[GiNaC-devel] [PATCH] [BUGFIX] pgcd(): detect relatively prime polynomials properly...

[Alexei Sheplyakov]

... so pgcd() doesn't loop forever any more. Division test does not handle
relatively prime polynomials (because C = 1 divides any polynomial). So we
should stop interpolation (and return gcd of contents) if we got relatively
prime images (we should do that for efficiency reasons too).

Thanks to Jörg Arndt for a bug report.
---
 check/Makefile.am                   |    3 +++
 check/pgcd_relatively_prime_bug.cpp |   31 +++++++++++++++++++++++++++++++
 ginac/polynomial/pgcd.cpp           |    4 ++--
 3 files changed, 36 insertions(+), 2 deletions(-)
 create mode 100644 check/pgcd_relatively_prime_bug.cpp

diff --git a/check/Makefile.am b/check/Makefile.am
index 5b9dd58..3751613 100644
--- a/check/Makefile.am
+++ b/check/Makefile.am
@@ -30,6 +30,7 @@ EXAMS = exam_paranoia \
 	exam_misc \
 	exam_mod_gcd \
 	bugme_chinrem_gcd \
+	pgcd_relatively_prime_bug \
 	exam_cra
 
 TIMES = time_dennyfliegner \
@@ -255,6 +256,8 @@ time_parser_LDADD = ../ginac/libginac.la
 bugme_chinrem_gcd_SOURCES = bugme_chinrem_gcd.cpp
 bugme_chinrem_gcd_LDADD = ../ginac/libginac.la
 
+pgcd_relatively_prime_bug_SOURCES = pgcd_relatively_prime_bug.cpp
+pgcd_relatively_prime_bug_LDADD = ../ginac/libginac.la
 
 AM_CPPFLAGS = -I$(srcdir)/../ginac -I../ginac -DIN_GINAC
 
diff --git a/check/pgcd_relatively_prime_bug.cpp b/check/pgcd_relatively_prime_bug.cpp
new file mode 100644
index 0000000..1d4c287
--- /dev/null
+++ b/check/pgcd_relatively_prime_bug.cpp
@@ -0,0 +1,31 @@
+/** @file pgcd_relatively_prime_bug.cpp
+ *
+ * A program exposing historical bug in the pgcd() function. 
+ */
+#include <string>
+#include <iostream>
+#include <utility>
+#include "ginac.h"
+using namespace std;
+using namespace GiNaC;
+
+int main(int argc, char** argv)
+{
+	cout << "Checking for pgcd() bug regarding relatively prime polynomials: " << flush;
+	const symbol q("q");
+	parser reader;
+	reader.get_syms().insert(make_pair(string("q"), q));
+
+	ex t = reader("-E20^16*E4^8*E5^8*E1^16*q^4"
+		      "-(E10^24-E20^8*E5^16)*E4^16*E1^8"
+		      "+E2^24*E20^16*E5^8*q^4");
+	ex g = gcd(t.expand(), t.diff(q).expand()) - 1;
+	if (!g.is_zero()) {
+		cout << " oops!" << endl <<
+			"** Error: should be 0, got " << g << endl << flush;
+		throw std::logic_error("gcd was miscalculated");
+	}
+	cout << "not found" << endl << flush;
+	return 0;
+}
+
diff --git a/ginac/polynomial/pgcd.cpp b/ginac/polynomial/pgcd.cpp
index 6e5e6b2..ef3748d 100644
--- a/ginac/polynomial/pgcd.cpp
+++ b/ginac/polynomial/pgcd.cpp
@@ -121,6 +121,8 @@ ex pgcd(const ex& A, const ex& B, const exvector& vars, const long p)
 			// evaluation point is bad. Skip it.
 			continue;
 		}
+		if (img_gcd_deg == 0)
+			return cont_gcd;
 
 		// Image has the same degree as the previous one
 		// (or at least not higher than the limit)
@@ -145,8 +147,6 @@ ex pgcd(const ex& A, const ex& B, const exvector& vars, const long p)
 			if (divide_in_z_p(Aprim, C, dummy1, vars, p) &&
 					divide_in_z_p(Bprim, C, dummy2, vars, p))
 				return (cont_gcd*C).expand().smod(pn);
-			else if (img_gcd_deg == 0)
-				return cont_gcd;
 			// else continue building the candidate
 		} 
 	} while(true);
-- 
1.6.6.1