### diff libtommath/bn_mp_div_2d.c @ 284:eed26cff980b

author Matt Johnston Wed, 08 Mar 2006 13:23:49 +0000 5ff8218bcee9
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```--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libtommath/bn_mp_div_2d.c	Wed Mar 08 13:23:49 2006 +0000
@@ -0,0 +1,93 @@
+#include <tommath.h>
+#ifdef BN_MP_DIV_2D_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, [email protected], http://math.libtomcrypt.org
+ */
+
+/* shift right by a certain bit count (store quotient in c, optional remainder in d) */
+int mp_div_2d (mp_int * a, int b, mp_int * c, mp_int * d)
+{
+  mp_digit D, r, rr;
+  int     x, res;
+  mp_int  t;
+
+
+  /* if the shift count is <= 0 then we do no work */
+  if (b <= 0) {
+    res = mp_copy (a, c);
+    if (d != NULL) {
+      mp_zero (d);
+    }
+    return res;
+  }
+
+  if ((res = mp_init (&t)) != MP_OKAY) {
+    return res;
+  }
+
+  /* get the remainder */
+  if (d != NULL) {
+    if ((res = mp_mod_2d (a, b, &t)) != MP_OKAY) {
+      mp_clear (&t);
+      return res;
+    }
+  }
+
+  /* copy */
+  if ((res = mp_copy (a, c)) != MP_OKAY) {
+    mp_clear (&t);
+    return res;
+  }
+
+  /* shift by as many digits in the bit count */
+  if (b >= (int)DIGIT_BIT) {
+    mp_rshd (c, b / DIGIT_BIT);
+  }
+
+  /* shift any bit count < DIGIT_BIT */
+  D = (mp_digit) (b % DIGIT_BIT);
+  if (D != 0) {
+    register mp_digit *tmpc, mask, shift;
+
+    mask = (((mp_digit)1) << D) - 1;
+
+    /* shift for lsb */
+    shift = DIGIT_BIT - D;
+
+    /* alias */
+    tmpc = c->dp + (c->used - 1);
+
+    /* carry */
+    r = 0;
+    for (x = c->used - 1; x >= 0; x--) {
+      /* get the lower  bits of this word in a temp */
+      rr = *tmpc & mask;
+
+      /* shift the current word and mix in the carry bits from the previous word */
+      *tmpc = (*tmpc >> D) | (r << shift);
+      --tmpc;
+
+      /* set the carry to the carry bits of the current word found above */
+      r = rr;
+    }
+  }
+  mp_clamp (c);
+  if (d != NULL) {
+    mp_exch (&t, d);
+  }
+  mp_clear (&t);
+  return MP_OKAY;
+}
+#endif```