Adds lineSlice method (#158)

* Implement lineSlice with tests

* add line_slice link

---------

Co-authored-by: Lukas Himsel <lukas@himsel.me>

README.md

@@ -149,7 +149,7 @@ Any new benchmarks must be named `*_benchmark.dart` and reside in the
 - [ ] [lineIntersect](https://github.com/dartclub/turf_dart/blob/main/lib/src/line_intersect.dart)
 - [ ] lineOverlap
 - [x] [lineSegment](https://github.com/dartclub/turf_dart/blob/main/lib/src/line_segment.dart)
-- [ ] lineSlice
+- [x] [lineSlice](https://github.com/dartclub/turf_dart/blob/main/lib/src/line_slice.dart)
 - [ ] lineSliceAlong
 - [ ] lineSplit
 - [ ] mask

lib/line_slice.dart

@@ -0,0 +1,3 @@
+library turf_along;
+
+export "src/line_slice.dart";

lib/src/line_slice.dart

@@ -0,0 +1,47 @@
+import 'package:turf/helpers.dart';
+import 'package:turf/nearest_point_on_line.dart';
+import 'package:turf/src/invariant.dart';
+
+/// Takes a [line], at a start point [startPt], and a stop point [stopPt]
+/// and returns a subsection of the line in-between those points.
+/// The start & stop points don't need to fall exactly on the line.
+///
+/// If [startPt] and [stopPt] resolve to the same point on [line], null is returned
+/// as the resolved line would only contain one point which isn't supported by LineString.
+///
+/// This can be useful for extracting only the part of a route between waypoints.
+Feature<LineString> lineSlice(
+    Feature<Point> startPt, Feature<Point> stopPt, Feature<LineString> line) {
+  final coords = line.geometry;
+  final startPtGeometry = startPt.geometry;
+  final stopPtGeometry = stopPt.geometry;
+  if (coords == null) {
+    throw Exception('line has no geometry');
+  }
+  if (startPtGeometry == null) {
+    throw Exception('startPt has no geometry');
+  }
+  if (stopPtGeometry == null) {
+    throw Exception('stopPt has no geometry');
+  }
+
+  final startVertex = nearestPointOnLine(coords, startPtGeometry);
+  final stopVertex = nearestPointOnLine(coords, stopPtGeometry);
+  late final List<Feature<Point>> ends;
+  if (startVertex.properties!['index'] <= stopVertex.properties!['index']) {
+    ends = [startVertex, stopVertex];
+  } else {
+    ends = [stopVertex, startVertex];
+  }
+  final List<Position> clipCoords = [getCoord(ends[0])];
+  for (var i = ends[0].properties!['index'] + 1;
+      i < ends[1].properties!['index'];
+      i++) {
+    clipCoords.add(coords.coordinates[i]);
+  }
+  clipCoords.add(getCoord(ends[1]));
+  return Feature<LineString>(
+    geometry: LineString(coordinates: clipCoords),
+    properties: line.properties,
+  );
+}

test/components/line_slice_test.dart

@@ -0,0 +1,78 @@
+import 'package:test/test.dart';
+import 'package:turf/along.dart';
+import 'package:turf/distance.dart';
+import 'package:turf/helpers.dart';
+import 'package:turf/length.dart';
+import 'package:turf/src/line_slice.dart';
+
+void main() {
+  test('lineSlice - exact points', () {
+    final slice = lineSlice(startFeature, viaFeature, lineFeature);
+    expect(slice, isNotNull);
+    expect(slice.properties, isNotNull);
+    expect(slice.properties!.keys, contains(propName));
+    expect(slice.properties![propName], equals(propValue));
+
+    final expectedLineFeature = Feature<LineString>(
+      geometry: LineString(coordinates: [start, via]),
+    );
+    expect(slice.geometry, isNotNull);
+    expect(slice.geometry!.coordinates, hasLength(2));
+    expect(length(slice).round(), equals(length(expectedLineFeature).round()));
+  });
+  test('lineSlice - interpolation', () {
+    const skipDist = 10;
+
+    final sliceFrom = along(lineFeature, skipDist, Unit.meters);
+    expect(sliceFrom, isNotNull);
+
+    final slice = lineSlice(sliceFrom, viaFeature, lineFeature);
+    expect(slice, isNotNull);
+    expect(slice.properties, isNotNull);
+    expect(slice.properties!.keys, contains(propName));
+    expect(slice.properties![propName], equals(propValue));
+
+    final expectedLine = Feature<LineString>(
+      geometry: LineString(coordinates: [start, via]),
+    );
+    expect(slice.geometry, isNotNull);
+    expect(slice.geometry!.coordinates, hasLength(2));
+    expect(
+      length(slice, Unit.meters).round(),
+      equals(length(expectedLine, Unit.meters).round() - skipDist),
+    );
+
+    // Sanity check of test data. No interpolation occurs if start and via are skipDist apart.
+    expect(distance(Point(coordinates: start), Point(coordinates: via)).round(),
+        isNot(equals(skipDist)));
+  });
+}
+
+final start = Position.named(
+  lat: 55.7090430186194,
+  lng: 13.184645393920405,
+);
+final via = Position.named(
+  lat: 55.70901279569489,
+  lng: 13.185546616182755,
+);
+final end = Position.named(
+  lat: 55.70764669578079,
+  lng: 13.187563637197076,
+);
+const propName = 'prop1';
+const propValue = 1;
+final lineFeature = Feature<LineString>(
+  geometry: LineString(
+    coordinates: [
+      start,
+      via,
+      end,
+    ],
+  ),
+  properties: {
+    propName: propValue,
+  },
+);
+final startFeature = Feature<Point>(geometry: Point(coordinates: start));
+final viaFeature = Feature<Point>(geometry: Point(coordinates: via));