libmp: add mpfield() function for fast field arithmetic

instead of testing for special field primes each time in mpmod(),
make it explicit with a mpfiled() function that tests a modulus N
to be of some special form that can be reduced more efficiently with
some precalculation, and replaces N with a Mfield* when it can. the
Mfield*'s are recognized by mpmod() as they have the MPfield flag
set and provide a function pointer that executes the fast reduction.

1 files changed, 21 insertions, 0 deletions

diff --git a/sys/src/libmp/port/mpfield.c b/sys/src/libmp/port/mpfield.c
new file mode 100644
index 000000000..b5f8a9b00
--- /dev/null
+++ b/sys/src/libmp/port/mpfield.c
@@ -0,0 +1,21 @@
+#include "os.h"
+#include <mp.h>
+#include "dat.h"
+
+mpint*
+mpfield(mpint *N)
+{
+	Mfield *f;
+
+	if(N == nil || N->flags & (MPfield|MPstatic))
+		return N;
+	if((f = cnfield(N)) != nil)
+		goto Exchange;
+	if((f = gmfield(N)) != nil)
+		goto Exchange;
+	return N;
+Exchange:
+	setmalloctag(f, getcallerpc(&N));
+	mpfree(N);
+	return f;
+}