Add MoreGatherers.distinctByKeepLast() (#26)

README.md

@@ -26,5 +26,6 @@ Provided `Gatherers`:
 - `MoreGatherers.zipWithIterable(Iterable<T2>)`
 - `MoreGatherers.zipWithIterable(Iterable<T2>, BiFunction<T1,T2>)`
 - `MoreGatherers.distinctBy(Function<T, R>)`
+- `MoreGatherers.distinctByKeepLast(Function<T, R>)`
 - `MoreGatherers.distinctUntilChanged()`
 - `MoreGatherers.distinctUntilChanged(Function<T, R>)`

src/main/java/com/pivovarit/gatherers/MoreGatherers.java

@@ -2,6 +2,7 @@
 
 import java.util.HashSet;
 import java.util.Iterator;
+import java.util.LinkedHashMap;
 import java.util.LinkedList;
 import java.util.Map;
 import java.util.Objects;
@@ -56,6 +57,22 @@ private MoreGatherers() {
           ));
     }
 
+    public static <T, U> Gatherer<T, ?, T> distinctByKeepLast(Function<? super T, ? extends U> keyExtractor) {
+        Objects.requireNonNull(keyExtractor, "keyExtractor can't be null");
+        return Gatherer.ofSequential(
+          LinkedHashMap<U, T>::new,
+          (state, element, _) -> {
+              state.put(keyExtractor.apply(element), element);
+              return true;
+          },
+          (state, downstream) -> {
+              for (T element : state.values()) {
+                  downstream.push(element);
+              }
+          }
+        );
+    }
+
     public static <T, U> Gatherer<T, ?, T> distinctBy(Function<? super T, ? extends U> keyExtractor) {
         Objects.requireNonNull(keyExtractor, "keyExtractor can't be null");
         return Gatherer.ofSequential(

src/test/java/com/pivovarit/gatherers/DistinctByKeepLastTest.java

@@ -0,0 +1,29 @@
+package com.pivovarit.gatherers;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.stream.Stream;
+
+import static com.pivovarit.gatherers.MoreGatherers.distinctByKeepLast;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+class DistinctByKeepLastTest {
+
+    @Test
+    void shouldDistinctByEmptyStream() {
+        assertThat(Stream.empty().gather(distinctByKeepLast(i -> i))).isEmpty();
+    }
+
+    @Test
+    void shouldDistinctBy() {
+        assertThat(Stream.of("a", "bb", "cc", "ddd")
+          .gather(distinctByKeepLast(String::length)))
+          .containsExactly("a", "cc", "ddd");
+    }
+
+    @Test
+    void shouldRejectNullExtractor() {
+        assertThatThrownBy(() -> distinctByKeepLast(null)).isInstanceOf(NullPointerException.class);
+    }
+}