Fix list partitioning helper.

Fixes #2934

// FREEBIE

androidTest/java/org/thoughtcrime/securesms/util/ListPartitionTest.java

@@ -0,0 +1,83 @@
+package org.thoughtcrime.securesms.util;
+
+import java.util.LinkedList;
+import java.util.List;
+
+import static junit.framework.Assert.assertEquals;
+
+public class ListPartitionTest {
+
+  public void testPartitionEven() {
+    List<Integer> list = new LinkedList<>();
+
+    for (int i=0;i<100;i++) {
+      list.add(i);
+    }
+
+    List<List<Integer>> partitions = Util.partition(list, 10);
+
+    assertEquals(partitions.size(), 10);
+
+    int counter = 0;
+
+    for (int i=0;i<partitions.size();i++) {
+      List<Integer> partition = partitions.get(i);
+      assertEquals(partition.size(), 10);
+
+      for (int j=0;j<partition.size();j++) {
+        assertEquals((int)partition.get(j), counter++);
+      }
+    }
+  }
+
+  public void testPartitionOdd() {
+    List<Integer> list = new LinkedList<>();
+
+    for (int i=0;i<100;i++) {
+      list.add(i);
+    }
+
+    list.add(100);
+
+    List<List<Integer>> partitions = Util.partition(list, 10);
+
+    assertEquals(partitions.size(), 11);
+
+    int counter = 0;
+
+    for (int i=0;i<partitions.size()-1;i++) {
+      List<Integer> partition = partitions.get(i);
+      assertEquals(partition.size(), 10);
+
+      for (int j=0;j<partition.size();j++) {
+        assertEquals((int)partition.get(j), counter++);
+      }
+    }
+
+    assertEquals(partitions.get(10).size(), 1);
+    assertEquals((int)partitions.get(10).get(0), 100);
+  }
+
+  public void testPathological() {
+    List<Integer> list = new LinkedList<>();
+
+    for (int i=0;i<100;i++) {
+      list.add(i);
+    }
+
+    List<List<Integer>> partitions = Util.partition(list, 1);
+
+    assertEquals(partitions.size(), 100);
+
+    int counter = 0;
+
+    for (int i=0;i<partitions.size();i++) {
+      List<Integer> partition = partitions.get(i);
+      assertEquals(partition.size(), 1);
+
+      for (int j=0;j<partition.size();j++) {
+        assertEquals((int)partition.get(j), counter++);
+      }
+    }
+  }
+}

src/org/thoughtcrime/securesms/util/Util.java

@@ -201,10 +201,9 @@ public class Util {
     List<List<T>> results = new LinkedList<>();
 
     for (int index=0;index<list.size();index+=partitionSize) {
-      int offset      = index * partitionSize;
-      int subListSize = Math.min(partitionSize, list.size() - offset);
+      int subListSize = Math.min(partitionSize, list.size() - index);
 
-      results.add(list.subList(offset, offset + subListSize));
+      results.add(list.subList(index, index + subListSize));
     }
 
     return results;