Add Way#affix

js/id/core/way.js

@@ -32,6 +32,11 @@ _.extend(iD.Way.prototype, {
         return this.nodes.indexOf(node) >= 0;
     },
 
+    affix: function(node) {
+        if (this.nodes[0] === node) return 'prefix';
+        if (this.nodes[this.nodes.length - 1] === node) return 'suffix';
+    },
+
     isOneWay: function() {
         return this.tags.oneway === 'yes' ||
             this.tags.oneway === '1' ||

js/id/modes/add_line.js

@@ -23,7 +23,7 @@ iD.modes.AddLine = function(context) {
             iD.actions.AddEntity(way),
             iD.actions.AddVertex(way.id, node.id));
 
-        context.enter(iD.modes.DrawLine(context, way.id, 'forward', graph));
+        context.enter(iD.modes.DrawLine(context, way.id, graph));
     }
 
     function startFromWay(loc, edge) {
@@ -37,7 +37,7 @@ iD.modes.AddLine = function(context) {
             iD.actions.AddVertex(way.id, node.id),
             iD.actions.AddMidpoint({ loc: loc, edge: edge }, node));
 
-        context.enter(iD.modes.DrawLine(context, way.id, 'forward', graph));
+        context.enter(iD.modes.DrawLine(context, way.id, graph));
     }
 
     function startFromNode(node) {
@@ -47,7 +47,7 @@ iD.modes.AddLine = function(context) {
             iD.actions.AddEntity(way),
             iD.actions.AddVertex(way.id, node.id));
 
-        context.enter(iD.modes.DrawLine(context, way.id, 'forward', context.graph()));
+        context.enter(iD.modes.DrawLine(context, way.id, context.graph()));
     }
 
     mode.enter = function() {

js/id/modes/draw_line.js

@@ -1,4 +1,4 @@
-iD.modes.DrawLine = function(context, wayId, direction, baseGraph) {
+iD.modes.DrawLine = function(context, wayId, baseGraph, affix) {
     var mode = {
         button: 'line',
         id: 'draw-line'
@@ -8,8 +8,8 @@ iD.modes.DrawLine = function(context, wayId, direction, baseGraph) {
 
     mode.enter = function() {
         var way = context.entity(wayId),
-            index = (direction === 'forward') ? undefined : 0,
-            headId = (direction === 'forward') ? way.last() : way.first();
+            index = (affix === 'prefix') ? 0 : undefined,
+            headId = (affix === 'prefix') ? way.first() : way.last();
 
         behavior = iD.behavior.DrawWay(context, wayId, index, mode, baseGraph)
             .tail(t('modes.draw_line.tail'));

js/id/operations/continue.js

@@ -8,7 +8,7 @@ iD.operations.Continue = function(selectedIDs, context) {
     function candidateWays() {
         return graph.parentWays(vertex).filter(function(parent) {
             return parent.geometry(graph) === 'line' &&
-                (parent.first() === vertex.id || parent.last() === vertex.id) &&
+                parent.affix(vertex.id) &&
                 (geometries.line.length === 0 || geometries.line[0] === parent);
         });
     }
@@ -18,8 +18,8 @@ iD.operations.Continue = function(selectedIDs, context) {
         context.enter(iD.modes.DrawLine(
             context,
             candidate.id,
-            candidate.first() === vertex.id ? 'backward' : 'forward',
-            context.graph()));
+            context.graph(),
+            candidate.affix(vertex.id)));
     };
 
     operation.available = function() {

test/spec/core/way.js

@@ -48,6 +48,21 @@ describe('iD.Way', function() {
         });
     });
 
+    describe("#affix", function () {
+        it("returns 'prefix' if the way starts with the given node", function () {
+            expect(iD.Way({nodes: ['a', 'b', 'c']}).affix('a')).to.equal('prefix');
+        });
+
+        it("returns 'suffix' if the way ends with the given node", function () {
+            expect(iD.Way({nodes: ['a', 'b', 'c']}).affix('c')).to.equal('suffix');
+        });
+
+        it("returns falsy if the way does not start or end with the given node", function () {
+            expect(iD.Way({nodes: ['a', 'b', 'c']}).affix('b')).not.to.be.ok;
+            expect(iD.Way({nodes: []}).affix('b')).not.to.be.ok;
+        });
+    });
+
     describe("#extent", function () {
         it("returns the minimal extent containing all member nodes", function () {
             var node1 = iD.Node({loc: [0, 0]}),