Add multi-track GPX export option (splitByDay)

generateGpx() gains a splitByDay option that splits the track into
separate <trk> elements per day, each named "Day N: Start - End".
Track points are split at the closest points to overnight waypoints.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/gpx/src/generate.ts

@@ -13,6 +13,8 @@ export function generateGpx(options: {
   waypoints?: Waypoint[];
   tracks?: TrackPoint[][];
   noGoAreas?: NoGoArea[];
+  /** When true, splits tracks at overnight waypoints into separate <trk> elements per day */
+  splitByDay?: boolean;
 }): string {
   const hasExtensions = options.noGoAreas && options.noGoAreas.length > 0;
   const lines: string[] = [
@@ -41,19 +43,44 @@ export function generateGpx(options: {
   }
 
   if (options.tracks) {
-    for (const track of options.tracks) {
-      lines.push("  <trk>", "    <trkseg>");
-      for (const pt of track) {
-        lines.push(`      <trkpt lat="${pt.lat}" lon="${pt.lon}">`);
-        if (pt.ele !== undefined) {
-          lines.push(`        <ele>${pt.ele}</ele>`);
+    const allPoints = options.tracks.flat();
+
+    if (options.splitByDay && options.waypoints) {
+      // Split track into per-day segments at overnight waypoints
+      const dayTracks = splitTrackByDays(allPoints, options.waypoints);
+      for (const dayTrack of dayTracks) {
+        lines.push("  <trk>");
+        if (dayTrack.name) {
+          lines.push(`    <name>${escapeXml(dayTrack.name)}</name>`);
         }
-        if (pt.time) {
-          lines.push(`        <time>${escapeXml(pt.time)}</time>`);
+        lines.push("    <trkseg>");
+        for (const pt of dayTrack.points) {
+          lines.push(`      <trkpt lat="${pt.lat}" lon="${pt.lon}">`);
+          if (pt.ele !== undefined) {
+            lines.push(`        <ele>${pt.ele}</ele>`);
+          }
+          if (pt.time) {
+            lines.push(`        <time>${escapeXml(pt.time)}</time>`);
+          }
+          lines.push("      </trkpt>");
         }
-        lines.push("      </trkpt>");
+        lines.push("    </trkseg>", "  </trk>");
+      }
+    } else {
+      for (const track of options.tracks) {
+        lines.push("  <trk>", "    <trkseg>");
+        for (const pt of track) {
+          lines.push(`      <trkpt lat="${pt.lat}" lon="${pt.lon}">`);
+          if (pt.ele !== undefined) {
+            lines.push(`        <ele>${pt.ele}</ele>`);
+          }
+          if (pt.time) {
+            lines.push(`        <time>${escapeXml(pt.time)}</time>`);
+          }
+          lines.push("      </trkpt>");
+        }
+        lines.push("    </trkseg>", "  </trk>");
       }
-      lines.push("    </trkseg>", "  </trk>");
     }
   }
 
@@ -73,6 +100,54 @@ export function generateGpx(options: {
   return lines.join("\n");
 }
 
+function splitTrackByDays(
+  points: TrackPoint[],
+  waypoints: Waypoint[],
+): Array<{ name: string; points: TrackPoint[] }> {
+  if (points.length === 0 || waypoints.length === 0) return [];
+
+  // Find closest track point index for each waypoint
+  const wpTrackIndices = waypoints.map((wp) => {
+    let bestIdx = 0;
+    let bestDist = Infinity;
+    for (let i = 0; i < points.length; i++) {
+      const dx = points[i]!.lat - wp.lat;
+      const dy = points[i]!.lon - wp.lon;
+      const d = dx * dx + dy * dy;
+      if (d < bestDist) {
+        bestDist = d;
+        bestIdx = i;
+      }
+    }
+    return bestIdx;
+  });
+
+  // Build day boundaries from overnight waypoints
+  const breakWpIndices = waypoints
+    .map((wp, i) => (wp.isDayBreak ? i : -1))
+    .filter((i) => i >= 0);
+
+  if (breakWpIndices.length === 0) {
+    return [{ name: "Day 1", points }];
+  }
+
+  const boundaries = [0, ...breakWpIndices.map((i) => wpTrackIndices[i]!), points.length];
+  const dayTracks: Array<{ name: string; points: TrackPoint[] }> = [];
+
+  for (let d = 0; d < boundaries.length - 1; d++) {
+    const start = boundaries[d]!;
+    const end = boundaries[d + 1]!;
+    const startWp = d === 0 ? waypoints[0] : waypoints[breakWpIndices[d - 1]!];
+    const endWp = d < breakWpIndices.length ? waypoints[breakWpIndices[d]!] : waypoints[waypoints.length - 1];
+    const startName = startWp?.name ?? "";
+    const endName = endWp?.name ?? "";
+    const name = startName && endName ? `Day ${d + 1}: ${startName} - ${endName}` : `Day ${d + 1}`;
+    dayTracks.push({ name, points: points.slice(start, end) });
+  }
+
+  return dayTracks;
+}
+
 function escapeXml(str: string): string {
   return str
     .replace(/&/g, "&amp;")