/**
 * The behaviors entered by double-clicking call nodes, root first. Either empty or at least two
 * long, and always ending at the behavior the graph shows – anything else opened it otherwise.
 */
export type Trail = readonly string[];

/** Opening `to` from a call node in `from`. A behavior already on the trail is a step back to it. */
export function descend(trail: Trail, from: string, to: string): Trail {
  const base = trail.at(-1) === from ? trail : [from];
  const known = base.indexOf(to);
  return known >= 0 ? backTo(base, known) : [...base, to];
}

/** Clicking crumb `index`; back at the root there is nothing left to show. */
export function backTo(trail: Trail, index: number): Trail {
  const next = trail.slice(0, index + 1);
  return next.length > 1 ? next : [];
}

/** What survives the graph showing `shown`: only a trail that still leads there, every step still alive. */
export function follow(trail: Trail, shown: string, exists: (id: string) => boolean): Trail {
  return trail.at(-1) === shown && trail.every(exists) ? trail : [];
}