package pier

import (
	"context"
	"errors"
	"slices"
	"strconv"
	"strings"
	"sync"
	"time"

	"git.golem15.com/golem15/summercms/modules/backpack"
	"git.golem15.com/golem15/summercms/modules/bouncer"
	"git.golem15.com/golem15/summercms/modules/cabana"
	"git.golem15.com/golem15/summercms/modules/pact"
)

// OAuthTokenPrefix is the prefix of the OAuth access tokens the published
// pact.MCPTokenVerifier verifies; every other bearer is a personal token.
const OAuthTokenPrefix = "oas_at_"

// Token kinds of an Identity.
const (
	TokenKindOAuth    = "oauth"
	TokenKindPersonal = "pat"
)

// LinkCacheTTL bounds how long a resolved backend link is reused on one
// instance before it is read again (P-07); a link event drops it at once.
const LinkCacheTTL = 5 * time.Second

// ErrUnauthorized is what identity resolution answers for a missing,
// unknown, expired or revoked bearer; the endpoint turns it into a 401 with
// the protected resource metadata challenge.
var ErrUnauthorized = errors.New("pier: unauthorized")

// Identity is who one MCP request acts as: the frontend subject (the OAuth
// grant's user or a frontend personal token), the backend subject (the
// linked admin when the grant carries the backend scope, or an admin
// personal token), the scopes of the credential, the session binding key
// (D-35: oauth:<client_id>:<user_id>, pat:user:<token_id> or
// pat:backend:<token_id>), the credential's expiry and its kind. Every
// request resolves its own Identity from its bearer; a session never
// replaces it.
type Identity struct {
	Frontend  *pact.MCPSubject
	Backend   *pact.MCPSubject
	Scopes    []string
	Binding   string
	ExpiresAt time.Time
	TokenKind string

	// ceiling says the read/write ceiling (D-20) applies: OAuth grants and
	// frontend personal tokens carry read and write; an admin personal
	// token is capped through its cabana scope limit instead.
	ceiling bool
	// credential is the admin personal token row stored next to the
	// backend principal on a dispatched call.
	credential any
}

// subjectFor returns the subject acting under guard, nil when the identity
// has none for it.
func (id Identity) subjectFor(guard string) *pact.MCPSubject {
	if id.Backend != nil && id.Backend.Guard == guard {
		return id.Backend
	}
	if id.Frontend != nil && id.Frontend.Guard == guard {
		return id.Frontend
	}
	return nil
}

// writes reports whether the identity may call tools that are not
// read-only: no ceiling, or the write scope granted.
func (id Identity) writes() bool {
	return !id.ceiling || slices.Contains(id.Scopes, "write")
}

// fingerprint encodes the authorization-relevant parts of the identity
// (P-26): the sorted scopes, both subjects (guard, user, client, token and
// their sorted scopes), the ceiling and the token kind. Two requests of one
// binding with different fingerprints may see different tools, so a cached
// server is re-filtered when it changes (A-WR-04).
func (id Identity) fingerprint() string {
	var b strings.Builder
	sorted := func(in []string) string {
		out := slices.Clone(in)
		slices.Sort(out)
		return strings.Join(out, ",")
	}
	subject := func(s *pact.MCPSubject) {
		if s == nil {
			b.WriteString("-|")
			return
		}
		b.WriteString(s.Guard)
		b.WriteByte(':')
		b.WriteString(strconv.FormatUint(uint64(s.UserID), 10))
		b.WriteByte(':')
		b.WriteString(s.ClientID)
		b.WriteByte(':')
		b.WriteString(strconv.FormatUint(uint64(s.TokenID), 10))
		b.WriteByte(':')
		b.WriteString(sorted(s.Scopes))
		b.WriteByte('|')
	}
	b.WriteString(sorted(id.Scopes))
	b.WriteByte('|')
	subject(id.Frontend)
	subject(id.Backend)
	b.WriteString(strconv.FormatBool(id.ceiling))
	b.WriteByte('|')
	b.WriteString(id.TokenKind)
	return b.String()
}

// subjects lists the identity's subjects (frontend first).
func (id Identity) subjects() []pact.MCPSubject {
	var out []pact.MCPSubject
	if id.Frontend != nil {
		out = append(out, *id.Frontend)
	}
	if id.Backend != nil {
		out = append(out, *id.Backend)
	}
	return out
}

// has reports whether the identity includes the subject (guard and user id).
func (id Identity) has(guard string, userID uint) bool {
	for _, s := range id.subjects() {
		if s.Guard == guard && s.UserID == userID {
			return true
		}
	}
	return false
}

// backendTokenFunc resolves an admin personal token; tests inject one in
// place of cabana.ResolveBackendToken, which needs a database.
type backendTokenFunc func(ctx context.Context, app *backpack.App, raw string) (*bouncer.Principal, *cabana.BackendAPIToken, []string, error)

// identityResolver turns a bearer into an Identity through the published
// verifier and resolvers, with the per-instance link cache.
type identityResolver struct {
	app           *backpack.App
	cfg           Config
	frontendGuard string
	backendTokens backendTokenFunc

	mu    sync.Mutex
	links map[uint]linkEntry
	now   func() time.Time
}

type linkEntry struct {
	link pact.LinkedBackendUser
	ok   bool
	at   time.Time
}

func newIdentityResolver(app *backpack.App, cfg Config, frontendGuard string) *identityResolver {
	return &identityResolver{
		app:           app,
		cfg:           cfg,
		frontendGuard: frontendGuard,
		backendTokens: cabana.ResolveBackendToken,
		links:         map[uint]linkEntry{},
		now:           time.Now,
	}
}

// resolve verifies raw afresh: an OAuth access token through the published
// pact.MCPTokenVerifier (missing verifier: unauthorized); with
// mcp.allow_personal_tokens true, an admin g15_ token through cabana and
// any other prefix through the published pact.MCPPersonalTokenResolver;
// with mcp.allow_personal_tokens false (the default) any bearer that is
// not an OAuth token is unauthorized without a lookup (D-19). A
// backend subject for an OAuth grant exists only with the backend scope and
// a link the published pact.MCPLinkedBackendResolver confirms (D-08).
func (r *identityResolver) resolve(ctx context.Context, raw string) (Identity, error) {
	raw = strings.TrimSpace(raw)
	if raw == "" {
		return Identity{}, ErrUnauthorized
	}
	if strings.HasPrefix(raw, OAuthTokenPrefix) {
		return r.resolveOAuth(ctx, raw)
	}
	if !r.cfg.AllowPersonalTokens {
		return Identity{}, ErrUnauthorized
	}
	if strings.HasPrefix(raw, cabana.APITokenPrefix) {
		return r.resolveBackendToken(ctx, raw)
	}
	return r.resolvePersonalToken(ctx, raw)
}

func (r *identityResolver) resolveOAuth(ctx context.Context, raw string) (Identity, error) {
	verifier, ok := r.app.Lookup[pact.MCPTokenVerifier]()
	if !ok || verifier == nil {
		return Identity{}, ErrUnauthorized
	}
	grant, err := verifier.MCPVerifyToken(ctx, raw)
	if err != nil || grant.UserID == 0 {
		return Identity{}, ErrUnauthorized
	}
	scopes := append([]string(nil), grant.Scopes...)
	id := Identity{
		Scopes:    scopes,
		Binding:   "oauth:" + grant.ClientID + ":" + strconv.FormatUint(uint64(grant.UserID), 10),
		ExpiresAt: grant.ExpiresAt,
		TokenKind: TokenKindOAuth,
		ceiling:   true,
	}
	if r.frontendGuard != "" {
		id.Frontend = &pact.MCPSubject{
			Guard:    r.frontendGuard,
			UserID:   grant.UserID,
			Scopes:   scopes,
			ClientID: grant.ClientID,
			TokenID:  grant.TokenID,
		}
	}
	if slices.Contains(scopes, "backend") {
		if link, ok := r.linkedBackend(ctx, grant.UserID); ok {
			id.Backend = &pact.MCPSubject{
				Guard:    BackendGuard,
				UserID:   link.ID,
				Scopes:   scopes,
				ClientID: grant.ClientID,
				TokenID:  grant.TokenID,
			}
		}
	}
	return id, nil
}

func (r *identityResolver) resolveBackendToken(ctx context.Context, raw string) (Identity, error) {
	principal, row, scopes, err := r.backendTokens(ctx, r.app, raw)
	if err != nil || principal == nil || row == nil {
		return Identity{}, ErrUnauthorized
	}
	id := Identity{
		Scopes:    append([]string(nil), scopes...),
		Binding:   "pat:backend:" + strconv.FormatUint(uint64(row.ID), 10),
		TokenKind: TokenKindPersonal,
		Backend: &pact.MCPSubject{
			Guard:   BackendGuard,
			UserID:  principal.ID,
			Scopes:  append([]string(nil), scopes...),
			TokenID: row.ID,
		},
		credential: row,
	}
	if row.ExpiresAt != nil {
		id.ExpiresAt = *row.ExpiresAt
	}
	return id, nil
}

func (r *identityResolver) resolvePersonalToken(ctx context.Context, raw string) (Identity, error) {
	resolver, ok := r.app.Lookup[pact.MCPPersonalTokenResolver]()
	if !ok || resolver == nil {
		return Identity{}, ErrUnauthorized
	}
	subject, ok, err := resolver.MCPResolvePersonalToken(ctx, raw)
	// TokenID keys the binding (pat:user:<token_id>): a subject without one
	// would put every user of the resolver on one binding (A-WR-09).
	if err != nil || !ok || subject.UserID == 0 || subject.TokenID == 0 {
		return Identity{}, ErrUnauthorized
	}
	if subject.Guard == "" {
		subject.Guard = r.frontendGuard
	}
	if subject.Guard == "" || subject.Guard == BackendGuard {
		return Identity{}, ErrUnauthorized
	}
	return Identity{
		Frontend:  &subject,
		Scopes:    append([]string(nil), subject.Scopes...),
		Binding:   "pat:user:" + strconv.FormatUint(uint64(subject.TokenID), 10),
		TokenKind: TokenKindPersonal,
		ceiling:   true,
	}, nil
}

// linkedBackend answers the backend account linked to a frontend user
// through the published resolver, reusing an answer younger than
// LinkCacheTTL on this instance (P-07).
func (r *identityResolver) linkedBackend(ctx context.Context, userID uint) (pact.LinkedBackendUser, bool) {
	resolver, ok := r.app.Lookup[pact.MCPLinkedBackendResolver]()
	if !ok || resolver == nil {
		return pact.LinkedBackendUser{}, false
	}
	now := r.now()
	r.mu.Lock()
	if e, ok := r.links[userID]; ok && now.Sub(e.at) < LinkCacheTTL {
		r.mu.Unlock()
		return e.link, e.ok
	}
	r.mu.Unlock()
	link, found, err := resolver.LinkedBackendUser(ctx, userID)
	if err != nil {
		return pact.LinkedBackendUser{}, false
	}
	if link.ID == 0 {
		found = false
	}
	r.mu.Lock()
	r.links[userID] = linkEntry{link: link, ok: found, at: now}
	r.mu.Unlock()
	return link, found
}

// forgetLink drops the cached link of one frontend user (a link event).
func (r *identityResolver) forgetLink(userID uint) {
	r.mu.Lock()
	delete(r.links, userID)
	r.mu.Unlock()
}