Function mergeStatements

packages/lang-core/src/parser/merge.ts:141–184

(existing: string, patch: string, rootId = "root")

 * Returns the merged program as a string.
 */
export function mergeStatements(existing: string, patch: string, rootId = "root"): string {
  const existingStmts = parseStatements(existing);
  const patchStmts = parseStatements(stripFences(patch));

  if (!existingStmts.length) {
    return patchStmts.map((stmt) => stmt.raw).join("\n");
  }
  if (!patchStmts.length) return existing;

  // Merge: patch statements override existing by name
  const merged = new Map<string, string>();
  const asts = new Map<string, ASTNode>();
  const order: string[] = [];

  for (const stmt of existingStmts) {
    merged.set(stmt.id, stmt.raw);
    asts.set(stmt.id, stmt.ast);
    order.push(stmt.id);
  }

  for (const stmt of patchStmts) {
    if (stmt.ast.k === "Null") {
      // `name = null` in a patch means "delete this statement"
      merged.delete(stmt.id);
      asts.delete(stmt.id);
      const idx = order.indexOf(stmt.id);
      if (idx !== -1) order.splice(idx, 1);
      continue;
    }
    if (!merged.has(stmt.id)) {
      order.push(stmt.id);
    }
    merged.set(stmt.id, stmt.raw);
    asts.set(stmt.id, stmt.ast);
  }

  // GC: remove statements unreachable from root
  gcUnreachable(order, merged, asts, rootId);

  return order
    .filter((id) => merged.has(id))
    .map((id) => merged.get(id)!)
    .join("\n");
}