Function gcUnreachable

packages/lang-core/src/parser/merge.ts:92–133

* Remove statements unreachable from `root` (garbage collection). * Walks the AST graph from root, collecting all referenced statement IDs. * $state variables are always kept (they're referenced at runtime, not by Ref nodes).

(
  order: string[],
  merged: Map<string, string>,
  asts: Map<string, ASTNode>,
  rootId = "root",
)

 * $state variables are always kept (they're referenced at runtime, not by Ref nodes).
 */
function gcUnreachable(
  order: string[],
  merged: Map<string, string>,
  asts: Map<string, ASTNode>,
  rootId = "root",
): void {
  const rootAst = asts.get(rootId);
  if (!rootAst) return; // no root → can't GC

  // BFS from root to find all reachable statements
  const reachable = new Set<string>([rootId]);
  const queue: string[] = [rootId];

  while (queue.length > 0) {
    const id = queue.pop()!;
    const ast = asts.get(id);
    if (!ast) continue;

    const refs = new Set<string>();
    collectRefs(ast, refs);

    for (const ref of refs) {
      if (!reachable.has(ref) && asts.has(ref)) {
        reachable.add(ref);
        queue.push(ref);
      }
    }
  }

  // Keep $state variables — they're bound at runtime, not via Ref
  for (const id of order) {
    if (id.startsWith("$")) reachable.add(id);
  }

  // Remove unreachable statements
  for (let i = order.length - 1; i >= 0; i--) {
    if (!reachable.has(order[i])) {
      merged.delete(order[i]);
      order.splice(i, 1);
    }
  }
}

/**
 * Merge an existing program with a patch (partial update).