Changeset 4fb4f3 in git

Timestamp:

Jul 22, 2010, 1:13:53 PM (13 years ago)

Author:

Frank Seelisch <seelisch@…>

Branches:

(u'jengelh-datetime', 'ceac47cbc86fe4a15902392bdbb9bd2ae0ea02c6')(u'spielwiese', 'f875bbaccd0831e36aaed09ff6adeb3eb45aeb94')

Children:

c9208c6142738462cca70025492057ebce264baf

Parents:

ad0c5c42970dfab59d4ca1c9afc9fe369b894106

Message:

bug fix: ATTENTION: nr2mDivBy and nr2mIntDiv do not do what their names suggest

git-svn-id: file:///usr/local/Singular/svn/trunk@13046 2c84dea3-7e68-4137-9b89-c4e89433aadc

File:

• kernel/rmodulo2m.cc (modified) (3 diffs)

kernel/rmodulo2m.cc

@@ -151 +151 @@
 {
   NATNUMBER nn = (unsigned long)(NATNUMBER)n & r->nr2mModul;
-  unsigned long l = r->nr2mModul >> 1; l++;
-  if (l == 0)
-    return (int)(signed long)(NATNUMBER)nn;
-  else if ((NATNUMBER)nn > l)
+  unsigned long l = r->nr2mModul >> 1; l++; /* now: l = 2^(m-1) */
+  if ((NATNUMBER)nn > l)
     return (int)((NATNUMBER)nn - r->nr2mModul - 1);
   else
@@ -210 +208 @@
 }
 
+/* This does not seem to be the predicate whether a
+   is divisible by b in Z/2^m: if a is NULL then
+   the answer is not necessarily TRUE. */
 BOOLEAN nr2mDivBy (number a, number b)
 {
-  if ((NATNUMBER)a == 0) return TRUE;
-  if ((NATNUMBER)b == 0) return FALSE;
-  return ((NATNUMBER)a % (NATNUMBER)b) == 0;
+  if (a == NULL)
+  {
+    NATNUMBER c = currRing->nr2mModul + 1;
+    if (c != 0) /* i.e., if no overflow */
+      return (c % (NATNUMBER)b) == 0;
+    else
+    {
+      /* overflow: we need to check whether b
+         is a power of 2: */
+      c = (NATNUMBER)b;
+      while (c != 0)
+      {
+        if ((c % 2) != 0) return FALSE;
+        c = c >> 1;
+      }
+      return TRUE;
+    }
+  }
+  else
+    return ((NATNUMBER)a % (NATNUMBER)b) == 0;
 }
 
@@ -391 +409 @@
 }
 
-number nr2mIntDiv (number a,number b)
-{
-  assume((NATNUMBER)b != 0);
-  return (number) ((NATNUMBER) a / (NATNUMBER) b);
+number nr2mIntDiv (number a, number b)
+{
+  if ((NATNUMBER)a == 0)
+  {
+    if ((NATNUMBER)b == 0)
+      return (number)1;
+    if ((NATNUMBER)b == 1)
+      return (number)0;
+    NATNUMBER c = currRing->nr2mModul + 1;
+    if (c != 0) /* i.e., if no overflow */
+      return (number)(c / (NATNUMBER)b);
+    else
+    {
+      /* overflow: c = 2^32 resp. 2^64, depending on platform */
+      int_number cc = (int_number)omAlloc(sizeof(mpz_t));
+      mpz_init_set_ui(cc, currRing->nr2mModul); mpz_add_ui(cc, cc, 1);
+      mpz_div_ui(cc, cc, (unsigned long)(NATNUMBER)b);
+      unsigned long s = mpz_get_ui(cc);
+      mpz_clear(cc); omFree((ADDRESS)cc);
+      return (number)(NATNUMBER)s;
+    }
+  }
+  else
+  {
+    if ((NATNUMBER)b == 0)
+      return (number)0;
+    return (number)((NATNUMBER) a / (NATNUMBER) b);
+  }
 }