garmin_fit: add handling of global UTC offset

Some devices may record timestamps in system time (i.e. boot time),
rather than system time. As mentioned in Github issue #54 and in the
upstream documentation, we can recognize such a scenario when
timestamp is < 0x10000000.

Field 4 under the global ID for device settings records the difference
between system time and UTC time (this is called global UTC offset in
the upstream documentation).

Add code to store the global UTC offset in a new global variable, and
correctly handle all timestamps with the new information.

diff --git a/garmin_fit.cc b/garmin_fit.cc
index 57c8a2a1fea8b08d5972f2ab86eee2e632ae00d3..3fb4712952fb781605812ac82f9e75f0082373ba 100644 (file)
--- a/garmin_fit.cc
+++ b/garmin_fit.cc
@@ -57,6 +57,7 @@ static struct {
   int endian;
   route_head* track;
   uint32_t last_timestamp;
+  uint32_t global_utc_offset;
   fit_message_def message_def[16];
 } fit_data;
 
@@ -135,6 +136,8 @@ fit_parse_header(void)
     // Unused according to Ingo Arndt
     gbfgetuint16(fin);
   }
+
+  fit_data.global_utc_offset = 0;
 }
 
 static uint8_t
@@ -352,9 +355,31 @@ fit_parse_data(fit_message_def* def, int time_offset)
       if (global_opts.debug_level >= 7) {
         debug_print(7,"%s: parsing fit data: timestamp=%d\n", MYNAME, val);
       }
-      fit_data.last_timestamp = timestamp = val;
+      timestamp = val;
+      // if the timestamp is < 0x10000000, this value represents
+      // system time; to convert it to UTC, add the global utc offset to it
+      if (timestamp < 0x10000000)
+        timestamp += fit_data.global_utc_offset;
+      fit_data.last_timestamp = timestamp;
     } else {
       switch (def->global_id) {
+      case 0: // device settings message
+        switch (f->id) {
+        case 4:
+          if (global_opts.debug_level >= 7) {
+            debug_print(7,"%s: parsing fit data: global utc_offset=%d\n", MYNAME, val);
+          }
+          fit_data.global_utc_offset = val;
+          break;
+        default:
+          if (global_opts.debug_level >= 1) {
+            debug_print(1, "%s: unrecognized data type in GARMIN FIT device settings: f->id=%d\n", MYNAME, f->id);
+          }
+          break;
+        } // switch (f->id)
+        // end of case def->global_id = 0
+        break;
+
       case 20: // record message - trkType is a track
         switch (f->id) {
         case 0: