/**
 * Graphology Operators To Undirected Caster
 * ==========================================
 *
 * Function used to cast any graph to an undirected one.
 */
var isGraph = require('graphology-utils/is-graph');
var copyEdge = require('graphology-utils/add-edge').copyEdge;

module.exports = function toUndirected(graph, options) {
  if (!isGraph(graph))
    throw new Error(
      'graphology-operators/to-undirected: expecting a valid graphology instance.'
    );

  if (typeof options === 'function') options = {mergeEdge: options};

  options = options || {};

  var mergeEdge =
    typeof options.mergeEdge === 'function' ? options.mergeEdge : null;

  if (graph.type === 'undirected') return graph.copy();

  var undirectedGraph = graph.emptyCopy({type: 'undirected'});

  // Adding undirected edges
  graph.forEachUndirectedEdge(function (edge, attr, source, target) {
    copyEdge(undirectedGraph, true, edge, source, target, attr);
  });

  // TODO: one loop for better performance

  // Merging directed edges
  graph.forEachDirectedEdge(function (edge, attr, source, target) {
    if (!graph.multi) {
      var existingEdge = undirectedGraph.edge(source, target);

      if (existingEdge) {
        // We need to merge
        if (mergeEdge)
          undirectedGraph.replaceEdgeAttributes(
            existingEdge,
            mergeEdge(undirectedGraph.getEdgeAttributes(existingEdge), attr)
          );

        return;
      }
    }

    copyEdge(undirectedGraph, true, null, source, target, attr);
  });

  return undirectedGraph;
};