// The member URN an attendee list records for a participant, and the secret it is derived from
// (decision_2026-10-29_participation-privacy-mechanism).
//
// THE URN. `urn:festipod:userref:` + the lowercase-hex sha256 of the string
// "<participant WebID> <event URI> <secret>" (single spaces), where the event URI is the meeting
// point's anchor (`as:context`). It is the same for one person on every meeting point of one event —
// so the per-event count of distinct URNs is publicly computable — and unrelated from one event to
// another. Only whoever can read the secret can compute it: the person, their contacts, the app. The
// backend computes it from the secret; a client never supplies one, so it cannot inflate a count.
// ("festipod" here is the URN namespace, unrelated to the http://festipod.org/ vocabulary.)
//
// THE SECRET lives in the participant's `as:Profile` under `http://festipod.org/participationSecret`:
// 32 random bytes as 64 lowercase hex characters, written by the participant's browser before the
// first sign-up. Anything else — absent, several values, another length, uppercase — is no secret.

const crypto = require('crypto');

const MEMBER_URN_PREFIX = 'urn:festipod:userref:';
const PARTICIPATION_SECRET = 'http://festipod.org/participationSecret';
const SECRET_PATTERN = /^[0-9a-f]{64}$/;

const listOf = value => (value === undefined || value === null ? [] : Array.isArray(value) ? value : [value]);

/** The string a JSON-LD value holds: a string, or a value node's `@value`; undefined otherwise. */
const stringOf = value => {
  if (typeof value === 'string') return value;
  if (value && typeof value === 'object' && typeof value['@value'] === 'string') return value['@value'];
  return undefined;
};

/**
 * participationSecretOf(profile) → the participant's secret (64 lowercase hex characters), or
 * undefined when the profile carries none that is usable. Reads the predicate under its full IRI or
 * the `fp:` CURIE; exactly one value is accepted.
 */
function participationSecretOf(profile) {
  if (!profile || typeof profile !== 'object') return undefined;
  const values = listOf(profile[PARTICIPATION_SECRET] ?? profile['fp:participationSecret']);
  if (values.length !== 1) return undefined;
  const secret = stringOf(values[0]);
  return secret !== undefined && SECRET_PATTERN.test(secret) ? secret : undefined;
}

/** memberUrn({ participantUri, eventUri, secret }) → `urn:festipod:userref:<sha256 hex>`. */
function memberUrn({ participantUri, eventUri, secret }) {
  for (const [name, value] of Object.entries({ participantUri, eventUri, secret })) {
    if (typeof value !== 'string' || value.length === 0) throw new Error(`memberUrn needs a ${name}`);
  }
  const digest = crypto.createHash('sha256').update(`${participantUri} ${eventUri} ${secret}`, 'utf8').digest('hex');
  return MEMBER_URN_PREFIX + digest;
}

module.exports = { MEMBER_URN_PREFIX, PARTICIPATION_SECRET, participationSecretOf, memberUrn };