Add tests for functional utility methods

pgpainless-core/src/main/java/org/pgpainless/util/CollectionUtils.java

@@ -100,6 +100,13 @@ public final class CollectionUtils {
         };
     }
 
+    public static <T, N> N reduce(Iterator<T> iterator, Reducer<T, N> reducer) {
+        while (iterator.hasNext()) {
+            reducer.accumulate(iterator.next());
+        }
+        return reducer.getResult();
+    }
+
     @FunctionalInterface
     interface Filter<T> {
         boolean accept(T t);
@@ -110,6 +117,12 @@ public final class CollectionUtils {
         N mapValue(T item);
     }
 
+    interface Reducer<T, N> {
+        void accumulate(T item);
+
+        N getResult();
+    }
+
     /**
      * Add all items from the iterator to the collection.
      *

pgpainless-core/src/test/java/org/pgpainless/util/CollectionUtilsTest.java

@@ -0,0 +1,98 @@
+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package org.pgpainless.util;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Iterator;
+import java.util.List;
+import java.util.NoSuchElementException;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class CollectionUtilsTest {
+
+    @Test
+    public void testIteratorToList() {
+        List<String> list = Arrays.asList("Hello", "World");
+        Iterator<String> iterator = list.iterator();
+
+        List<String> toList = CollectionUtils.iteratorToList(iterator);
+        assertEquals(list, toList);
+    }
+
+    @Test
+    public void testEmptyIteratorToList() {
+        Iterator<String> emptyIterator = new Iterator<String>() {
+            @Override
+            public boolean hasNext() {
+                return false;
+            }
+
+            @Override
+            public String next() {
+                throw new NoSuchElementException();
+            }
+        };
+
+        List<String> list = CollectionUtils.iteratorToList(emptyIterator);
+        assertTrue(list.isEmpty());
+    }
+
+    @Test
+    public void testIteratorContains() {
+        Iterator<String> iterator = Arrays.asList("Alpha", "Bravo", "Charlie", "Delta").iterator();
+        assertTrue(CollectionUtils.contains(iterator,"Charlie"));
+    }
+
+    @Test
+    public void testIteratorNotContains() {
+        Iterator<String> iterator = Arrays.asList("Alpha", "Bravo", "Charlie", "Delta").iterator();
+        assertFalse(CollectionUtils.contains(iterator,"Echo"));
+    }
+
+    @Test
+    public void testMapStringsToNumbers() {
+        Iterator<String> phonetics = Arrays.asList("Foxtrot", "Uniform", "Charlie", "Kilo").iterator();
+        CollectionUtils.Mapper<String, Character> phoneticsMapper =
+                item -> item.toUpperCase().charAt(0);
+        Iterator<Character> characters = CollectionUtils.map(phonetics, phoneticsMapper);
+        CollectionUtils.Reducer<Character, String> concat = new CollectionUtils.Reducer<Character, String>() {
+            final StringBuilder sb = new StringBuilder();
+            @Override
+            public void accumulate(Character item) {
+                sb.append(item);
+            }
+
+            @Override
+            public String getResult() {
+                return sb.toString();
+            }
+        };
+        String explicit = CollectionUtils.reduce(characters, concat);
+        assertEquals(rot13("SHPX"), explicit);
+    }
+
+    private String rot13(String string) {
+        char[] chars = string.toCharArray();
+        for (int i = 0; i < chars.length; i++) {
+            chars[i] = (char) (65 + ((chars[i] - 52) % 26));
+        }
+        return new String(chars);
+    }
+
+    @Test
+    public void testAddAll() {
+        List<String> list = new ArrayList<>(Arrays.asList("Alpha", "Bravo"));
+        Iterator<String> iterator = Arrays.asList("Charlie", "Delta", "Echo").iterator();
+
+        CollectionUtils.addAll(iterator, list);
+        assertEquals(Arrays.asList("Alpha", "Bravo", "Charlie", "Delta", "Echo"), list);
+    }
+}