Add libclipper, a library to 'smoothly' clip overdriven audio levels

libclipper/Makefile.am

@@ -0,0 +1,6 @@
+AM_CPPFLAGS = -Wall -Wextra -g $(all_includes)
+
+noinst_LIBRARIES = libclipper.a
+
+libclipper_a_SOURCES = \
+	clipper.c

libclipper/clipper.c

@@ -0,0 +1,81 @@
+/* Clipper implementation, based on code by Jonathan Olds
+ *
+ * (C) 2017 by Andreas Eversberg <jolly@eversberg.eu>
+ * All Rights Reserved
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <stdio.h>
+#include <math.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include "../libsample/sample.h"
+#include "clipper.h"
+
+static double clipper_lut[6000];
+
+static double clipper_point = NAN;
+
+void clipper_init(double point)
+{
+	double a;
+	int i;
+
+	if (point > 0.99)
+		point = 0.99;
+	if (point < 0.01)
+		point = 0.01;
+	clipper_point = point;
+
+	a = M_PI / (2.0 * (1.0 - clipper_point));
+
+	for (i = 0; i < 6000; i++)
+		clipper_lut[i] = clipper_point + atan(a * i / 1000.0) / a;
+}
+
+void clipper_process(sample_t *samples, int length)
+{
+	int i;
+	double val, inv, shiftmultval;
+	int n, q;
+
+	if (isnan(clipper_point)) {
+		fprintf(stderr, "Clipper not initialized, aborting!\n");
+		abort();
+	}
+
+	for (i = 0; i < length; i++) {
+		val = samples[i];
+		if (val < 0) {
+			inv = -1.0;
+			val = -val;
+		} else
+			inv = 1.0;
+		shiftmultval = (val - clipper_point) * 1000.0;
+		/* no clipping up to clipping point */
+		if (shiftmultval <= 0.0)
+			continue;
+		n = (int)shiftmultval;
+		q = n + 1;
+		if (q >= 6000) {
+			samples[i] = inv;
+			continue;
+		}
+		/* get clipped value from lut, interpolate between table entries */
+		val = clipper_lut[n] + (shiftmultval - (double)n) * (clipper_lut[q] - clipper_lut[n]);
+		samples[i] = val * inv;
+	}
+}
+

libclipper/clipper.h

@@ -0,0 +1,4 @@
+
+void clipper_init(double point);
+void clipper_process(sample_t *samples, int length);
+