// Package session implements signed admin-session tokens.
//
// Tokens are opaque strings of the form
//
//	base64url(payload) . base64url(HMAC-SHA256(payload))
//
// where payload is a small JSON ({actor, role, iat, exp}). A 32-byte
// secret on disk (0600 edgeguard:edgeguard, generated on first use)
// keys the HMAC. No DB round-trip for verification — handlers
// validate the token and trust the payload.
//
// Pattern 1:1 nach mail-gateway/internal/services/session/. Audience-
// Splitting (admin vs portal) und API-Key-Synthese sind bewusst nicht
// im v1-Scope (kein Quarantine-Portal in EdgeGuard).
package session

import (
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"time"
)

const (
	DefaultSecretPath = "/var/lib/edgeguard/.jwt_fingerprint"

	defaultTTL = 24 * time.Hour
	secretSize = 32
)

type Token struct {
	Actor string `json:"actor"`
	Role  string `json:"role,omitempty"`
	Iat   int64  `json:"iat"`
	Exp   int64  `json:"exp"`
}

type Signer struct {
	Secret []byte
	Now    func() time.Time
	TTL    time.Duration
}

// NewSignerFromPath loads or creates a 32-byte secret at path. Parent
// dir gets 0o700, file is 0o600.
func NewSignerFromPath(path string) (*Signer, error) {
	if path == "" {
		path = DefaultSecretPath
	}
	secret, err := loadOrCreateSecret(path)
	if err != nil {
		return nil, err
	}
	return &Signer{
		Secret: secret,
		Now:    func() time.Time { return time.Now().UTC() },
		TTL:    defaultTTL,
	}, nil
}

// NewSigner builds a signer with an in-memory secret — for tests.
func NewSigner(secret []byte, now func() time.Time, ttl time.Duration) *Signer {
	if now == nil {
		now = func() time.Time { return time.Now().UTC() }
	}
	if ttl == 0 {
		ttl = defaultTTL
	}
	return &Signer{Secret: secret, Now: now, TTL: ttl}
}

func loadOrCreateSecret(path string) ([]byte, error) {
	if b, err := os.ReadFile(path); err == nil {
		if len(b) < secretSize {
			return nil, fmt.Errorf("%s is shorter than %d bytes", path, secretSize)
		}
		return b[:secretSize], nil
	}
	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
		return nil, err
	}
	secret := make([]byte, secretSize)
	if _, err := rand.Read(secret); err != nil {
		return nil, err
	}
	if err := os.WriteFile(path, secret, 0o600); err != nil {
		return nil, err
	}
	return secret, nil
}

// IssueWithRole returns a signed token for the given actor + role.
func (s *Signer) IssueWithRole(actor, role string) (string, *Token, error) {
	now := s.Now()
	t := Token{
		Actor: actor,
		Role:  role,
		Iat:   now.Unix(),
		Exp:   now.Add(s.TTL).Unix(),
	}
	data, err := json.Marshal(t)
	if err != nil {
		return "", nil, err
	}
	mac := hmac.New(sha256.New, s.Secret)
	mac.Write(data)
	signature := mac.Sum(nil)
	encoded := base64.RawURLEncoding.EncodeToString(data) + "." +
		base64.RawURLEncoding.EncodeToString(signature)
	return encoded, &t, nil
}

// Issue is IssueWithRole with empty role.
func (s *Signer) Issue(actor string) (string, *Token, error) {
	return s.IssueWithRole(actor, "")
}

// Verify checks a token. Returns ErrInvalidToken or ErrExpiredToken.
func (s *Signer) Verify(raw string) (*Token, error) {
	if raw == "" {
		return nil, ErrInvalidToken
	}
	var payloadB64, sigB64 string
	for i := 0; i < len(raw); i++ {
		if raw[i] == '.' {
			payloadB64 = raw[:i]
			sigB64 = raw[i+1:]
			break
		}
	}
	if payloadB64 == "" || sigB64 == "" {
		return nil, ErrInvalidToken
	}
	payload, err := base64.RawURLEncoding.DecodeString(payloadB64)
	if err != nil {
		return nil, ErrInvalidToken
	}
	sig, err := base64.RawURLEncoding.DecodeString(sigB64)
	if err != nil {
		return nil, ErrInvalidToken
	}
	mac := hmac.New(sha256.New, s.Secret)
	mac.Write(payload)
	if subtle.ConstantTimeCompare(mac.Sum(nil), sig) != 1 {
		return nil, ErrInvalidToken
	}
	var t Token
	if err := json.Unmarshal(payload, &t); err != nil {
		return nil, ErrInvalidToken
	}
	if s.Now().Unix() >= t.Exp {
		return nil, ErrExpiredToken
	}
	return &t, nil
}

var (
	ErrInvalidToken = errors.New("invalid session token")
	ErrExpiredToken = errors.New("session token expired")
)