From 5cb1407dfa199bdf32d982e9d18f6e0530bc06d5 Mon Sep 17 00:00:00 2001
From: Connor <cloningdonor@gmail.com>
Date: Wed, 18 Jun 2014 05:46:12 -0700
Subject: [PATCH 1/2]

---
 .dummy | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 .dummy

diff --git a/.dummy b/.dummy
new file mode 100644
index 0000000..945c9b4
--- /dev/null
+++ b/.dummy
@@ -0,0 +1 @@
+.
\ No newline at end of file

From a61ff7660c73c76d0a803b9c384611c96f1909f0 Mon Sep 17 00:00:00 2001
From: Connor Olding <cloningdonor@gmail.com>
Date: Wed, 18 Jun 2014 05:48:19 -0700
Subject: [PATCH 2/2] .

---
 .dummy     |  1 -
 halfband.c | 98 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 98 insertions(+), 1 deletion(-)
 delete mode 100644 .dummy
 create mode 100644 halfband.c

diff --git a/.dummy b/.dummy
deleted file mode 100644
index 945c9b4..0000000
--- a/.dummy
+++ /dev/null
@@ -1 +0,0 @@
-.
\ No newline at end of file
diff --git a/halfband.c b/halfband.c
new file mode 100644
index 0000000..6254452
--- /dev/null
+++ b/halfband.c
@@ -0,0 +1,98 @@
+// C port of  http://ldesoras.free.fr/prod.html#src_hiir
+#include <stdlib.h>
+#include <stdio.h>
+#include <math.h>
+#include <errno.h>
+
+const double PI = 3.1415926535897932384626433832795;
+
+void
+compute_transition_param(double *kp, double *qp, double transition)
+{
+	double k = *kp;
+	double q = *qp;
+	k = tan((1 - transition*2)*PI/4);
+	k *= k;
+	double kksqrt = pow(1 - k*k, 0.25);
+	double e = 0.5*(1 - kksqrt)/(1 + kksqrt);
+	double e4 = e*e*e*e;
+	q = e*(1 + e4*(2 + e4*(15 + 150*e4)));
+	*kp = k;
+	*qp = q;
+}
+
+double
+compute_acc_numden(double q, int order, int c, int den)
+{
+	den = den == 1;
+	int i = den;
+	int sign = den ? -1 : 1;
+	double sum = 0;
+	double q_ii;
+	do {
+		int i2 = i + 1 - den;
+		q_ii = pow(q, i*i2);
+		q_ii *= sin((i + i2)*c*PI/order + den*PI/2);
+		q_ii *= sign;
+		sum += q_ii;
+
+		sign = -sign;
+		++i;
+	} while (fabs(q_ii) > 1e-100);
+	return sum;
+}
+
+double
+compute_coef(int c, double k, double q, int order)
+{
+	double num = compute_acc_numden(q, order, c, 0)*pow(q, 0.25);
+	double den = compute_acc_numden(q, order, c, 1) + 0.5;
+	double ww = num/den;
+	ww *= ww;
+
+	double x = sqrt((1 - ww*k)*(1 - ww/k))/(1 + ww);
+	double coef = (1 - x)/(1 + x);
+
+	return coef;
+}
+
+void
+compute_coefs_spec_order_tbw(double coef_arr[], int n, double transition)
+{
+	double k;
+	double q;
+	compute_transition_param(&k, &q, transition);
+	const int order = n*2 + 1;
+
+	/*printf("k: %.18f\nq: %.18f\n", k, q);*/
+
+	for (int i = 0; i < n; i++)
+		coef_arr[i] = compute_coef(i + 1, k, q, order);
+}
+
+int
+main(int argc, char **argv)
+{
+	if (argc != 3) {
+		fputs("usage: halfband COUNT TRANSITION\n", stderr);
+		return 1;
+	}
+
+	double tv[2];
+	for (int i = 1; i <= 2; i++) {
+		tv[i - 1]=strtold(argv[i], NULL);
+		if (errno) {
+			fprintf(stderr, "arg #%i failed to convert to double\n", i);
+			return 1;
+		}
+	}
+	int count = (int) tv[0];
+
+	double *coefs = malloc(count*sizeof(double));
+	compute_coefs_spec_order_tbw(coefs, count, tv[1]);
+	for (int i = 0; i < count; i++)
+		printf("%.18f\n", coefs[i]);
+	free(coefs);
+
+	return 0;
+}