Add MoreGatherers.zipWithIndex() (#27)

README.md

@@ -25,6 +25,8 @@ Provided `Gatherers`:
 - `MoreGatherers.zip(Stream<T2>, BiFunction<T1,T2>)`
 - `MoreGatherers.zipWithIterable(Iterable<T2>)`
 - `MoreGatherers.zipWithIterable(Iterable<T2>, BiFunction<T1,T2>)`
+- `MoreGatherers.zipWithIndex()`
+- `MoreGatherers.zipWithIndex(BiFunction<Long,T>)`
 - `MoreGatherers.distinctBy(Function<T, R>)`
 - `MoreGatherers.distinctByKeepLast(Function<T, R>)`
 - `MoreGatherers.distinctUntilChanged()`

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

@@ -153,6 +153,18 @@ class State {
           });
     }
 
+    public static <T, R> Gatherer<T, ?, R> zipWithIndex(BiFunction<Long, ? super T, ? extends R> mapper) {
+        Objects.requireNonNull(mapper, "mapper can't be null");
+
+        return ofSequential(
+          AtomicLong::new,
+          ofGreedy((state, element, downstream) -> {
+              downstream.push(mapper.apply(state.getAndIncrement(), element));
+              return true;
+          })
+        );
+    }
+
     public static <T> Gatherer<T, ?, Map.Entry<Long, T>> zipWithIndex() {
         return ofSequential(
           AtomicLong::new,

src/test/java/com/pivovarit/gatherers/ZipWithIndexMapperTest.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.zipWithIndex;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+class ZipWithIndexMapperTest {
+
+    @Test
+    void shouldRejectNullMapper() {
+        assertThatThrownBy(() -> zipWithIndex(null)).isInstanceOf(NullPointerException.class);
+    }
+
+    @Test
+    void shouldZipEmptyStream() {
+        assertThat(Stream.empty().gather(zipWithIndex((idx, i) -> "" + idx + i))).isEmpty();
+    }
+
+    @Test
+    void shouldZipWithIndex() {
+        assertThat(Stream.of("a", "b", "c")
+          .gather(zipWithIndex((idx, i) -> idx + i)))
+          .containsExactly("0a", "1b", "2c");
+    }
+}