More functional utilities from Mr Greenwell (with some name tweaks by MDB). He

so wants to be using Scala. For example, in Java:

 Folds.sum(0, values)

where values must implement Iterable and all you get is sum. In Scala:

 scala> Array(1, 2, 3).getClass // a real array
 res1: java.lang.Class[_] = class [I

 scala> Array(1, 2, 3).sum     
 res2: Int = 6

 scala> Vector(1, 2, 3).sum // more like ArrayList
 res3: Int = 6

 scala> Map(1 -> 2, 3 -> 4).keysIterator.sum // even works on Iterator
 res4: Int = 4

and you can do arbitrary folds just as easily:

 scala> Array(1, 2, 3).reduceLeft(math.max)
 res5: Int = 3

 scala> Array(2, 3, 4).reduceLeft(_*_)                 
 res6: Int = 24

Want to specify the type of the result? Can do:

 scala> Array(1, 2, 3).foldLeft(0L)(_+_)
 res7: Long = 6

Have a list of maps you need to merge? No problem:

 scala> List(Map(1 -> 2), Map(2 -> 3), Map(3 -> 4)).reduceLeft(_++_)
 res8: scala.collection.immutable.Map[Int,Int] = Map((1,2), (2,3), (3,4))

Functional programming truly does start with fun!


git-svn-id: https://samskivert.googlecode.com/svn/trunk@2895 6335cc39-0255-0410-8fd6-9bcaacd3b74c

src/main/java/com/samskivert/util/Folds.java

@@ -0,0 +1,112 @@
+//
+// $Id$
+//
+// samskivert library - useful routines for java programs
+// Copyright (C) 2001-2010 Michael Bayne, et al.
+// 
+// This library is free software; you can redistribute it and/or modify it
+// under the terms of the GNU Lesser General Public License as published
+// by the Free Software Foundation; either version 2.1 of the License, or
+// (at your option) any later version.
+//
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+// Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public
+// License along with this library; if not, write to the Free Software
+// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+
+package com.samskivert.util;
+
+import java.math.BigInteger;
+import java.util.Collection;
+import java.util.Map;
+
+/**
+ * Various useful folds over iterables.
+ */
+public class Folds
+{
+    /**
+     * Sums the supplied collection of numbers to an int with a left to right fold.
+     */
+    public static int sum (int zero, Iterable<? extends Number> values)
+    {
+        for (Number value : values) {
+            zero += value.intValue();
+        }
+        return zero;
+    }
+
+    /**
+     * Sums the supplied collection of numbers to a long with a left to right fold.
+     */
+    public static long sum (long zero, Iterable<? extends Number> values)
+    {
+        for (Number value : values) {
+            zero += value.longValue();
+        }
+        return zero;
+    }
+
+    /**
+     * Sums the supplied collection of numbers to a float with a left to right fold.
+     */
+    public static float sum (float zero, Iterable<? extends Number> values)
+    {
+        for (Number value : values) {
+            zero += value.floatValue();
+        }
+        return zero;
+    }
+
+    /**
+     * Sums the supplied collection of numbers to a double with a left to right fold.
+     */
+    public static double sum (double zero, Iterable<? extends Number> values)
+    {
+        for (Number value : values) {
+            zero += value.doubleValue();
+        }
+        return zero;
+    }
+
+    /**
+     * Sums the supplied collection of numbers to a bigint with a left to right fold.
+     */
+    public static BigInteger sum (BigInteger zero, Iterable<? extends BigInteger> values)
+    {
+        for (BigInteger value : values) {
+            zero = zero.add(value);
+        }
+        return zero;
+    }
+
+    /**
+     * Merges the supplied collections into <code>zero</code> using a left to right fold of
+     * {@link Collection#addAll}.
+     */
+    public static <T, C extends Collection<T>> C addAll (
+        C zero, Iterable<Collection<? extends T>> values)
+    {
+        for (Collection<? extends T> value : values) {
+            zero.addAll(value);
+        }
+        return zero;
+    }
+
+    /**
+     * Merges the supplied maps into <code>zero</code> using a left to right fold of
+     * {@link Map#putAll}.
+     */
+    public static <K, V, M extends Map<K, V>> M putAll (
+        M zero, Iterable<Map<? extends K, ? extends V>> values)
+    {
+        for (Map<? extends K, ? extends V> value : values) {
+            zero.putAll(value);
+        }
+        return zero;
+    }
+}