teach lowranceusr about track segments (#1306)

* teach lowranceusr about track segments.

lowranceusr versions 2 & 3 have a continuous flag used to
indicate dicontinuities in a trail.
The lowranceusr version 2 & 3 reader now flags trail discontinuities
as new track segements.
The lowranceusr version 2 & 3 writer now flags new track segments as
discontinuities.

Also fix two subtle bugs:
1) when reading a version 2 or 3 trail with the break option we would
fail to break the trail if the discontinuity was at the start of a
section other than the first section.
2) when writing a version 2 or 3 trail with the merge option if the
number of points exceeded MAX_TRAIL_POINTS we would write a header
that indicated MAX_TRAIL_POINTS but actually write MAX_TRAIL_POINTS + 1.

* add lowranceusr test for track segments.

lowranceusr.cc

@@ -989,16 +989,20 @@ LowranceusrFormat::lowranceusr_parse_trail(int* trail_num)
         wpt_tmp->latitude = lat_mm_to_deg(gbfgetint32(file_in));
         wpt_tmp->longitude = lon_mm_to_deg(gbfgetint32(file_in));
 
-        // It's not clear if this should be the continuous global or
-        // a local continuous_flag.
         char continuous_flag = gbfgetc(file_in);
-        if (!continuous_flag && opt_seg_break && j) {
-          /* option to break trails into segments was specified */
-          auto* trk_tmp = new route_head;
-          trk_tmp->rte_num = ++(*trail_num);
-          trk_tmp->rte_name = trk_head->rte_name;
-          track_add_head(trk_tmp);
-          trk_head = trk_tmp;
+        if (!continuous_flag) {
+          if (opt_seg_break) {
+            /* option to break trails into segments was specified */
+            if (!trk_head->rte_waypt_empty()) {
+              auto* trk_tmp = new route_head;
+              trk_tmp->rte_num = ++(*trail_num);
+              trk_tmp->rte_name = trk_head->rte_name;
+              track_add_head(trk_tmp);
+              trk_head = trk_tmp;
+            }
+          } else {
+            wpt_tmp->wpt_flags.new_trkseg = 1;
+          }
         }
 
         /* Track Point */
@@ -1589,7 +1593,7 @@ LowranceusrFormat::lowranceusr_trail_hdr(const route_head* trk)
   gbfputint16(num_trail_points, file_out);
   gbfputint16(max_trail_size, file_out);
   gbfputint16(num_section_points, file_out);
-  trail_point_count=1;
+  trail_point_count=0;
 }
 
 void
@@ -1689,7 +1693,8 @@ LowranceusrFormat::lowranceusr4_route_trl(const route_head* /*unused*/) const
 void
 LowranceusrFormat::lowranceusr_trail_disp(const Waypoint* wpt)
 {
-  if (trail_point_count <= MAX_TRAIL_POINTS) {
+  if (trail_point_count < MAX_TRAIL_POINTS) {
+    trail_point_count++;
     int lat = lat_deg_to_mm(wpt->latitude);
     int lon = lon_deg_to_mm(wpt->longitude);
 
@@ -1699,11 +1704,15 @@ LowranceusrFormat::lowranceusr_trail_disp(const Waypoint* wpt)
 
     gbfputint32(lat, file_out);
     gbfputint32(lon, file_out);
-    gbfwrite(&continuous, 1, 1, file_out);
-    if (!continuous) {
-      continuous = 1;
-    }
-    trail_point_count++;
+    /* If this isn't the first point in the outgoing trail, and
+     *   i) the source wpt was the start of a new track segment or
+     *  ii) the source wpt is the first waypoint of a track that is being merged
+     * then set the continuous flag to 0 to indicate a discontinuity.
+     * Otherwise set the continous flag to 1.
+     */
+    char continuous_flag = !((trail_point_count > 1) && (wpt->wpt_flags.new_trkseg || merge_new_track));
+    merge_new_track = false;
+    gbfwrite(&continuous_flag, 1, 1, file_out);
   }
 }
 
@@ -1757,7 +1766,7 @@ LowranceusrFormat::lowranceusr_merge_trail_tlr(const route_head* /*unused*/)
 void
 LowranceusrFormat::lowranceusr_merge_trail_hdr_2(const route_head* /*unused*/)
 {
-  continuous = 0;
+  merge_new_track = true;
 }
 
 void
@@ -2038,6 +2047,7 @@ LowranceusrFormat::write()
       gbfputint16(NumTrails, file_out);
       if (NumTrails) {
         trail_count=0;
+        merge_new_track = false;
         auto lowranceusr_trail_disp_lambda = [this](const Waypoint* waypointp)->void {
           lowranceusr_trail_disp(waypointp);
         };

lowranceusr.h

@@ -457,7 +457,7 @@ class LowranceusrFormat : public Format
   unsigned short waypt_out_count{};
   int            trail_count{}, lowrance_route_count{};
   int            trail_point_count{};
-  char           continuous = 1;
+  bool           merge_new_track{false};
   short          num_section_points{};
   char*          merge{};
   int            reading_version{};