main: Add helper functions for mount matrices

One function to parse the mount matrix string in the format exported by
the iio core in Linux to a 3x3 matrix, and another to apply the mount
matrix to an existing 3-dimensional vector.

src/Makefile.am

@@ -26,6 +26,8 @@ iio_sensor_proxy_SOURCES =			\
 	drv-iio-buffer-compass.c		\
 	iio-buffer-utils.h			\
 	iio-buffer-utils.c			\
+	accel-mount-matrix.h			\
+	accel-mount-matrix.c			\
 	$(BUILT_SOURCES)
 
 iio_sensor_proxy_CPPFLAGS =			\

src/accel-mount-matrix.c

@@ -0,0 +1,72 @@
+/*
+ * Copyright (c) 2016 Bastien Nocera <hadess@hadess.net>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 3 as published by
+ * the Free Software Foundation.
+ *
+ */
+
+#include <glib.h>
+#include <stdio.h>
+
+#include "accel-mount-matrix.h"
+
+/* The format is the same used in the iio core to export the values:
+ * https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/iio/industrialio-core.c?id=dfc57732ad38f93ae6232a3b4e64fd077383a0f1#n431
+ */
+
+static IioAccelVec3 id_matrix[3] = {
+	{ 1.0, 0.0, 0.0 },
+	{ 0.0, 1.0, 0.0 },
+	{ 0.0, 0.0, 1.0 }
+};
+
+gboolean
+parse_mount_matrix (const char   *mtx,
+		    IioAccelVec3 *vecs[3])
+{
+	IioAccelVec3 *ret;
+
+	g_return_val_if_fail (vecs != NULL, FALSE);
+
+
+	/* Empty string means we use the identity matrix */
+	if (mtx == NULL || *mtx == '\0') {
+		*vecs = g_memdup (id_matrix, sizeof(id_matrix));
+		return TRUE;
+	}
+
+	ret = g_new0 (IioAccelVec3, 3);
+	if (sscanf (mtx, "%f, %f, %f; %f, %f, %f; %f, %f, %f",
+		    &ret[0].x, &ret[0].y, &ret[0].z,
+		    &ret[1].x, &ret[1].y, &ret[1].z,
+		    &ret[2].x, &ret[2].y, &ret[2].z) != 9) {
+		g_free (ret);
+		g_warning ("Failed to parse '%s' as a mount matrix", mtx);
+		return FALSE;
+	}
+
+	*vecs = ret;
+
+	return TRUE;
+}
+
+gboolean
+apply_mount_matrix (const IioAccelVec3 vecs[3],
+		    IioAccelVec3       *accel)
+{
+	float _x, _y, _z;
+
+	g_return_val_if_fail (accel != NULL, FALSE);
+
+	_x = accel->x * vecs[0].x + accel->y * vecs[0].y + accel->z * vecs[0].z;
+	_y = accel->x * vecs[1].x + accel->y * vecs[1].y + accel->z * vecs[1].z;
+	_z = accel->x * vecs[2].x + accel->y * vecs[2].y + accel->z * vecs[2].z;
+
+	accel->x = _x;
+	accel->y = _y;
+	accel->z = _z;
+
+	return TRUE;
+}

src/accel-mount-matrix.h

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2016 Bastien Nocera <hadess@hadess.net>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 3 as published by
+ * the Free Software Foundation.
+ *
+ */
+
+#include <glib.h>
+
+typedef struct {
+	float x;
+	float y;
+	float z;
+} IioAccelVec3;
+
+gboolean parse_mount_matrix (const char   *mtx,
+                             IioAccelVec3 *vecs[3]);
+
+gboolean apply_mount_matrix (const IioAccelVec3 vecs[3],
+                             IioAccelVec3       *accel);