const sectionKey = (key) =>
  key
    .split(",")
    .map((value) => Math.floor(Number(value) / 16))
    .join(",");

/** Block positions indexed by their 16³ section without copying block values. */
export class SectionBlockMap extends Map {
  #sections = new Map();

  constructor(entries) {
    super();
    if (entries != null)
      for (const [key, value] of entries) this.set(key, value);
  }

  set(key, value) {
    if (!super.has(key)) this.#index(key, sectionKey(key));
    return super.set(key, value);
  }

  /** Insert using a section ID already validated by the caller. */
  setInSection(key, value, section) {
    if (!super.has(key)) this.#index(key, section);
    return super.set(key, value);
  }

  #index(key, section) {
    let keys = this.#sections.get(section);
    if (!keys) this.#sections.set(section, (keys = new Set()));
    keys.add(key);
  }

  delete(key) {
    if (!super.delete(key)) return false;
    const section = sectionKey(key),
      keys = this.#sections.get(section);
    keys.delete(key);
    if (!keys.size) this.#sections.delete(section);
    return true;
  }

  /** Remove every stored block in a section and return the number removed. */
  deleteSection(section) {
    const keys = this.#sections.get(section);
    if (!keys) return 0;
    const count = keys.size;
    for (const key of keys) super.delete(key);
    keys.clear();
    this.#sections.delete(section);
    return count;
  }

  clear() {
    super.clear();
    this.#sections.clear();
  }

  /** Live key iterator; an absent or empty section contains no stored blocks. */
  keysInSection(section) {
    return this.#sections.get(section)?.keys() ?? [][Symbol.iterator]();
  }

  /** Sections containing stored blocks; empty loaded sections have no entries. */
  loadedSectionKeys() {
    return this.#sections.keys();
  }
}