Convert stateful predicate distinctByKey into a list method

src/ext/java/org/opentripplanner/ext/geocoder/LuceneIndex.java

@@ -33,7 +33,6 @@
 import org.apache.lucene.search.FuzzyQuery;
 import org.apache.lucene.search.PrefixQuery;
 import org.apache.lucene.search.TermQuery;
-import org.apache.lucene.search.TopScoreDocCollector;
 import org.apache.lucene.search.suggest.document.Completion90PostingsFormat;
 import org.apache.lucene.search.suggest.document.CompletionAnalyzer;
 import org.apache.lucene.search.suggest.document.ContextQuery;

src/ext/java/org/opentripplanner/ext/geocoder/StopClusterMapper.java

@@ -1,11 +1,11 @@
 package org.opentripplanner.ext.geocoder;
 
+import com.google.common.collect.Iterables;
 import java.util.Collection;
 import java.util.Optional;
-import java.util.stream.Stream;
+import org.opentripplanner.framework.collection.ListUtils;
 import org.opentripplanner.framework.geometry.WgsCoordinate;
 import org.opentripplanner.framework.i18n.I18NString;
-import org.opentripplanner.framework.lang.PredicateUtils;
 import org.opentripplanner.transit.model.site.StopLocation;
 import org.opentripplanner.transit.model.site.StopLocationsGroup;
 import org.opentripplanner.transit.service.TransitService;
@@ -29,7 +29,7 @@ class StopClusterMapper {
    * - of "identical" stops which are very close to each other and have an identical name, only one
    *   is chosen (at random)
    */
-  Stream<StopCluster> generateStopClusters(
+  Iterable<StopCluster> generateStopClusters(
     Collection<StopLocation> stopLocations,
     Collection<StopLocationsGroup> stopLocationsGroups
   ) {
@@ -39,16 +39,20 @@ Stream<StopCluster> generateStopClusters(
       .filter(sl -> sl.getParentStation() == null)
       // stops without a name (for example flex areas) are useless for searching, so we remove them, too
       .filter(sl -> sl.getName() != null)
-      // if they are very close to each other and have the same name, only one is chosen (at random)
-      .filter(
-        PredicateUtils.distinctByKey(sl ->
-          new DeduplicationKey(sl.getName(), sl.getCoordinate().roundToApproximate100m())
-        )
+      .toList();
+
+    // if they are very close to each other and have the same name, only one is chosen (at random)
+    var deduplicatedStops = ListUtils
+      .distinctByKey(
+        stops,
+        sl -> new DeduplicationKey(sl.getName(), sl.getCoordinate().roundToApproximate100m())
       )
-      .flatMap(sl -> this.map(sl).stream());
-    var stations = stopLocationsGroups.stream().map(this::map);
+      .stream()
+      .flatMap(s -> this.map(s).stream())
+      .toList();
+    var stations = stopLocationsGroups.stream().map(this::map).toList();
 
-    return Stream.concat(stops, stations);
+    return Iterables.concat(deduplicatedStops, stations);
   }
 
   StopCluster map(StopLocationsGroup g) {

src/main/java/org/opentripplanner/framework/collection/ListUtils.java

@@ -1,8 +1,12 @@
 package org.opentripplanner.framework.collection;
 
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
+import java.util.function.Function;
 
 public class ListUtils {
 
@@ -13,4 +17,28 @@ public class ListUtils {
   public static <T> List<T> combine(Collection<T>... lists) {
     return Arrays.stream(lists).flatMap(Collection::stream).toList();
   }
+
+  /**
+   * Take a collection and a {@code keyExtractor} to remove duplicates where the key to be compared
+   * is not the entity itself but a field of it.
+   * <p>
+   * Note: Duplicate check is based on equality not identity.
+   */
+  public static <T> List<T> distinctByKey(
+    Collection<T> original,
+    Function<? super T, ?> keyExtractor
+  ) {
+    Set<Object> seen = new HashSet<>();
+    var ret = new ArrayList<T>();
+
+    original.forEach(elem -> {
+      var key = keyExtractor.apply(elem);
+      if (!seen.contains(key)) {
+        seen.add(key);
+        ret.add(elem);
+      }
+    });
+
+    return ret;
+  }
 }

src/main/java/org/opentripplanner/routing/graphfinder/StopFinderTraverseVisitor.java

@@ -4,7 +4,7 @@
 import java.util.List;
 import org.opentripplanner.astar.spi.SkipEdgeStrategy;
 import org.opentripplanner.astar.spi.TraverseVisitor;
-import org.opentripplanner.framework.lang.PredicateUtils;
+import org.opentripplanner.framework.collection.ListUtils;
 import org.opentripplanner.street.model.edge.Edge;
 import org.opentripplanner.street.model.vertex.TransitStopVertex;
 import org.opentripplanner.street.model.vertex.Vertex;
@@ -42,7 +42,7 @@ public void visitEnqueue() {}
    * @return A de-duplicated list of nearby stops found by this visitor.
    */
   public List<NearbyStop> stopsFound() {
-    return stopsFound.stream().filter(PredicateUtils.distinctByKey(ns -> ns.stop)).toList();
+    return ListUtils.distinctByKey(stopsFound, ns -> ns.stop);
   }
 
   /**

src/test/java/org/opentripplanner/framework/collection/ListUtilsTest.java

@@ -12,4 +12,30 @@ void combine() {
     var combined = ListUtils.combine(List.of(1, 2, 3), List.of(5, 6, 7));
     assertEquals(List.of(1, 2, 3, 5, 6, 7), combined);
   }
+
+  private static String makeHello() {
+    return new String("HELLO");
+  }
+
+  @Test
+  void distinctByKey() {
+    var first = new Wrapper(10, makeHello());
+    var second = new Wrapper(20, makeHello());
+    var third = new Wrapper(20, "HI");
+
+    // the first and second elements have equal values for the "string" property but different
+    // values for the "i" property
+    var duplicates = List.of(first, second, third);
+
+    // with a normal stream().distinct() or Set.copyOf no item would be removed but when
+    // we tell the deduplication logic to only look at a single property those with equal
+    // "string" values are de-duplicated
+    var deduplicated = ListUtils.distinctByKey(duplicates, w -> w.string);
+
+    // as a result the second element is removed from the result as its "string" value is
+    // equal to the first element's
+    assertEquals(List.of(first, third), deduplicated);
+  }
+
+  private record Wrapper(int i, String string) {}
 }