import { isDocument, isNode, isPair, isCollection, isMap, isSeq, isScalar, isAlias } from './nodes/Node.mjs';

const BREAK = Symbol('break visit');
const SKIP = Symbol('skip children');
const REMOVE = Symbol('remove node');
/**
 * Apply a visitor to an AST node or document.
 *
 * Walks through the tree (depth-first) starting from `node`, calling a
 * `visitor` function with three arguments:
 *   - `key`: For sequence values and map `Pair`, the node's index in the
 *     collection. Within a `Pair`, `'key'` or `'value'`, correspondingly.
 *     `null` for the root node.
 *   - `node`: The current node.
 *   - `path`: The ancestry of the current node.
 *
 * The return value of the visitor may be used to control the traversal:
 *   - `undefined` (default): Do nothing and continue
 *   - `visit.SKIP`: Do not visit the children of this node, continue with next
 *     sibling
 *   - `visit.BREAK`: Terminate traversal completely
 *   - `visit.REMOVE`: Remove the current node, then continue with the next one
 *   - `Node`: Replace the current node, then continue by visiting it
 *   - `number`: While iterating the items of a sequence or map, set the index
 *     of the next step. This is useful especially if the index of the current
 *     node has changed.
 *
 * If `visitor` is a single function, it will be called with all values
 * encountered in the tree, including e.g. `null` values. Alternatively,
 * separate visitor functions may be defined for each `Map`, `Pair`, `Seq`,
 * `Alias` and `Scalar` node. To define the same visitor function for more than
 * one node type, use the `Collection` (map and seq), `Value` (map, seq & scalar)
 * and `Node` (alias, map, seq & scalar) targets. Of all these, only the most
 * specific defined one will be used for each node.
 */
function visit(node, visitor) {
  const visitor_ = initVisitor(visitor);
  if (isDocument(node)) {
    const cd = visit_(null, node.contents, visitor_, Object.freeze([node]));
    if (cd === REMOVE) node.contents = null;
  } else visit_(null, node, visitor_, Object.freeze([]));
}
// Without the `as symbol` casts, TS declares these in the `visit`
// namespace using `var`, but then complains about that because
// `unique symbol` must be `const`.
/** Terminate visit traversal completely */
visit.BREAK = BREAK;
/** Do not visit the children of the current node */
visit.SKIP = SKIP;
/** Remove the current node */
visit.REMOVE = REMOVE;
function visit_(key, node, visitor, path) {
  const ctrl = callVisitor(key, node, visitor, path);
  if (isNode(ctrl) || isPair(ctrl)) {
    replaceNode(key, path, ctrl);
    return visit_(key, ctrl, visitor, path);
  }
  if (typeof ctrl !== 'symbol') {
    if (isCollection(node)) {
      path = Object.freeze(path.concat(node));
      for (let i = 0; i < node.items.length; ++i) {
        const ci = visit_(i, node.items[i], visitor, path);
        if (typeof ci === 'number') i = ci - 1;else if (ci === BREAK) return BREAK;else if (ci === REMOVE) {
          node.items.splice(i, 1);
          i -= 1;
        }
      }
    } else if (isPair(node)) {
      path = Object.freeze(path.concat(node));
      const ck = visit_('key', node.key, visitor, path);
      if (ck === BREAK) return BREAK;else if (ck === REMOVE) node.key = null;
      const cv = visit_('value', node.value, visitor, path);
      if (cv === BREAK) return BREAK;else if (cv === REMOVE) node.value = null;
    }
  }
  return ctrl;
}
function initVisitor(visitor) {
  if (typeof visitor === 'object' && (visitor.Collection || visitor.Node || visitor.Value)) {
    return Object.assign({
      Alias: visitor.Node,
      Map: visitor.Node,
      Scalar: visitor.Node,
      Seq: visitor.Node
    }, visitor.Value && {
      Map: visitor.Value,
      Scalar: visitor.Value,
      Seq: visitor.Value
    }, visitor.Collection && {
      Map: visitor.Collection,
      Seq: visitor.Collection
    }, visitor);
  }
  return visitor;
}
function callVisitor(key, node, visitor, path) {
  if (typeof visitor === 'function') return visitor(key, node, path);
  if (isMap(node)) return visitor.Map?.(key, node, path);
  if (isSeq(node)) return visitor.Seq?.(key, node, path);
  if (isPair(node)) return visitor.Pair?.(key, node, path);
  if (isScalar(node)) return visitor.Scalar?.(key, node, path);
  if (isAlias(node)) return visitor.Alias?.(key, node, path);
  return undefined;
}
function replaceNode(key, path, node) {
  const parent = path[path.length - 1];
  if (isCollection(parent)) {
    parent.items[key] = node;
  } else if (isPair(parent)) {
    if (key === 'key') parent.key = node;else parent.value = node;
  } else if (isDocument(parent)) {
    parent.contents = node;
  } else {
    const pt = isAlias(parent) ? 'alias' : 'scalar';
    throw new Error(`Cannot replace node with ${pt} parent`);
  }
}

export { visit };
