// Package linkstore binds an existing local person to an independently verified
// Home identity. It creates no people, sessions, membership or document grants.
package linkstore

import (
	"context"
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha256"
	"database/sql"
	"embed"
	"encoding/base64"
	"encoding/binary"
	"encoding/hex"
	"errors"
	"net/url"
	"strings"
	"time"
	"unicode/utf8"

	"amadan.net/oficina/home/handoff"
	"amadan.net/oficina/home/handoff/seal"
)

//go:embed migrations/*.sql
var migrations embed.FS

// Migrations must be applied before construction.
var Migrations = migrations

const SchemaNamespace = "oficina_connect_link"

var (
	ErrRefused = errors.New("home link: refused")
	ErrStorage = errors.New("home link: storage unavailable")
)

const maxFlows = 10000
const maxAge = 300

// Local is supplied only after the caller verifies the actual local session
// and its method-aware authentication time. Home assertions cannot supply it.
// SessionToken is the actual cookie value, never a subject or a minted alias.
type Local struct {
	PersonID     int64
	SessionToken string
	AuthTime     time.Time
}

// Home contains already verified, consumed shared-handoff claims. The caller
// must not construct it from query parameters or an unverified assertion.
type Home struct {
	Issuer, Subject, Nonce string
	AuthTime               time.Time
}

// Pending is safe server-side continuation state, not an authentication token.
type Pending struct {
	PersonID     int64
	ReturnPath   string
	HomeAuthTime time.Time
}

type Store struct {
	db             *sql.DB
	seal           *seal.Sealer
	origin, issuer string
	lookup         LookupPerson
	bindingKey     []byte
	namespace      string
}

// New uses a dedicated handoff/storage DEK, never an application content key.
// Apply Schema first, then call Sweep at boot before accepting requests.
func New(db *sql.DB, dek []byte, origin, issuer, namespace string, lookup LookupPerson) (*Store, error) {
	if lookup == nil || db == nil || !validOrigin(origin) || !validOrigin(issuer) {
		return nil, ErrRefused
	}
	sealer, err := seal.New(dek, namespace)
	if err != nil {
		return nil, ErrRefused
	}
	return &Store{db: db, seal: sealer, origin: origin, issuer: issuer, lookup: lookup, bindingKey: append([]byte(nil), dek...), namespace: namespace}, nil
}

func validOrigin(v string) bool {
	u, err := url.Parse(v)
	return err == nil && len(v) <= 2048 && u.Scheme == "https" && u.Host != "" && u.Hostname() != "" && u.User == nil && u.Path == "" && u.RawQuery == "" && !u.ForceQuery && u.Fragment == "" && u.Opaque == "" && u.String() == v && !strings.ContainsAny(v, "\\\r\n\t ")
}

func hash(v string) string { sum := sha256.Sum256([]byte(v)); return hex.EncodeToString(sum[:]) }
func secretHash(v string) (string, bool) {
	if len(v) != 43 {
		return "", false
	}
	b, err := base64.RawURLEncoding.Strict().DecodeString(v)
	return hash(v), err == nil && len(b) == 32 && base64.RawURLEncoding.EncodeToString(b) == v
}
func newSecret() (string, error) {
	b := make([]byte, 32)
	if _, err := rand.Read(b); err != nil {
		return "", ErrStorage
	}
	return base64.RawURLEncoding.EncodeToString(b), nil
}
func fresh(at int64, now, high int64) bool { return at > 0 && at <= now && high-at <= maxAge }
func validLocal(local Local, now, high int64) bool {
	return local.PersonID > 0 && local.SessionToken != "" && len(local.SessionToken) <= 4096 && fresh(local.AuthTime.Unix(), now, high)
}
func validSubject(v string) bool {
	if v == "" || len(v) > 512 || !utf8.ValidString(v) {
		return false
	}
	for _, r := range v {
		if r <= 32 || r == 127 {
			return false
		}
	}
	return true
}

// begin obtains the write lock before any reads, including capacity checks.
// A persisted clock prevents stale evidence becoming fresh after a restart or
// a clock rollback. Match the shared handoff's 30-second rollback tolerance.
func (s *Store) begin(ctx context.Context, now time.Time) (*sql.Tx, int64, error) {
	if now.Unix() <= 0 {
		return nil, 0, ErrRefused
	}
	tx, err := s.db.BeginTx(ctx, nil)
	if err != nil {
		return nil, 0, ErrStorage
	}
	if _, err = tx.ExecContext(ctx, "UPDATE home_link_clock SET high_water = MAX(high_water, ?) WHERE id = 1", now.Unix()); err != nil {
		tx.Rollback()
		return nil, 0, ErrStorage
	}
	var high int64
	if err = tx.QueryRowContext(ctx, "SELECT high_water FROM home_link_clock WHERE id = 1").Scan(&high); err != nil {
		tx.Rollback()
		return nil, 0, ErrStorage
	}
	if now.Unix() < high-30 {
		tx.Rollback()
		return nil, 0, ErrRefused
	}
	return tx, high, nil
}
func refuse(tx *sql.Tx) error {
	if err := tx.Commit(); err != nil {
		return ErrStorage
	}
	return ErrRefused
}
func commit(tx *sql.Tx) error {
	if err := tx.Commit(); err != nil {
		return ErrStorage
	}
	return nil
}
func sweep(ctx context.Context, tx *sql.Tx, high int64) error {
	if _, err := tx.ExecContext(ctx, "DELETE FROM home_link_flows WHERE expires_at <= ?", high); err != nil {
		return ErrStorage
	}
	return nil
}
func (s *Store) Sweep(ctx context.Context, now time.Time) error {
	tx, high, err := s.begin(ctx, now)
	if err != nil {
		return err
	}
	defer tx.Rollback()
	if err := sweep(ctx, tx, high); err != nil {
		return err
	}
	return commit(tx)
}

// Resolve reads the current existing person, never EnsurePerson or a cached
// address. A deleted or unlinked person is refused without creating anything.
func (s *Store) Resolve(ctx context.Context, issuer, subject string) (int64, string, error) {
	if issuer != s.issuer || !validSubject(subject) {
		return 0, "", ErrRefused
	}
	var id int64
	var address, binding string
	err := s.db.QueryRowContext(ctx, `SELECT person_id, person_binding FROM home_identity_links WHERE issuer = ? AND subject = ?`, issuer, subject).Scan(&id, &binding)
	if errors.Is(err, sql.ErrNoRows) {
		return 0, "", ErrRefused
	}
	if err != nil {
		return 0, "", ErrStorage
	}
	address, err = s.lookup(ctx, s.db, id)
	if errors.Is(err, sql.ErrNoRows) {
		return 0, "", ErrRefused
	}
	if err != nil {
		return 0, "", ErrStorage
	}
	if id <= 0 || address == "" || !hmac.Equal([]byte(binding), []byte(s.personBinding(id, address))) {
		return 0, "", ErrRefused
	}
	return id, address, nil
}

func safePath(v string) string {
	if handoff.SafePath(v) {
		return v
	}
	return "/"
}

// Queryer permits lookups inside the same transaction as link writes.
type Queryer interface {
	QueryRowContext(context.Context, string, ...any) *sql.Row
}

// LookupPerson returns the canonical verified local address, never a display name.
// IDs must never be reused; deletion must remove links in the same transaction. It must never
// create one, and must use q rather than opening a separate transaction.
// Missing or currently inadmissible people return sql.ErrNoRows.
type LookupPerson func(ctx context.Context, q Queryer, id int64) (string, error)

// personBinding prevents a stale numeric link from resolving a different address.
// Length-prefixed fields keep the namespace, origin, ID and address unambiguous.
func (s *Store) personBinding(id int64, address string) string {
	m := hmac.New(sha256.New, s.bindingKey)
	for _, field := range []string{"home-connect/person-binding/v1", s.namespace, s.origin, s.issuer} {
		_ = binary.Write(m, binary.BigEndian, uint64(len(field)))
		_, _ = m.Write([]byte(field))
	}
	_ = binary.Write(m, binary.BigEndian, id)
	_ = binary.Write(m, binary.BigEndian, uint64(len(address)))
	_, _ = m.Write([]byte(address))
	return hex.EncodeToString(m.Sum(nil))
}