package service

import (
	"context"
	"errors"
	"log/slog"
	"strings"
	"time"

	"gitea.stevedudenhoeffer.com/steve/pansy/internal/domain"
)

// maxPasswordLen caps accepted password length. It bounds argon2 input and
// request work, and sits far above any real password.
const maxPasswordLen = 1024

// RegisterInput is the payload for local self-service signup.
type RegisterInput struct {
	Email       string
	DisplayName string
	Password    string
}

// Providers reports which login methods the server offers, so the login page
// (#6) can render the right controls. OIDCLabel is only meaningful when OIDC is
// true.
type Providers struct {
	Local     bool   `json:"local"`
	OIDC      bool   `json:"oidc"`
	OIDCLabel string `json:"oidcLabel"`
}

// OIDCIdentity is the set of claims the API layer extracts from a verified ID
// token and hands to LoginOIDC for provisioning. Issuer and Subject come from
// the verified token (not raw claims); Email/Name come from claims.
type OIDCIdentity struct {
	Issuer        string
	Subject       string
	Email         string
	EmailVerified bool
	Name          string
}

// Providers returns the enabled auth methods, so the login page renders the
// right controls. OIDC is reported only when it can actually be offered (issuer,
// client ID, and a BaseURL for the redirect URI), matching the routes that
// api.New registers.
func (s *Service) Providers() Providers {
	return Providers{
		Local:     s.cfg.LocalAuth,
		OIDC:      s.cfg.OIDCReady(),
		OIDCLabel: s.cfg.OIDC.ButtonLabel,
	}
}

// LoginOIDC provisions and returns the pansy user for a verified OIDC identity,
// issuing no session itself (the caller does). The IdP has already gated access,
// so PANSY_REGISTRATION does not apply. Resolution order:
//  1. an existing user with the same (issuer, subject) — a returning OIDC user;
//  2. else an existing user with the same *verified* email — linked to this
//     identity (so one person isn't split across a local and an OIDC account);
//  3. else a new just-in-time account stamped with the identity.
//
// An unverified email that collides with an existing account is refused (it
// would let anyone who can assert that email at the IdP take over the account).
// An identity with no email can't be provisioned (email is the account key).
func (s *Service) LoginOIDC(ctx context.Context, id OIDCIdentity) (*domain.User, error) {
	if id.Issuer == "" || id.Subject == "" {
		return nil, domain.ErrInvalidInput
	}

	// 1. Returning OIDC user.
	u, err := s.store.GetUserByOIDC(ctx, id.Issuer, id.Subject)
	if err == nil {
		return u, nil
	}
	if !errors.Is(err, domain.ErrNotFound) {
		return nil, err
	}

	email := normalizeEmail(id.Email)
	if email == "" {
		return nil, domain.ErrOIDCNoEmail
	}

	// 2. Link to an existing account by email — but only a verified one.
	existing, err := s.store.GetUserByEmail(ctx, email)
	switch {
	case err == nil:
		if !id.EmailVerified {
			return nil, domain.ErrOIDCEmailUnverified
		}
		return s.store.LinkOIDC(ctx, existing.ID, id.Issuer, id.Subject)
	case !errors.Is(err, domain.ErrNotFound):
		return nil, err
	}

	// 3. Just-in-time provisioning.
	name := strings.TrimSpace(id.Name)
	if name == "" {
		name = emailLocalPart(email)
	}
	return s.store.CreateUser(ctx, &domain.User{
		Email:       email,
		DisplayName: name,
		OIDCIssuer:  &id.Issuer,
		OIDCSubject: &id.Subject,
	}, true) // allowSignup: the IdP gates access, so registration policy is bypassed.
}

// Register creates a local (password) account and returns it. The first user on
// a fresh instance becomes admin and may always register (bootstrap), even when
// PANSY_REGISTRATION=closed; afterward, closed registration is enforced.
func (s *Service) Register(ctx context.Context, in RegisterInput) (*domain.User, error) {
	if !s.cfg.LocalAuth {
		return nil, domain.ErrLocalAuthDisabled
	}

	email := normalizeEmail(in.Email)
	displayName := strings.TrimSpace(in.DisplayName)
	if email == "" || displayName == "" || in.Password == "" || len(in.Password) > maxPasswordLen {
		return nil, domain.ErrInvalidInput
	}

	// Cheap best-effort gate so a closed instance doesn't burn argon2 work on
	// signups it will reject anyway. The authoritative, race-free gate — plus
	// atomic first-user-is-admin assignment and duplicate-email detection — lives
	// in store.CreateUser's single INSERT.
	if !s.cfg.RegistrationOpen() {
		n, err := s.store.CountUsers(ctx)
		if err != nil {
			return nil, err
		}
		if n > 0 {
			return nil, domain.ErrRegistrationClosed
		}
	}

	hash, err := hashPassword(in.Password)
	if err != nil {
		return nil, err
	}

	return s.store.CreateUser(ctx, &domain.User{
		Email:        email,
		DisplayName:  displayName,
		PasswordHash: &hash,
	}, s.cfg.RegistrationOpen())
}

// Login verifies an email/password pair and returns the user. Unknown-email and
// wrong-password both return domain.ErrInvalidCredentials, and both spend the
// same argon2 work, so neither the error nor the timing reveals which failed.
func (s *Service) Login(ctx context.Context, email, password string) (*domain.User, error) {
	if !s.cfg.LocalAuth {
		return nil, domain.ErrLocalAuthDisabled
	}
	if len(password) > maxPasswordLen {
		// No stored password is this long; reject without spending argon2 work.
		return nil, domain.ErrInvalidCredentials
	}

	u, err := s.store.GetUserByEmail(ctx, normalizeEmail(email))
	switch {
	case errors.Is(err, domain.ErrNotFound):
		// Spend comparable time so timing can't distinguish a missing account.
		_, _ = verifyPassword(s.dummyHash, password)
		return nil, domain.ErrInvalidCredentials
	case err != nil:
		return nil, err
	case u.PasswordHash == nil:
		// OIDC-only account: no local password to check, but equalize timing.
		_, _ = verifyPassword(s.dummyHash, password)
		return nil, domain.ErrInvalidCredentials
	}

	ok, err := verifyPassword(*u.PasswordHash, password)
	if err != nil {
		// A stored hash we can't parse is a data problem, not a wrong password;
		// surface it so a corrupt row doesn't silently lock a user out unnoticed.
		slog.Error("service: malformed stored password hash", "user_id", u.ID, "error", err)
		return nil, domain.ErrInvalidCredentials
	}
	if !ok {
		return nil, domain.ErrInvalidCredentials
	}
	return u, nil
}

// CreateSession issues a new session for a user and returns the raw bearer token
// (to place in the cookie) and its expiry.
func (s *Service) CreateSession(ctx context.Context, userID int64) (token string, expiresAt time.Time, err error) {
	raw, err := newSessionToken()
	if err != nil {
		return "", time.Time{}, err
	}
	exp := s.now().Add(sessionTTL)
	if err := s.store.CreateSession(ctx, &domain.Session{
		TokenHash: hashToken(raw),
		UserID:    userID,
		ExpiresAt: formatTime(exp),
	}); err != nil {
		return "", time.Time{}, err
	}
	return raw, exp, nil
}

// ResolveSession validates a raw bearer token and returns its user and the
// session's current expiry (so the caller can slide the client cookie in
// lockstep with the server). An expired session is deleted and treated as absent
// (domain.ErrNotFound). A still-valid session has its expiry slid forward, but
// only when that moves it by more than an hour, so a busy client doesn't write
// on every request.
func (s *Service) ResolveSession(ctx context.Context, rawToken string) (*domain.User, time.Time, error) {
	if rawToken == "" {
		return nil, time.Time{}, domain.ErrNotFound
	}
	hash := hashToken(rawToken)
	sess, err := s.store.GetSession(ctx, hash)
	if err != nil {
		return nil, time.Time{}, err
	}

	exp, err := parseTime(sess.ExpiresAt)
	if err != nil {
		// A corrupt expiry means we can't trust the session; drop it.
		if delErr := s.store.DeleteSession(ctx, hash); delErr != nil {
			slog.Warn("service: deleting session with corrupt expiry", "error", delErr)
		}
		return nil, time.Time{}, domain.ErrNotFound
	}

	now := s.now()
	if !now.Before(exp) {
		if delErr := s.store.DeleteSession(ctx, hash); delErr != nil {
			slog.Warn("service: deleting expired session", "error", delErr)
		}
		return nil, time.Time{}, domain.ErrNotFound
	}

	if newExp := now.Add(sessionTTL); newExp.Sub(exp) > time.Hour {
		if err := s.store.TouchSession(ctx, hash, formatTime(newExp)); err != nil {
			// Non-fatal: the session is still valid at its current expiry.
			slog.Warn("service: sliding session expiry", "error", err)
		} else {
			exp = newExp
		}
	}

	user, err := s.store.GetUserByID(ctx, sess.UserID)
	if err != nil {
		return nil, time.Time{}, err
	}
	return user, exp, nil
}

// Logout deletes the session behind a raw bearer token. It is idempotent.
func (s *Service) Logout(ctx context.Context, rawToken string) error {
	if rawToken == "" {
		return nil
	}
	return s.store.DeleteSession(ctx, hashToken(rawToken))
}

// CleanupExpiredSessions deletes all sessions that have passed their expiry and
// returns the count. Called periodically; expired sessions are also dropped
// lazily on access by ResolveSession.
func (s *Service) CleanupExpiredSessions(ctx context.Context) (int64, error) {
	return s.store.DeleteExpiredSessions(ctx, formatTime(s.now()))
}

// normalizeEmail trims and lowercases an email for consistent storage and
// lookup. (The users.email column is also NOCASE, so lookups are robust either
// way; normalizing keeps stored values tidy.)
func normalizeEmail(email string) string {
	return strings.ToLower(strings.TrimSpace(email))
}

// emailLocalPart returns the portion of an email before '@', used as a fallback
// display name for JIT-provisioned OIDC users whose token carried no name.
func emailLocalPart(email string) string {
	if i := strings.IndexByte(email, '@'); i > 0 {
		return email[:i]
	}
	return email
}