now version controlled.

1 files changed, 63 insertions, 0 deletions

diff --git a/bincoeff.c b/bincoeff.c
new file mode 100644
index 0000000..624bb19
--- /dev/null
+++ b/bincoeff.c
@@ -0,0 +1,63 @@
+#include <u.h>
+#include <libc.h>
+
+vlong t0, Δt;
+
+double
+fac(double n)
+{
+	double Π;
+
+	Π = 1;
+	assert(n > 0);
+	while(n > 1)
+		Π *= n--;
+	return Π;
+}
+
+double
+bincoeff(double n, double k)
+{
+	assert(k <= n);
+	return fac(n)/(fac(k)*fac(n-k));
+}
+
+double
+bincoeffmul(double n, double k)
+{
+	double Π;
+	int i;
+
+	assert(k <= n);
+	Π = 1;
+	for(i = 1; i <= k; i++)
+		Π *= (n + 1 - i)/i;
+	return Π;
+}
+
+void
+usage(void)
+{
+	fprint(2, "usage: bincoeff n k\n");
+	exits("usage");
+}
+
+void
+main(int argc, char *argv[])
+{
+	double n, k, bc;
+
+	if(argc != 3)
+		usage();
+	n = strtod(argv[1], nil);
+	k = strtod(argv[2], nil);
+	t0 = nsec();
+	bc = bincoeff(n, k);
+	Δt = nsec()-t0;
+	print("method 1: %g (%lldns)\n", bc, Δt);
+	t0 = nsec();
+	bc = bincoeffmul(n, k);
+	Δt = nsec()-t0;
+	print("method 2: %g (%lldns)\n", bc, Δt);
+	exits(nil);
+}