package api

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

	"github.com/go-chi/chi/v5"
	"golang.org/x/crypto/bcrypt"
)

const (
	// sessionCookie carries a web UI session. It is HttpOnly, so page script
	// never sees the token; the page learns who it is from GET /api/me.
	sessionCookie = "terdut_session"

	// sessionTTL is how long a session lives without being used. It slides, so
	// a phone that opens the UI now and then stays signed in indefinitely.
	sessionTTL = 30 * 24 * time.Hour

	// sessionTouchEvery bounds how often a request may slide the expiry.
	sessionTouchEvery = time.Hour

	minPasswordLen = 10
	// maxPasswordLen is bcrypt's limit; it rejects longer input outright.
	maxPasswordLen = 72

	loginWindow      = 15 * time.Minute
	loginMaxPerUser  = 10
	loginMaxPerAddr  = 30
	passwordHashCost = bcrypt.DefaultCost
)

// dummyHash is compared against when the username is unknown or has no
// password, so a failed login takes as long whichever way it failed.
var dummyHash = sync.OnceValue(func() []byte {
	h, _ := bcrypt.GenerateFromPassword([]byte("terdut-dummy-password"), passwordHashCost)
	return h
})

// loginLimiter counts failed logins in a fixed window, per username and per
// client address. The username limit is what stops guessing one account; the
// address limit is looser because every user behind the same gateway or NAT
// shares it.
type loginLimiter struct {
	mu       sync.Mutex
	failures map[string]*loginWindowCount
}

type loginWindowCount struct {
	start time.Time
	n     int
}

func newLoginLimiter() *loginLimiter {
	return &loginLimiter{failures: map[string]*loginWindowCount{}}
}

func (l *loginLimiter) blocked(key string, max int) bool {
	l.mu.Lock()
	defer l.mu.Unlock()
	c, ok := l.failures[key]
	if !ok || time.Since(c.start) > loginWindow {
		return false
	}
	return c.n >= max
}

func (l *loginLimiter) fail(keys ...string) {
	l.mu.Lock()
	defer l.mu.Unlock()
	now := time.Now()
	for k, c := range l.failures {
		if now.Sub(c.start) > loginWindow {
			delete(l.failures, k)
		}
	}
	for _, key := range keys {
		c, ok := l.failures[key]
		if !ok {
			c = &loginWindowCount{start: now}
			l.failures[key] = c
		}
		c.n++
	}
}

func (l *loginLimiter) clear(key string) {
	l.mu.Lock()
	defer l.mu.Unlock()
	delete(l.failures, key)
}

// clientAddr is the address a login is counted against. Behind the gateway
// RemoteAddr is the gateway itself, so the first X-Forwarded-For hop is used
// when present. It can be forged, but only to dodge the address limit; the
// per-username limit does not depend on it.
func clientAddr(r *http.Request) string {
	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
		first, _, _ := strings.Cut(xff, ",")
		return strings.TrimSpace(first)
	}
	host, _, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		return r.RemoteAddr
	}
	return host
}

// cookieSecure decides the cookie's Secure flag. TLS terminates at the gateway,
// so the server usually sees plain HTTP; the public URL is what says whether
// browsers reach it over HTTPS.
func cookieSecure(publicURL string, r *http.Request) bool {
	return strings.HasPrefix(publicURL, "https://") ||
		r.TLS != nil ||
		r.Header.Get("X-Forwarded-Proto") == "https"
}

// validatePassword returns a message for the client, or "" when acceptable.
func validatePassword(pw string) string {
	switch {
	case len(pw) < minPasswordLen:
		return "password must be at least " + strconv.Itoa(minPasswordLen) + " characters"
	case len(pw) > maxPasswordLen:
		return "password must be at most " + strconv.Itoa(maxPasswordLen) + " bytes"
	}
	return ""
}

func hashPassword(pw string) (string, error) {
	h, err := bcrypt.GenerateFromPassword([]byte(pw), passwordHashCost)
	return string(h), err
}

// startSession mints a session and sets the cookie. Shared by login and
// sign-up: somebody who has just chosen a password is signed in, rather than
// being sent to a form to type the same credential again.
func startSession(w http.ResponseWriter, r *http.Request, db *sql.DB, userID int64, publicURL string) error {
	raw, tokenHash, err := randomToken()
	if err != nil {
		return err
	}
	now := time.Now()
	if _, err := db.ExecContext(r.Context(), `
		INSERT INTO sessions (token_hash, user_id, created_at, last_seen_at, expires_at, user_agent)
		VALUES ($1, $2, $3, $4, $5, $6)`,
		tokenHash, userID, now.Unix(), now.Unix(), now.Add(sessionTTL).Unix(), r.UserAgent()); err != nil {
		return err
	}

	http.SetCookie(w, &http.Cookie{
		Name:     sessionCookie,
		Value:    raw,
		Path:     "/",
		MaxAge:   int(sessionTTL.Seconds()),
		HttpOnly: true,
		Secure:   cookieSecure(publicURL, r),
		SameSite: http.SameSiteLaxMode,
	})
	return nil
}

// handleLogin exchanges a username and password for a session cookie.
func handleLogin(db *sql.DB, limiter *loginLimiter, publicURL string) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		var req struct {
			Username string `json:"username"`
			Password string `json:"password"`
		}
		if err := decodeJSON(r, &req); err != nil {
			respond(w, http.StatusBadRequest, errResp("invalid request body"))
			return
		}
		username := strings.TrimSpace(req.Username)
		userKey := "user:" + strings.ToLower(username)
		addrKey := "addr:" + clientAddr(r)

		if limiter.blocked(userKey, loginMaxPerUser) || limiter.blocked(addrKey, loginMaxPerAddr) {
			w.Header().Set("Retry-After", strconv.Itoa(int(loginWindow.Seconds())))
			respond(w, http.StatusTooManyRequests, errResp("too many failed attempts, try again later"))
			return
		}

		var userID int64
		var hash sql.NullString
		err := db.QueryRowContext(r.Context(),
			"SELECT id, password_hash FROM users WHERE username = $1", username,
		).Scan(&userID, &hash)
		if err != nil && !errors.Is(err, sql.ErrNoRows) {
			respond(w, http.StatusInternalServerError, errResp("internal error"))
			return
		}

		stored := dummyHash()
		if hash.Valid {
			stored = []byte(hash.String)
		}
		match := bcrypt.CompareHashAndPassword(stored, []byte(req.Password)) == nil
		if !match || !hash.Valid {
			limiter.fail(userKey, addrKey)
			respond(w, http.StatusUnauthorized, errResp("invalid username or password"))
			return
		}
		limiter.clear(userKey)

		if err := startSession(w, r, db, userID, publicURL); err != nil {
			respond(w, http.StatusInternalServerError, errResp("internal error"))
			return
		}

		user, err := fetchUser(r.Context(), db, userID)
		if err != nil {
			respond(w, http.StatusInternalServerError, errResp("internal error"))
			return
		}
		respond(w, http.StatusOK, meResponse{User: user, HasPassword: true})
	}
}

// handleLogout ends the browser's session. It sits outside AuthMiddleware so
// that a browser holding an already-expired cookie can still clear it.
func handleLogout(db *sql.DB, publicURL string) http.HandlerFunc {
	crossOrigin := http.NewCrossOriginProtection()
	return func(w http.ResponseWriter, r *http.Request) {
		if err := crossOrigin.Check(r); err != nil {
			respond(w, http.StatusForbidden, errResp("cross-origin request rejected"))
			return
		}
		if c, err := r.Cookie(sessionCookie); err == nil && c.Value != "" {
			db.ExecContext(r.Context(), "DELETE FROM sessions WHERE token_hash = $1", hashToken(c.Value))
		}
		http.SetCookie(w, &http.Cookie{
			Name:     sessionCookie,
			Value:    "",
			Path:     "/",
			MaxAge:   -1,
			HttpOnly: true,
			Secure:   cookieSecure(publicURL, r),
			SameSite: http.SameSiteLaxMode,
		})
		w.WriteHeader(http.StatusNoContent)
	}
}

type meResponse struct {
	User        any  `json:"user"`
	HasPassword bool `json:"has_password"`

	// OnboardingDismissed is whether this person has put the first-run
	// checklist away. Per user rather than per browser: somebody who finishes
	// setting up on a laptop should not be nagged again on their phone.
	OnboardingDismissed bool `json:"onboarding_dismissed"`
}

// handleMe says who the caller is. The web UI calls it on load to decide
// between the login form and the app, since it cannot read its own cookie.
func handleMe(db *sql.DB) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		caller, _ := userFromContext(r.Context())
		user, err := fetchUser(r.Context(), db, caller.ID)
		if err != nil {
			respond(w, http.StatusInternalServerError, errResp("internal error"))
			return
		}
		var hash sql.NullString
		var dismissed *int64
		db.QueryRowContext(r.Context(),
			"SELECT password_hash, onboarding_dismissed_at FROM users WHERE id = $1",
			caller.ID).Scan(&hash, &dismissed)
		respond(w, http.StatusOK, meResponse{
			User:                user,
			HasPassword:         hash.Valid,
			OnboardingDismissed: dismissed != nil,
		})
	}
}

// handleSetPassword sets a user's web UI password.
//
// Changing your own password takes the current one, when there is one, so an
// unattended signed-in browser cannot be used to take the account over. Setting
// somebody else's is how an admin gives a user their first password, and is
// restricted to administrators: it hands over an account outright, without
// knowing the password it replaces.
//
// Every other session of the target is ended: a password change is what you
// do when you think someone else is signed in.
func handleSetPassword(db *sql.DB) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		id, err := strconv.ParseInt(chi.URLParam(r, "id"), 10, 64)
		if err != nil {
			respond(w, http.StatusBadRequest, errResp("invalid user id"))
			return
		}
		if !requireSelfOrAdmin(w, r, id) {
			return
		}
		var req struct {
			Password        string `json:"password"`
			CurrentPassword string `json:"current_password"`
		}
		if err := decodeJSON(r, &req); err != nil {
			respond(w, http.StatusBadRequest, errResp("invalid request body"))
			return
		}
		if msg := validatePassword(req.Password); msg != "" {
			respond(w, http.StatusBadRequest, errResp(msg))
			return
		}

		var existing sql.NullString
		err = db.QueryRowContext(r.Context(),
			"SELECT password_hash FROM users WHERE id = $1", id).Scan(&existing)
		if errors.Is(err, sql.ErrNoRows) {
			respond(w, http.StatusNotFound, errResp("user not found"))
			return
		}
		if err != nil {
			respond(w, http.StatusInternalServerError, errResp("internal error"))
			return
		}

		caller, _ := userFromContext(r.Context())
		if caller.ID == id && existing.Valid &&
			bcrypt.CompareHashAndPassword([]byte(existing.String), []byte(req.CurrentPassword)) != nil {
			respond(w, http.StatusForbidden, errResp("current password is incorrect"))
			return
		}

		hash, err := hashPassword(req.Password)
		if err != nil {
			respond(w, http.StatusInternalServerError, errResp("internal error"))
			return
		}

		tx, err := db.BeginTx(r.Context(), nil)
		if err != nil {
			respond(w, http.StatusInternalServerError, errResp("internal error"))
			return
		}
		defer tx.Rollback()

		if _, err := tx.ExecContext(r.Context(),
			"UPDATE users SET password_hash = $1 WHERE id = $2", hash, id); err != nil {
			respond(w, http.StatusInternalServerError, errResp("internal error"))
			return
		}
		keep, _ := sessionFromContext(r.Context()) // zero when changed with an API key
		if _, err := tx.ExecContext(r.Context(),
			"DELETE FROM sessions WHERE user_id = $1 AND id != $2", id, keep); err != nil {
			respond(w, http.StatusInternalServerError, errResp("internal error"))
			return
		}
		if err := tx.Commit(); err != nil {
			respond(w, http.StatusInternalServerError, errResp("internal error"))
			return
		}
		w.WriteHeader(http.StatusNoContent)
	}
}

// purgeSessions deletes sessions that have expired, from the sweeper.
func purgeSessions(ctx context.Context, db *sql.DB) {
	res, err := db.ExecContext(ctx,
		"DELETE FROM sessions WHERE expires_at < $1", time.Now().Unix())
	if err != nil {
		log.Printf("sweeper: purge sessions: %v", err)
		return
	}
	if n, _ := res.RowsAffected(); n > 0 {
		log.Printf("sweeper: purged %d expired session(s)", n)
	}
}