — so the bytes exist before anything is handed
// over, and building them again to send would risk sending something other than
// what was shown.
func sendRaw(creds smtpCreds, from string, to []Recipient, raw []byte) error {
	addr := fmt.Sprintf("%s:%d", creds.host, creds.port)
	auth := sasl.NewPlainClient("", creds.user, creds.pass)
	if err := gosmtp.SendMailTLS(addr, auth, from, recipientAddresses(to), bytes.NewReader(raw)); err != nil {
		return fmt.Errorf("smtp send: %w", err)
	}
	log.Printf("smtp: sent message from %s to %v", from, recipientAddresses(to))
	return nil
}

// ============================================================================
// Send via SMTP
// ============================================================================

// sendMail builds and sends an email via SMTP implicit TLS (port 465).
// contentType is passed through to buildMessage ("text/plain" or "text/html").
// unsubscribeFor, if non-empty, sets the X-Mailweb-Unsubscribe header.
// Returns the raw message bytes so the caller can append to Sent.
func sendMail(creds smtpCreds, from, to, subject, body, contentType, unsubscribeFor string) ([]byte, error) {
	raw, err := buildMessage(from, to, subject, body, contentType, unsubscribeFor)
	if err != nil {
		return nil, fmt.Errorf("build message: %w", err)
	}

	addr := fmt.Sprintf("%s:%d", creds.host, creds.port)
	log.Printf("smtp: connecting to %s ...", addr)

	auth := sasl.NewPlainClient("", creds.user, creds.pass)
	if err := gosmtp.SendMailTLS(addr, auth, from, []string{to}, bytes.NewReader(raw)); err != nil {
		return nil, fmt.Errorf("smtp send: %w", err)
	}

	log.Printf("smtp: sent message from %s to %s", from, to)
	return raw, nil
}

// ============================================================================
// Append to Sent mailbox via IMAP
// ============================================================================

// appendToSent appends raw message bytes to the IMAP Sent mailbox.
// Opens a fresh connection so it does not disturb the pool's SELECT state.
func appendToSent(creds imapCreds, raw []byte) error {
	c, err := connectIMAP(creds)
	if err != nil {
		return fmt.Errorf("imap connect: %w", err)
	}
	defer c.Logout()

	sentName, err := findSentMailbox(c)
	if err != nil {
		return err
	}

	flags := []string{imap.SeenFlag}
	if err := c.Append(sentName, flags, time.Now(), strings.NewReader(string(raw))); err != nil {
		return fmt.Errorf("imap append to %s: %w", sentName, err)
	}
	log.Printf("smtp: appended message to IMAP %s", sentName)
	return nil
}

// findSentMailbox lists all mailboxes and returns the one with the \Sent
// special-use attribute, falling back to names "Sent" or "Sent Messages".
func findSentMailbox(c *client.Client) (string, error) {
	mboxes := make(chan *imap.MailboxInfo, 20)
	done := make(chan error, 1)
	go func() { done <- c.List("", "*", mboxes) }()

	var sentByAttr, sentByName string
	for m := range mboxes {
		for _, attr := range m.Attributes {
			if strings.EqualFold(attr, `\Sent`) {
				sentByAttr = m.Name
			}
		}
		lower := strings.ToLower(m.Name)
		if lower == "sent" || lower == "sent messages" {
			sentByName = m.Name
		}
	}
	if err := <-done; err != nil {
		return "", fmt.Errorf("list mailboxes: %w", err)
	}

	if sentByAttr != "" {
		return sentByAttr, nil
	}
	if sentByName != "" {
		return sentByName, nil
	}
	// Fall back to "Sent" and let APPEND fail with a clear error.
	return "Sent", nil
}

// ============================================================================
// Synthetic mailweb action messages
// ============================================================================

// buildActionMessage constructs a synthetic RFC 5322 message recording a
// mailweb action (e.g. URL unsubscribe intent). The message is addressed
// from/to the user's own address and carries X-Mailweb-Unsubscribe so it
// appears in the contact's conversation thread.
//
// extraHeaders is an optional map of additional header name → value pairs
// (e.g. "X-Mailweb-Unsubscribe-URL" → "https://...").
func buildActionMessage(from, contact, subject, body string, extraHeaders map[string]string) ([]byte, error) {
	var buf bytes.Buffer

	var h mail.Header
	h.SetDate(time.Now())
	setMessageID(&h, from)
	h.SetAddressList("From", []*mail.Address{{Address: from}})
	h.SetAddressList("To", []*mail.Address{{Address: from}})
	h.Set("Subject", subject)
	h.Set("Content-Type", "text/plain; charset=utf-8")
	h.Set("X-Mailweb-Unsubscribe", contact)
	for k, v := range extraHeaders {
		h.Set(k, v)
	}

	wc, err := mail.CreateSingleInlineWriter(&buf, h)
	if err != nil {
		return nil, fmt.Errorf("create writer: %w", err)
	}
	if _, err := wc.Write([]byte(body)); err != nil {
		return nil, fmt.Errorf("write body: %w", err)
	}
	if err := wc.Close(); err != nil {
		return nil, fmt.Errorf("close writer: %w", err)
	}
	return buf.Bytes(), nil
}

// appendToMailweb appends raw message bytes to the mailweb synthetic folder.
// If the folder does not exist it is created first.
func appendToMailweb(creds imapCreds, folder string, raw []byte) error {
	c, err := connectIMAP(creds)
	if err != nil {
		return fmt.Errorf("imap connect: %w", err)
	}
	defer c.Logout()

	// Create folder if it doesn't exist (error silently ignored).
	if err := c.Create(folder); err != nil {
		log.Printf("imap: CREATE %q: %v (may already exist)", folder, err)
	}

	flags := []string{imap.SeenFlag}
	if err := c.Append(folder, flags, time.Now(), strings.NewReader(string(raw))); err != nil {
		return fmt.Errorf("imap append to %s: %w", folder, err)
	}
	log.Printf("mailweb: appended action message to IMAP %s", folder)
	return nil
}

// ============================================================================
// Spam report: multipart/mixed with message/rfc822 attachments
// ============================================================================

// buildSpamReport constructs a multipart/mixed email containing:
//   - a text/plain cover letter (coverNote)
//   - one message/rfc822 part per raw message in attachments
func buildSpamReport(from, to, subject, coverNote string, attachments [][]byte) ([]byte, error) {
	var buf bytes.Buffer

	var h mail.Header
	h.SetDate(time.Now())
	setMessageID(&h, from)
	h.SetAddressList("From", []*mail.Address{{Address: from}})
	h.SetAddressList("To", []*mail.Address{{Address: to}})
	h.Set("Subject", subject)

	mw, err := mail.CreateWriter(&buf, h)
	if err != nil {
		return nil, fmt.Errorf("create writer: %w", err)
	}

	// Part 1: plain-text cover letter.
	var ih mail.InlineHeader
	ih.Set("Content-Type", "text/plain; charset=utf-8")
	tw, err := mw.CreateSingleInline(ih)
	if err != nil {
		return nil, fmt.Errorf("create inline part: %w", err)
	}
	if _, err := tw.Write([]byte(coverNote)); err != nil {
		return nil, fmt.Errorf("write cover note: %w", err)
	}
	if err := tw.Close(); err != nil {
		return nil, fmt.Errorf("close cover note: %w", err)
	}

	// Parts 2+: one .eml attachment per message. message/rfc822 is what an
	// attached mail is, and is what the recipient's tooling reads to recover
	// the original headers.
	for i, raw := range attachments {
		var ah gomessage.Header
		ah.Set("Content-Type", "message/rfc822")
		ah.Set("Content-Disposition", fmt.Sprintf(`attachment; filename="spam-%d.eml"`, i+1))
		ah.Set("Content-Transfer-Encoding", "base64")
		aw, err := mw.CreateAttachment(mail.AttachmentHeader{Header: ah})
		if err != nil {
			return nil, fmt.Errorf("create attachment %d: %w", i+1, err)
		}
		if _, err := aw.Write(raw); err != nil {
			return nil, fmt.Errorf("write attachment %d: %w", i+1, err)
		}
		if err := aw.Close(); err != nil {
			return nil, fmt.Errorf("close attachment %d: %w", i+1, err)
		}
	}

	if err := mw.Close(); err != nil {
		return nil, fmt.Errorf("close message: %w", err)
	}
	return buf.Bytes(), nil
}