// Package mailsync reads Gmail messages for a project's client domain using a
// Google Workspace service account with DOMAIN-WIDE DELEGATION — stdlib only, no
// third-party SDK (mirrors internal/push for the JWT/OAuth dance).
//
// It is OPTIONAL: with no credentials configured the Service is "disabled" and
// list calls return empty. This lets spin run without Google until the Workspace
// admin provisions a service account + domain-wide delegation (gmail.readonly).
//
// For a project the handler passes the set of member emails to impersonate; for
// each we mint a delegated token (sub=member) and search that mailbox for the
// client domain, then aggregate + dedup by RFC822 Message-ID across the team.
package mailsync

import (
	"context"
	"crypto"
	"crypto/rsa"
	"crypto/sha256"
	"crypto/x509"
	"encoding/base64"
	"encoding/json"
	"encoding/pem"
	"errors"
	"fmt"
	"log"
	"net/http"
	"net/url"
	"os"
	"sort"
	"strings"
	"sync"
	"time"
)

const gmailScope = "https://www.googleapis.com/auth/gmail.readonly"

type serviceAccount struct {
	ProjectID   string `json:"project_id"`
	PrivateKey  string `json:"private_key"`
	ClientEmail string `json:"client_email"`
	TokenURI    string `json:"token_uri"`
}

// Message is a slim mail summary surfaced to the project mail tab.
type Message struct {
	ID       string `json:"id"`       // RFC822 Message-ID (stable dedup key)
	ThreadID string `json:"threadId"`
	From     string `json:"from"`
	To       string `json:"to"`
	Cc       string `json:"cc"`
	Subject  string `json:"subject"`
	Date     string `json:"date"`    // raw Date header
	Snippet  string `json:"snippet"`
	Mailbox  string `json:"mailbox"` // whose mailbox surfaced it
	TS       int64  `json:"ts"`      // internalDate (ms) for sorting/formatting
}

// Involves reports whether the given email appears in From/To/Cc (case-insensitive).
// Used to gate per-viewer visibility: a member only sees mail they're a party to.
func (m Message) Involves(email string) bool {
	if email == "" {
		return false
	}
	hay := strings.ToLower(m.From + " " + m.To + " " + m.Cc)
	return strings.Contains(hay, strings.ToLower(email))
}

type tokenEntry struct {
	token   string
	expires time.Time
}

// Service holds the delegated service-account credentials and a per-subject
// (impersonated user) access-token cache.
type Service struct {
	sa     *serviceAccount
	key    *rsa.PrivateKey
	client *http.Client
	mu     sync.Mutex
	tokens map[string]tokenEntry
}

// New builds a Service from a service-account JSON file path. Empty path or a
// read/parse error yields a disabled Service.
func New(credsPath string) *Service {
	if strings.TrimSpace(credsPath) == "" {
		log.Printf("mailsync: Gmail disabled (no credentials configured)")
		return &Service{}
	}
	raw, err := os.ReadFile(credsPath)
	if err != nil {
		log.Printf("mailsync: Gmail disabled (cannot read %s: %v)", credsPath, err)
		return &Service{}
	}
	var sa serviceAccount
	if err := json.Unmarshal(raw, &sa); err != nil || sa.PrivateKey == "" || sa.ClientEmail == "" {
		log.Printf("mailsync: Gmail disabled (invalid service account JSON)")
		return &Service{}
	}
	key, err := parsePrivateKey(sa.PrivateKey)
	if err != nil {
		log.Printf("mailsync: Gmail disabled (bad private key: %v)", err)
		return &Service{}
	}
	if sa.TokenURI == "" {
		sa.TokenURI = "https://oauth2.googleapis.com/token"
	}
	log.Printf("mailsync: Gmail enabled (sa=%s)", sa.ClientEmail)
	return &Service{sa: &sa, key: key, client: &http.Client{Timeout: 15 * time.Second}, tokens: map[string]tokenEntry{}}
}

// Enabled reports whether real Gmail access is configured.
func (s *Service) Enabled() bool { return s != nil && s.sa != nil }

// ListForDomain impersonates each mailbox in turn, searches it for the client
// domain, and returns the aggregated, de-duplicated, newest-first list (<= max).
func (s *Service) ListForDomain(ctx context.Context, mailboxes []string, domain string, max int) ([]Message, error) {
	if !s.Enabled() {
		return nil, nil
	}
	domain = strings.TrimSpace(strings.TrimPrefix(domain, "@"))
	if domain == "" {
		return nil, nil
	}
	perBox := 25
	q := fmt.Sprintf("(from:%s OR to:%s OR cc:%s) -in:chats", domain, domain, domain)
	seen := map[string]bool{}
	var all []Message
	var firstErr error
	for _, box := range mailboxes {
		box = strings.TrimSpace(box)
		if box == "" {
			continue
		}
		ids, err := s.listIDs(ctx, box, q, perBox)
		if err != nil {
			if firstErr == nil {
				firstErr = err
			}
			continue
		}
		for _, id := range ids {
			m, err := s.getMeta(ctx, box, id)
			if err != nil {
				continue
			}
			key := m.ID
			if key == "" {
				key = box + "/" + id
			}
			if seen[key] {
				continue
			}
			seen[key] = true
			all = append(all, m)
		}
	}
	sort.Slice(all, func(i, j int) bool { return all[i].TS > all[j].TS })
	if max > 0 && len(all) > max {
		all = all[:max]
	}
	return all, firstErr
}

func (s *Service) listIDs(ctx context.Context, subject, q string, max int) ([]string, error) {
	tok, err := s.accessToken(ctx, subject)
	if err != nil {
		return nil, err
	}
	u := fmt.Sprintf("https://gmail.googleapis.com/gmail/v1/users/me/messages?maxResults=%d&q=%s", max, url.QueryEscape(q))
	req, _ := http.NewRequestWithContext(ctx, http.MethodGet, u, nil)
	req.Header.Set("Authorization", "Bearer "+tok)
	resp, err := s.client.Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	if resp.StatusCode >= 300 {
		return nil, fmt.Errorf("gmail list %d for %s", resp.StatusCode, subject)
	}
	var out struct {
		Messages []struct {
			ID string `json:"id"`
		} `json:"messages"`
	}
	if err := json.NewDecoder(resp.Body).Decode(&out); err != nil {
		return nil, err
	}
	ids := make([]string, 0, len(out.Messages))
	for _, m := range out.Messages {
		ids = append(ids, m.ID)
	}
	return ids, nil
}

func (s *Service) getMeta(ctx context.Context, subject, id string) (Message, error) {
	tok, err := s.accessToken(ctx, subject)
	if err != nil {
		return Message{}, err
	}
	u := fmt.Sprintf("https://gmail.googleapis.com/gmail/v1/users/me/messages/%s?format=metadata&metadataHeaders=From&metadataHeaders=To&metadataHeaders=Cc&metadataHeaders=Subject&metadataHeaders=Date&metadataHeaders=Message-ID", id)
	req, _ := http.NewRequestWithContext(ctx, http.MethodGet, u, nil)
	req.Header.Set("Authorization", "Bearer "+tok)
	resp, err := s.client.Do(req)
	if err != nil {
		return Message{}, err
	}
	defer resp.Body.Close()
	if resp.StatusCode >= 300 {
		return Message{}, fmt.Errorf("gmail get %d", resp.StatusCode)
	}
	var out struct {
		ID           string `json:"id"`
		ThreadID     string `json:"threadId"`
		Snippet      string `json:"snippet"`
		InternalDate string `json:"internalDate"`
		Payload      struct {
			Headers []struct {
				Name  string `json:"name"`
				Value string `json:"value"`
			} `json:"headers"`
		} `json:"payload"`
	}
	if err := json.NewDecoder(resp.Body).Decode(&out); err != nil {
		return Message{}, err
	}
	m := Message{ThreadID: out.ThreadID, Snippet: out.Snippet, Mailbox: subject}
	for _, h := range out.Payload.Headers {
		switch strings.ToLower(h.Name) {
		case "from":
			m.From = h.Value
		case "to":
			m.To = h.Value
		case "cc":
			m.Cc = h.Value
		case "subject":
			m.Subject = h.Value
		case "date":
			m.Date = h.Value
		case "message-id":
			m.ID = h.Value
		}
	}
	if m.ID == "" {
		m.ID = subject + "/" + out.ID
	}
	fmt.Sscan(out.InternalDate, &m.TS)
	return m, nil
}

// accessToken returns a cached delegated token for the given subject (impersonated
// user), minting a new one via the service-account JWT grant when expired.
func (s *Service) accessToken(ctx context.Context, subject string) (string, error) {
	s.mu.Lock()
	defer s.mu.Unlock()
	if e, ok := s.tokens[subject]; ok && e.token != "" && time.Now().Before(e.expires.Add(-60*time.Second)) {
		return e.token, nil
	}
	now := time.Now()
	claims := map[string]any{
		"iss":   s.sa.ClientEmail,
		"sub":   subject, // domain-wide delegation: act as this Workspace user
		"scope": gmailScope,
		"aud":   s.sa.TokenURI,
		"iat":   now.Unix(),
		"exp":   now.Add(time.Hour).Unix(),
	}
	assertion, err := s.signJWT(claims)
	if err != nil {
		return "", err
	}
	form := url.Values{}
	form.Set("grant_type", "urn:ietf:params:oauth:grant-type:jwt-bearer")
	form.Set("assertion", assertion)
	req, _ := http.NewRequestWithContext(ctx, http.MethodPost, s.sa.TokenURI, strings.NewReader(form.Encode()))
	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
	resp, err := s.client.Do(req)
	if err != nil {
		return "", err
	}
	defer resp.Body.Close()
	var out struct {
		AccessToken string `json:"access_token"`
		ExpiresIn   int    `json:"expires_in"`
		Error       string `json:"error"`
		ErrorDesc   string `json:"error_description"`
	}
	if err := json.NewDecoder(resp.Body).Decode(&out); err != nil {
		return "", err
	}
	if out.AccessToken == "" {
		return "", fmt.Errorf("token exchange failed for %s: %s %s", subject, out.Error, out.ErrorDesc)
	}
	s.tokens[subject] = tokenEntry{token: out.AccessToken, expires: now.Add(time.Duration(out.ExpiresIn) * time.Second)}
	return out.AccessToken, nil
}

func (s *Service) signJWT(claims map[string]any) (string, error) {
	header := map[string]string{"alg": "RS256", "typ": "JWT"}
	hb, _ := json.Marshal(header)
	cb, _ := json.Marshal(claims)
	signingInput := b64(hb) + "." + b64(cb)
	h := sha256.Sum256([]byte(signingInput))
	sig, err := rsa.SignPKCS1v15(nil, s.key, crypto.SHA256, h[:])
	if err != nil {
		return "", err
	}
	return signingInput + "." + base64.RawURLEncoding.EncodeToString(sig), nil
}

func b64(b []byte) string { return base64.RawURLEncoding.EncodeToString(b) }

func parsePrivateKey(pemStr string) (*rsa.PrivateKey, error) {
	block, _ := pem.Decode([]byte(pemStr))
	if block == nil {
		return nil, errors.New("no PEM block")
	}
	if k, err := x509.ParsePKCS1PrivateKey(block.Bytes); err == nil {
		return k, nil
	}
	keyAny, err := x509.ParsePKCS8PrivateKey(block.Bytes)
	if err != nil {
		return nil, err
	}
	k, ok := keyAny.(*rsa.PrivateKey)
	if !ok {
		return nil, errors.New("not an RSA private key")
	}
	return k, nil
}