1 files changed, 58 insertions, 0 deletions
diff --git a/base/intmath.cc b/base/intmath.cc
new file mode 100644
index 000000000..8d08e59a8
--- /dev/null
+++ b/base/intmath.cc
@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2003 The Regents of The University of Michigan
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "intmath.h"
+
+int
+PrevPrime(int n)
+{
+  int decr;
+
+  // If the number is even, let's start with the previous odd number.
+  if (!(n & 1))
+    --n;
+
+  // Lets test for divisibility by 3.  Then we will be able to easily
+  // avoid numbers that are divisible by 3 in the future.
+  decr = n % 3;
+  if (decr == 0) {
+    n -= 2;
+    decr = 2;
+  }
+  else if (decr == 1)
+    decr = 4;
+
+  for(;;) {
+    if (IsPrime(n))
+      return n;
+    n -= decr;
+    // Toggle between 2 and 4 to prevent trying numbers that are known
+    // to be divisible by 3.
+    decr = 6 - decr;
+  }
+}