/**
 * Lightweight request tracing, toggled via TRACE_REQUESTS=true.
 *
 * Usage:
 *   const trace = createTrace();
 *   const result = await span(trace, 'db:settings', () => getCachedSettings(...));
 *   logTrace(trace, 'GET /index.html', logger);
 */

export const TRACE_ENABLED = process.env.TRACE_REQUESTS === 'true'

export interface Span {
	name: string
	durationMs: number
}

export interface RequestTrace {
	spans: Span[]
	startMs: number
}

export function createTrace(): RequestTrace | null {
	if (!TRACE_ENABLED) return null
	return { spans: [], startMs: performance.now() }
}

export async function span<T>(trace: RequestTrace | null | undefined, name: string, fn: () => Promise<T>): Promise<T> {
	if (!trace) return fn()
	const t0 = performance.now()
	try {
		return await fn()
	} finally {
		trace.spans.push({ name, durationMs: +(performance.now() - t0).toFixed(2) })
	}
}

export function logTrace(
	trace: RequestTrace | null,
	label: string,
	logger: { info: (msg: string, ctx?: Record<string, unknown>) => void },
) {
	if (!trace) return
	const totalMs = +(performance.now() - trace.startMs).toFixed(2)
	const breakdown: Record<string, unknown> = { total: totalMs }
	for (const s of trace.spans) {
		breakdown[s.name] = s.durationMs
	}
	logger.info(`TRACE ${label}`, breakdown)
}