Add Gatherers.zipWithIndex()

Author: pivovarit

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

@@ -1,4 +1,24 @@
 package com.pivovarit.gatherers;
 
-class Gatherers {
+import java.util.Map;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.stream.Gatherer;
+
+import static java.util.stream.Gatherer.Integrator.ofGreedy;
+import static java.util.stream.Gatherer.ofSequential;
+
+public final class Gatherers {
+
+    private Gatherers() {
+    }
+
+    public static <T> Gatherer<T, ?, Map.Entry<Long, T>> zipWithIndex() {
+        return ofSequential(
+          AtomicLong::new,
+          ofGreedy((state, element, downstream) -> {
+              downstream.push(Map.entry(state.getAndIncrement(), element));
+              return true;
+          })
+        );
+    }
 }

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

@@ -0,0 +1,28 @@
+package com.pivovarit.gatherers;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.stream.Stream;
+
+import static com.pivovarit.gatherers.Gatherers.zipWithIndex;
+import static java.util.Map.entry;
+import static org.assertj.core.api.Assertions.assertThat;
+
+class ZipWithIndexTest {
+
+    @Test
+    void shouldZipEmptyStream() {
+        assertThat(Stream.empty().gather(zipWithIndex())).isEmpty();
+    }
+
+    @Test
+    void shouldZipWithIndex() {
+        assertThat(Stream.of("a", "b", "c")
+          .gather(zipWithIndex()))
+          .containsExactly(
+            entry(0L, "a"),
+            entry(1L, "b"),
+            entry(2L, "c")
+          );
+    }
+}