export const TRAVERSAL_PATH = {
  ArrayExpression: ['elements'],
  ArrayPattern: ['elements'],
  AssignmentExpression: ['left', 'right'],
  AwaitExpression: ['argument'],
  BinaryExpression: ['left', 'right'],
  BlockStatement: ['body'],
  CallExpression: ['arguments', 'callee'],
  ConditionalExpression: ['alternate', 'consequent', 'test'],
  DoWhileStatement: ['body', 'test'],
  ExpressionStatement: ['expression'],
  ForStatement: ['body', 'init', 'test', 'update'],
  Identifier: [],
  IfStatement: ['alternate', 'consequent', 'test'],
  MemberExpression: ['object', 'property'],
  Literal: [],
  ObjectExpression: ['properties'],
  ObjectPattern: ['properties'],
  Program: ['body'],
  Property: ['key', 'value'],
  ReturnStatement: ['argument'],
  UnaryExpression: ['argument'],
  UpdateExpression: ['argument'],
  VariableDeclaration: ['declarations'],
  VariableDeclarator: ['id', 'init'],
  WhileStatement: ['body', 'test'],
};

export default function traverse(node, visitor) {
  /* v8 ignore next 3 */
  if (!(node.type in TRAVERSAL_PATH)) {
    throw new Error(`node type ${node.type} not traversable. (${Object.keys(node).join(', ')})`);
  }
  visitor(node, 'enter');
  const path = TRAVERSAL_PATH[node.type];
  let children;
  if (path instanceof Function) {
    children = path(node);
  }
  else if (Array.isArray(path)) {
    children = [];
    for (const key of path) {
      children.push(...(Array.isArray(node[key]) ? node[key] : [node[key]]));
    }
  }
  for (const child of children) {
    if (child) {
      traverse(child, visitor);
    }
  }
  visitor(node, 'exit');
}