From d4734356e92d31f4c1a3b235e4370f39a87a3b60 Mon Sep 17 00:00:00 2001
From: Marshall Clow <mclow.lists@gmail.com>
Date: Wed, 3 Dec 2014 15:15:15 -0800
Subject: [PATCH] Added more general power functionality as requested by Sean
 Parent. Also added enable_if to make sure the exponent is an integral type.

---
 include/boost/algorithm/algorithm.hpp | 70 ++++++++++++++++++++-------
 test/Jamfile.v2                       |  1 +
 test/power_fail1.cpp                  | 24 +++++++++
 test/power_test.cpp                   | 27 +++++------
 4 files changed, 91 insertions(+), 31 deletions(-)
 create mode 100644 test/power_fail1.cpp

diff --git a/include/boost/algorithm/algorithm.hpp b/include/boost/algorithm/algorithm.hpp
index 3304831..0cf6c45 100644
--- a/include/boost/algorithm/algorithm.hpp
+++ b/include/boost/algorithm/algorithm.hpp
@@ -18,32 +18,68 @@
 #define BOOST_ALGORITHM_HPP
 
 #include <boost/utility/enable_if.hpp> // for boost::disable_if
+#include <boost/type_traits/is_integral.hpp>
 
 namespace boost { namespace algorithm {
 
+template <typename T>
+T identity_operation ( std::multiplies<T> ) { return T(1); }
+
+template <typename T>
+T identity_operation ( std::plus<T> ) { return T(0); }
+
+
 /// \fn power ( T x, Integer n )
 /// \return the value "x" raised to the power "n"
 /// 
 /// \param x     The value to be exponentiated
-/// \param n     The exponent
+/// \param n     The exponent (must be >= 0)
 ///
-//	\remark Taken from Knuth, The Art of Computer Programming, Volume 2:
-//		Seminumerical Algorithms, Section 4.6.3
+//  \remark Taken from Knuth, The Art of Computer Programming, Volume 2:
+//  Seminumerical Algorithms, Section 4.6.3
 template <typename T, typename Integer>
-T power (T x, Integer n) {
-	T y = 1; // Should be "T y{1};" 
-	if (n == 0) return y;
-	while (true) {
-		if (n % 2 == 1) {
-			y = x * y;
-			if (n == 1)
-				return y;
-			}
-		n = n / 2;
-		x = x * x;
-		}
-	return y;
-	}
+typename boost::enable_if<boost::is_integral<Integer>, T>::type
+power (T x, Integer n) {
+    T y = 1; // Should be "T y{1};" 
+    if (n == 0) return y;
+    while (true) {
+        if (n % 2 == 1) {
+            y = x * y;
+            if (n == 1)
+                return y;
+            }
+        n = n / 2;
+        x = x * x;
+        }
+    return y;
+    }
+
+/// \fn power ( T x, Integer n, Operation op )
+/// \return the value "x" raised to the power "n"
+/// using the operaton "op".
+/// 
+/// \param x     The value to be exponentiated
+/// \param n     The exponent (must be >= 0)
+/// \param op    The operation used
+///
+//  \remark Taken from Knuth, The Art of Computer Programming, Volume 2:
+//  Seminumerical Algorithms, Section 4.6.3
+template <typename T, typename Integer, typename Operation>
+typename boost::enable_if<boost::is_integral<Integer>, T>::type
+power (T x, Integer n, Operation op) {
+    T y = identity_operation(op);
+    if (n == 0) return y;
+    while (true) {
+        if (n % 2 == 1) {
+            y = op(x, y);
+            if (n == 1)
+                return y;
+            }
+        n = n / 2;
+        x = op(x, x);
+        }
+    return y;
+    }
 
 }}
 
diff --git a/test/Jamfile.v2 b/test/Jamfile.v2
index 727cada..9b5d30b 100644
--- a/test/Jamfile.v2
+++ b/test/Jamfile.v2
@@ -30,6 +30,7 @@ alias unit_test_framework
 # Misc tests
      [ run clamp_test.cpp unit_test_framework         : : : : clamp_test ]
      [ run power_test.cpp unit_test_framework         : : : : power_test ]
+     [ compile-fail power_fail1.cpp  : : : : ]
    
 # Cxx11 tests
      [ run all_of_test.cpp unit_test_framework         : : : : all_of_test ]
diff --git a/test/power_fail1.cpp b/test/power_fail1.cpp
new file mode 100644
index 0000000..6a3bf5f
--- /dev/null
+++ b/test/power_fail1.cpp
@@ -0,0 +1,24 @@
+/* 
+   Copyright (c) Marshall Clow 2014.
+
+   Distributed under the Boost Software License, Version 1.0. (See accompanying
+   file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+    For more information, see http://www.boost.org
+*/
+
+#include <iostream>
+
+#include <boost/config.hpp>
+#include <boost/algorithm/algorithm.hpp>
+
+#define BOOST_TEST_MAIN
+#include <boost/test/unit_test.hpp>
+
+namespace ba = boost::algorithm;
+
+BOOST_AUTO_TEST_CASE( test_main )
+{
+//  Second argument must be an integral value
+    BOOST_CHECK ( ba::power(1, 1.0) == 1);
+}
diff --git a/test/power_test.cpp b/test/power_test.cpp
index b4d8cf9..20204c1 100644
--- a/test/power_test.cpp
+++ b/test/power_test.cpp
@@ -15,22 +15,21 @@
 #define BOOST_TEST_MAIN
 #include <boost/test/unit_test.hpp>
 
-#include <string>
-#include <vector>
-#include <list>
-
 namespace ba = boost::algorithm;
 
 BOOST_AUTO_TEST_CASE( test_main )
 {
-	BOOST_CHECK ( ba::power(0, 0) == 1);
-	BOOST_CHECK ( ba::power(5, 0) == 1);
-	BOOST_CHECK ( ba::power(1, 1) == 1);
-	BOOST_CHECK ( ba::power(1, 4) == 1);
-	BOOST_CHECK ( ba::power(3, 2) == 9);
-	BOOST_CHECK ( ba::power(2, 3) == 8);
-	BOOST_CHECK ( ba::power(3, 3) == 27);
-	BOOST_CHECK ( ba::power(2, 30) == 0x40000000);
-	BOOST_CHECK ( ba::power(5L, 10) == 3125*3125);
-	BOOST_CHECK ( ba::power(18, 3) == 18*18*18);
+    BOOST_CHECK ( ba::power(0, 0) == 1);
+    BOOST_CHECK ( ba::power(5, 0) == 1);
+    BOOST_CHECK ( ba::power(1, 1) == 1);
+    BOOST_CHECK ( ba::power(1, 4) == 1);
+    BOOST_CHECK ( ba::power(3, 2) == 9);
+    BOOST_CHECK ( ba::power(2, 3) == 8);
+    BOOST_CHECK ( ba::power(3, 3) == 27);
+    BOOST_CHECK ( ba::power(2, 30) == 0x40000000);
+    BOOST_CHECK ( ba::power(5L, 10) == 3125*3125);
+    BOOST_CHECK ( ba::power(18, 3) == 18*18*18);
+    
+    BOOST_CHECK ( ba::power(3,2) == ba::power(3,2, std::multiplies<int>()));
+    BOOST_CHECK ( ba::power(3,2, std::plus<int>()) == 6);
 }