package server

import (
	"errors"
	"fmt"
	"net"
	"net/http"
	"strconv"
	"strings"
	"sync"
	"time"

	"cc.tiredsysadmin.keeporg/internal/secret"
	"cc.tiredsysadmin.keeporg/internal/store"
	"cc.tiredsysadmin.keeporg/internal/totp"
)

// Proving who someone is: every password and one-time code the server
// checks goes through verify, wherever it was typed — the API, the sign-in
// form, the device page, a password change, a second-factor change or a
// vault deletion. Two things follow from that being one function.
//
// The password hash is checked outside the store's lock. It is deliberately
// slow, and under the write lock it stalled every other request, so a few
// guesses a second were a denial of service. verify reads what it needs,
// checks it, and commit re-checks inside the write that nothing changed in
// between.
//
// Failures are counted per client address and per account, and a client or
// account over its budget is refused before any hash is computed. The
// counts live in memory: a restart forgets them, which costs an attacker a
// restart they cannot cause.

// errThrottled means too many failures; RetryAfter says how long to wait.
type errThrottled struct{ RetryAfter time.Duration }

func (e *errThrottled) Error() string {
	return fmt.Sprintf("too many failed attempts; try again in %s",
		e.RetryAfter.Round(time.Minute))
}

// credentials is a password, and code, that checked out against an account
// as it stood when verify read it.
type credentials struct {
	userID string
	hash   string
	step   int64 // the one-time code's time step, when one was needed
	totp   bool
}

// dummyHash is verified against when there is no real one, so that an
// unknown, disabled or password-less account takes as long to refuse as a
// wrong password: the timing would otherwise list the usernames.
var dummyHash = func() string {
	h, err := secret.Hash("keeporg timing equaliser, never a real password")
	if err != nil {
		panic(err)
	}
	return h
}()

// verify checks a username, password and one-time code.
func (s *Server) verify(r *http.Request, username, password, code string) (*credentials, error) {
	ip := "ip:" + s.clientIP(r)
	acct := "user:" + strings.ToLower(strings.TrimSpace(username))
	if wait := s.limits.blocked(ip, acct); wait > 0 {
		return nil, &errThrottled{RetryAfter: wait}
	}
	var (
		u     store.User
		found bool
	)
	s.st.Read(func(st *store.State) {
		if p := st.UserByName(username); p != nil {
			u, found = *p, true
		}
	})
	if !found || u.Disabled || u.PassHash == "" {
		secret.Verify(dummyHash, password)
		s.limits.fail(ip, acct)
		return nil, errBadCredentials
	}
	if !secret.Verify(u.PassHash, password) {
		s.limits.fail(ip, acct)
		return nil, errBadCredentials
	}
	c := &credentials{userID: u.ID, hash: u.PassHash}
	if u.TOTPConfirmed {
		if strings.TrimSpace(code) == "" {
			return nil, errTOTPRequired
		}
		step, ok := totp.Match(u.TOTPSecret, code, time.Now())
		if !ok || step <= u.TOTPLastStep {
			s.limits.fail(ip, acct)
			return nil, errTOTPInvalid
		}
		c.step, c.totp = step, true
	}
	s.limits.reset(acct)
	return c, nil
}

// commit re-checks verified credentials inside a write: the account must
// still exist, be enabled and have the same password, and the one-time code
// is spent. It returns the account to act on.
func (c *credentials) commit(st *store.State) (*store.User, error) {
	u := st.Users[c.userID]
	if u == nil || u.Disabled || u.PassHash != c.hash {
		return nil, errBadCredentials
	}
	if c.totp && u.TOTPConfirmed {
		if c.step <= u.TOTPLastStep {
			return nil, errTOTPInvalid
		}
		u.TOTPLastStep = c.step
	}
	return u, nil
}

// signIn verifies credentials and, inside one write, re-checks them and runs
// then on the account. It is the shape of every sign-in.
func (s *Server) signIn(r *http.Request, username, password, code string,
	then func(st *store.State, u *store.User) error) error {
	c, err := s.verify(r, username, password, code)
	if err != nil {
		return err
	}
	return s.st.Update(func(st *store.State) error {
		u, err := c.commit(st)
		if err != nil {
			return err
		}
		return then(st, u)
	})
}

// reconfirm proves, for a change that weakens or replaces how an account
// signs in, that the person holding the session also holds the password and
// the current code. Unlike a sign-in it does not say which of the two was
// wrong: a stolen session must not become a password oracle.
func (s *Server) reconfirm(r *http.Request, u *store.User, password, code string,
	then func(st *store.State, u *store.User) error) error {
	// The account as stored now, not as the session saw it at the start of
	// the request.
	s.st.Read(func(st *store.State) {
		if cur := st.Users[u.ID]; cur != nil {
			cp := *cur
			u = &cp
		}
	})
	if u.PassHash == "" {
		return errNoPassword
	}
	if password == "" {
		return errReauth
	}
	if u.TOTPConfirmed && strings.TrimSpace(code) == "" {
		// Asked before the password is checked, so it says nothing about
		// whether the password was right.
		return errTOTPRequired
	}
	err := s.signIn(r, u.Username, password, code, then)
	if errors.Is(err, errTOTPInvalid) || errors.Is(err, errTOTPRequired) {
		return errBadCredentials
	}
	return err
}

// writeThrottled answers a refused attempt with 429 and Retry-After.
func writeThrottled(w http.ResponseWriter, err error) bool {
	var t *errThrottled
	if !errors.As(err, &t) {
		return false
	}
	w.Header().Set("Retry-After", strconv.Itoa(int(t.RetryAfter.Seconds())+1))
	fail(w, http.StatusTooManyRequests, "throttled", t.Error())
	return true
}

// ---- limits ----

// limitWindow is how long failures are remembered.
const limitWindow = 15 * time.Minute

// Budgets per window. An address may fail more often than an account, since
// several people may share one address; an account fails rarely when its
// owner is the one typing.
var limitBudget = map[string]int{
	"ip":    30,
	"user":  10,
	"start": 30, // unauthenticated device and single sign-on starts, per address
}

type attempts struct {
	n     int
	since time.Time
}

// limiter counts failures per key ("ip:…", "user:…", "start:…").
type limiter struct {
	mu  sync.Mutex
	m   map[string]*attempts
	now func() time.Time
}

func newLimiter() *limiter { return &limiter{m: map[string]*attempts{}, now: time.Now} }

func budget(key string) int {
	kind, _, _ := strings.Cut(key, ":")
	return limitBudget[kind]
}

// blocked says how long until every key is under budget again, or 0.
func (l *limiter) blocked(keys ...string) time.Duration {
	l.mu.Lock()
	defer l.mu.Unlock()
	now := l.now()
	var wait time.Duration
	for _, k := range keys {
		a := l.m[k]
		if a == nil || now.Sub(a.since) >= limitWindow || a.n < budget(k) {
			continue
		}
		if left := limitWindow - now.Sub(a.since); left > wait {
			wait = left
		}
	}
	return wait
}

// fail counts one failure against each key.
func (l *limiter) fail(keys ...string) {
	l.mu.Lock()
	defer l.mu.Unlock()
	now := l.now()
	if len(l.m) > 50_000 {
		for k, a := range l.m {
			if now.Sub(a.since) >= limitWindow {
				delete(l.m, k)
			}
		}
	}
	for _, k := range keys {
		a := l.m[k]
		if a == nil || now.Sub(a.since) >= limitWindow {
			a = &attempts{since: now}
			l.m[k] = a
		}
		a.n++
	}
}

// take counts one use of key and reports whether it was within budget, for
// endpoints limited by use rather than by failure.
func (l *limiter) take(key string) time.Duration {
	if wait := l.blocked(key); wait > 0 {
		return wait
	}
	l.fail(key)
	return 0
}

func (l *limiter) reset(key string) {
	l.mu.Lock()
	delete(l.m, key)
	l.mu.Unlock()
}

// ---- client addresses ----

// clientIP is the address a request came from. Behind a reverse proxy every
// request comes from the proxy, so for a peer listed in trusted_proxies the
// address is taken from X-Forwarded-For, right to left, skipping trusted
// hops; nobody else gets to choose their own address with a header.
func (s *Server) clientIP(r *http.Request) string {
	host, _, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		host = r.RemoteAddr
	}
	if !s.trusted(host) {
		return host
	}
	hops := strings.Split(r.Header.Get("X-Forwarded-For"), ",")
	for i := len(hops) - 1; i >= 0; i-- {
		h := strings.TrimSpace(hops[i])
		if h == "" {
			continue
		}
		if !s.trusted(h) {
			return h
		}
		host = h
	}
	if real := strings.TrimSpace(r.Header.Get("X-Real-IP")); real != "" && net.ParseIP(real) != nil {
		return real
	}
	return host
}

func (s *Server) trusted(addr string) bool {
	ip := net.ParseIP(addr)
	if ip == nil {
		return false
	}
	for _, n := range s.proxies {
		if n.Contains(ip) {
			return true
		}
	}
	return false
}

// parseProxies reads trusted_proxies: addresses or CIDR ranges.
func parseProxies(list []string) ([]*net.IPNet, error) {
	var out []*net.IPNet
	for _, p := range list {
		p = strings.TrimSpace(p)
		if p == "" {
			continue
		}
		if !strings.Contains(p, "/") {
			if ip := net.ParseIP(p); ip != nil && ip.To4() != nil {
				p += "/32"
			} else {
				p += "/128"
			}
		}
		_, n, err := net.ParseCIDR(p)
		if err != nil {
			return nil, fmt.Errorf("trusted_proxies: %q: %w", p, err)
		}
		out = append(out, n)
	}
	return out, nil
}

// maxLabel bounds a session or device label a client chooses: it is shown
// in lists and stored with every session, and nothing needs more.
const maxLabel = 64

// clampLabel trims a client-chosen label to something displayable.
func clampLabel(label string) string {
	label = strings.Map(func(r rune) rune {
		if r < 0x20 || r == 0x7f {
			return -1
		}
		return r
	}, strings.TrimSpace(label))
	if r := []rune(label); len(r) > maxLabel {
		label = string(r[:maxLabel])
	}
	return label
}