create 'follow' feature for drawing ways

data/core.yaml

@@ -346,6 +346,19 @@ en:
       not_downloaded:
         single: This feature can't be moved because parts of it have not yet been downloaded.
         multiple: These features can't be moved because parts of them have not yet been downloaded.
+    follow:
+      key: F
+      error:
+        needs_more_initial_nodes: This line can't follow a way because it isn't connected to enough consecutive points along a way. Add another point manually to continue.
+        intersection_of_mutiple_ways:
+          line: This line can't follow a way because multiple lines are connected to the line's last two points. Add another vertex manually to continue.
+          area: This line can't follow a way because multiple areas are connected to the line's last two points. Add another vertex manually to continue.
+          generic: This line can't follow a way because multiple features are connected to the line's last two points. Add another vertex manually to continue.
+        intersection_of_different_ways:
+          line: This line can't follow a way because the line is only connected to the line at a single point. Add another point manually to continue.
+          area: This line can't follow a way because the line is only connected to the area at a single point. Add another point manually to continue.
+          generic: This line can't follow a way because the line is only connected to the way at a single point. Add another point manually to continue.
+        unknown: This line can't follow a way.
     reflect:
       title:
         long: Flip Long
@@ -783,7 +796,7 @@ en:
   map_data:
     title: Map Data
     description: Map Data
-    key: F
+    key: U
     data_layers: Data Layers
     layers:
       osm:
@@ -2332,6 +2345,7 @@ en:
         split: "Split features at the selected points"
         reverse: "Reverse selected features"
         move: "Move selected features"
+        follow: "Follow a line or area"
         nudge: "Nudge selected features"
         nudge_more: "Nudge selected features by a lot"
         scale: "Scale selected features"

data/shortcuts.json

@@ -283,6 +283,10 @@
             "shortcuts": ["operations.move.key"],
             "text": "shortcuts.editing.operations.move"
           },
+          {
+            "shortcuts": ["operations.follow.key"],
+            "text": "shortcuts.editing.operations.follow"
+          },
           {
             "modifiers": ["⇧"],
             "shortcuts": ["↓", "↑", "←", "→"],

modules/behavior/draw_way.js

@@ -18,6 +18,7 @@ import { utilRebind } from '../util/rebind';
 import { utilKeybinding } from '../util';
 
 export function behaviorDrawWay(context, wayID, mode, startGraph) {
+    const keybinding = utilKeybinding('drawWay');
 
     var dispatch = d3_dispatch('rejectedSelfIntersection');
 
@@ -412,6 +413,96 @@ export function behaviorDrawWay(context, wayID, mode, startGraph) {
         });
     };
 
+    /**
+     * @param {(typeof osmWay)[]} ways
+     * @returns {"line" | "area" | "generic"}
+     */
+    function getFeatureType(ways) {
+        if (ways.every(way => way.isClosed())) return 'area';
+        if (ways.every(way => !way.isClosed())) return 'line';
+        return 'generic';
+    }
+
+    /** see PR #8671 */
+    function followMode() {
+        if (_didResolveTempEdit) return;
+
+        try {
+
+            // get the last 2 added nodes.
+            // check if they are both part of only oneway (the same one)
+            // check if the ways that they're part of are the same way
+            // find index of the last two nodes, to determine the direction to travel around the existing way
+            // add the next node to the way we are drawing
+
+            // if we're drawing an area, the first node = last node.
+            const isDrawingArea = _origWay.nodes[0] === _origWay.nodes.slice(-1)[0];
+
+            const [secondLastNodeId, lastNodeId] = _origWay.nodes.slice(isDrawingArea ? -3 : -2);
+
+            if (!lastNodeId || !secondLastNodeId || !startGraph.hasEntity(lastNodeId) || !startGraph.hasEntity(secondLastNodeId)) {
+                context.ui().flash
+                    .duration(4000)
+                    .iconName('#iD-icon-no')
+                    .label(t('operations.follow.error.needs_more_initial_nodes'))();
+                return;
+            }
+
+            const lastNodesParents = startGraph.parentWays(startGraph.entity(lastNodeId));
+            const secondLastNodesParents = startGraph.parentWays(startGraph.entity(secondLastNodeId));
+
+            const featureType = getFeatureType(lastNodesParents);
+
+            if (lastNodesParents.length !== 1 || secondLastNodesParents.length === 0) {
+                context.ui().flash
+                    .duration(4000)
+                    .iconName('#iD-icon-no')
+                    .label(t(`operations.follow.error.intersection_of_mutiple_ways.${featureType}`))();
+                return;
+            }
+
+            // Check if the last node's parent is also the parent of the second last node.
+            // The last node must only have one parent, but the second last node can have
+            // multiple parents.
+            if (!secondLastNodesParents.some(n => n.id === lastNodesParents[0].id)) {
+                context.ui().flash
+                    .duration(4000)
+                    .iconName('#iD-icon-no')
+                    .label(t(`operations.follow.error.intersection_of_different_ways.${featureType}`))();
+                return;
+            }
+
+            const way = lastNodesParents[0];
+
+            const indexOfLast = way.nodes.indexOf(lastNodeId);
+            const indexOfSecondLast = way.nodes.indexOf(secondLastNodeId);
+
+            // for a closed way, the first/last node is the same so it appears twice in the array,
+            // but indexOf always finds the first occurance. This is only an issue when following a way
+            // in descending order
+            const isDescendingPastZero = indexOfLast === way.nodes.length - 2 && indexOfSecondLast === 0;
+
+            let nextNodeIndex = indexOfLast + (indexOfLast > indexOfSecondLast && !isDescendingPastZero ? 1 : -1);
+            // if we're following a closed way and we pass the first/last node, the  next index will be -1
+            if (nextNodeIndex === -1) nextNodeIndex = indexOfSecondLast === 1 ? way.nodes.length - 2 : 1;
+
+            const nextNode = startGraph.entity(way.nodes[nextNodeIndex]);
+
+            drawWay.addNode(nextNode, {
+                geometry: { type: 'Point', coordinates: nextNode.loc },
+                id: nextNode.id,
+                properties: { target: true, entity: nextNode },
+            });
+        } catch (ex) {
+            context.ui().flash
+                .duration(4000)
+                .iconName('#iD-icon-no')
+                .label(t('operations.follow.error.unknown'))();
+        }
+    }
+
+    keybinding.on(t('operations.follow.key'), followMode);
+    d3_select(document).call(keybinding);
 
     // Finish the draw operation, removing the temporary edit.
     // If the way has enough nodes to be valid, it's selected.