add signal utility functions

Author: jacobmcnamee

include/libswiftnav/signal.h

@@ -33,6 +33,8 @@ enum band {
 #define GPS_FIRST_PRN 0
 #define SBAS_FIRST_PRN 120
 
+#define SID_STR_LEN_MAX 16
+
 typedef struct __attribute__((packed)) {
   u16 sat;
   u8 band;
@@ -59,5 +61,23 @@ static inline bool sid_is_equal(const gnss_signal_t a, const gnss_signal_t b)
   return sid_compare(a, b) == 0;
 }
 
+/** Print a string representation of sid to the buffer s of capacity n.
+ *  Returns the number of characters printed, excluding the terminating null.
+ */
+int sid_to_string(char *s, int n, gnss_signal_t sid);
+
+/** Returns true if sid corresponds to a known constellation, band, and
+ *  satellite identifier.
+ */
+bool sid_valid(gnss_signal_t sid);
+
+/** Converts the global index i in [0, NUM_SATS) to the corresponding sid
+ */
+gnss_signal_t sid_from_index(u32 i);
+
+/** Converts sid to a global index in [0, NUM_SATS)
+ */
+u32 sid_to_index(gnss_signal_t sid);
+
 #endif /* LIBSWIFTNAV_SIGNAL_H */
 

src/CMakeLists.txt

@@ -41,6 +41,7 @@ set(libswiftnav_SRCS
   ambiguity_test.c
   printing_utils.c
   filter_utils.c
+  signal.c
   ${plover_SRCS}
 
   CACHE INTERNAL ""

src/signal.c

@@ -0,0 +1,127 @@
+/*
+ * Copyright (c) 2015 Swift Navigation Inc.
+ * Contact: Jacob McNamee <jacob@swiftnav.com>
+ *
+ * This source is subject to the license found in the file 'LICENSE' which must
+ * be be distributed together with this source. All other rights reserved.
+ *
+ * THIS CODE AND INFORMATION IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND,
+ * EITHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND/OR FITNESS FOR A PARTICULAR PURPOSE.
+ */
+
+#include "signal.h"
+
+#include <stdio.h>
+#include <assert.h>
+
+typedef struct {
+  u8 constellation;
+  u8 band;
+  u16 sat_count;
+  u16 (*sat_get)(u32 local_index);
+  u32 (*local_index_get)(u16 sat);
+} sat_group_t;
+
+static const char * constellation_strs[CONSTELLATION_COUNT] = {
+  [CONSTELLATION_GPS] = "GPS",
+  [CONSTELLATION_SBAS] = "SBAS"
+};
+
+static const char * band_strs[BAND_COUNT] = {
+  [BAND_L1] = "L1"
+};
+
+static const char * unknown_str = "?";
+
+static u16 sat_get_gps(u32 local_index);
+static u16 sat_get_sbas(u32 local_index);
+static u32 local_index_get_gps(u16 sat);
+static u32 local_index_get_sbas(u16 sat);
+
+static const sat_group_t sat_groups[CONSTELLATION_COUNT] = {
+  [CONSTELLATION_GPS] = {
+    CONSTELLATION_GPS, BAND_L1, NUM_SATS_GPS,
+     sat_get_gps, local_index_get_gps
+  },
+  [CONSTELLATION_SBAS] = {
+    CONSTELLATION_SBAS, BAND_L1, NUM_SATS_SBAS,
+    sat_get_sbas, local_index_get_sbas
+  }
+};
+
+static u16 sat_get_gps(u32 local_index)
+{
+  return local_index + GPS_FIRST_PRN;
+}
+
+static u16 sat_get_sbas(u32 local_index)
+{
+  return local_index + SBAS_FIRST_PRN;
+}
+
+static u32 local_index_get_gps(u16 sat)
+{
+  return sat - GPS_FIRST_PRN;
+}
+
+static u32 local_index_get_sbas(u16 sat)
+{
+  return sat - SBAS_FIRST_PRN;
+}
+
+int sid_to_string(char *s, int n, gnss_signal_t sid)
+{
+  const char *constellation_str = (sid.constellation < CONSTELLATION_COUNT) ?
+      constellation_strs[sid.constellation] : unknown_str;
+  const char *band_str = (sid.band < BAND_COUNT) ?
+      band_strs[sid.band] : unknown_str;
+
+  int nchars = snprintf(s, n, "%s %s %u", constellation_str, band_str, sid.sat);
+  s[n-1] = 0;
+  return nchars;
+}
+
+bool sid_valid(gnss_signal_t sid)
+{
+  if (sid.constellation >= CONSTELLATION_COUNT)
+    return false;
+  if (sid.band >= BAND_COUNT)
+    return false;
+  if (sat_groups[sid.constellation].local_index_get(sid.sat)
+        >= sat_groups[sid.constellation].sat_count)
+    return false;
+  return true;
+}
+
+gnss_signal_t sid_from_index(u32 i)
+{
+  assert(i < NUM_SATS);
+  gnss_signal_t sid = {0, 0, 0};
+  u32 offset = i;
+  u32 group_index = 0;
+  while (group_index < sizeof(sat_groups) / sizeof(sat_groups[0])) {
+    if (offset >= sat_groups[group_index].sat_count) {
+      offset -= sat_groups[group_index].sat_count;
+      group_index++;
+    } else {
+      sid = (gnss_signal_t) {
+        .constellation = sat_groups[group_index].constellation,
+        .band = sat_groups[group_index].band,
+        .sat = sat_groups[group_index].sat_get(offset)
+      };
+      return sid;
+    }
+  }
+  assert("sid_from_index() failed");
+  return sid;
+}
+
+u32 sid_to_index(gnss_signal_t sid)
+{
+  assert(sid_valid(sid));
+  u32 offset = 0;
+  for (u32 i=0; i<sid.constellation; i++)
+    offset += sat_groups[i].sat_count;
+  return offset + sat_groups[sid.constellation].local_index_get(sid.sat);
+}