// Package auth implements the service's own session login (AGENTS.md §9 Auth:
// a separate session login, not SSO). Sessions are stateless, signed cookies.
// A random CSRF cookie uses the double-submit pattern for forms.
package auth

import (
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"errors"
	"fmt"
	"net/http"
	"strconv"
	"strings"
	"time"

	"golang.org/x/crypto/bcrypt"
)

const (
	sessionCookie = "school_session"
	csrfCookie    = "school_csrf"
)

// Manager issues and validates sessions.
type Manager struct {
	// Secret signs every cookie. Required.
	Secret []byte
	// TTL is the session lifetime.
	TTL time.Duration
	// Secure sets the Secure cookie attribute. Keep true behind HTTPS.
	Secure bool
	// Username is the single allowed login name.
	Username string
	// passwordHash is a bcrypt hash, or nil when only a plaintext password is set.
	passwordHash []byte
	// plaintext is the fallback password, or empty when a hash is set.
	plaintext string
}

// New builds a Manager. Exactly one of bcryptHash or plaintext must be set.
func New(secret []byte, ttl time.Duration, secure bool, username, bcryptHash, plaintext string) (*Manager, error) {
	if len(secret) < 16 {
		return nil, errors.New("session secret too short")
	}
	m := &Manager{Secret: secret, TTL: ttl, Secure: secure, Username: username}
	switch {
	case bcryptHash != "":
		if _, err := bcrypt.Cost([]byte(bcryptHash)); err != nil {
			return nil, fmt.Errorf("SCHOOL_ADMIN_PASSWORD_HASH is not a valid bcrypt hash: %w", err)
		}
		m.passwordHash = []byte(bcryptHash)
	case plaintext != "":
		m.plaintext = plaintext
	default:
		return nil, errors.New("no admin password configured")
	}
	return m, nil
}

// Hashed reports whether the manager verifies against a bcrypt hash.
func (m *Manager) Hashed() bool { return m.passwordHash != nil }

// CheckCredentials validates a username and password in constant time where
// possible. It never reveals which part was wrong.
func (m *Manager) CheckCredentials(username, password string) bool {
	userOK := subtle.ConstantTimeCompare([]byte(username), []byte(m.Username)) == 1
	passOK := false
	if m.passwordHash != nil {
		passOK = bcrypt.CompareHashAndPassword(m.passwordHash, []byte(password)) == nil
	} else {
		passOK = subtle.ConstantTimeCompare([]byte(password), []byte(m.plaintext)) == 1
	}
	return userOK && passOK
}

// Issue writes a signed session cookie and the matching CSRF cookie.
func (m *Manager) Issue(w http.ResponseWriter) {
	csrf := randomToken()
	expires := time.Now().Add(m.TTL).Unix()
	payload := strings.Join([]string{m.Username, csrf, strconv.FormatInt(expires, 10)}, "\n")

	http.SetCookie(w, &http.Cookie{
		Name:     sessionCookie,
		Value:    m.sign(payload),
		Path:     "/",
		HttpOnly: true,
		Secure:   m.Secure,
		SameSite: http.SameSiteLaxMode,
		Expires:  time.Unix(expires, 0),
		MaxAge:   int(m.TTL.Seconds()),
	})
	http.SetCookie(w, &http.Cookie{
		Name:     csrfCookie,
		Value:    csrf,
		Path:     "/",
		HttpOnly: true,
		Secure:   m.Secure,
		SameSite: http.SameSiteLaxMode,
		Expires:  time.Unix(expires, 0),
		MaxAge:   int(m.TTL.Seconds()),
	})
}

// Clear removes the session and CSRF cookies.
func (m *Manager) Clear(w http.ResponseWriter) {
	for _, name := range []string{sessionCookie, csrfCookie} {
		http.SetCookie(w, &http.Cookie{
			Name:     name,
			Value:    "",
			Path:     "/",
			HttpOnly: true,
			Secure:   m.Secure,
			SameSite: http.SameSiteLaxMode,
			MaxAge:   -1,
			Expires:  time.Unix(0, 0),
		})
	}
}

// Authenticated reports whether the request carries a valid, unexpired session.
func (m *Manager) Authenticated(r *http.Request) bool {
	c, err := r.Cookie(sessionCookie)
	if err != nil {
		return false
	}
	payload, ok := m.verify(c.Value)
	if !ok {
		return false
	}
	parts := strings.Split(payload, "\n")
	if len(parts) != 3 || parts[0] != m.Username {
		return false
	}
	expires, err := strconv.ParseInt(parts[2], 10, 64)
	if err != nil || time.Now().Unix() > expires {
		return false
	}
	return true
}

// EnsureCSRF returns the CSRF token for a form, setting the cookie if absent.
func (m *Manager) EnsureCSRF(w http.ResponseWriter, r *http.Request) string {
	if c, err := r.Cookie(csrfCookie); err == nil && c.Value != "" {
		return c.Value
	}
	token := randomToken()
	http.SetCookie(w, &http.Cookie{
		Name:     csrfCookie,
		Value:    token,
		Path:     "/",
		HttpOnly: true,
		Secure:   m.Secure,
		SameSite: http.SameSiteLaxMode,
		MaxAge:   int(m.TTL.Seconds()),
	})
	return token
}

// CheckCSRF compares the submitted form token with the CSRF cookie.
func (m *Manager) CheckCSRF(r *http.Request) bool {
	c, err := r.Cookie(csrfCookie)
	if err != nil || c.Value == "" {
		return false
	}
	form := r.PostFormValue("csrf")
	return subtle.ConstantTimeCompare([]byte(form), []byte(c.Value)) == 1
}

func (m *Manager) sign(payload string) string {
	mac := hmac.New(sha256.New, m.Secret)
	mac.Write([]byte(payload))
	sig := mac.Sum(nil)
	return base64.RawURLEncoding.EncodeToString([]byte(payload)) + "." +
		base64.RawURLEncoding.EncodeToString(sig)
}

func (m *Manager) verify(token string) (string, bool) {
	encoded, sigEncoded, ok := strings.Cut(token, ".")
	if !ok {
		return "", false
	}
	payload, err := base64.RawURLEncoding.DecodeString(encoded)
	if err != nil {
		return "", false
	}
	gotSig, err := base64.RawURLEncoding.DecodeString(sigEncoded)
	if err != nil {
		return "", false
	}
	mac := hmac.New(sha256.New, m.Secret)
	mac.Write(payload)
	if !hmac.Equal(gotSig, mac.Sum(nil)) {
		return "", false
	}
	return string(payload), true
}

func randomToken() string {
	b := make([]byte, 32)
	if _, err := rand.Read(b); err != nil {
		panic("auth: random source failed: " + err.Error())
	}
	return base64.RawURLEncoding.EncodeToString(b)
}