1 files changed, 140 insertions, 0 deletions
diff --git a/src/lib/math/bigint/divide.cpp b/src/lib/math/bigint/divide.cpp
new file mode 100644
index 000000000..8b21bce13
--- /dev/null
+++ b/src/lib/math/bigint/divide.cpp
@@ -0,0 +1,140 @@
+/*
+* Division Algorithm
+* (C) 1999-2007,2012 Jack Lloyd
+*
+* Distributed under the terms of the Botan license
+*/
+
+#include <botan/divide.h>
+#include <botan/internal/mp_core.h>
+#include <botan/internal/mp_madd.h>
+
+namespace Botan {
+
+namespace {
+
+/*
+* Handle signed operands, if necessary
+*/
+void sign_fixup(const BigInt& x, const BigInt& y, BigInt& q, BigInt& r)
+   {
+   if(x.sign() == BigInt::Negative)
+      {
+      q.flip_sign();
+      if(r.is_nonzero()) { --q; r = y.abs() - r; }
+      }
+   if(y.sign() == BigInt::Negative)
+      q.flip_sign();
+   }
+
+bool division_check(word q, word y2, word y1,
+                    word x3, word x2, word x1)
+   {
+   // Compute (y3,y2,y1) = (y2,y1) * q
+
+   word y3 = 0;
+   y1 = word_madd2(q, y1, &y3);
+   y2 = word_madd2(q, y2, &y3);
+
+   // Return (y3,y2,y1) >? (x3,x2,x1)
+
+   if(y3 > x3) return true;
+   if(y3 < x3) return false;
+
+   if(y2 > x2) return true;
+   if(y2 < x2) return false;
+
+   if(y1 > x1) return true;
+   if(y1 < x1) return false;
+
+   return false;
+   }
+
+}
+
+/*
+* Solve x = q * y + r
+*/
+void divide(const BigInt& x, const BigInt& y_arg, BigInt& q, BigInt& r)
+   {
+   if(y_arg.is_zero())
+      throw BigInt::DivideByZero();
+
+   BigInt y = y_arg;
+   const size_t y_words = y.sig_words();
+
+   r = x;
+   q = 0;
+
+   r.set_sign(BigInt::Positive);
+   y.set_sign(BigInt::Positive);
+
+   s32bit compare = r.cmp(y);
+
+   if(compare == 0)
+      {
+      q = 1;
+      r = 0;
+      }
+   else if(compare > 0)
+      {
+      size_t shifts = 0;
+      word y_top = y.word_at(y.sig_words()-1);
+      while(y_top < MP_WORD_TOP_BIT) { y_top <<= 1; ++shifts; }
+      y <<= shifts;
+      r <<= shifts;
+
+      const size_t n = r.sig_words() - 1, t = y_words - 1;
+
+      if(n < t)
+         throw Internal_Error("BigInt division word sizes");
+
+      q.grow_to(n - t + 1);
+
+      word* q_words = q.mutable_data();
+
+      if(n <= t)
+         {
+         while(r > y) { r -= y; ++q; }
+         r >>= shifts;
+         sign_fixup(x, y_arg, q, r);
+         return;
+         }
+
+      BigInt temp = y << (MP_WORD_BITS * (n-t));
+
+      while(r >= temp) { r -= temp; q_words[n-t] += 1; }
+
+      for(size_t j = n; j != t; --j)
+         {
+         const word x_j0  = r.word_at(j);
+         const word x_j1 = r.word_at(j-1);
+         const word y_t  = y.word_at(t);
+
+         if(x_j0 == y_t)
+            q_words[j-t-1] = MP_WORD_MAX;
+         else
+            q_words[j-t-1] = bigint_divop(x_j0, x_j1, y_t);
+
+         while(division_check(q_words[j-t-1],
+                              y_t, y.word_at(t-1),
+                              x_j0, x_j1, r.word_at(j-2)))
+            {
+            q_words[j-t-1] -= 1;
+            }
+
+         r -= (q_words[j-t-1] * y) << (MP_WORD_BITS * (j-t-1));
+
+         if(r.is_negative())
+            {
+            r += y << (MP_WORD_BITS * (j-t-1));
+            q_words[j-t-1] -= 1;
+            }
+         }
+      r >>= shifts;
+      }
+
+   sign_fixup(x, y_arg, q, r);
+   }
+
+}