Add distinctByKeepLast for Seq (#3006)

Author: sam-chakrabarty

vavr/src/main/java/io/vavr/collection/Array.java

@@ -746,6 +746,18 @@ public <U> Array<T> distinctBy(Function<? super T, ? extends U> keyExtractor) {
         return filter(t -> seen.add(keyExtractor.apply(t)));
     }
 
+    @Override
+    public Array<T> distinctByKeepLast(Comparator<? super T> comparator) {
+        Objects.requireNonNull(comparator, "comparator is null");
+        return ofAll(iterator().distinctByKeepLast(comparator));
+    }
+
+    @Override
+    public <U> Array<T> distinctByKeepLast(Function<? super T, ? extends U> keyExtractor) {
+        Objects.requireNonNull(keyExtractor, "keyExtractor is null");
+        return ofAll(iterator().distinctByKeepLast(keyExtractor));
+    }
+
     @Override
     public Array<T> drop(int n) {
         if (n <= 0) {

vavr/src/main/java/io/vavr/collection/CharSeq.java

@@ -471,6 +471,18 @@ public <U> CharSeq distinctBy(Function<? super Character, ? extends U> keyExtrac
         return filter(t -> seen.add(keyExtractor.apply(t)));
     }
 
+    @Override
+    public CharSeq distinctByKeepLast(Comparator<? super Character> comparator) {
+        Objects.requireNonNull(comparator, "comparator is null");
+        return ofAll(iterator().distinctByKeepLast(comparator));
+    }
+
+    @Override
+    public <U> CharSeq distinctByKeepLast(Function<? super Character, ? extends U> keyExtractor) {
+        Objects.requireNonNull(keyExtractor, "keyExtractor is null");
+        return ofAll(iterator().distinctByKeepLast(keyExtractor));
+    }
+
     @Override
     public CharSeq drop(int n) {
         if (n <= 0) {

vavr/src/main/java/io/vavr/collection/IndexedSeq.java

@@ -105,6 +105,12 @@ public boolean isDefinedAt(Integer index) {
     @Override
     <U> IndexedSeq<T> distinctBy(Function<? super T, ? extends U> keyExtractor);
 
+    @Override
+    IndexedSeq<T> distinctByKeepLast(Comparator<? super T> comparator);
+
+    @Override
+    <U> IndexedSeq<T> distinctByKeepLast(Function<? super T, ? extends U> keyExtractor);
+
     @Override
     IndexedSeq<T> drop(int n);
 

vavr/src/main/java/io/vavr/collection/Iterator.java

@@ -1373,6 +1373,30 @@ default <U> Iterator<T> distinctBy(Function<? super T, ? extends U> keyExtractor
         }
     }
 
+    default Iterator<T> distinctByKeepLast(Comparator<? super T> comparator) {
+        Objects.requireNonNull(comparator, "comparator is null");
+        if (!hasNext()) {
+            return empty();
+        } else {
+            return Collections.reverseIterator(new DistinctIterator<>(
+                    Collections.reverseIterator(this),
+                    TreeSet.empty(comparator),
+                    Function.identity()));
+        }
+    }
+
+    default <U> Iterator<T> distinctByKeepLast(Function<? super T, ? extends U> keyExtractor) {
+        Objects.requireNonNull(keyExtractor, "keyExtractor is null");
+        if (!hasNext()) {
+            return empty();
+        } else {
+            return Collections.reverseIterator(new DistinctIterator<>(
+                    Collections.reverseIterator(this),
+                    io.vavr.collection.HashSet.empty(),
+                    keyExtractor));
+        }
+    }
+
     /**
      * Removes up to n elements from this iterator.
      *

vavr/src/main/java/io/vavr/collection/LinearSeq.java

@@ -105,6 +105,12 @@ public boolean isDefinedAt(Integer index) {
     @Override
     <U> LinearSeq<T> distinctBy(Function<? super T, ? extends U> keyExtractor);
 
+    @Override
+    LinearSeq<T> distinctByKeepLast(Comparator<? super T> comparator);
+
+    @Override
+    <U> LinearSeq<T> distinctByKeepLast(Function<? super T, ? extends U> keyExtractor);
+
     @Override
     LinearSeq<T> drop(int n);
 

vavr/src/main/java/io/vavr/collection/List.java

@@ -817,6 +817,18 @@ default <U> List<T> distinctBy(Function<? super T, ? extends U> keyExtractor) {
         return filter(t -> seen.add(keyExtractor.apply(t)));
     }
 
+    @Override
+    default List<T> distinctByKeepLast(Comparator<? super T> comparator) {
+        Objects.requireNonNull(comparator, "comparator is null");
+        return ofAll(iterator().distinctByKeepLast(comparator));
+    }
+
+    @Override
+    default <U> List<T> distinctByKeepLast(Function<? super T, ? extends U> keyExtractor) {
+        Objects.requireNonNull(keyExtractor, "keyExtractor is null");
+        return ofAll(iterator().distinctByKeepLast(keyExtractor));
+    }
+
     @Override
     default List<T> drop(int n) {
         if (n <= 0) {

vavr/src/main/java/io/vavr/collection/Queue.java

@@ -737,6 +737,18 @@ public <U> Queue<T> distinctBy(Function<? super T, ? extends U> keyExtractor) {
         return ofAll(toList().distinctBy(keyExtractor));
     }
 
+    @Override
+    public Queue<T> distinctByKeepLast(Comparator<? super T> comparator) {
+        Objects.requireNonNull(comparator, "comparator is null");
+        return ofAll(toList().distinctByKeepLast(comparator));
+    }
+
+    @Override
+    public <U> Queue<T> distinctByKeepLast(Function<? super T, ? extends U> keyExtractor) {
+        Objects.requireNonNull(keyExtractor, "keyExtractor is null");
+        return ofAll(toList().distinctByKeepLast(keyExtractor));
+    }
+
     @Override
     public Queue<T> drop(int n) {
         if (n <= 0) {

vavr/src/main/java/io/vavr/collection/Seq.java

@@ -1126,6 +1126,31 @@ default boolean startsWith(Iterable<? extends T> that, int offset) {
     @Override
     <U> Seq<T> distinctBy(Function<? super T, ? extends U> keyExtractor);
 
+    /**
+     * Returns a sequential collection where duplicate elements, as determined by the comparator,
+     * are removed. If duplicates are encountered, the last occurrence of the element is
+     * retained in the resulting sequence.
+     *
+     * @param comparator the comparator used to determine equality of elements for the purpose
+     *                   of identifying duplicates. The comparator defines if two elements are
+     *                   considered equal.
+     * @return a new sequence where duplicates are removed, retaining the last occurrence of
+     *         each element as per the comparator provided.
+     */
+    Seq<T> distinctByKeepLast(Comparator<? super T> comparator);
+
+    /**
+     * Returns a sequential Stream-like sequence where the elements are distinct based on a key
+     * derived from the provided keyExtractor function. When duplicate elements are found,
+     * the last encountered element is kept in the sequence.
+     *
+     * @param <U> the type of key extracted from elements of this sequence
+     * @param keyExtractor the function to extract the key used for determining uniqueness
+     * @return a sequence consisting of distinct elements, keeping the last occurrence of each
+     *         duplicate based on the extracted key
+     */
+    <U> Seq<T> distinctByKeepLast(Function<? super T, ? extends U> keyExtractor);
+
     @Override
     Seq<T> drop(int n);
 

vavr/src/main/java/io/vavr/collection/Stream.java

@@ -992,6 +992,18 @@ default <U> Stream<T> distinctBy(Function<? super T, ? extends U> keyExtractor)
         return filter(t -> seen.add(keyExtractor.apply(t)));
     }
 
+    @Override
+    default Stream<T> distinctByKeepLast(Comparator<? super T> comparator) {
+        Objects.requireNonNull(comparator, "comparator is null");
+        return ofAll(iterator().distinctByKeepLast(comparator));
+    }
+
+    @Override
+    default <U> Stream<T> distinctByKeepLast(Function<? super T, ? extends U> keyExtractor) {
+        Objects.requireNonNull(keyExtractor, "keyExtractor is null");
+        return ofAll(iterator().distinctByKeepLast(keyExtractor));
+    }
+
     @Override
     default Stream<T> drop(int n) {
         Stream<T> stream = this;

vavr/src/main/java/io/vavr/collection/Vector.java

@@ -685,6 +685,18 @@ public <U> Vector<T> distinctBy(Function<? super T, ? extends U> keyExtractor) {
         return filter(t -> seen.add(keyExtractor.apply(t)));
     }
 
+    @Override
+    public Vector<T> distinctByKeepLast(Comparator<? super T> comparator) {
+        Objects.requireNonNull(comparator, "comparator is null");
+        return ofAll(iterator().distinctByKeepLast(comparator));
+    }
+
+    @Override
+    public <U> Vector<T> distinctByKeepLast(Function<? super T, ? extends U> keyExtractor) {
+        Objects.requireNonNull(keyExtractor, "keyExtractor is null");
+        return ofAll(iterator().distinctByKeepLast(keyExtractor));
+    }
+
     @Override
     public Vector<T> drop(int n) {
         return wrap(trie.drop(n));

vavr/src/test/java/io/vavr/collection/AbstractSeqTest.java

@@ -24,12 +24,15 @@
 import java.math.BigDecimal;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Comparator;
 import java.util.Spliterator;
 import java.util.function.Function;
 import java.util.function.Predicate;
 import java.util.stream.Collector;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.TestTemplate;
 
+import static java.util.Comparator.comparingInt;
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
@@ -2286,6 +2289,57 @@ public void shouldTestIndexedSeqEndsWithNonIndexedSeq() {
         assertThat(of(1, 2, 3, 4).endsWith(Stream.of(2, 3, 5))).isFalse();
     }
 
+    // -- distinctByKeepLast(Comparator)
+
+    @TestTemplate
+    public void shouldComputeDistinctByKeepLastOfEmptySeqUsingComparator() {
+        final Comparator<Integer> comparator = comparingInt(i -> i);
+        if (useIsEqualToInsteadOfIsSameAs()) {
+            assertThat(this.<Integer>empty().distinctByKeepLast(comparator)).isEqualTo(empty());
+        } else {
+            assertThat(this.<Integer>empty().distinctByKeepLast(comparator)).isSameAs(empty());
+        }
+    }
+
+    @TestTemplate
+    public void shouldComputeDistinctByKeepLastOfNonEmptySeqUsingComparator() {
+        final Comparator<String> comparator = comparingInt(s -> (s.charAt(1)));
+        final Seq<String> distinct = of("1a", "2a", "3b", "4b", "3a", "5c")
+                .distinctByKeepLast(comparator);
+        assertThat(distinct).isEqualTo(of("4b", "3a", "5c"));
+    }
+
+    @TestTemplate
+    public void shouldReturnSameInstanceWhenDistinctByKeepLastComparatorEmptySeq() {
+        final Seq<?> empty = empty();
+        assertThat(empty.distinctByKeepLast(Comparators.naturalComparator())).isSameAs(empty);
+    }
+
+    // -- distinctByKeepLast(Function)
+
+    @TestTemplate
+    public void shouldComputeDistinctByKeepLastOfEmptySeqUsingKeyExtractor() {
+        if (useIsEqualToInsteadOfIsSameAs()) {
+            assertThat(empty().distinctByKeepLast(Function.identity())).isEqualTo(empty());
+        } else {
+            assertThat(empty().distinctByKeepLast(Function.identity())).isSameAs(empty());
+        }
+    }
+
+    @TestTemplate
+    public void shouldComputeDistinctByKeepLastOfNonEmptySeqUsingKeyExtractor() {
+        final Function<String, Character> function = c -> c.charAt(1);
+        final Seq<String> distinct = of("1a", "2a", "3b", "4b", "3a", "5c")
+                .distinctByKeepLast(function);
+        assertThat(distinct).isEqualTo(of("4b", "3a", "5c"));
+    }
+
+    @TestTemplate
+    public void shouldReturnSameInstanceWhenDistinctByKeepLastFunctionEmptySeq() {
+        final Seq<?> empty = empty();
+        assertThat(empty.distinctByKeepLast(Function.identity())).isSameAs(empty);
+    }
+
     private interface SomeInterface {
     }
 

vavr/src/test/java/io/vavr/collection/CharSeqTest.java

@@ -664,6 +664,34 @@ public void shouldComputeDistinctByOfNonEmptyTraversableUsingKeyExtractor() {
           .isEqualTo(CharSeq.of('1', '2', '3', '4', '5'));
     }
 
+    // -- distinctByKeepLast(Comparator)
+
+    @Test
+    public void shouldComputeDistinctByKeepLastOfEmptyTraversableUsingComparator() {
+        final Comparator<Character> comparator = Comparator.comparingInt(i -> i);
+        assertThat(CharSeq.empty().distinctByKeepLast(comparator)).isSameAs(CharSeq.empty());
+    }
+
+    @Test
+    public void shouldComputeDistinctByKeepLastOfNonEmptyTraversableUsingComparator() {
+        final Comparator<Character> comparator = (s1, s2) -> (s1 - s2);
+        assertThat(CharSeq.of('1', '2', '3', '3', '4', '5').distinctByKeepLast(comparator))
+                .isEqualTo(CharSeq.of('1', '2', '3', '4', '5'));
+    }
+
+    // -- distinctByKeepLast(Function)
+
+    @Test
+    public void shouldComputeDistinctByKeepLastOfEmptyTraversableUsingKeyExtractor() {
+        assertThat(CharSeq.empty().distinctByKeepLast(Function.identity())).isSameAs(CharSeq.empty());
+    }
+
+    @Test
+    public void shouldComputeDistinctByKeepLastOfNonEmptyTraversableUsingKeyExtractor() {
+        assertThat(CharSeq.of('1', '2', '3', '2', '4', '5').distinctByKeepLast(c -> c))
+                .isEqualTo(CharSeq.of('1', '3', '2', '4', '5'));
+    }
+
     // -- drop
 
     @Test

vavr/src/test/java/io/vavr/collection/IteratorTest.java

@@ -604,6 +604,8 @@ public void multipleHasNext() {
         multipleHasNext(() -> Iterator.of(1, 2, 1, 2, 1, 2).distinct());
         multipleHasNext(() -> Iterator.of(1, 2, 1, 2, 1, 2).distinctBy(e -> e % 2));
         multipleHasNext(() -> Iterator.of(1, 2, 1, 2, 1, 2).distinctBy(Comparator.comparingInt(e -> e % 2)));
+        multipleHasNext(() -> Iterator.of(1, 2, 1, 2, 1, 2).distinctByKeepLast(e -> e % 2));
+        multipleHasNext(() -> Iterator.of(1, 2, 1, 2, 1, 2).distinctByKeepLast(Comparator.comparingInt(e -> e % 2)));
         multipleHasNext(() -> Iterator.of(1, 2, 3, 4).drop(1));
         multipleHasNext(() -> Iterator.of(1, 2, 3, 4).dropRight(1));
         multipleHasNext(() -> Iterator.of(1, 2, 3, 4).dropUntil(e -> e == 3));