ManyToManyMap.ts

function saveKeys<P, S>(set: Map<P | S, Set<P | S>>, fromKey: P, toKey: S) {
  if (set.has(fromKey))
    set.get(fromKey)!.add(toKey)
  else
    set.set(fromKey, new Set([toKey]))
}

function deleteKeys<P, S>(set: Map<P | S, Set<P | S>>, fromKey: P, toKey: S) {
  if (set.has(fromKey)) {
    const dependantSet = set.get(fromKey)!
    dependantSet.delete(toKey)
    if (dependantSet.size === 0)
      set.delete(fromKey)
  }
}

/**
 * Data structure for managing bidirectional
 * many-to-many mappings between primary and secondary keys.
 */
export class ManyToManyMap<P, S> {
  private p2s = new Map<P, Set<S>>()
  private s2p = new Map<S, Set<P>>()

  add = (primary: P, secondary: S) => {
    saveKeys(this.p2s, primary, secondary)
    saveKeys(this.s2p, secondary, primary)
  }

  remove = (primary: P, secondary: S) => {
    deleteKeys(this.p2s, primary, secondary)
    deleteKeys(this.s2p, secondary, primary)
  }

  getSecondaries = (primary: P) => this.p2s.get(primary)

  getPrimaries = (secondary: S) => this.s2p.get(secondary)

  hasPrimary = (primary: P) => this.p2s.has(primary)

  hasSecondary = (secondary: S) => this.s2p.has(secondary)

  clear = () => {
    this.p2s.clear()
    this.s2p.clear()
  }

  // TODO: implement iterator methods if you need)
}