From 0e27d6eaef26d2630785622cbbfad1573396aedc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ullrich=20Sch=C3=A4fer?= <hi+github@ullrich.is>
Date: Thu, 2 Apr 2026 22:36:13 +0100
Subject: [PATCH] Use Douglas-Peucker simplification for single-segment GPX
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For GPX files with one long track segment, extracting only start/end
gives just 2 waypoints. Now uses Douglas-Peucker line simplification
(epsilon=0.05° ≈ 5km) to find significant turning points.

Result: berlin-dresden-radweg (249km, 5660 points) → 10 waypoints
that capture the route's key direction changes.

Multi-segment GPX files still use segment endpoints as before.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 packages/gpx/src/waypoints.ts | 84 ++++++++++++++++++++++++++++++-----
 1 file changed, 74 insertions(+), 10 deletions(-)

diff --git a/packages/gpx/src/waypoints.ts b/packages/gpx/src/waypoints.ts
index eca8da4..4d4da9b 100644
--- a/packages/gpx/src/waypoints.ts
+++ b/packages/gpx/src/waypoints.ts
@@ -2,32 +2,96 @@ import type { GpxData } from "./types.ts";
 
 /**
  * Extract waypoints from parsed GPX data.
- * Uses explicit <wpt> elements if present, otherwise extracts the start
- * of each track segment + end of the last segment.
+ * Uses explicit <wpt> elements if present, otherwise simplifies the
+ * track using Douglas-Peucker to find significant turning points.
  */
 export function extractWaypoints(gpxData: GpxData): Array<{ lat: number; lon: number; name?: string }> {
   if (gpxData.waypoints.length > 0) return gpxData.waypoints;
   if (gpxData.tracks.length === 0) return [];
 
-  const result: Array<{ lat: number; lon: number }> = [];
+  // Collect start of each segment + end of last (for multi-segment GPX)
+  const segmentEndpoints: Array<{ lat: number; lon: number }> = [];
   for (const seg of gpxData.tracks) {
     if (seg.length === 0) continue;
     const first = seg[0]!;
-    const prev = result[result.length - 1];
+    const prev = segmentEndpoints[segmentEndpoints.length - 1];
     if (!prev || prev.lat !== first.lat || prev.lon !== first.lon) {
-      result.push({ lat: first.lat, lon: first.lon });
+      segmentEndpoints.push({ lat: first.lat, lon: first.lon });
     }
   }
-
-  // Add end of last segment
   const lastSeg = gpxData.tracks[gpxData.tracks.length - 1]!;
   if (lastSeg.length > 0) {
     const last = lastSeg[lastSeg.length - 1]!;
-    const prev = result[result.length - 1];
+    const prev = segmentEndpoints[segmentEndpoints.length - 1];
     if (!prev || prev.lat !== last.lat || prev.lon !== last.lon) {
-      result.push({ lat: last.lat, lon: last.lon });
+      segmentEndpoints.push({ lat: last.lat, lon: last.lon });
     }
   }
 
-  return result;
+  // If multi-segment already gives us enough waypoints, use those
+  if (segmentEndpoints.length > 2) return segmentEndpoints;
+
+  // Single segment: simplify the track to find key turning points
+  const allPoints = gpxData.tracks.flat().map((p) => ({ lat: p.lat, lon: p.lon }));
+  if (allPoints.length < 2) return allPoints;
+
+  return douglasPeucker(allPoints, 0.05);
+}
+
+/**
+ * Douglas-Peucker line simplification.
+ * Epsilon is in degrees (~0.005° ≈ 500m at mid-latitudes).
+ * Recursively finds the point with maximum perpendicular distance
+ * from the line between start and end, keeping significant turns.
+ */
+function douglasPeucker(
+  points: Array<{ lat: number; lon: number }>,
+  epsilon: number,
+): Array<{ lat: number; lon: number }> {
+  if (points.length <= 2) return points;
+
+  let maxDist = 0;
+  let maxIdx = 0;
+
+  const start = points[0]!;
+  const end = points[points.length - 1]!;
+
+  for (let i = 1; i < points.length - 1; i++) {
+    const dist = perpendicularDistance(points[i]!, start, end);
+    if (dist > maxDist) {
+      maxDist = dist;
+      maxIdx = i;
+    }
+  }
+
+  if (maxDist > epsilon) {
+    const left = douglasPeucker(points.slice(0, maxIdx + 1), epsilon);
+    const right = douglasPeucker(points.slice(maxIdx), epsilon);
+    return [...left.slice(0, -1), ...right];
+  }
+
+  return [start, end];
+}
+
+function perpendicularDistance(
+  point: { lat: number; lon: number },
+  lineStart: { lat: number; lon: number },
+  lineEnd: { lat: number; lon: number },
+): number {
+  const dx = lineEnd.lon - lineStart.lon;
+  const dy = lineEnd.lat - lineStart.lat;
+  const lenSq = dx * dx + dy * dy;
+  if (lenSq === 0) {
+    const px = point.lon - lineStart.lon;
+    const py = point.lat - lineStart.lat;
+    return Math.sqrt(px * px + py * py);
+  }
+  const t = Math.max(0, Math.min(1,
+    ((point.lon - lineStart.lon) * dx + (point.lat - lineStart.lat) * dy) / lenSq,
+  ));
+  const projLon = lineStart.lon + t * dx;
+  const projLat = lineStart.lat + t * dy;
+  const px = point.lon - projLon;
+  const py = point.lat - projLat;
+  return Math.sqrt(px * px + py * py);
 }