Add sumRight for Semigroup using a NonEmptyList

puffnfresh · puffnfresh · commit 4fa4e82a05ff · 2016-05-16T20:13:06.000+10:00
diff --git a/core/src/main/java/fj/Semigroup.java b/core/src/main/java/fj/Semigroup.java
@@ -98,6 +98,12 @@ static <A> A multiply1p(F<A, F<A, A>> sum, int n, A a) {
   }
 
   /**
+   * Sums the given values with right-fold.
+   */
+  public A sumRight(final NonEmptyList<A> as) {
+    return as.foldRight1(sum);
+  }
+
   /**
    * Swaps the arguments when summing.
    */
diff --git a/core/src/main/java/fj/data/NonEmptyList.java b/core/src/main/java/fj/data/NonEmptyList.java
@@ -6,6 +6,7 @@
 import java.util.Collection;
 import java.util.Iterator;
 
+import static fj.Function.flip;
 import static fj.Function.identity;
 import static fj.data.Option.some;
 import static fj.data.Option.somes;
@@ -87,6 +88,26 @@ public NonEmptyList<A> append(final NonEmptyList<A> as) {
     return nel(head, bb);
   }
 
+  /**
+   * Performs a right-fold reduction across this list. This function uses O(length) stack space.
+   */
+  public final A foldRight1(final F<A, F<A, A>> f) {
+    return reverse().foldLeft1(flip(f));
+  }
+
+  /**
+   * Performs a left-fold reduction across this list. This function runs in constant space.
+   */
+  public final A foldLeft1(final F<A, F<A, A>> f) {
+    A x = head;
+
+    for (List<A> xs = tail; !xs.isEmpty(); xs = xs.tail()) {
+      x = f.f(x).f(xs.head());
+    }
+
+    return x;
+  }
+
   /**
    * Maps the given function across this list.
    *
