Add GREIS (Javad) GPS driver.

All functional changes inside "#ifdef GREIS_ENABLE"

Includes new regression tests.  All regressions tests
pass.

Developed by Gregory Fong, with help and support from
Virgin Orbit.

Signed-off-by: Gary E. Miller <gem@rellim.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/driver_greis_checksum.c b/driver_greis_checksum.c
new file mode 100644
index 00000000..e7f69518
--- /dev/null
+++ b/driver_greis_checksum.c
@@ -0,0 +1,24 @@
+/*
+ * Checksum for the GNSS Receiver External Interface Specification (GREIS).
+ *
+ * This file is Copyright (c) 2017 Virgin Orbit
+ * SPDX-License-Identifier: BSD-2-clause
+ */
+
+#include <limits.h>
+
+#include "driver_greis.h"
+
+static inline unsigned char greis_rotate_left(unsigned char val)
+{
+    /* left circular rotation by two bits */
+    return (val << 2) | (val >> (CHAR_BIT - 2));
+}
+
+unsigned char greis_checksum(const unsigned char *src, int count)
+{
+    unsigned char res = 0;
+    while (count--)
+	res = greis_rotate_left(res) ^ *src++;
+    return greis_rotate_left(res);
+}