altos: Add conversion between Pa and meters

[fw/altos] / src / core / ao_convert_pa.c

diff --git a/src/core/ao_convert_pa.c b/src/core/ao_convert_pa.c
new file mode 100644 (file)
index 0000000..0c93cae
--- /dev/null
+++ b/src/core/ao_convert_pa.c
@@ -0,0 +1,72 @@
+/*
+ * Copyright © 2012 Keith Packard <keithp@keithp.com>
+ *
+ * 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; version 2 of the License.
+ *
+ * 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, write to the Free Software Foundation, Inc.,
+ * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
+ */
+
+#if !defined(AO_CONVERT_TEST) && !defined(AO_FLIGHT_TEST)
+#include "ao.h"
+#endif
+
+static const int32_t altitude_table[] = {
+#include "altitude-pa.h"
+};
+
+#define ALT_SCALE      (1 << ALT_SHIFT)
+#define ALT_MASK       (ALT_SCALE - 1)
+
+int32_t
+ao_pa_to_altitude(int32_t pa)
+{
+       int16_t o;
+       int16_t part;
+       int32_t low, high;
+
+       if (pa < 0)
+               pa = 0;
+       if (pa > 120000)
+               pa = 120000;
+       o = pa >> ALT_SHIFT;
+       part = pa & ALT_MASK;
+
+       low = (int32_t) altitude_table[o] * (ALT_SCALE - part);
+       high = (int32_t) altitude_table[o+1] * part + (ALT_SCALE >> 1);
+       return (low + high) >> ALT_SHIFT;
+}
+
+int32_t
+ao_altitude_to_pa(int32_t alt)
+{
+       int32_t         span, sub_span;
+       uint16_t        l, h, m;
+       int32_t         pa;
+
+       l = 0;
+       h = NALT - 1;
+       while ((h - l) != 1) {
+               m = (l + h) >> 1;
+               if (altitude_table[m] < alt)
+                       h = m;
+               else
+                       l = m;
+       }
+       span = altitude_table[l] - altitude_table[h];
+       sub_span = altitude_table[l] - alt;
+       pa = ((((int32_t) l * (span - sub_span) + (int32_t) h * sub_span) << ALT_SHIFT) + (span >> 1)) / span;
+       if (pa > 120000)
+               pa = 120000;
+       if (pa < 0)
+               pa = 0;
+       return pa;
+}