From: Matthias Maier <tamiko@43-1.org>
Date: Thu, 5 Sep 2019 23:08:55 +0000 (-0500)
Subject: Utilities::fixed_power - use exponentiation by squaring
X-Git-Tag: v9.2.0-rc1~1136^2~3
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=a7d76ae20aed647673dce37334b04917fbe41012;p=dealii.git

Utilities::fixed_power - use exponentiation by squaring
---

diff --git a/include/deal.II/base/utilities.h b/include/deal.II/base/utilities.h
index 00442d9d9d..4c9ae69d84 100644
--- a/include/deal.II/base/utilities.h
+++ b/include/deal.II/base/utilities.h
@@ -1030,27 +1030,15 @@ namespace Utilities
 {
   template <int N, typename T>
   inline T
-  fixed_power(const T n)
+  fixed_power(const T x)
   {
-    Assert(N >= 0, ExcNotImplemented());
-    switch (N)
-      {
-        case 0:
-          return dealii::internal::NumberType<T>::value(1);
-        case 1:
-          return n;
-        case 2:
-          return n * n;
-        case 3:
-          return n * n * n;
-        case 4:
-          return n * n * n * n;
-        default:
-          T result = n;
-          for (int d = 1; d < N; ++d)
-            result *= n;
-          return result;
-      }
+    if (N == 0)
+      return T(1.);
+    else if (N < 0)
+      return T(1.) / fixed_power<-N>(x);
+    else
+      // Use exponentiation by squaring:
+      return ((N % 2 == 1) ? x * fixed_power<N / 2>(x * x) : fixed_power<N / 2>(x * x));
   }