Method _mergeLineStrings

modules/services/VectorTileService.js:656–795

* _mergeLineStrings * Merge LineString features that share a prophash and have near-coincident * endpoints along a tile edge. This is the LineString equivalent of * `_mergePolygons` — it stitches road segments split by MVT tile clipping. * * @param {Object} cache * @param {stri

(cache, prophash, featureIDs, lowTile, highTile)

   * @param  {Tile}    highTile
   */
  _mergeLineStrings(cache, prophash, featureIDs, lowTile, highTile) {
    const features = Array.from(featureIDs)
      .map(id => cache.features.get(id))
      .filter(f => f && f.geojson?.geometry?.type === 'LineString');

    if (features.length < 2) return;

    const SNAP_TOL = 5e-5;   // ~5.5 m at equator

    // Build a working list of coordinate arrays (+ a reference to source properties)
    let lines = features.map(f => ({
      coords: f.geojson.geometry.coordinates.slice(),   // shallow copy of array
      properties: f.geojson.properties
    }));

    // Iteratively merge pairs of lines whose endpoints are close together
    // AND whose approach directions are compatible (prevents merging perpendicular roads)
    const MAX_MERGE_ANGLE = 30;  // degrees — roads must approach within 30° of each other
    let didMerge = true;
    while (didMerge) {
      didMerge = false;
      for (let i = 0; i < lines.length && !didMerge; i++) {
        for (let j = i + 1; j < lines.length && !didMerge; j++) {
          const a = lines[i].coords;
          const b = lines[j].coords;
          const aFirst = a[0];
          const aLast  = a[a.length - 1];
          const bFirst = b[0];
          const bLast  = b[b.length - 1];

          let merged = null;

          if (this._pointsClose(aLast, bFirst, SNAP_TOL) && this._directionsCompatible(a, b, MAX_MERGE_ANGLE)) {
            merged = this._joinLineCoords(a, b, SNAP_TOL);              // A ──→ B
          } else if (this._pointsClose(aFirst, bLast, SNAP_TOL) && this._directionsCompatible(b, a, MAX_MERGE_ANGLE)) {
            merged = this._joinLineCoords(b, a, SNAP_TOL);              // B ──→ A
          } else if (this._pointsClose(aLast, bLast, SNAP_TOL)) {
            const bRev = b.slice().reverse();
            if (this._directionsCompatible(a, bRev, MAX_MERGE_ANGLE)) {
              merged = this._joinLineCoords(a, bRev, SNAP_TOL);         // A ──→ rev(B)
            }
          } else if (this._pointsClose(aFirst, bFirst, SNAP_TOL)) {
            const aRev = a.slice().reverse();
            if (this._directionsCompatible(aRev, b, MAX_MERGE_ANGLE)) {
              merged = this._joinLineCoords(aRev, b, SNAP_TOL);         // rev(A) ──→ B
            }
          }

          if (merged) {
            lines[i] = { coords: merged, properties: lines[i].properties };
            lines.splice(j, 1);
            didMerge = true;
          }
        }
      }
    }

    // ── Re-cache results ───────────────────────────────────────────────────