From 3f12dd510704025bdbe27553af40c71c5f953f24 Mon Sep 17 00:00:00 2001
From: Kyle Hensel <k.y.l.e@outlook.co.nz>
Date: Tue, 21 Sep 2021 12:34:32 +1200
Subject: [PATCH] keep the oldest Way when merging

---
 modules/actions/join.js   | 16 +++++++++-------
 test/spec/actions/join.js | 23 +++++++++++++++++++++++
 2 files changed, 32 insertions(+), 7 deletions(-)

diff --git a/modules/actions/join.js b/modules/actions/join.js
index 75c7469c9..3515a243b 100644
--- a/modules/actions/join.js
+++ b/modules/actions/join.js
@@ -27,7 +27,6 @@ export function actionJoin(ids) {
 
     var action = function(graph) {
         var ways = ids.map(graph.entity, graph);
-        var survivorID = ways[0].id;
 
         // if any of the ways are sided (e.g. coastline, cliff, kerb)
         // sort them first so they establish the overall order - #6033
@@ -40,12 +39,15 @@ export function actionJoin(ids) {
         });
 
         // Prefer to keep an existing way.
-        for (var i = 0; i < ways.length; i++) {
-            if (!ways[i].isNew()) {
-                survivorID = ways[i].id;
-                break;
-            }
-        }
+        // if there are mulitple existing ways, keep the oldest one
+        // the oldest way is determined by the ID of the way
+        const survivorID = (
+            ways
+                .filter((way) => !way.isNew())
+                .sort((a, b) => +a.osmId() - +b.osmId())[0] || ways[0]
+        ).id;
+
+
 
         var sequences = osmJoinWays(ways, graph);
         var joined = sequences[0];
diff --git a/test/spec/actions/join.js b/test/spec/actions/join.js
index deb30e77f..fecc421de 100644
--- a/test/spec/actions/join.js
+++ b/test/spec/actions/join.js
@@ -424,6 +424,29 @@ describe('iD.actionJoin', function () {
         expect(graph.hasEntity('w-2')).to.be.undefined;
     });
 
+    it('prefers to keep the oldest way', function () {
+        // n1 ==> n2 ++> n3 --> n4
+        // ==> is existing, ++> is existing, --> is new
+        // Expected result:
+        // n1 ==> n2 ==> n3 ==> n4
+        var graph = iD.coreGraph([
+            iD.osmNode({ id: 'n1', loc: [0,0] }),
+            iD.osmNode({ id: 'n2', loc: [2,0] }),
+            iD.osmNode({ id: 'n3', loc: [4,0] }),
+            iD.osmNode({ id: 'n4', loc: [6,0] }),
+            iD.osmWay({ id: 'w1', nodes: ['n2', 'n3'] }),
+            iD.osmWay({ id: 'w2', nodes: ['n1', 'n2'] }),
+            iD.osmWay({ id: 'w-1', nodes: ['n3', 'n4'] })
+        ]);
+
+        graph = iD.actionJoin(['w1', 'w2', 'w-1'])(graph);
+
+        // way 1 is the oldest (it has the lower id) so it kept that one
+        expect(graph.entity('w1').nodes).to.eql(['n1', 'n2', 'n3', 'n4']);
+        expect(graph.hasEntity('w2')).to.be.undefined;
+        expect(graph.hasEntity('w-1')).to.be.undefined;
+    });
+
     it('merges tags', function () {
         var graph = iD.coreGraph([
             iD.osmNode({id: 'a', loc: [0,0]}),