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diff --git a/libtommath/bn_mp_dr_setup.c b/libtommath/bn_mp_dr_setup.c
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+#include <tommath.h>
+#ifdef BN_MP_DR_SETUP_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis
+ *
+ * LibTomMath is a library that provides multiple-precision
+ * integer arithmetic as well as number theoretic functionality.
+ *
+ * The library was designed directly after the MPI library by
+ * Michael Fromberger but has been written from scratch with
+ * additional optimizations in place.
+ *
+ * The library is free for all purposes without any express
+ * guarantee it works.
+ *
+ * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com
+ */
+
+/* determines the setup value */
+void mp_dr_setup(mp_int *a, mp_digit *d)
+{
+ /* the casts are required if DIGIT_BIT is one less than
+ * the number of bits in a mp_digit [e.g. DIGIT_BIT==31]
+ */
+ *d = (mp_digit)((((mp_word)1) << ((mp_word)DIGIT_BIT)) -
+ ((mp_word)a->dp[0]));
+}
+
+#endif