Function getSessionId

apps/api/src/utils/ids.ts:146–196

* Deterministic session id for (projectId, deviceId) within a time window, * with a grace period at the *start* of each window to avoid boundary splits. * * - windowMs: 30 minutes by default * - graceMs: 1 minute by default (events in first minute of a bucket map to previous bucket) * - Output:

(params: {
  projectId: string;
  deviceId: string;
  eventMs?: number; // use event timestamp; defaults to Date.now()
  windowMs?: number; // default 5 min
  graceMs?: number; // default 1 min
  bytes?: number; // default 16 (128-bit). You can set 24 or 32 for longer ids.
})

 * - Output: base64url, 128-bit (16 bytes) truncated from SHA-256
 */
function getSessionId(params: {
  projectId: string;
  deviceId: string;
  eventMs?: number; // use event timestamp; defaults to Date.now()
  windowMs?: number; // default 5 min
  graceMs?: number; // default 1 min
  bytes?: number; // default 16 (128-bit). You can set 24 or 32 for longer ids.
}): string {
  const {
    projectId,
    deviceId,
    eventMs = Date.now(),
    windowMs = 5 * 60 * 1000,
    graceMs = 60 * 1000,
    bytes = 16,
  } = params;

  if (!projectId) {
    throw new Error('projectId is required');
  }
  if (!deviceId) {
    throw new Error('deviceId is required');
  }
  if (windowMs <= 0) {
    throw new Error('windowMs must be > 0');
  }
  if (graceMs < 0 || graceMs >= windowMs) {
    throw new Error('graceMs must be >= 0 and < windowMs');
  }
  if (bytes < 8 || bytes > 32) {
    throw new Error('bytes must be between 8 and 32');
  }

  const bucket = Math.floor(eventMs / windowMs);
  const offset = eventMs - bucket * windowMs;

  // Grace at the start of the bucket: stick to the previous bucket.
  const chosenBucket = offset < graceMs ? bucket - 1 : bucket;

  const input = `sess:v1:${projectId}:${deviceId}:${chosenBucket}`;

  const digest = crypto.createHash('sha256').update(input).digest();
  const truncated = digest.subarray(0, bytes);

  // base64url
  return truncated
    .toString('base64')
    .replace(/\+/g, '-')
    .replace(/\//g, '_')
    .replace(/=+$/g, '');
}