package mailstore

import (
	"bytes"
	"crypto/rand"
	"encoding/hex"
	"errors"
	"fmt"
	"net/mail"
	"os"
	"path/filepath"
	"strings"
	"time"

	"mailgoserver/internal/mailview"
)

// extractHeaderValue reads a single header out of raw without parsing the body — used
// to compute StoreMessage's cached_to column cheaply (no MIME/multipart walk needed
// just to cache a header for fast folder-listing display). Returns "" on any parse
// failure or if the header is absent, never an error — this is a display convenience,
// not something delivery should ever fail over.
func extractHeaderValue(raw []byte, name string) string {
	msg, err := mail.ReadMessage(bytes.NewReader(raw))
	if err != nil {
		return ""
	}
	return msg.Header.Get(name)
}

const previewSnippetLen = 150

// previewSnippet extracts up to previewSnippetLen characters of the plain-text body
// for the folder list's preview line — cached in plain text alongside cached_from/
// cached_subject (see schema.go's comment on cached_preview for why that's consistent
// with the existing cached_* columns, not a new exposure). HTML-only mail (no
// text/plain part) gets no preview rather than a crude tag-stripped approximation —
// an accepted scope limit, not a bug: most real mail includes a text/plain
// alternative regardless of whether the sender expects it to be shown.
func previewSnippet(raw []byte) string {
	parsed, err := mailview.Parse(raw)
	if err != nil {
		return ""
	}
	text := strings.Join(strings.Fields(parsed.TextBody), " ")
	// Rune-safe truncation — a plain byte slice could split a multi-byte UTF-8
	// character in half and produce invalid text.
	if runes := []rune(text); len(runes) > previewSnippetLen {
		text = string(runes[:previewSnippetLen])
	}
	return text
}

// ErrQuotaExceeded is returned by StoreMessage when storing raw would push the
// mailbox over its quota. No row, file, or used_bytes change occurs in that case.
var ErrQuotaExceeded = errors.New("mailstore: mailbox quota exceeded")

// StoreMessage encrypts raw with the mailbox's own data encryption key and persists
// it to disk, then indexes it in esrv_mailbox_messages and updates the mailbox's
// cached used_bytes. from/subject are cached in the DB in plain text by design (see
// schema.go) so IMAP LIST/basic SEARCH don't need to decrypt every message.
func (s *Store) StoreMessage(mailboxID int64, folder string, raw []byte, messageIDHeader, from, subject string) (uid int64, err error) {
	mbox, err := s.DB.GetMailboxByID(mailboxID)
	if err != nil {
		return 0, err
	}
	if mbox == nil {
		return 0, fmt.Errorf("mailstore: mailbox %d not found", mailboxID)
	}
	if mbox.UsedBytes+int64(len(raw)) > mbox.QuotaBytes {
		return 0, ErrQuotaExceeded
	}

	dek, err := s.UnwrapDEK(mbox.DEKWrapped, mbox.DEKNonce)
	if err != nil {
		return 0, err
	}
	ciphertext, nonce, err := sealAESGCM(dek, raw)
	if err != nil {
		return 0, err
	}

	now := time.Now()
	dir := filepath.Join(s.BasePath, sanitizePathSegment(mbox.Email), folder, now.Format("2006-02-Jan"))
	if err := os.MkdirAll(dir, 0o755); err != nil {
		return 0, err
	}
	name := make([]byte, 8)
	rand.Read(name)
	storagePath := filepath.Join(dir, hex.EncodeToString(name)+".eml.enc")
	if err := os.WriteFile(storagePath, ciphertext, 0o600); err != nil {
		return 0, err
	}

	uid, err = s.DB.InsertMessage(mailboxID, folder, messageIDHeader, "", now, int64(len(raw)), storagePath, nonce, from, extractHeaderValue(raw, "To"), subject, previewSnippet(raw))
	if err != nil {
		os.Remove(storagePath)
		return 0, err
	}
	if err := s.DB.AddMailboxUsedBytes(mailboxID, int64(len(raw))); err != nil {
		return 0, err
	}
	return uid, nil
}

// RebuildMessageCache re-derives cached_from/cached_to/cached_subject/cached_preview
// for every message already stored in mailboxID, from each message's own decrypted
// content — these fields are otherwise only ever computed once, at delivery time
// (see StoreMessage), so mail stored before a caching fix or addition landed (e.g.
// caching the From: header's display name instead of the bare envelope address, or
// the cached_preview column itself) keeps showing the old/blank value forever unless
// something re-derives it. Returns how many rows actually changed; a message that
// fails to decrypt/parse is skipped (counted in the error map, not fatal to the rest).
func (s *Store) RebuildMessageCache(mailboxID int64) (updated int, skipped map[int64]error) {
	skipped = map[int64]error{}
	msgs, err := s.DB.ListMessagesForMailbox(mailboxID)
	if err != nil {
		skipped[0] = err
		return 0, skipped
	}
	for _, m := range msgs {
		raw, err := s.FetchMessage(mailboxID, m.ID)
		if err != nil {
			skipped[m.ID] = err
			continue
		}
		from := extractHeaderValue(raw, "From")
		if from == "" {
			from = m.CachedFrom
		}
		to := extractHeaderValue(raw, "To")
		if to == "" {
			to = m.CachedTo
		}
		subject := extractHeaderValue(raw, "Subject")
		if subject == "" {
			subject = m.CachedSubject
		}
		preview := previewSnippet(raw)
		if from == m.CachedFrom && to == m.CachedTo && subject == m.CachedSubject && preview == m.CachedPreview {
			continue // already correct — don't churn a write for nothing
		}
		if err := s.DB.UpdateMessageCachedFields(m.ID, from, to, subject, preview); err != nil {
			skipped[m.ID] = err
			continue
		}
		updated++
	}
	return updated, skipped
}

// FetchMessage decrypts a stored message on demand. Plaintext is never written to disk
// or cached — only returned to the caller.
func (s *Store) FetchMessage(mailboxID, uid int64) ([]byte, error) {
	msg, err := s.DB.GetMessageByUID(mailboxID, uid)
	if err != nil {
		return nil, err
	}
	if msg == nil {
		return nil, fmt.Errorf("mailstore: message %d not found in mailbox %d", uid, mailboxID)
	}
	mbox, err := s.DB.GetMailboxByID(mailboxID)
	if err != nil {
		return nil, err
	}
	if mbox == nil {
		return nil, fmt.Errorf("mailstore: mailbox %d not found", mailboxID)
	}
	dek, err := s.UnwrapDEK(mbox.DEKWrapped, mbox.DEKNonce)
	if err != nil {
		return nil, err
	}
	ciphertext, err := os.ReadFile(msg.StoragePath)
	if err != nil {
		return nil, err
	}
	return openAESGCM(dek, ciphertext, msg.Nonce)
}

// DeleteMessage removes the on-disk ciphertext, the index row, and frees the quota.
func (s *Store) DeleteMessage(mailboxID, uid int64) error {
	msg, err := s.DB.GetMessageByUID(mailboxID, uid)
	if err != nil {
		return err
	}
	if msg == nil {
		return nil
	}
	if err := os.Remove(msg.StoragePath); err != nil && !os.IsNotExist(err) {
		return err
	}
	if err := s.DB.DeleteMessage(mailboxID, uid); err != nil {
		return err
	}
	return s.DB.AddMailboxUsedBytes(mailboxID, -msg.SizeBytes)
}