perf: cache the location index when the distance matrix is filled first

service/maps/api/src/main/java/ai/timefold/solver/service/maps/impl/LongArrayDistanceMatrix.java

@@ -161,9 +161,11 @@ private short getShortId(Location location) {
         if (shortId == null) {
             shortId = idGenerator;
             locationToId.put(location, shortId);
-            location.setIndex(this, shortId);
             idGenerator++;
         }
+        // The matrix is usually filled before the location is attached to it,
+        // so the id may already exist while the location has not cached it yet.
+        location.setIndex(this, shortId);
         return shortId;
     }
 

service/maps/api/src/test/java/ai/timefold/solver/service/maps/impl/LongArrayDistanceMatrixTest.java

@@ -64,6 +64,26 @@ void cacheShortIndexInLocation() {
         assertThat((int) b.getIndex(distanceMatrix)).isEqualTo(1);
     }
 
+    @Test
+    void cacheShortIndexInLocationAttachedAfterMatrixIsFilled() {
+        LongArrayDistanceMatrix distanceMatrix = new LongArrayDistanceMatrix(2);
+        // The converters fill the matrix with their own location instances before the model's locations are attached.
+        Location responseA = new Location(0.0, 1.0);
+        Location responseB = new Location(1.0, 0.0);
+        distanceMatrix.put(responseA, responseB, 7L);
+        distanceMatrix.put(responseB, responseA, 9L);
+
+        Location a = new Location(0.0, 1.0);
+        Location b = new Location(1.0, 0.0);
+        a.setDistanceMatrix(distanceMatrix);
+        b.setDistanceMatrix(distanceMatrix);
+
+        assertThat((int) a.getIndex(distanceMatrix)).isZero();
+        assertThat((int) b.getIndex(distanceMatrix)).isEqualTo(1);
+        assertThat(distanceMatrix.get(a, b)).isEqualTo(7L);
+        assertThat(distanceMatrix.get(b, a)).isEqualTo(9L);
+    }
+
     @Test
     void putNoResize() {
         LongArrayDistanceMatrix distanceMatrix = new LongArrayDistanceMatrix(4);