For node-way merge, preserve original node if possible

(closes #3683)

modules/actions/merge.js

@@ -17,15 +17,34 @@ export function actionMerge(ids) {
 
         points.forEach(function(point) {
             target = target.mergeTags(point.tags);
+            graph = graph.replace(target);
 
             graph.parentRelations(point).forEach(function(parent) {
                 graph = graph.replace(parent.replaceMember(point, target));
             });
 
-            graph = graph.remove(point);
-        });
+            var nodes = _.uniq(graph.childNodes(target)),
+                removeNode = point;
 
-        graph = graph.replace(target);
+            for (var i = 0; i < nodes.length; i++) {
+                var node = nodes[i];
+                if (graph.parentWays(node).length > 1 ||
+                    graph.parentRelations(node).length ||
+                    node.hasInterestingTags()) {
+                    continue;
+                }
+
+                // Found an uninteresting child node on the target way.
+                // Move orig point into its place to preserve point's history. #3683
+                graph = graph.replace(point.update({ tags: {}, loc: node.loc }));
+                target = target.replaceNode(node.id, point.id);
+                graph = graph.replace(target);
+                removeNode = node;
+                break;
+            }
+
+            graph = graph.remove(removeNode);
+        });
 
         return graph;
     };

modules/operations/merge.js

@@ -27,7 +27,10 @@ export function operationMerge(selectedIDs, context) {
         }
 
         context.perform(action, annotation);
-        var ids = selectedIDs.filter(function(id) { return context.hasEntity(id); });
+        var ids = selectedIDs.filter(function(id) {
+            var entity = context.hasEntity(id);
+            return entity && entity.type !== 'node';
+        });
         context.enter(modeSelect(context, ids).suppressMenu(true));
     };
 

test/spec/actions/merge.js

@@ -36,4 +36,23 @@ describe('iD.actionMerge', function () {
         expect(graph.entity('w').tags).to.eql({a: 'a', b: 'b', area: 'yes'});
         expect(graph.entity('r').members).to.eql([{id: 'w', role: 'r', type: 'way'}]);
     });
+
+    it('preserves original point if possible', function () {
+        var graph = iD.Graph([
+                iD.Node({id: 'a', loc: [1, 0], tags: {a: 'a'}}),
+                iD.Node({id: 'p', loc: [0, 0], tags: {p: 'p'}}),
+                iD.Node({id: 'q', loc: [0, 1]}),
+                iD.Way({id: 'w', nodes: ['p', 'q'], tags: {w: 'w'}})
+            ]),
+            action = iD.actionMerge(['a', 'w']);
+
+        graph = action(graph);
+        expect(graph.hasEntity('a')).to.be.ok;
+        expect(graph.hasEntity('p')).to.be.ok;
+        expect(graph.hasEntity('q')).to.be.undefined;
+        expect(graph.entity('w').tags).to.eql({a: 'a', w: 'w'});
+        expect(graph.entity('w').nodes).to.eql(['p', 'a']);
+        expect(graph.entity('a').loc[0]).to.eql(0);
+        expect(graph.entity('a').loc[1]).to.eql(1);
+    });
 });