Introduce dedicated ZipWithIndexMappingGatherer

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

@@ -124,15 +124,7 @@ 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;
-          })
-        );
+        return new ZipWithIndexMappingGatherer<>(mapper);
     }
 
     public static <T> Gatherer<T, ?, Map.Entry<Long, T>> zipWithIndex() {

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

@@ -0,0 +1,28 @@
+package com.pivovarit.gatherers;
+
+import java.util.Objects;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.function.BiFunction;
+import java.util.function.Supplier;
+import java.util.stream.Gatherer;
+
+record ZipWithIndexMappingGatherer<T, R>(BiFunction<Long, ? super T, ? extends R> mapper)
+  implements Gatherer<T, AtomicLong, R> {
+
+    ZipWithIndexMappingGatherer {
+        Objects.requireNonNull(mapper, "mapper can't be null");
+    }
+
+    @Override
+    public Supplier<AtomicLong> initializer() {
+        return AtomicLong::new;
+    }
+
+    @Override
+    public Integrator<AtomicLong, T, R> integrator() {
+        return Integrator.ofGreedy((state, element, downstream) -> {
+            downstream.push(mapper.apply(state.getAndIncrement(), element));
+            return true;
+        });
+    }
+}