var _ = require('../lodash')

module.exports = dfs

/*
 * A helper that preforms a pre- or post-order traversal on the input graph
 * and returns the nodes in the order they were visited. If the graph is
 * undirected then this algorithm will navigate using neighbors. If the graph
 * is directed then this algorithm will navigate using successors.
 *
 * Order must be one of "pre" or "post".
 */
function dfs (g, vs, order) {
  if (!_.isArray(vs)) {
    vs = [vs]
  }

  var navigation = (g.isDirected() ? g.successors : g.neighbors).bind(g)

  const acc = []
  const visited = {}
  _.each(vs, function (v) {
    if (!g.hasNode(v)) {
      throw new Error('Graph does not have node: ' + v)
    }

    doDfs(g, v, order === 'post', visited, navigation, acc)
  })
  return acc
}

function doDfs (g, v, postorder, visited, navigation, acc) {
  if (!_.has(visited, v)) {
    visited[v] = true

    if (!postorder) { acc.push(v) }
    _.each(navigation(v), function (w) {
      doDfs(g, w, postorder, visited, navigation, acc)
    })
    if (postorder) { acc.push(v) }
  }
}