// Package audit is MyDay's append-only security trail (A11).
//
// Every auth-sensitive event — bootstrap, enroll, login, logout, recovery
// request/approve/deny, revoke, role change — is recorded here, together with
// the client IP and (IP-derived, D-031) location. The trail outlives the users
// it names, so it carries no foreign keys.
//
// Spec: docs/specs/spec-004-identity-sessions.md §5.
package audit

import (
	"context"
	"database/sql"
	"log/slog"
	"time"
)

// Event is one audit entry. Secrets are NEVER placed in Detail or Subject
// (A9) — those columns are for identifiers and outcomes only.
type Event struct {
	// ActorUserID is the authenticated principal responsible for the event;
	// 0 means anonymous/system (stored as NULL).
	ActorUserID int64
	// Action is the event kind, e.g. "user.create", "identity.rotate".
	Action string
	// Subject is what the event acted on, e.g. "user:2" or "identity:5".
	Subject string
	// Detail is non-secret context, e.g. "role=manager".
	Detail string
	// ClientIP is the resolved client address (A6); optional.
	ClientIP string
	// Location is the approximate, IP-derived location — AUDIT ONLY (D-031).
	Location string
	// CorrelationID ties the event to its request (SPEC-003).
	CorrelationID string
}

// Audit records events. The contract is deliberately fire-and-forget — the
// audit trail must never break a request.
//
// Per ruling R-1 (SPEC-004 §5) the event is recorded WITH the request context:
// the door attaches Meta before any service call, and services write their
// audit rows through it, so actor · client IP · correlation are never lost.
type Audit interface {
	Record(ctx context.Context, e Event)
}

// Meta is the per-request audit metadata the door attaches to the
// context.Context before calling any service (SPEC-004 §5, ruling R-1;
// SPEC-003 §4). It is IDENTIFICATION and CONTEXT only — never a credential.
type Meta struct {
	// ActorUserID is the authenticated principal responsible for the request;
	// 0 means anonymous/system.
	ActorUserID int64
	// ClientIP is the resolved client address (A6).
	ClientIP string
	// CorrelationID ties every event of one request together (SPEC-003 §4).
	CorrelationID string
}

// metaKey is the unexported context key for Meta; using a struct type prevents
// collisions with other packages' keys.
type metaKey struct{}

// WithMeta attaches m to ctx. The door calls this once, before handing off to a
// module or a service.
func WithMeta(ctx context.Context, m Meta) context.Context {
	return context.WithValue(ctx, metaKey{}, m)
}

// MetaFrom returns the Meta attached to ctx, if any.
func MetaFrom(ctx context.Context) (Meta, bool) {
	m, ok := ctx.Value(metaKey{}).(Meta)
	return m, ok
}

// Merge fills an event's empty context fields from the metadata on ctx, so a
// service never has to re-thread actor · client IP · correlation by hand.
//
// Explicit event values WIN — a service that knows the actor better than the
// ambient request context (e.g. the identity owner) may set it and it is kept
// (ruling R-3).
func Merge(ctx context.Context, e Event) Event {
	m, ok := MetaFrom(ctx)
	if !ok {
		return e
	}
	if e.ActorUserID == 0 {
		e.ActorUserID = m.ActorUserID
	}
	if e.ClientIP == "" {
		e.ClientIP = m.ClientIP
	}
	if e.CorrelationID == "" {
		e.CorrelationID = m.CorrelationID
	}
	return e
}

// Log writes events to the audit_log table.
type Log struct {
	db  *sql.DB
	now func() time.Time
}

// New returns an Audit writing to db. time.Now is injectable for tests via the
// unexported field (same package).
func New(db *sql.DB) *Log {
	return &Log{db: db, now: time.Now}
}

// Record inserts one audit row. A failure is logged, never returned: an
// un-writable audit trail must not take the application down (but it is loud).
//
// The event is merged with the Meta carried on ctx first (ruling R-1), so the
// actor · client IP · correlation attached by the door reach the row.
func (l *Log) Record(ctx context.Context, e Event) {
	e = Merge(ctx, e)

	var actor any
	if e.ActorUserID != 0 {
		actor = e.ActorUserID
	}

	_, err := l.db.Exec(
		`INSERT INTO audit_log
			(at, actor_user_id, action, subject, detail, client_ip, location, correlation_id)
		 VALUES (?, ?, ?, ?, ?, ?, ?, ?)`,
		l.now().UTC().Truncate(time.Second).Format(time.RFC3339),
		actor,
		e.Action,
		nullString(e.Subject),
		nullString(e.Detail),
		nullString(e.ClientIP),
		nullString(e.Location),
		nullString(e.CorrelationID),
	)
	if err != nil {
		slog.Error("audit: record failed", "action", e.Action, "error", err)
	}
}

// nullString maps "" to NULL so the trail distinguishes "absent" from "empty".
func nullString(s string) any {
	if s == "" {
		return nil
	}
	return s
}

var _ Audit = (*Log)(nil)