iD/modules/actions/reverse.js

/*
  Order the nodes of a way in reverse order and reverse any direction dependent tags
  other than `oneway`. (We assume that correcting a backwards oneway is the primary
  reason for reversing a way.)

  The following transforms are performed:

    Keys:
          *:right=* ⟺ *:left=*
        *:forward=* ⟺ *:backward=*
       direction=up ⟺ direction=down
         incline=up ⟺ incline=down
            *=right ⟺ *=left

    Relation members:
       role=forward ⟺ role=backward
         role=north ⟺ role=south
          role=east ⟺ role=west

   In addition, numeric-valued `incline` tags are negated.

   The JOSM implementation was used as a guide, but transformations that were of unclear benefit
   or adjusted tags that don't seem to be used in practice were omitted.

   References:
      http://wiki.openstreetmap.org/wiki/Forward_%26_backward,_left_%26_right
      http://wiki.openstreetmap.org/wiki/Key:direction#Steps
      http://wiki.openstreetmap.org/wiki/Key:incline
      http://wiki.openstreetmap.org/wiki/Route#Members
      http://josm.openstreetmap.de/browser/josm/trunk/src/org/openstreetmap/josm/corrector/ReverseWayTagCorrector.java
 */
export function Reverse(wayId, options) {
    var replacements = [
            [/:right$/, ':left'], [/:left$/, ':right'],
            [/:forward$/, ':backward'], [/:backward$/, ':forward']
        ],
        numeric = /^([+\-]?)(?=[\d.])/,
        roleReversals = {
            forward: 'backward',
            backward: 'forward',
            north: 'south',
            south: 'north',
            east: 'west',
            west: 'east'
        };

    function reverseKey(key) {
        for (var i = 0; i < replacements.length; ++i) {
            var replacement = replacements[i];
            if (replacement[0].test(key)) {
                return key.replace(replacement[0], replacement[1]);
            }
        }
        return key;
    }

    function reverseValue(key, value) {
        if (key === 'incline' && numeric.test(value)) {
            return value.replace(numeric, function(_, sign) { return sign === '-' ? '' : '-'; });
        } else if (key === 'incline' || key === 'direction') {
            return {up: 'down', down: 'up'}[value] || value;
        } else if (options && options.reverseOneway && key === 'oneway') {
            return {yes: '-1', '1': '-1', '-1': 'yes'}[value] || value;
        } else {
            return {left: 'right', right: 'left'}[value] || value;
        }
    }

    return function(graph) {
        var way = graph.entity(wayId),
            nodes = way.nodes.slice().reverse(),
            tags = {}, key, role;

        for (key in way.tags) {
            tags[reverseKey(key)] = reverseValue(key, way.tags[key]);
        }

        graph.parentRelations(way).forEach(function(relation) {
            relation.members.forEach(function(member, index) {
                if (member.id === way.id && (role = roleReversals[member.role])) {
                    relation = relation.updateMember({role: role}, index);
                    graph = graph.replace(relation);
                }
            });
        });

        return graph.replace(way.update({nodes: nodes, tags: tags}));
    };
}