import type { MetricRegistry } from "../../observability/metric-registry.ts";
import type { HistogramPoint, MetricLabels, MetricLabelValue, MetricPoint } from "../../observability/metrics-primitives.ts";
import type { RunUiSnapshot } from "../snapshot-types.ts";

export interface MetricsPaneOptions {
	registry?: MetricRegistry;
	maxCounters?: number;
}

// Metric label values come from an unvalidated `Record` (disk/producer fed) —
// every interpolation is guarded so `undefined` can never reach the pane.
function labelValue(value: MetricLabelValue | undefined): string {
	if (value === undefined || value === null) return "?";
	if (typeof value === "number" && !Number.isFinite(value)) return "?";
	return `${value}`;
}

function labelsText(labels: MetricLabels | undefined): string {
	const entries = Object.entries(labels ?? {});
	return entries.length ? `{${entries.map(([key, value]) => `${key}=${labelValue(value)}`).join(",")}}` : "";
}

function isHistogramPoint(point: MetricPoint | HistogramPoint): point is HistogramPoint {
	return "quantiles" in point;
}

export function renderMetricsPane(_snapshot: RunUiSnapshot | undefined, opts: MetricsPaneOptions = {}): string[] {
	if (!opts.registry) return ["Metrics pane: registry unavailable"];
	const snapshots = opts.registry.snapshot();
	if (!snapshots.length) return ["Metrics pane: no metrics recorded"];
	const lines = ["Metrics pane: top metrics"];
	for (const snapshot of snapshots.slice(0, opts.maxCounters ?? 10)) {
		const first = snapshot.values[0];
		if (!first) {
			lines.push(`${snapshot.name ?? "?"}: empty`);
			continue;
		}
		if (isHistogramPoint(first))
			lines.push(
				`${snapshot.name ?? "?"}${labelsText(first.labels)} count=${first.count ?? 0} p95=${Number.isFinite(first.quantiles?.p95) ? Math.round(first.quantiles.p95) : "n/a"}`,
			);
		else lines.push(`${snapshot.name ?? "?"}${labelsText(first.labels)} ${labelValue(first.value)}`);
	}
	return lines;
}