CRDTs and Client Sync

// two small CRDTs: a G-counter and an observed-remove set
type GCounter = Record<string, number>;
const inc = (c: GCounter, me: string, n = 1): GCounter => ({ ...c, [me]: (c[me] ?? 0) + n });
const value = (c: GCounter) => Object.values(c).reduce((a, b) => a + b, 0);
const mergeG = (a: GCounter, b: GCounter): GCounter => {
  const out = { ...a };
  for (const [k, v] of Object.entries(b)) out[k] = Math.max(out[k] ?? 0, v);   // commutative, idempotent
  return out;
};

type ORSet<T> = { adds: Map<string, T>; removed: Set<string> };          // tag → element; removed tags
const add = <T,>(s: ORSet<T>, x: T): ORSet<T> => ({ ...s, adds: new Map(s.adds).set(crypto.randomUUID(), x) });
const remove = <T,>(s: ORSet<T>, x: T): ORSet<T> =>                       // removes only the tags it has seen
  ({ ...s, removed: new Set([...s.removed, ...[...s.adds].filter(([, v]) => v === x).map(([t]) => t)]) });
const mergeS = <T,>(a: ORSet<T>, b: ORSet<T>): ORSet<T> =>
  ({ adds: new Map([...a.adds, ...b.adds]), removed: new Set([...a.removed, ...b.removed]) });
const elements = <T,>(s: ORSet<T>) => new Set([...s.adds].filter(([t]) => !s.removed.has(t)).map(([, v]) => v));