code: function computeNodeLinks (graph) {
        graph.nodes.forEach(function (node, i) {
          node.index = i
          node.sourceLinks = []
          node.targetLinks = []
        })
        var nodeById = map(graph.nodes, id)
        graph.links.forEach(function (link, i) {
          link.index = i
          var source = link.source
          var target = link.target
          if (typeof source !== 'object') {
            source = link.source = find(nodeById, source)
          }
          if (typeof target !== 'object') {
            target = link.target = find(nodeById, target)
          }
          source.sourceLinks.push(link)
          target.targetLinks.push(link)
        })
        return graph
      }
docs: Populate the sourceLinks and targetLinks for each node. Also, if the source and target are not objects, assume they are indices.