add method keyedByIndex (#18)

Author: tobbber

src/main/java/com/bakdata/util/seq2/Seq2.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c), 2024 bakdata GmbH
+ * Copyright (c), 2025 bakdata GmbH
  * 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
@@ -1332,10 +1332,26 @@ default <U> PairSeq<T, U> zip(final Seq2<? extends U> other) {
         return this.zip(other.toSeq());
     }
 
+    /**
+     * Zips each element of this sequence with its zero-based index.
+     *
+     * @return a {@link PairSeq} whose elements are the original elements paired with their respective index (element,
+     * index)
+     * @see Seq#zipWithIndex()
+     */
     default PairSeq<T, Long> zipWithIndex() {
         return PairSeq.seq(this.toSeq().zipWithIndex());
     }
 
+    /**
+     * Similar to {@link #zipWithIndex()}, but index is used as {@link PairSeq} key.
+     *
+     * @return a {@link PairSeq} whose elements are the index paired with the original element (index, element)
+     */
+    default PairSeq<Long, T> keyedByIndex() {
+        return this.zipWithIndex().swapped();
+    }
+
     /**
      * @see Seq#zipWithIndex()
      */

src/test/java/com/bakdata/util/seq2/Seq2Test.java

@@ -92,4 +92,26 @@ void shouldFlatMapToIterablePair() {
                         new Tuple2<>(2, 2), new Tuple2<>(3, 3));
     }
 
+    @Test
+    void shouldZipWithIndex() {
+        assertThat((Stream<Tuple2<Integer, Long>>) Seq2.seq(List.of(10, 11, 12))
+                .zipWithIndex()
+        ).containsExactlyInAnyOrder(
+                new Tuple2<>(10, 0L),
+                new Tuple2<>(11, 1L),
+                new Tuple2<>(12, 2L)
+        );
+    }
+
+    @Test
+    void shouldPairWithIndexKey() {
+        assertThat((Stream<Tuple2<Long, Integer>>) Seq2.seq(List.of(10, 11, 12))
+                .keyedByIndex()
+        ).containsExactlyInAnyOrder(
+                new Tuple2<>(0L, 10),
+                new Tuple2<>(1L, 11),
+                new Tuple2<>(2L, 12)
+        );
+    }
+
 }