package mail

import (
	"fmt"
	"io"
	"sort"
	"strings"
	"time"

	"github.com/emersion/go-imap"
	"github.com/emersion/go-imap/client"
)

type IMAPConfig struct {
	Host     string
	Port     int
	Security string // tls | starttls
	Username string
	Password string
}

type Client struct {
	c *client.Client
}

func Dial(cfg IMAPConfig) (*Client, error) {
	addr := fmt.Sprintf("%s:%d", cfg.Host, cfg.Port)
	var (
		ic  *client.Client
		err error
	)
	switch cfg.Security {
	case "tls":
		ic, err = client.DialTLS(addr, nil)
	case "starttls":
		ic, err = client.Dial(addr)
		if err == nil {
			err = ic.StartTLS(nil)
		}
	default:
		return nil, fmt.Errorf("unknown imap security %q", cfg.Security)
	}
	if err != nil {
		return nil, err
	}
	if err := ic.Login(cfg.Username, cfg.Password); err != nil {
		ic.Logout()
		return nil, err
	}
	return &Client{c: ic}, nil
}

func (c *Client) Logout() error { return c.c.Logout() }

func (c *Client) SelectFolder(folder string) (uint32, uint32, error) {
	mbox, err := c.c.Select(folder, true) // read-only select
	if err != nil {
		return 0, 0, err
	}
	var maxUID uint32
	if mbox.Messages > 0 {
		// UIDNext-1 is an upper bound for the highest existing UID.
		if mbox.UidNext > 0 {
			maxUID = mbox.UidNext - 1
		}
	}
	return mbox.UidValidity, maxUID, nil
}

// hasAttachment reports whether a BODYSTRUCTURE tree contains a part marked
// Content-Disposition: attachment — the same notion of "attachment" that
// ParseMessage uses (go-message routes attachment-disposition parts to
// AttachmentHeader). This lets list/search report has_attachments without
// downloading any body.
func hasAttachment(bs *imap.BodyStructure) bool {
	if bs == nil {
		return false
	}
	if strings.EqualFold(bs.Disposition, "attachment") {
		return true
	}
	for _, p := range bs.Parts {
		if hasAttachment(p) {
			return true
		}
	}
	return false
}

// fetchHeadersByUIDSet fetches only message headers (BODY.PEEK[HEADER]) plus the
// BODYSTRUCTURE, never the body, so listing is cheap on large mailboxes.
func (c *Client) fetchHeadersByUIDSet(folder string, set *imap.SeqSet) ([]Header, error) {
	if _, err := c.c.Select(folder, true); err != nil {
		return nil, err
	}
	section := &imap.BodySectionName{
		BodyPartName: imap.BodyPartName{Specifier: imap.HeaderSpecifier},
		Peek:         true, // BODY.PEEK[HEADER] — headers only, no \Seen side-effect
	}
	items := []imap.FetchItem{imap.FetchUid, imap.FetchBodyStructure, section.FetchItem()}

	msgCh := make(chan *imap.Message, 16)
	done := make(chan error, 1)
	go func() { done <- c.c.UidFetch(set, items, msgCh) }()

	var out []Header
	for m := range msgCh {
		r := m.GetBody(section)
		if r == nil {
			continue
		}
		raw, err := io.ReadAll(r)
		if err != nil {
			return nil, fmt.Errorf("uid %d: read header: %w", m.Uid, err)
		}
		h, err := ParseHeaderBytes(m.Uid, raw)
		if err != nil {
			return nil, err
		}
		h.HasAttachments = hasAttachment(m.BodyStructure)
		out = append(out, h)
	}
	if err := <-done; err != nil {
		return nil, err
	}
	sort.Slice(out, func(i, j int) bool { return out[i].UID > out[j].UID })
	return out, nil
}

// fetchFullByUID fetches one message in full (body + attachments).
func (c *Client) fetchFullByUID(folder string, uid uint32) (Message, error) {
	if _, err := c.c.Select(folder, true); err != nil {
		return Message{}, err
	}
	section := &imap.BodySectionName{Peek: true} // entire message, no \Seen side-effect
	items := []imap.FetchItem{imap.FetchUid, section.FetchItem()}

	set := new(imap.SeqSet)
	set.AddNum(uid)
	msgCh := make(chan *imap.Message, 1)
	done := make(chan error, 1)
	go func() { done <- c.c.UidFetch(set, items, msgCh) }()

	var (
		msg   Message
		found bool
	)
	for m := range msgCh {
		r := m.GetBody(section)
		if r == nil {
			continue
		}
		raw, err := io.ReadAll(r)
		if err != nil {
			return Message{}, fmt.Errorf("uid %d: read body: %w", m.Uid, err)
		}
		parsed, err := ParseMessage(m.Uid, raw)
		if err != nil {
			return Message{}, err
		}
		msg, found = parsed, true
	}
	if err := <-done; err != nil {
		return Message{}, err
	}
	if !found {
		return Message{}, fmt.Errorf("uid %d not found in %s", uid, folder)
	}
	return msg, nil
}

func (c *Client) FetchHeaders(folder string, uids []uint32) ([]Header, error) {
	set := new(imap.SeqSet)
	if len(uids) == 0 {
		set.AddRange(1, 0) // 1:* — all
	} else {
		for _, u := range uids {
			set.AddNum(u)
		}
	}
	return c.fetchHeadersByUIDSet(folder, set)
}

func (c *Client) FetchHeadersRange(folder string, sinceUID, beforeUID uint32, limit int) ([]Header, error) {
	set := new(imap.SeqSet)
	lo := sinceUID + 1
	if sinceUID == 0 {
		lo = 1
	}
	hi := uint32(0) // '*'
	if beforeUID > 1 {
		hi = beforeUID - 1
	}
	set.AddRange(lo, hi)
	h, err := c.fetchHeadersByUIDSet(folder, set)
	if err != nil {
		return nil, err
	}
	if limit > 0 && len(h) > limit {
		h = h[:limit]
	}
	return h, nil
}

func (c *Client) FetchFull(folder string, uid uint32) (Message, error) {
	return c.fetchFullByUID(folder, uid)
}

type SearchCriteria struct {
	From            string
	SubjectContains string
	Text            string
	Since           time.Time
	Before          time.Time
}

func (c *Client) Search(folder string, sc SearchCriteria, limit int) ([]Header, error) {
	if _, err := c.c.Select(folder, true); err != nil {
		return nil, err
	}
	crit := imap.NewSearchCriteria()
	if sc.From != "" {
		crit.Header.Add("From", sc.From)
	}
	if sc.SubjectContains != "" {
		crit.Header.Add("Subject", sc.SubjectContains)
	}
	if sc.Text != "" {
		crit.Text = []string{sc.Text}
	}
	if !sc.Since.IsZero() {
		crit.Since = sc.Since
	}
	if !sc.Before.IsZero() {
		crit.Before = sc.Before
	}
	uids, err := c.c.UidSearch(crit)
	if err != nil {
		return nil, err
	}
	if len(uids) == 0 {
		return nil, nil
	}
	if limit > 0 && len(uids) > limit {
		uids = uids[len(uids)-limit:] // keep highest (most recent) UIDs
	}
	return c.FetchHeaders(folder, uids)
}