Bug fix to MPFR mod operation for negative numerator.

6 files changed, 55 insertions, 4 deletions

diff --git a/ChangeLog b/ChangeLog
index 0abecbf4..53aa90e0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,13 +2,21 @@
 
 	Bug fix: For MPFR sqrt(), need to set precision of result to be
 	the same as that of the argument. Doesn't hurt other functions.
-	See test/mpfrsqrt.awk.
+	See test/mpfrsqrt.awk. Thank to Katie Wasserman <katie@wass.net>
+	for the bug report.
 
 	* mpfr.c (do_mpfr_func): New function. Runs code for MPFR functions
 	while still enabling debugging. Add call here to mpfr_set_prec().
 	Original code from SPEC_MATH macro.
 	(SPEC_MATH): Change macro to call do_mpfr_func().
 
+	Next MPFR bug fix: The % operator gave strange results for negative
+	numerator. Thanks again to Katie Wasserman for the bug report.
+
+	* mpfr.c (mpg_mod): Use mpz_tdiv_qr() instead of mpz_mod(). From
+	the GMP doc, mpz_mod() should have worked; it's not clear why
+	it doesn't.
+
 2014-08-03         Arnold D. Robbins     <arnold@skeeve.com>
 
 	* builtin.c (format_tree): Don't need to check return value of
diff --git a/mpfr.c b/mpfr.c
index 37a12175..e53af616 100644
--- a/mpfr.c
+++ b/mpfr.c
@@ -1376,8 +1376,27 @@ mpg_mod(NODE *t1, NODE *t2)
 	int tval;
 
 	if (is_mpg_integer(t1) && is_mpg_integer(t2)) {
+		/*
+		 * 8/2014: Originally, this was just
+		 *
+		 * r = mpg_integer();
+		 * mpz_mod(r->mpg_i, t1->mpg_i, t2->mpg_i);
+		 *
+		 * But that gave very strange results with negative numerator:
+		 *
+		 *	$ ./gawk -M 'BEGIN { print -15 % 7 }'
+		 *	6
+		 *
+		 * So instead we use mpz_tdiv_qr() to get the correct result
+		 * and just throw away the quotient. We could not find any
+		 * reason why mpz_mod() wasn't working correctly.
+		 */
+		NODE *dummy_quotient;
+
 		r = mpg_integer();
-		mpz_mod(r->mpg_i, t1->mpg_i, t2->mpg_i);
+		dummy_quotient = mpg_integer();
+		mpz_tdiv_qr(dummy_quotient->mpg_i, r->mpg_i, t1->mpg_i, t2->mpg_i);
+		unref(dummy_quotient);
 	} else {
 		mpfr_ptr p1, p2;
 		p1 = MP_FLOAT(t1);
diff --git a/test/Makefile.am b/test/Makefile.am
index 8349a73e..ede44523 100644
--- a/test/Makefile.am
+++ b/test/Makefile.am
@@ -529,6 +529,8 @@ EXTRA_DIST = \
 	mpfrnr.awk \
 	mpfrnr.in \
 	mpfrnr.ok \
+	mpfrrem.awk \
+	mpfrrem.ok \
 	mpfrrnd.awk \
 	mpfrrnd.ok \
 	mpfrsort.awk \
@@ -1024,7 +1026,7 @@ INET_TESTS = inetdayu inetdayt inetechu inetecht
 
 MACHINE_TESTS = double1 double2 fmtspcl intformat
 
-MPFR_TESTS = mpfrnr mpfrnegzero mpfrrnd mpfrieee mpfrexprange \
+MPFR_TESTS = mpfrnr mpfrnegzero mpfrrem mpfrrnd mpfrieee mpfrexprange \
 	mpfrsort mpfrsqrt mpfrbigint
 
 LOCALE_CHARSET_TESTS = \
@@ -1752,6 +1754,11 @@ mpfrsqrt:
 	@$(AWK) -M -f "$(srcdir)"/$@.awk > _$@ 2>&1
 	@-$(CMP) "$(srcdir)"/$@.ok _$@ && rm -f _$@
 
+mpfrrem:
+	@echo $@
+	@$(AWK) -M -f "$(srcdir)"/$@.awk > _$@ 2>&1
+	@-$(CMP) "$(srcdir)"/$@.ok _$@ && rm -f _$@
+
 jarebug::
 	@echo $@
 	@"$(srcdir)"/$@.sh "$(AWKPROG)" "$(srcdir)"/$@.awk "$(srcdir)"/$@.in "_$@"
diff --git a/test/Makefile.in b/test/Makefile.in
index e8fd8844..196e125c 100644
--- a/test/Makefile.in
+++ b/test/Makefile.in
@@ -775,6 +775,8 @@ EXTRA_DIST = \
 	mpfrnr.awk \
 	mpfrnr.in \
 	mpfrnr.ok \
+	mpfrrem.awk \
+	mpfrrem.ok \
 	mpfrrnd.awk \
 	mpfrrnd.ok \
 	mpfrsort.awk \
@@ -1266,7 +1268,7 @@ GAWK_EXT_TESTS = \
 EXTRA_TESTS = inftest regtest
 INET_TESTS = inetdayu inetdayt inetechu inetecht
 MACHINE_TESTS = double1 double2 fmtspcl intformat
-MPFR_TESTS = mpfrnr mpfrnegzero mpfrrnd mpfrieee mpfrexprange \
+MPFR_TESTS = mpfrnr mpfrnegzero mpfrrem mpfrrnd mpfrieee mpfrexprange \
 	mpfrsort mpfrsqrt mpfrbigint
 
 LOCALE_CHARSET_TESTS = \
@@ -2176,6 +2178,11 @@ mpfrsqrt:
 	@$(AWK) -M -f "$(srcdir)"/$@.awk > _$@ 2>&1
 	@-$(CMP) "$(srcdir)"/$@.ok _$@ && rm -f _$@
 
+mpfrrem:
+	@echo $@
+	@$(AWK) -M -f "$(srcdir)"/$@.awk > _$@ 2>&1
+	@-$(CMP) "$(srcdir)"/$@.ok _$@ && rm -f _$@
+
 jarebug::
 	@echo $@
 	@"$(srcdir)"/$@.sh "$(AWKPROG)" "$(srcdir)"/$@.awk "$(srcdir)"/$@.in "_$@"
diff --git a/test/mpfrrem.awk b/test/mpfrrem.awk
new file mode 100644
index 00000000..fd8bc4d5
--- /dev/null
+++ b/test/mpfrrem.awk
@@ -0,0 +1,6 @@
+BEGIN {
+	print "15 % 7 =", 15 % 7
+	print "15 % -7 =", 15 % -7
+	print "-15 % 7 =", -15 % 7
+	print "-15 % -7 =", -15 % -7
+}
diff --git a/test/mpfrrem.ok b/test/mpfrrem.ok
new file mode 100644
index 00000000..91010457
--- /dev/null
+++ b/test/mpfrrem.ok
@@ -0,0 +1,4 @@
+15 % 7 = 1
+15 % -7 = 1
+-15 % 7 = -1
+-15 % -7 = -1