// Package auth: argon2id password hashing, cookie sessions, login rate limiting.
package auth

import (
	"crypto/rand"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"encoding/hex"
	"errors"
	"fmt"
	"regexp"
	"sort"
	"strings"
	"sync"
	"time"

	"golang.org/x/crypto/argon2"

	"hamlab/internal/config"
	"hamlab/internal/store"
)

const (
	argonTime    = 3
	argonMemory  = 64 * 1024
	argonThreads = 2
	argonKeyLen  = 32

	SessionTTL = 30 * 24 * time.Hour
)

func HashPassword(pw string) (string, error) {
	salt := make([]byte, 16)
	if _, err := rand.Read(salt); err != nil {
		return "", err
	}
	key := argon2.IDKey([]byte(pw), salt, argonTime, argonMemory, argonThreads, argonKeyLen)
	enc := base64.RawStdEncoding.EncodeToString
	return fmt.Sprintf("$argon2id$v=19$m=%d,t=%d,p=%d$%s$%s",
		argonMemory, argonTime, argonThreads, enc(salt), enc(key)), nil
}

func VerifyPassword(hash, pw string) bool {
	parts := strings.Split(hash, "$")
	if len(parts) != 6 || parts[1] != "argon2id" {
		return false
	}
	var m, t uint32
	var p uint8
	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &m, &t, &p); err != nil {
		return false
	}
	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
	if err != nil {
		return false
	}
	want, err := base64.RawStdEncoding.DecodeString(parts[5])
	if err != nil {
		return false
	}
	got := argon2.IDKey([]byte(pw), salt, t, m, p, uint32(len(want)))
	return subtle.ConstantTimeCompare(got, want) == 1
}

// ---- API tokens (hub → node) ----

// NewToken returns a random bearer token and its SHA-256 hex, which is what gets stored.
func NewToken() (token, hash string, err error) {
	b := make([]byte, 32)
	if _, err := rand.Read(b); err != nil {
		return "", "", err
	}
	token = base64.RawURLEncoding.EncodeToString(b)
	return token, HashToken(token), nil
}

func HashToken(token string) string {
	sum := sha256.Sum256([]byte(token))
	return hex.EncodeToString(sum[:])
}

// TokenMatches compares a presented token against stored hashes in constant time.
func TokenMatches(token string, hashes []string) bool { return TokenIndex(token, hashes) >= 0 }

// TokenIndex is which of hashes the presented token matches, -1 for none; every hash is
// compared whatever the answer.
func TokenIndex(token string, hashes []string) int {
	h := []byte(HashToken(token))
	idx := -1
	for i, want := range hashes {
		if subtle.ConstantTimeCompare(h, []byte(want)) == 1 {
			idx = i
		}
	}
	return idx
}

// ---- users ----

// UserNameRe is what an account may be called: like a Unix login, lower-case.
var UserNameRe = regexp.MustCompile(`^[a-z0-9][a-z0-9._-]{0,31}$`)

const MinPasswordLen = 12

// dummyHash is verified against when the named user does not exist, so a login attempt
// takes the same time either way and the form cannot be used to list accounts.
var dummyHash = func() string {
	h, err := HashPassword("not-a-real-password")
	if err != nil {
		return ""
	}
	return h
}()

// CheckUser verifies a password for one of users. A missing user — or a guest login whose
// time is up — fails in the same time as a wrong password.
func CheckUser(users []config.User, name, pw string) bool {
	for _, u := range users {
		if u.Name == name {
			if u.Expired(time.Now()) {
				break
			}
			return VerifyPassword(u.Hash, pw)
		}
	}
	VerifyPassword(dummyHash, pw)
	return false
}

// ---- guest logins ----

// guestAlphabet leaves out every letter and digit that can be misread in a screenshot or
// over the phone: i, l, o, 0 and 1. A guest login is read aloud and typed by hand.
const guestAlphabet = "abcdefghjkmnpqrstuvwxyz23456789"

// randomFrom returns n characters of guestAlphabet, chosen without modulo bias.
func randomFrom(n int) (string, error) {
	out := make([]byte, 0, n)
	buf := make([]byte, n)
	limit := byte(256 - 256%len(guestAlphabet)) // the top of the last whole run of the alphabet
	for len(out) < n {
		if _, err := rand.Read(buf); err != nil {
			return "", err
		}
		for _, b := range buf {
			if b >= limit {
				continue // would favour the first few letters; ask for another byte instead
			}
			out = append(out, guestAlphabet[int(b)%len(guestAlphabet)])
			if len(out) == n {
				break
			}
		}
	}
	return string(out), nil
}

// GuestPassword is a password for a guest login: four groups of four, ~78 bits, in an
// alphabet with nothing ambiguous in it.
func GuestPassword() (string, error) {
	var groups []string
	for i := 0; i < 4; i++ {
		g, err := randomFrom(4)
		if err != nil {
			return "", err
		}
		groups = append(groups, g)
	}
	return strings.Join(groups, "-"), nil
}

// GuestSuffix is the tail of a generated guest name.
func GuestSuffix() (string, error) { return randomFrom(4) }

// ---- sessions ----

// Sessions is a token store backed by the state file. Expiry is sliding.
type Sessions struct {
	st *store.Store
}

func NewSessions(st *store.Store) *Sessions {
	s := &Sessions{st: st}
	_ = st.Update(func(state *store.State) { pruneExpired(state) })
	return s
}

func pruneExpired(state *store.State) {
	now := time.Now()
	for tok, ses := range state.Sessions {
		if ses.Expires.Before(now) {
			delete(state.Sessions, tok)
		}
	}
}

func randomHex(n int) (string, error) {
	b := make([]byte, n)
	if _, err := rand.Read(b); err != nil {
		return "", err
	}
	return hex.EncodeToString(b), nil
}

// Create signs user in from ip with agent, returning the cookie token.
func (s *Sessions) Create(user, ip, agent string) (string, error) {
	tok, err := randomHex(32)
	if err != nil {
		return "", err
	}
	id, err := randomHex(6)
	if err != nil {
		return "", err
	}
	if len(agent) > 200 {
		agent = agent[:200]
	}
	now := time.Now()
	err = s.st.Update(func(state *store.State) {
		pruneExpired(state)
		state.Sessions[tok] = store.Session{ID: id, User: user, Expires: now.Add(SessionTTL), Created: now, LastSeen: now, IP: ip, Agent: agent}
	})
	return tok, err
}

// Lookup reports whether tok is live and whose it is, sliding its expiry and noting the
// visit (persisted at most hourly, so a busy console is not writing the file all day).
func (s *Sessions) Lookup(tok, ip string) (store.Session, bool) {
	if tok == "" {
		return store.Session{}, false
	}
	var ses store.Session
	ok, touch := false, false
	s.st.View(func(state *store.State) {
		found, have := state.Sessions[tok]
		if have && found.Expires.After(time.Now()) {
			ses, ok = found, true
			touch = time.Until(found.Expires) < SessionTTL-time.Hour || time.Since(found.LastSeen) > time.Hour
		}
	})
	if ok && touch {
		_ = s.st.Update(func(state *store.State) {
			if x, found := state.Sessions[tok]; found {
				x.Expires = time.Now().Add(SessionTTL)
				x.LastSeen = time.Now()
				if ip != "" {
					x.IP = ip
				}
				state.Sessions[tok] = x
			}
		})
	}
	return ses, ok
}

func (s *Sessions) Delete(tok string) {
	_ = s.st.Update(func(state *store.State) { delete(state.Sessions, tok) })
}

// List is every live session of user, newest first.
func (s *Sessions) List(user string) []store.Session {
	var out []store.Session
	s.st.View(func(state *store.State) {
		now := time.Now()
		for _, ses := range state.Sessions {
			if ses.User == user && ses.Expires.After(now) {
				out = append(out, ses)
			}
		}
	})
	sort.Slice(out, func(i, j int) bool { return out[i].LastSeen.After(out[j].LastSeen) })
	return out
}

// Count is how many live sessions each user has.
func (s *Sessions) Count() map[string]int {
	n := map[string]int{}
	s.st.View(func(state *store.State) {
		now := time.Now()
		for _, ses := range state.Sessions {
			if ses.Expires.After(now) {
				n[ses.User]++
			}
		}
	})
	return n
}

// Revoke ends the session of user with this ID. Reports whether there was one.
func (s *Sessions) Revoke(user, id string) bool {
	found := false
	_ = s.st.Update(func(state *store.State) {
		for tok, ses := range state.Sessions {
			if ses.User == user && ses.ID == id {
				delete(state.Sessions, tok)
				found = true
			}
		}
	})
	return found
}

// RevokeOthers ends every session of user except the one holding keep. Returns how many.
func (s *Sessions) RevokeOthers(user, keep string) int {
	n := 0
	_ = s.st.Update(func(state *store.State) {
		for tok, ses := range state.Sessions {
			if ses.User == user && tok != keep {
				delete(state.Sessions, tok)
				n++
			}
		}
	})
	return n
}

// RevokeUser ends every session of user (the account is going away).
func (s *Sessions) RevokeUser(user string) { s.RevokeOthers(user, "") }

// Limiter locks out a client after repeated failed logins, per client address, with a
// global backstop so a botnet can't burn CPU on argon2 indefinitely. A lockout for one
// address never blocks another. Past perClientLock failures, each further failure doubles a
// lockout starting at 30s, capped at 10 minutes.
type Limiter struct {
	mu      sync.Mutex
	clients map[string]*bucket
	global  bucket
}

type bucket struct {
	failures  int
	lockUntil time.Time
	last      time.Time
}

var ErrLocked = errors.New("too many failed logins")

const (
	// perClientLock is how many failures one address gets before it is made to wait. Set high
	// on purpose: a guest login is a generated password read off a message and typed by hand
	// on a phone, and being locked out for fumbling it is worse than the brute-force risk —
	// which the doubling backoff past this point, and the global backstop, still cover.
	perClientLock = 20
	globalLock    = 100 // failures (all clients) in the window before everyone waits a minute
	globalWindow  = 5 * time.Minute
)

func (l *Limiter) get(key string) *bucket {
	if l.clients == nil {
		l.clients = map[string]*bucket{}
	}
	b, ok := l.clients[key]
	if !ok {
		b = &bucket{}
		l.clients[key] = b
	}
	// forget clients that have been quiet for an hour
	if len(l.clients) > 256 {
		for k, x := range l.clients {
			if time.Since(x.last) > time.Hour && time.Now().After(x.lockUntil) {
				delete(l.clients, k)
			}
		}
	}
	return b
}

func (l *Limiter) Allow(key string) error {
	l.mu.Lock()
	defer l.mu.Unlock()
	now := time.Now()
	if now.Before(l.global.lockUntil) {
		return ErrLocked
	}
	if now.Sub(l.global.last) > globalWindow {
		l.global.failures = 0
	}
	if b := l.get(key); now.Before(b.lockUntil) {
		return ErrLocked
	}
	return nil
}

func (l *Limiter) Fail(key string) {
	l.mu.Lock()
	defer l.mu.Unlock()
	now := time.Now()
	b := l.get(key)
	b.failures++
	b.last = now
	if b.failures >= perClientLock {
		d := 30 * time.Second << uint(b.failures-perClientLock)
		if d > 10*time.Minute {
			d = 10 * time.Minute
		}
		b.lockUntil = now.Add(d)
	}
	if now.Sub(l.global.last) > globalWindow {
		l.global.failures = 0
	}
	l.global.failures++
	l.global.last = now
	if l.global.failures >= globalLock {
		l.global.lockUntil = now.Add(time.Minute)
		l.global.failures = 0
	}
}

func (l *Limiter) Reset(key string) {
	l.mu.Lock()
	defer l.mu.Unlock()
	delete(l.clients, key)
}