iD/js/id/actions/split.js

// Split a way at the given node.
//
// This is the inverse of `iD.actions.Join`.
//
// For testing convenience, accepts an ID to assign to the new way.
// Normally, this will be undefined and the way will automatically
// be assigned a new ID.
//
// Reference:
//   https://github.com/systemed/potlatch2/blob/master/net/systemeD/halcyon/connection/actions/SplitWayAction.as
//
iD.actions.Split = function(nodeId, newWayId) {
    function candidateWays(graph) {
        var node = graph.entity(nodeId),
            parents = graph.parentWays(node);

        return parents.filter(function(parent) {
            return parent.isClosed() ||
                (parent.first() !== nodeId &&
                 parent.last()  !== nodeId);
        });
    }

    var action = function(graph) {
        var wayA = candidateWays(graph)[0],
            wayB = iD.Way({id: newWayId, tags: wayA.tags}),
            nodesA,
            nodesB,
            isArea = wayA.isArea();

        if (wayA.isClosed()) {
            var nodes = wayA.nodes.slice(0, -1),
                idxA = _.indexOf(nodes, nodeId),
                idxB = idxA + Math.floor(nodes.length / 2);

            if (idxB >= nodes.length) {
                idxB %= nodes.length;
                nodesA = nodes.slice(idxA).concat(nodes.slice(0, idxB + 1));
                nodesB = nodes.slice(idxB, idxA + 1);
            } else {
                nodesA = nodes.slice(idxA, idxB + 1);
                nodesB = nodes.slice(idxB).concat(nodes.slice(0, idxA + 1));
            }
        } else {
            var idx = _.indexOf(wayA.nodes, nodeId);
            nodesA = wayA.nodes.slice(0, idx + 1);
            nodesB = wayA.nodes.slice(idx);
        }

        wayA = wayA.update({nodes: nodesA});
        wayB = wayB.update({nodes: nodesB});

        graph = graph.replace(wayA);
        graph = graph.replace(wayB);

        graph.parentRelations(wayA).forEach(function(relation) {
            if (relation.isRestriction()) {
                var via = relation.memberByRole('via');
                if (via && wayB.contains(via.id)) {
                    relation = relation.updateMember({id: wayB.id}, relation.memberById(wayA.id).index);
                    graph = graph.replace(relation);
                }
            } else {
                var role = relation.memberById(wayA.id).role,
                    last = wayB.last(),
                    i = relation.memberById(wayA.id).index,
                    j;

                for (j = 0; j < relation.members.length; j++) {
                    var entity = graph.entity(relation.members[j].id);
                    if (entity && entity.type === 'way' && entity.contains(last)) {
                        break;
                    }
                }

                relation = relation.addMember({id: wayB.id, type: 'way', role: role}, i <= j ? i + 1 : i);
                graph = graph.replace(relation);
            }
        });

        if (isArea) {
            var multipolygon = iD.Relation({
                tags: _.extend({}, wayA.tags, {type: 'multipolygon'}),
                members: [
                    {id: wayA.id, role: 'outer', type: 'way'},
                    {id: wayB.id, role: 'outer', type: 'way'}
                ]});

            graph = graph.replace(multipolygon);
            graph = graph.replace(wayA.update({tags: {}}));
            graph = graph.replace(wayB.update({tags: {}}));
        }

        return graph;
    };

    action.enabled = function(graph) {
        return candidateWays(graph).length === 1;
    };

    return action;
};