From 7cced98dd282c3b21b83f9383d47e00e82d1dd12 Mon Sep 17 00:00:00 2001
From: Christoph Ruegg <git@cdrnet.ch>
Date: Mon, 10 Oct 2011 00:20:05 +0200
Subject: [PATCH] Threading: CommonParallel now supports empty loops. Closes
 gh-19.

CommonParallel did break on empty loops with an argument exception instead
of just doing nothing. It now exits early if there's nothing to do in
For and Invoke, no longer throwing an exception. Also, it now special cases
single-element iterations and inlines their execution (trivial loop
unrolling), skipping all the threading/task overhead.
---
 src/Numerics/Threading/CommonParallel.cs | 28 ++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/src/Numerics/Threading/CommonParallel.cs b/src/Numerics/Threading/CommonParallel.cs
index 7ce9ce15..25a45d8b 100644
--- a/src/Numerics/Threading/CommonParallel.cs
+++ b/src/Numerics/Threading/CommonParallel.cs
@@ -72,6 +72,20 @@ namespace MathNet.Numerics.Threading
         /// <exception cref="AggregateException">At least one invocation of the body threw an exception.</exception>
         public static void For(int fromInclusive, int toExclusive, Action<int> body, bool parallel)
         {
+            // Special case: no action
+            if (fromInclusive >= toExclusive)
+            {
+                return;
+            }
+
+            // Special case: single action, inline
+            if (fromInclusive == (toExclusive - 1))
+            {
+                body(fromInclusive);
+                return;
+            }
+
+            // Common case
             if (parallel)
             {
 #if SILVERLIGHT
@@ -353,6 +367,20 @@ namespace MathNet.Numerics.Threading
         /// <exception cref="AggregateException">An action threw an exception.</exception>
         public static void Invoke(params Action[] actions)
         {
+            // Special case: no action
+            if (actions.Length == 0)
+            {
+                return;
+            }
+
+            // Special case: single action, inline
+            if (actions.Length == 1)
+            {
+                actions[0]();
+                return;
+            }
+
+            // Common case
             var maxThreads = Control.DisableParallelization ? 1 : Control.NumberOfParallelWorkerThreads;
 #if SILVERLIGHT
             Parallel.Invoke(actions);