source: frontend/node_modules/@jridgewell/gen-mapping/src/set-array.ts

type Key = string | number | symbol;

/**
 * SetArray acts like a `Set` (allowing only one occurrence of a string `key`), but provides the
 * index of the `key` in the backing array.
 *
 * This is designed to allow synchronizing a second array with the contents of the backing array,
 * like how in a sourcemap `sourcesContent[i]` is the source content associated with `source[i]`,
 * and there are never duplicates.
 */
export class SetArray<T extends Key = Key> {
  declare private _indexes: Record<T, number | undefined>;
  declare array: readonly T[];

  constructor() {
    this._indexes = { __proto__: null } as any;
    this.array = [];
  }
}

interface PublicSet<T extends Key> {
  array: T[];
  _indexes: SetArray<T>['_indexes'];
}

/**
 * Typescript doesn't allow friend access to private fields, so this just casts the set into a type
 * with public access modifiers.
 */
function cast<T extends Key>(set: SetArray<T>): PublicSet<T> {
  return set as any;
}

/**
 * Gets the index associated with `key` in the backing array, if it is already present.
 */
export function get<T extends Key>(setarr: SetArray<T>, key: T): number | undefined {
  return cast(setarr)._indexes[key];
}

/**
 * Puts `key` into the backing array, if it is not already present. Returns
 * the index of the `key` in the backing array.
 */
export function put<T extends Key>(setarr: SetArray<T>, key: T): number {
  // The key may or may not be present. If it is present, it's a number.
  const index = get(setarr, key);
  if (index !== undefined) return index;

  const { array, _indexes: indexes } = cast(setarr);

  const length = array.push(key);
  return (indexes[key] = length - 1);
}

/**
 * Pops the last added item out of the SetArray.
 */
export function pop<T extends Key>(setarr: SetArray<T>): void {
  const { array, _indexes: indexes } = cast(setarr);
  if (array.length === 0) return;

  const last = array.pop()!;
  indexes[last] = undefined;
}

/**
 * Removes the key, if it exists in the set.
 */
export function remove<T extends Key>(setarr: SetArray<T>, key: T): void {
  const index = get(setarr, key);
  if (index === undefined) return;

  const { array, _indexes: indexes } = cast(setarr);
  for (let i = index + 1; i < array.length; i++) {
    const k = array[i];
    array[i - 1] = k;
    indexes[k]!--;
  }
  indexes[key] = undefined;
  array.pop();
}