import {
    CallHandler,
    ExecutionContext,
    Injectable,
    NestInterceptor,
} from "@nestjs/common";
import { trace } from "@opentelemetry/api";
import { Response } from "express";
import { Observable, tap } from "rxjs";

/**
 * Interceptor that adds the OpenTelemetry trace ID to HTTP response headers.
 * This enables clients to correlate their requests with backend traces in
 * observability tools like Grafana Tempo.
 *
 * Response headers added:
 * - `X-Trace-Id`: The trace ID in hex format (e.g., "abc123def456...")
 *
 * Usage: Clients can use this header to search for traces in Grafana:
 * Explore → Tempo → TraceQL: `{ trace:id="<X-Trace-Id value>" }`
 */
@Injectable()
export class TraceIdInterceptor implements NestInterceptor {
    intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
        const response = context.switchToHttp().getResponse<Response>();

        return next.handle().pipe(
            tap(() => {
                const span = trace.getActiveSpan();
                if (span) {
                    const traceId = span.spanContext().traceId;
                    response.setHeader("X-Trace-Id", traceId);
                }
            }),
        );
    }
}