export const TELEMETRY_READINGS_MAX_ITEMS = 128;
export const TELEMETRY_READINGS_MAX_BYTES = 128 * 1024;

const READING_KEYS = new Set(["id", "label", "value", "unit", "observedAt"]);
const READING_ID = /^[a-z][a-z0-9._:-]{1,127}$/;

/**
 * Validate the provider-neutral bounded telemetry collection accepted inside
 * one current-position fact. Provider-specific payloads, raw sensor objects
 * and arbitrary recursive JSON are deliberately excluded.
 */
export function isBoundedTelemetryReadings(value) {
  if (!Array.isArray(value) || value.length > TELEMETRY_READINGS_MAX_ITEMS) return false;
  let encoded;
  try {
    encoded = new TextEncoder().encode(JSON.stringify(value));
  } catch {
    return false;
  }
  if (encoded.byteLength > TELEMETRY_READINGS_MAX_BYTES) return false;

  const ids = new Set();
  for (const reading of value) {
    if (!isPlainObject(reading) || Object.keys(reading).some((key) => !READING_KEYS.has(key))) return false;
    if (typeof reading.id !== "string" || !READING_ID.test(reading.id) || ids.has(reading.id)) return false;
    ids.add(reading.id);
    if (typeof reading.label !== "string" || !reading.label.trim() || reading.label.length > 160) return false;
    if (!isScalar(reading.value)) return false;
    if (typeof reading.value === "string" && reading.value.length > 512) return false;
    if (reading.unit !== undefined && (typeof reading.unit !== "string" || reading.unit.length > 32)) return false;
    if (reading.observedAt !== undefined && !isIsoTimestamp(reading.observedAt)) return false;
  }
  return true;
}

function isScalar(value) {
  return typeof value === "string"
    || typeof value === "boolean"
    || (typeof value === "number" && Number.isFinite(value));
}

function isIsoTimestamp(value) {
  return typeof value === "string" && !Number.isNaN(Date.parse(value));
}

function isPlainObject(value) {
  return Boolean(value) && typeof value === "object" && !Array.isArray(value);
}