diff --git a/CHANGELOG.md b/CHANGELOG.md
index ade26c0..f09e2f8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,5 @@
### 0.6.0
-+ TBD
++ Implement `dropLast(n)`
### 0.5.0
+ Implement `reverse()`
diff --git a/README.md b/README.md
index 7f09931..ed8cfab 100644
--- a/README.md
+++ b/README.md
@@ -37,6 +37,7 @@ implementation("com.ginsberg:gatherers4j:0.6.0")
| `dedupeConsecutive()` | Remove consecutive duplicates from a stream |
| `dedupeConsecutiveBy(fn)` | Remove consecutive duplicates from a stream as returned by `fn` |
| `distinctBy(fn)` | Emit only distinct elements from the stream, as measured by `fn` |
+| `dropLast(n)` | Keep all but the last `n` elements of the stream |
| `exactSize(n)` | Ensure the stream is exactly `n` elements long, or throw an `IllegalStateException` |
| `filterWithIndex(predicate)` | Filter the stream with the given `predicate`, which takes an `element` and its `index` |
| `interleave(stream)` | Creates a stream of alternating objects from the input stream and the argument stream |
@@ -139,6 +140,16 @@ Stream
// [Person("Todd", "Ginsberg"), Person("Emma", "Ginsberg")]
```
+#### Keep all but the last `n` elements
+
+```java
+Stream.of("A", "B", "C", "D", "E")
+ .gather(Gatherers4j.dropLast(2))
+ .toList();
+
+// ["A", "B", "C"]
+```
+
#### Ensure the stream is exactly `n` elements long
```java
diff --git a/src/main/java/com/ginsberg/gatherers4j/DropLastGatherer.java b/src/main/java/com/ginsberg/gatherers4j/DropLastGatherer.java
new file mode 100644
index 0000000..05bf1c2
--- /dev/null
+++ b/src/main/java/com/ginsberg/gatherers4j/DropLastGatherer.java
@@ -0,0 +1,61 @@
+/*
+ * Copyright 2024 Todd Ginsberg
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.ginsberg.gatherers4j;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.BiConsumer;
+import java.util.function.Supplier;
+import java.util.stream.Gatherer;
+
+public class DropLastGatherer implements Gatherer, INPUT> {
+
+ private final int count;
+
+ DropLastGatherer(final int count) {
+ if (count <= 0) {
+ throw new IllegalArgumentException("DropLast count must be positive");
+ }
+ this.count = count;
+ }
+
+ @Override
+ public Supplier> initializer() {
+ return State::new;
+ }
+
+ @Override
+ public Integrator, INPUT, INPUT> integrator() {
+ return (state, element, downstream) -> {
+ state.elements.add(element);
+ return !downstream.isRejecting();
+ };
+ }
+
+ @Override
+ public BiConsumer, Downstream> finisher() {
+ return (inputState, downstream) -> {
+ for (int i = 0; i < inputState.elements.size() - count && !downstream.isRejecting(); i++) {
+ downstream.push(inputState.elements.get(i));
+ }
+ };
+ }
+
+ public static class State {
+ final List elements = new ArrayList<>();
+ }
+}
diff --git a/src/main/java/com/ginsberg/gatherers4j/Gatherers4j.java b/src/main/java/com/ginsberg/gatherers4j/Gatherers4j.java
index 30b0305..167920f 100644
--- a/src/main/java/com/ginsberg/gatherers4j/Gatherers4j.java
+++ b/src/main/java/com/ginsberg/gatherers4j/Gatherers4j.java
@@ -75,6 +75,15 @@ public static ThrottlingGatherer debounce(final int amount, final
return new DistinctGatherer<>(function);
}
+ /**
+ * Keep all elements except the last count elements of the stream.
+ *
+ * @param count A positive number of elements to drop from the end of the stream
+ */
+ public static DropLastGatherer dropLast(final int count) {
+ return new DropLastGatherer<>(count);
+ }
+
/**
* Ensure the input stream is exactly size elements long, and emit all elements
* if so. If not, throw an IllegalStateException.
diff --git a/src/test/java/com/ginsberg/gatherers4j/DropLastGathererTest.java b/src/test/java/com/ginsberg/gatherers4j/DropLastGathererTest.java
new file mode 100644
index 0000000..5d34737
--- /dev/null
+++ b/src/test/java/com/ginsberg/gatherers4j/DropLastGathererTest.java
@@ -0,0 +1,60 @@
+/*
+ * Copyright 2024 Todd Ginsberg
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.ginsberg.gatherers4j;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+import java.util.stream.Stream;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+class DropLastGathererTest {
+
+ @Test
+ void countMustBePositive() {
+ assertThatThrownBy(() -> Stream.of("A")
+ .gather(Gatherers4j.dropLast(0)).toList())
+ .isInstanceOf(IllegalArgumentException.class);
+ }
+
+ @Test
+ void dropLast() {
+ // Arrange
+ final Stream input = Stream.of("A", "B", "C");
+
+ // Act
+ final List output = input.gather(Gatherers4j.dropLast(2)).toList();
+
+ // Assert
+ assertThat(output).containsExactly("A");
+ }
+
+ @Test
+ void dropLastLongerThanStreamReturnsEmpty() {
+ // Arrange
+ final Stream input = Stream.of("A", "B", "C");
+
+ // Act
+ final List output = input.gather(Gatherers4j.dropLast(4)).toList();
+
+ // Assert
+ assertThat(output).isEmpty();
+ }
+
+}
\ No newline at end of file