package api

import (
	"context"
	"database/sql"
	"errors"
	"log"
	"net/http"
	"net/url"
	"strconv"
	"strings"
	"time"

	"git.ryuvia.com/niklas/terdut-server/internal/config"
	"git.ryuvia.com/niklas/terdut-server/internal/models"
	"git.ryuvia.com/niklas/terdut-server/internal/oidc"
)

const (
	// oidcStateCookie ties an in-flight login to the browser that started it.
	// Without it anybody could start a login, and send the callback URL that
	// results to somebody else, who would be signed in as the attacker.
	oidcStateCookie = "terdut_oidc_state"

	// oidcLoginTTL is how long a login may take between the redirect to the
	// provider and the callback, which includes the person typing a password
	// and a second factor.
	oidcLoginTTL = 10 * time.Minute

	// oidcStartMaxPerAddr bounds unauthenticated logins started per address.
	// Each writes a row, so an unbounded endpoint is a way to grow the table.
	oidcStartMaxPerAddr = 30
)

// ssoError is a sign-in refusal the person can be told about. Its value is the
// code the web UI is sent back with, as ?sso_error=<code>; the detail stays in
// the server log, since it can name accounts.
type ssoError string

func (e ssoError) Error() string { return "sso: " + string(e) }

const (
	ssoDenied        ssoError = "denied"         // the provider reported an error, or the person declined
	ssoExpired       ssoError = "expired"        // unknown, used or expired state; start again
	ssoFailed        ssoError = "failed"         // the token exchange or its verification failed
	ssoUnavailable   ssoError = "unavailable"    // the provider could not be reached
	ssoNotAllowed    ssoError = "not_allowed"    // authenticated, but in none of the allowed groups
	ssoNoEmail       ssoError = "no_email"       // the provider sent no email address
	ssoEmailConflict ssoError = "email_conflict" // a local account has this email and cannot be linked
	ssoDisabled      ssoError = "disabled"       // the linked account is disabled
)

// handleAuthConfig says how this server can be signed in to, so the login form
// and the TUI can offer the right choices before anybody types anything. It is
// unauthenticated by necessity, and reveals nothing beyond what the login page
// shows anyway.
func handleAuthConfig(cfg config.Config) http.HandlerFunc {
	type oidcInfo struct {
		Enabled bool   `json:"enabled"`
		Name    string `json:"name,omitempty"`
	}
	type response struct {
		PasswordLogin bool     `json:"password_login"`
		OIDC          oidcInfo `json:"oidc"`

		// DeviceLogin is whether a client that cannot open a browser (the TUI)
		// can sign in by showing a code, through /api/oidc/device.
		DeviceLogin bool `json:"device_login"`
	}
	return func(w http.ResponseWriter, r *http.Request) {
		resp := response{PasswordLogin: !cfg.DisablePasswordLogin}
		if cfg.OIDC.Enabled() {
			resp.OIDC = oidcInfo{Enabled: true, Name: cfg.OIDC.Name}
			resp.DeviceLogin = true
		}
		respond(w, http.StatusOK, resp)
	}
}

// passwordLoginOnly refuses a route when password login is switched off.
func passwordLoginOnly(enabled bool) func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		if enabled {
			return next
		}
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			respond(w, http.StatusForbidden, errResp("password login is disabled on this server"))
		})
	}
}

// ssoRedirect sends the browser back to the web UI with the reason a sign-in
// failed. It is a redirect and not a JSON error because the browser arrived
// here by navigating from the provider: there is no page script to read one.
func ssoRedirect(w http.ResponseWriter, r *http.Request, code ssoError) {
	http.Redirect(w, r, "/?sso_error="+url.QueryEscape(string(code)), http.StatusFound)
}

// handleOIDCLogin starts a sign-in: it records the state, nonce and PKCE
// verifier the callback will need and sends the browser to the provider.
func handleOIDCLogin(db *sql.DB, prov *oidc.Provider, limiter *loginLimiter, publicURL string) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		addrKey := "oidc:" + clientAddr(r)
		if limiter.blocked(addrKey, oidcStartMaxPerAddr) {
			w.Header().Set("Retry-After", strconv.Itoa(int(loginWindow.Seconds())))
			respond(w, http.StatusTooManyRequests, errResp("too many sign-in attempts, try again later"))
			return
		}
		limiter.fail(addrKey)

		state, stateHash, err := randomToken()
		if err != nil {
			respond(w, http.StatusInternalServerError, errResp("internal error"))
			return
		}
		nonce, _, err := randomToken()
		if err != nil {
			respond(w, http.StatusInternalServerError, errResp("internal error"))
			return
		}
		verifier := oidc.NewVerifier()
		next := safeNext(r.URL.Query().Get("next"))

		// Abandoned logins are swept here rather than by the sweeper: this is
		// the only place they are made, so the table cannot outgrow its writers.
		now := time.Now()
		db.ExecContext(r.Context(), "DELETE FROM oidc_logins WHERE expires_at < $1", now.Unix())
		if _, err := db.ExecContext(r.Context(), `
			INSERT INTO oidc_logins (state_hash, nonce, pkce_verifier, next, expires_at)
			VALUES ($1, $2, $3, $4, $5)`,
			stateHash, nonce, verifier, next, now.Add(oidcLoginTTL).Unix()); err != nil {
			respond(w, http.StatusInternalServerError, errResp("internal error"))
			return
		}

		authURL, err := prov.AuthURL(r.Context(), state, nonce, verifier)
		if err != nil {
			log.Printf("oidc: start login: %v", err)
			ssoRedirect(w, r, ssoUnavailable)
			return
		}

		http.SetCookie(w, &http.Cookie{
			Name:     oidcStateCookie,
			Value:    state,
			Path:     "/api/oidc",
			MaxAge:   int(oidcLoginTTL.Seconds()),
			HttpOnly: true,
			Secure:   cookieSecure(publicURL, r),
			// Lax, not Strict: the callback is a top-level navigation from the
			// provider's site, which Strict would not send the cookie on.
			SameSite: http.SameSiteLaxMode,
		})
		http.Redirect(w, r, authURL, http.StatusFound)
	}
}

// handleOIDCCallback finishes a sign-in: it verifies the provider's answer,
// finds or creates the user, applies their groups and starts a session.
func handleOIDCCallback(db *sql.DB, prov *oidc.Provider, publicURL string) http.HandlerFunc {
	cfg := prov.Config()
	return func(w http.ResponseWriter, r *http.Request) {
		// The state cookie has done its job once the callback arrives, whatever
		// the outcome.
		http.SetCookie(w, &http.Cookie{
			Name: oidcStateCookie, Value: "", Path: "/api/oidc", MaxAge: -1,
			HttpOnly: true, Secure: cookieSecure(publicURL, r), SameSite: http.SameSiteLaxMode,
		})

		q := r.URL.Query()
		if e := q.Get("error"); e != "" {
			log.Printf("oidc: provider returned error %q: %s", e, q.Get("error_description"))
			ssoRedirect(w, r, ssoDenied)
			return
		}
		state := q.Get("state")
		cookie, err := r.Cookie(oidcStateCookie)
		if state == "" || q.Get("code") == "" || err != nil || cookie.Value != state {
			ssoRedirect(w, r, ssoExpired)
			return
		}

		// DELETE ... RETURNING makes the state single-use: a replayed callback
		// finds nothing.
		var nonce, verifier, next string
		err = db.QueryRowContext(r.Context(), `
			DELETE FROM oidc_logins WHERE state_hash = $1 AND expires_at > $2
			RETURNING nonce, pkce_verifier, next`,
			hashToken(state), time.Now().Unix()).Scan(&nonce, &verifier, &next)
		if errors.Is(err, sql.ErrNoRows) {
			ssoRedirect(w, r, ssoExpired)
			return
		}
		if err != nil {
			log.Printf("oidc: load login state: %v", err)
			ssoRedirect(w, r, ssoFailed)
			return
		}

		identity, err := prov.Exchange(r.Context(), q.Get("code"), verifier, nonce)
		if err != nil {
			log.Printf("oidc: %v", err)
			ssoRedirect(w, r, ssoFailed)
			return
		}

		grants := oidc.ComputeGrants(cfg, identity.Groups)
		if !grants.Admitted {
			log.Printf("oidc: %q (%s) is in none of the allowed groups", identity.Username, identity.Subject)
			ssoRedirect(w, r, ssoNotAllowed)
			return
		}

		teamGroups, err := loadTeamGroups(r.Context(), db)
		if err != nil {
			log.Printf("oidc: load team groups: %v", err)
			ssoRedirect(w, r, ssoFailed)
			return
		}
		teamGrants := oidc.ComputeTeamGrants(teamGroups, identity.Groups)

		userID, err := signInSSO(r.Context(), db, cfg, identity, grants, teamGrants)
		if err != nil {
			var se ssoError
			if errors.As(err, &se) {
				log.Printf("oidc: refused %q (%s): %v", identity.Username, identity.Subject, se)
				ssoRedirect(w, r, se)
				return
			}
			log.Printf("oidc: sign in %q: %v", identity.Username, err)
			ssoRedirect(w, r, ssoFailed)
			return
		}

		if err := startSessionCapped(w, r, db, userID, publicURL, cfg.SessionMaxAge); err != nil {
			log.Printf("oidc: start session: %v", err)
			ssoRedirect(w, r, ssoFailed)
			return
		}
		http.Redirect(w, r, safeNext(next), http.StatusFound)
	}
}

// safeNext returns where to send the browser after a sign-in: the path asked
// for, if it is one on this server, and the front page otherwise. It is the
// only thing standing between a login link and an open redirect, so it accepts
// a single leading slash and nothing that a browser could read as another host
// ("//evil.example", "/\evil.example"), and never an API path, which would
// land somebody on raw JSON.
func safeNext(next string) string {
	switch {
	case next == "", len(next) > 512,
		!strings.HasPrefix(next, "/"),
		strings.HasPrefix(next, "//"),
		strings.HasPrefix(next, "/api/"),
		strings.ContainsAny(next, "\\\r\n"):
		return "/"
	}
	return next
}

// signInSSO resolves the identity to a user and applies its grants, in one
// transaction: a login that fails half way must not leave memberships changed.
func signInSSO(ctx context.Context, db *sql.DB, cfg config.OIDC, id *oidc.Identity, g oidc.Grants, teamRoles map[int64]string) (int64, error) {
	tx, err := db.BeginTx(ctx, nil)
	if err != nil {
		return 0, err
	}
	defer tx.Rollback() //nolint:errcheck

	userID, err := resolveSSOUser(ctx, tx, cfg, id)
	if err != nil {
		return 0, err
	}
	var disabled bool
	if err := tx.QueryRowContext(ctx,
		"SELECT disabled_at IS NOT NULL FROM users WHERE id = $1", userID).Scan(&disabled); err != nil {
		return 0, err
	}
	if disabled {
		return 0, ssoDisabled
	}
	if err := syncGrants(ctx, tx, userID, g, teamRoles); err != nil {
		return 0, err
	}
	return userID, tx.Commit()
}

// loadTeamGroups reads every team's own OIDC group binding, for the sync to
// evaluate against one user's groups at a time. Teams are few, so this reads
// the whole table rather than filtering it.
func loadTeamGroups(ctx context.Context, db *sql.DB) ([]oidc.TeamGroup, error) {
	rows, err := db.QueryContext(ctx,
		"SELECT id, COALESCE(oidc_member_group, ''), COALESCE(oidc_owner_group, '') FROM teams")
	if err != nil {
		return nil, err
	}
	defer rows.Close()

	var out []oidc.TeamGroup
	for rows.Next() {
		var tg oidc.TeamGroup
		if err := rows.Scan(&tg.TeamID, &tg.MemberGroup, &tg.OwnerGroup); err != nil {
			return nil, err
		}
		out = append(out, tg)
	}
	return out, rows.Err()
}

// resolveSSOUser finds the user an identity belongs to, linking or creating one
// when this is its first sign-in.
//
// The order matters. The (issuer, subject) pair is the identity; email is only
// a way to recognise an existing local account the first time. Once linked, a
// changed email at the provider must not move the account to somebody else.
func resolveSSOUser(ctx context.Context, tx *sql.Tx, cfg config.OIDC, id *oidc.Identity) (int64, error) {
	now := time.Now().Unix()

	var userID int64
	err := tx.QueryRowContext(ctx,
		"SELECT user_id FROM user_identities WHERE issuer = $1 AND subject = $2",
		id.Issuer, id.Subject).Scan(&userID)
	if err == nil {
		if _, err := tx.ExecContext(ctx,
			"UPDATE user_identities SET last_login_at = $1 WHERE issuer = $2 AND subject = $3",
			now, id.Issuer, id.Subject); err != nil {
			return 0, err
		}
		return userID, refreshProfile(ctx, tx, userID, id)
	}
	if !errors.Is(err, sql.ErrNoRows) {
		return 0, err
	}

	// First sign-in with this identity.
	if id.Email == "" {
		return 0, ssoNoEmail
	}
	err = tx.QueryRowContext(ctx,
		"SELECT id FROM users WHERE lower(email) = lower($1)", id.Email).Scan(&userID)
	switch {
	case err == nil:
		if !id.EmailVerified && !cfg.TrustEmail {
			return 0, ssoEmailConflict
		}
		// A local account that already has an identity from this issuer is a
		// different person at the provider using a recycled address. Linking
		// them would hand one person's account to another.
		var linked bool
		if err := tx.QueryRowContext(ctx,
			"SELECT EXISTS (SELECT 1 FROM user_identities WHERE user_id = $1 AND issuer = $2)",
			userID, id.Issuer).Scan(&linked); err != nil {
			return 0, err
		}
		if linked {
			return 0, ssoEmailConflict
		}
	case errors.Is(err, sql.ErrNoRows):
		userID, err = createSSOUser(ctx, tx, id)
		if err != nil {
			return 0, err
		}
	default:
		return 0, err
	}

	if _, err := tx.ExecContext(ctx,
		"INSERT INTO user_identities (user_id, issuer, subject) VALUES ($1, $2, $3)",
		userID, id.Issuer, id.Subject); err != nil {
		return 0, err
	}
	return userID, nil
}

// createSSOUser inserts a user with no password. The username is the provider's,
// made unique with a numeric suffix when somebody local already has it.
func createSSOUser(ctx context.Context, tx *sql.Tx, id *oidc.Identity) (int64, error) {
	base := strings.TrimSpace(id.Username)
	if base == "" {
		base, _, _ = strings.Cut(id.Email, "@")
	}
	if base == "" {
		base = "user"
	}
	for n := 1; n <= 100; n++ {
		name := base
		if n > 1 {
			name = base + "-" + strconv.Itoa(n)
		}
		var userID int64
		err := tx.QueryRowContext(ctx, `
			INSERT INTO users (username, email) VALUES ($1, $2)
			ON CONFLICT (username) DO NOTHING RETURNING id`,
			name, id.Email).Scan(&userID)
		if errors.Is(err, sql.ErrNoRows) {
			continue // taken; try the next suffix
		}
		return userID, err
	}
	return 0, errors.New("no free username for " + base)
}

// refreshProfile brings a linked user's username and email in line with the
// provider. Each update is skipped, not failed, when another user already holds
// the value: both columns are unique, and a sign-in must not break over a name.
func refreshProfile(ctx context.Context, tx *sql.Tx, userID int64, id *oidc.Identity) error {
	if id.Username != "" {
		if _, err := tx.ExecContext(ctx, `
			UPDATE users SET username = $1
			 WHERE id = $2 AND username <> $1
			   AND NOT EXISTS (SELECT 1 FROM users WHERE username = $1)`,
			id.Username, userID); err != nil {
			return err
		}
	}
	if id.Email != "" {
		if _, err := tx.ExecContext(ctx, `
			UPDATE users SET email = $1
			 WHERE id = $2 AND email <> $1
			   AND NOT EXISTS (SELECT 1 FROM users WHERE lower(email) = lower($1))`,
			id.Email, userID); err != nil {
			return err
		}
	}
	return nil
}

// syncGrants makes the user's OIDC-sourced access match what their groups grant
// now, and touches nothing else.
//
// Rows the sync owns are marked source 'oidc'. It adds them, changes their role
// and removes them. The last-owner and last-administrator guards do not apply:
// they exist to stop a person's mistake, and the provider is the source of truth
// for the access it grants, so a team or an install can be left without an
// SSO-granted owner. Administrators can always repair a team, and the bootstrap
// administrator is a manual one. Rows added by hand are 'manual', and the sync
// only ever raises them (turning them into 'oidc' rows), never lowers or removes
// them.
//
// teamRoles is keyed by team ID, not name: a team must already exist, with its
// own oidc_member_group/oidc_owner_group set by its owner, before a group can
// grant access to it. The sync never creates a team.
func syncGrants(ctx context.Context, tx *sql.Tx, userID int64, g oidc.Grants, teamRoles map[int64]string) error {
	// Administrator. A manual administrator stays one whatever the groups say.
	if g.Admin {
		if _, err := tx.ExecContext(ctx,
			"UPDATE users SET is_admin = true, admin_source = 'oidc' WHERE id = $1 AND NOT is_admin",
			userID); err != nil {
			return err
		}
	} else if _, err := tx.ExecContext(ctx,
		"UPDATE users SET is_admin = false, admin_source = 'manual' WHERE id = $1 AND admin_source = 'oidc'",
		userID); err != nil {
		return err
	}

	// Teams. The result of the loop is the set of teams the groups grant.
	granted := make([]int64, 0, len(teamRoles))
	for teamID, role := range teamRoles {
		granted = append(granted, teamID)

		// A row the sync owns follows the groups in both directions. One added by
		// hand is only raised: a member the owner made an owner by hand is not
		// demoted because the group says member.
		if _, err := tx.ExecContext(ctx, `
			INSERT INTO team_members (team_id, user_id, role, source)
			VALUES ($1, $2, $3, 'oidc')
			ON CONFLICT (team_id, user_id) DO UPDATE
			   SET role = excluded.role, source = 'oidc'
			 WHERE team_members.source = 'oidc'
			    OR (excluded.role = $4 AND team_members.role = $5)`,
			teamID, userID, role, models.RoleOwner, models.RoleMember); err != nil {
			return err
		}
	}

	// Access the groups no longer grant. granted is never nil, or the ALL
	// comparison would be against NULL and delete nothing.
	_, err := tx.ExecContext(ctx,
		"DELETE FROM team_members WHERE user_id = $1 AND source = 'oidc' AND team_id <> ALL($2)",
		userID, granted)
	return err
}