[GiNaC-devel] [PATCH] Improve (fix?) smod: now it really converts into symmetric representation...

[Alexei Sheplyakov]

... instead of clumsy convention inspirited by some proprietary CAS.

---
 doc/tutorial/ginac.texi |    2 +-
 ginac/numeric.cpp       |   19 +++++++++++--------
 2 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/doc/tutorial/ginac.texi b/doc/tutorial/ginac.texi
index 3207a96..fe7a264 100644
--- a/doc/tutorial/ginac.texi
+++ b/doc/tutorial/ginac.texi
@@ -1479,7 +1479,7 @@ evaluated immediately:
 @tab modulus in positive representation (in the range @code{[0, abs(b)-1]} with the sign of b, or zero)
 @cindex @code{mod()}
 @item @code{smod(a, b)}
- at tab modulus in symmetric representation (in the range @code{[-iquo(abs(b)-1, 2), iquo(abs(b), 2)]})
+ at tab modulus in symmetric representation (in the range @code{[-iquo(abs(b), 2), iquo(abs(b), 2)]})
 @cindex @code{smod()}
 @item @code{irem(a, b)}
 @tab integer remainder (has the sign of @math{a}, or is zero)
diff --git a/ginac/numeric.cpp b/ginac/numeric.cpp
index 91721e1..50d873a 100644
--- a/ginac/numeric.cpp
+++ b/ginac/numeric.cpp
@@ -2337,15 +2337,18 @@ const numeric mod(const numeric &a, const numeric &b)
 
 
 /** Modulus (in symmetric representation).
- *  Equivalent to Maple's mods.
  *
- *  @return a mod b in the range [-iquo(abs(b)-1,2), iquo(abs(b),2)]. */
-const numeric smod(const numeric &a, const numeric &b)
-{
-	if (a.is_integer() && b.is_integer()) {
-		const cln::cl_I b2 = cln::ceiling1(cln::the<cln::cl_I>(b.to_cl_N()) >> 1) - 1;
-		return numeric(cln::mod(cln::the<cln::cl_I>(a.to_cl_N()) + b2,
-		                cln::the<cln::cl_I>(b.to_cl_N())) - b2);
+ *  @return a mod b in the range [-iquo(abs(b),2), iquo(abs(b),2)]. */
+const numeric smod(const numeric &a_, const numeric &b_)
+{
+	if (a_.is_integer() && b_.is_integer()) {
+		const cln::cl_I a = cln::the<cln::cl_I>(a_.to_cl_N());
+		const cln::cl_I b = cln::the<cln::cl_I>(b_.to_cl_N());
+		const cln::cl_I b2 = b >> 1;
+		const cln::cl_I m = cln::mod(a, b);
+		const cln::cl_I m_b = m - b;
+		const cln::cl_I ret = m > b2 ? m_b : m;
+		return numeric(ret);
 	} else
 		return *_num0_p;
 }
-- 
1.5.6.5

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