/**
 * EDR run-number pairs, keyed by the odd EXPORT run (Ethiopia → Djibouti). The
 * even IMPORT run (Djibouti → Ethiopia) is fixed by the export run.
 *
 * Run numbers are always 4 digits (8401, never 84001). Pairs are listed out
 * rather than computed from the 8001/+100/+1 pattern, so a run that ever breaks
 * the convention stays correct here.
 *
 * SeedWagonRunNumbers2280000000000 carries its own frozen copy on purpose: a
 * migration must keep doing what it did when it was applied, whereas this list
 * is live config for the update script. Add or retire runs HERE.
 */
export const TRAIN_RUN_PAIRS: Record<string, string> = {
  '8001': '8002',
  '8101': '8102',
  '8201': '8202',
  '8301': '8302',
  '8401': '8402',
  '8501': '8502',
  '8601': '8602',
  '8701': '8702',
  '8801': '8802',
  '8901': '8902',
  '9001': '9002',
};

/** Even IMPORT run -> its odd EXPORT run. Derived so the two cannot drift. */
export const EXPORT_BY_IMPORT: Record<string, string> = Object.fromEntries(
  Object.entries(TRAIN_RUN_PAIRS).map(([exportRun, importRun]) => [importRun, exportRun]),
);

/**
 * Normalise any run number to its EXPORT run. Accepts either half of a pair, so
 * a sheet listing "8002" and one listing "8001" both resolve to the same train.
 * Returns null when the number belongs to no known run.
 */
export const toExportRun = (run: string): string | null => {
  const value = run.trim();
  if (TRAIN_RUN_PAIRS[value]) return value;
  return EXPORT_BY_IMPORT[value] ?? null;
};