Merge pull request #161 from mperry/list-partition

mperry · mperry · commit 431126a12097 · 2015-06-24T22:40:39.000+10:00
Added List.partition
diff --git a/core/src/main/java/fj/data/List.java b/core/src/main/java/fj/data/List.java
@@ -854,6 +854,22 @@ public final List<List<A>> partition(final int n) {
     return unfold(as -> as.isEmpty() ? Option.<P2<List<A>, List<A>>>none() : some(as.splitAt(n)), this);
   }
 
+  /**
+   * Partitions the list into a tuple where the first element contains the
+   * items that satisfy the the predicate f and the second element contains the
+   * items that does not.  The relative order of the elements in the returned tuple
+   * is the same as the original list.
+   *
+   * @param f Predicate function.
+   */
+  public P2<List<A>, List<A>> partition(F<A, Boolean> f) {
+    P2<List<A>, List<A>> p2 = foldLeft(acc -> a ->
+      f.f(a) ? P.p(acc._1().cons(a), acc._2()) : P.p(acc._1(), acc._2().cons(a)),
+      P.p(nil(), nil())
+    );
+    return P.p(p2._1().reverse(), p2._2().reverse());
+  }
+
   /**
    * Returns the list of initial segments of this list, shortest first.
    *
diff --git a/core/src/test/java/fj/data/ListTest.java b/core/src/test/java/fj/data/ListTest.java
@@ -1,5 +1,7 @@
 package fj.data;
 
+import fj.Equal;
+import fj.P2;
 import org.junit.Test;
 
 import java.util.Arrays;
@@ -53,4 +55,13 @@ public void convertToString() {
         expected.append(')');
         assertEquals(expected.toString(), List.range(0, n).toString());
     }
+
+    @Test
+    public void partition() {
+        P2<List<Integer>, List<Integer>> p = List.range(1, 5).partition(i -> i % 2 == 0);
+        Equal<List<Integer>> e = Equal.listEqual(Equal.intEqual);
+        assertTrue(e.eq(p._1(), List.list(2, 4)));
+        assertTrue(e.eq(p._2(), List.list(1, 3)));
+    }
+
 }
