gods-eye-view/src/sources/live/contract.js

/**
 * Browser live-source contract. Coordinates are WGS84 degrees; lengths and
 * velocities are metres and metres/second; times are Unix milliseconds.
 * Barometric altitude and ellipsoid altitude are separate, nullable observations.
 * A missing timestamp is unknown, never the time the response was received.
 *
 * Sources expose getSnapshot(query, { signal }) and optionally
 * getTrack(reference, { signal }). References are opaque to consumers.
 * Snapshots describe coverage and completeness independently of freshness.
 * Records contain observation data only, with no scene objects or transport data.
 */
export class LiveSourceError extends Error {
  constructor(
    code,
    message,
    { status = null, retryAfterMs = 20000, source = null } = {},
  ) {
    super(message);
    this.name = 'LiveSourceError';
    this.code = code;
    this.status = status;
    this.retryAfterMs = retryAfterMs;
    this.source = source;
  }
}

export function finite(value) {
  if (value == null || value === '' || typeof value === 'boolean') return null;
  const number = Number(value);
  return Number.isFinite(number) ? number : null;
}

export function epoch(value, scale = 1) {
  const number = finite(value);
  return number != null && number > 0 ? number * scale : null;
}

export function cleanText(value) {
  return typeof value === 'string' ? value.trim() : '';
}

export function coordinates(latitude, longitude) {
  return (
    Number.isFinite(latitude) &&
    Math.abs(latitude) <= 90 &&
    Number.isFinite(longitude) &&
    Math.abs(longitude) <= 180
  );
}

/** Admit a snapshot atomically; an all-invalid nonempty feed is unavailable. */
export function admitRecords(rows, normalize, label) {
  if (!Array.isArray(rows))
    throw new LiveSourceError('malformed', `Malformed ${label} response`);
  const records = [];
  const ids = new Set();
  for (const row of rows) {
    const record = normalize(row);
    if (record && !ids.has(record.id)) {
      records.push(record);
      ids.add(record.id);
    }
  }
  if (rows.length && !records.length) {
    throw new LiveSourceError('malformed', `Malformed ${label} aircraft rows`);
  }
  return {
    records,
    complete: records.length === rows.length,
    rejectedCount: rows.length - records.length,
  };
}

/** Cancellation is checked after body parsing even when a transport ignores it. */
export async function readResponse(
  fetchImpl,
  url,
  { signal, ...init } = {},
  source = 'Live source',
) {
  signal?.throwIfAborted();
  let response;
  try {
    response = await fetchImpl(url, { ...init, signal });
    signal?.throwIfAborted();
  } catch (error) {
    signal?.throwIfAborted();
    if (error?.name === 'AbortError') throw error;
    throw new LiveSourceError('unavailable', `${source} network error`, {
      source,
    });
  }
  let payload = null;
  try {
    payload = await response.json();
  } catch {
    /* classified below */
  }
  signal?.throwIfAborted();
  return { response, payload };
}

export function httpError(response, source) {
  const status = response.status;
  const code =
    status === 429
      ? 'limited'
      : status === 401 || status === 403
        ? 'denied'
        : 'unavailable';
  return new LiveSourceError(
    code,
    status === 429 ? `${source} rate limited` : `${source} HTTP ${status}`,
    {
      status,
      source,
      retryAfterMs: code === 'limited' || code === 'denied' ? 45000 : 20000,
    },
  );
}