// Local view ordering and intent binding. Contexts are not credentials.
function sameContext(left, right) {
  return ['instance', 'connection', 'configuration', 'draft'].every(key => left?.[key] === right?.[key]);
}

class ViewSession {
  constructor({read, send, render, error, permissions, edited = () => {}}) {
    Object.assign(this, {read, send, render, error, permissions, edited});
    this.view = null;
    this.queue = [];
    this.ticket = 0;
    this.acceptedTicket = 0;
    this.retired = new Set();
    this.message = '';
  }

  get busy() { return this.queue.length !== 0; }
  get onlyEdits() { return this.queue.every(entry => entry.path === '/api/config/edit'); }
  editing(id) {
    return this.queue.some(entry => entry.body.field_id === id
      && entry.context.instance === this.view?.state.context.instance
      && entry.context.connection === this.view?.state.context.connection
      && entry.context.configuration === this.view?.state.context.configuration);
  }

  accept(view, ticket) {
    const current = this.view?.state;
    const next = view.state;
    if (current) {
      if (current.context.instance === next.context.instance) {
        if (BigInt(next.config_revision) < BigInt(current.config_revision)) return false;
      } else {
        if (ticket < this.acceptedTicket || this.retired.has(next.context.instance)) return false;
        this.retired.add(current.context.instance);
      }
    }
    this.acceptedTicket = Math.max(ticket, this.acceptedTicket);
    this.view = view;
    this.render(view);
    if (this.message) this.error(this.message);
    return true;
  }

  async refresh() {
    const ticket = ++this.ticket;
    const view = await this.read();
    return this.accept(view, ticket);
  }

  mutate(path, body = {}, context = this.view?.state.context) {
    if (!this.view) return Promise.resolve(null);
    return new Promise(resolve => {
      this.queue.push({path, body, context, resolve});
      this.permissions();
      if (this.queue.length === 1) this.drain();
    });
  }

  async drain() {
    while (this.queue.length) {
      const entry = this.queue[0];
      const ticket = ++this.ticket;
      let result = null;
      try {
        if (!sameContext(entry.context, this.view.state.context)) {
          const error = Error('View changed; queued action discarded. Review the current configuration.');
          error.staleIntent = true;
          throw error;
        }
        const view = await this.send(entry.path, {...entry.body, context: entry.context});
        if (this.accept(view, ticket)) {
          this.message = '';
          result = view;
          // Only our own accepted edits can advance already queued edits.
          // A poll, another editor, a reset, or a new connection cannot rebase intent.
          if (entry.path === '/api/config/edit'
            && view.state.context.instance === entry.context.instance
            && view.state.context.connection === entry.context.connection
            && view.state.context.configuration === entry.context.configuration) {
            for (const queued of this.queue.slice(1)) {
              if (sameContext(queued.context, entry.context)) queued.context = view.state.context;
            }
            this.edited(entry.context, view.state.context);
          }
        }
      } catch (error) {
        const relevant = error.view
          ? this.accept(error.view, ticket)
          : error.staleIntent || sameContext(entry.context, this.view.state.context);
        if (relevant) this.message = error.message;
      } finally {
        this.queue.shift();
        this.render(this.view);
        if (this.message) this.error(this.message);
        this.permissions();
        entry.resolve(result);
      }
    }
  }
}

if (typeof module !== 'undefined') module.exports = {ViewSession, sameContext};