Flatten unobservable AsyncContext.Variable values from all living snapshot chains

flatten.ts

/**
 * Flattens all "snapshots", removing unobservable nodes so they may free their
 * values.
 *
 * "snapshots" here is all Snapshot instances, every created
 * Generator/AsyncGenerator, every stored Job, etc.
 */
function flatten(snapshots: Snapshot[]) {
  const current = new Map<Variable>();
  const maybe = new Map<LinkNode, LinkNode>();
  const seen = new Set<LinkNode>();

  for (const snapshot of snapshots) {
    let c = snapshot.[[AsyncContextLinkNode]];
    let parent;

    while (c) {
      // If we've already seen this variable in this linked-list chain, then it's an unobservable
      const variable = c.[[AsyncContextVariable]];
      if (current.has(variable)) {
        if (!seen.has(c)) {
          maybe.set(c, parent);
        }
      } else {
        maybe.delete(c);
      }

      seen.add(c);
      current.add(variable);

      parent = c;
      c = c.[[AsyncContextLinkNode]];
    }

    current.clear();
  }

  // Anything still in maybe is unreachable across all snapshots.
  for (const [node, parent] of maybe) {
    parent.[[AsyncContextLinkNode]] = node.[[AsyncContextLinkNode]];
  }
}