mermaid/packages/mermaid-layout-elk/src/find-common-ancestor.ts

export interface TreeData {
  parentById: Record<string, string>;
  childrenById: Record<string, string[]>;
}

export const findCommonAncestor = (id1: string, id2: string, { parentById }: TreeData) => {
  const visited = new Set();
  let currentId = id1;

  // Edge case with self edges
  if (id1 === id2) {
    return parentById[id1] || 'root';
  }

  while (currentId) {
    visited.add(currentId);
    if (currentId === id2) {
      return currentId;
    }
    currentId = parentById[currentId];
  }

  currentId = id2;
  while (currentId) {
    if (visited.has(currentId)) {
      return currentId;
    }
    currentId = parentById[currentId];
  }

  return 'root';
};