Fix libsnark dependency build.

This changes libsnark to build in-place, instead of copying first to
a build directory. Previously, modifications made to the original
sources wouldn't get rebuilt without a 'make clean' because users
would be pointing to the copies.

This closes #2689.

src/snark/libsnark/algebra/exponentiation/exponentiation.hpp

@@ -0,0 +1,31 @@
+/** @file
+ *****************************************************************************
+
+ Declaration of interfaces for (square-and-multiply) exponentiation.
+
+ *****************************************************************************
+ * @author     This file is part of libsnark, developed by SCIPR Lab
+ *             and contributors (see AUTHORS).
+ * @copyright  MIT license (see LICENSE file)
+ *****************************************************************************/
+
+#ifndef EXPONENTIATION_HPP_
+#define EXPONENTIATION_HPP_
+
+#include <cstdint>
+
+#include "algebra/fields/bigint.hpp"
+
+namespace libsnark {
+
+template<typename FieldT, mp_size_t m>
+FieldT power(const FieldT &base, const bigint<m> &exponent);
+
+template<typename FieldT>
+FieldT power(const FieldT &base, const unsigned long exponent);
+
+} // libsnark
+
+#include "algebra/exponentiation/exponentiation.tcc"
+
+#endif // EXPONENTIATION_HPP_

src/snark/libsnark/algebra/exponentiation/exponentiation.tcc

@@ -0,0 +1,53 @@
+/** @file
+ *****************************************************************************
+
+ Implementation of interfaces for (square-and-multiply) exponentiation.
+
+ See exponentiation.hpp .
+
+ *****************************************************************************
+ * @author     This file is part of libsnark, developed by SCIPR Lab
+ *             and contributors (see AUTHORS).
+ * @copyright  MIT license (see LICENSE file)
+ *****************************************************************************/
+
+#ifndef EXPONENTIATION_TCC_
+#define EXPONENTIATION_TCC_
+
+#include "common/utils.hpp"
+
+namespace libsnark {
+
+template<typename FieldT, mp_size_t m>
+FieldT power(const FieldT &base, const bigint<m> &exponent)
+{
+    FieldT result = FieldT::one();
+
+    bool found_one = false;
+
+    for (long i = exponent.max_bits() - 1; i >= 0; --i)
+    {
+        if (found_one)
+        {
+            result = result * result;
+        }
+
+        if (exponent.test_bit(i))
+        {
+            found_one = true;
+            result = result * base;
+        }
+    }
+
+    return result;
+}
+
+template<typename FieldT>
+FieldT power(const FieldT &base, const unsigned long exponent)
+{
+    return power<FieldT>(base, bigint<1>(exponent));
+}
+
+} // libsnark
+
+#endif // EXPONENTIATION_TCC_