Implement polygon-smooth and its tests (dartclub#127)

* initial implementation, and refactor

* finished test and benchmark impl

* reference in README

Author: lukas-h

README.md

@@ -11,6 +11,7 @@ This includes a fully [RFC 7946](https://tools.ietf.org/html/rfc7946)-compliant
 Most of the implementation is a direct translation from [turf.js](https://github.com/Turfjs/turf).
 
 ## Get started
+
 - Get the [Dart tools](https://dart.dev/tools)
 - Install the library with `dart pub add turf`
 - Import the library in your code and use it. For example:
@@ -58,12 +59,14 @@ main() {
 ![polymorphism](https://user-images.githubusercontent.com/10634693/159876354-f9da2f37-02b3-4546-b32a-c0f82c372272.png)
 
 ## Notable Design Decisions
+
 - Nested `GeometryCollections` (as described in
   [RFC 7946 section 3.1.8](https://datatracker.ietf.org/doc/html/rfc7946#section-3.1.8))
   are _not supported_ which takes a slightly firmer stance than the "should
   avoid" language in the specification
 
 ## Tests and Benchmarks
+
 Tests are run with `dart test` and benchmarks can be run with
 `dart run benchmark`
 
@@ -73,6 +76,7 @@ Any new benchmarks must be named `*_benchmark.dart` and reside in the
 ## Components
 
 ### Measurement
+
 - [ ] along
 - [x] [area](https://github.com/dartclub/turf_dart/blob/main/lib/src/area.dart)
 - [x] [bbox](https://github.com/dartclub/turf_dart/blob/main/lib/src/bbox.dart)
@@ -96,13 +100,15 @@ Any new benchmarks must be named `*_benchmark.dart` and reside in the
 - [ ] greatCircle
 
 ### Coordinate Mutation
+
 - [x] [cleanCoords](https://github.com/dartclub/turf_dart/blob/main/lib/src/clean_coords.dart)
 - [ ] flip
 - [ ] rewind
 - [ ] round
 - [x] [truncate](https://github.com/dartclub/turf_dart/blob/main/lib/src/truncate.dart)
 
 ### Transformation
+
 - [ ] bboxClip
 - [ ] bezierSpline
 - [ ] buffer
@@ -114,17 +120,18 @@ Any new benchmarks must be named `*_benchmark.dart` and reside in the
 - [ ] dissolve
 - [ ] intersect
 - [ ] lineOffset
-- [ ] polygonSmooth
+- [x] [polygonSmooth](ttps://github.com/dartclub/turf_dart/blob/main/lib/src/polygon_smooth.dart)
 - [ ] simplify
 - [ ] tesselate
 - [ ] transformRotate
 - [ ] transformTranslate
 - [ ] transformScale
 - [ ] union
 - [ ] voronoi
-- [x] [polyLineDecode](https://github.com/Dennis-Mwea/turf_dart/blob/main/lib/src/polyline.dart)
+- [x] [polyLineDecode](https://github.com/dartclub/turf_dart/blob/main/lib/src/polyline.dart)
 
 ### Feature Conversion
+
 - [ ] combine
 - [x] [explode](https://github.com/dartclub/turf_dart/blob/main/lib/src/explode.dart)
 - [ ] flatten
@@ -133,6 +140,7 @@ Any new benchmarks must be named `*_benchmark.dart` and reside in the
 - [x] [polygonToLine](https://github.com/dartclub/turf_dart/blob/main/lib/src/polygon_to_line.dart)
 
 ### MISC
+
 - [ ] ellipse
 - [ ] kinks
 - [ ] lineArc
@@ -150,40 +158,48 @@ Any new benchmarks must be named `*_benchmark.dart` and reside in the
 - [ ] unkinkPolygon
 
 ### Random
+
 - [ ] randomPosition
 - [ ] randomPoint
 - [ ] randomLineString
 - [ ] randomPolygon
 
 ### Data
+
 - [ ] sample
 
 ### Interpolation
+
 - [ ] interpolate
 - [ ] isobands
 - [ ] isolines
 - [ ] planepoint
 - [ ] tin
 
 ### Joins
+
 - [ ] pointsWithinPolygon
 - [ ] tag
 
 ### Grids
+
 - [ ] hexGrid
 - [ ] pointGrid
 - [ ] squareGrid
 - [ ] triangleGrid
 
 ### Classification
+
 - [x] [nearestPoint](https://github.com/dartclub/turf_dart/blob/main/lib/src/nearest_point.dart)
 
 ### Aggregation
+
 - [ ] collect
 - [ ] clustersDbscan
 - [ ] clustersKmeans
 
 ### META
+
 - [x] [coordAll](https://github.com/dartclub/turf_dart/blob/main/lib/src/meta/coord.dart)
 - [x] [coordEach](https://github.com/dartclub/turf_dart/blob/main/lib/src/meta/coord.dart)
 - [x] [coordReduce](https://github.com/dartclub/turf_dart/blob/main/lib/src/meta/coord.dart)
@@ -204,6 +220,7 @@ Any new benchmarks must be named `*_benchmark.dart` and reside in the
 - [x] [clusterReduce](https://github.com/dartclub/turf_dart/blob/main/lib/src/meta/cluster.dart)
 
 ### Booleans
+
 - [ ] booleanClockwise
 - [ ] booleanConcave
 - [ ] booleanContains
@@ -218,6 +235,7 @@ Any new benchmarks must be named `*_benchmark.dart` and reside in the
 - [ ] booleanWithin
 
 ### Unit Conversion
+
 - [x] [bearingToAzimuth](https://github.com/dartclub/turf_dart/blob/main/lib/src/helpers.dart)
 - [x] [convertArea](https://github.com/dartclub/turf_dart/blob/main/lib/src/helpers.dart)
 - [x] [convertLength](https://github.com/dartclub/turf_dart/blob/main/lib/src/helpers.dart)

benchmark/polygon_smooth_benchmark.dart

@@ -0,0 +1,21 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:benchmark/benchmark.dart';
+import 'package:turf/polygon_smooth.dart';
+import 'package:turf/turf.dart';
+
+main() {
+  group("turf-polygon-smooth", () {
+    var inDir = Directory('./test/examples/polygonSmooth/in');
+    for (var file in inDir.listSync(recursive: true)) {
+      if (file is File && file.path.endsWith('.geojson')) {
+        benchmark(file.path, () {
+          var inSource = file.readAsStringSync();
+          var inGeom = GeoJSONObject.fromJson(jsonDecode(inSource));
+          polygonSmooth(inGeom, iterations: 3);
+        });
+      }
+    }
+  });
+}

lib/polygon_smooth.dart

@@ -0,0 +1,3 @@
+library turf_polygon_smooth;
+
+export 'src/polygon_smooth.dart';

lib/src/line_to_polygon.dart

@@ -143,26 +143,24 @@ Feature<Polygon> lineStringToPolygon(
     List<List<Position>> multiCoords = [];
     num largestArea = 0;
 
-    line.coordinates.forEach(
-      (coord) {
-        if (autoComplete) {
-          coord = _autoCompleteCoords(coord);
-        }
+    for (var coord in line.coordinates) {
+      if (autoComplete) {
+        coord = _autoCompleteCoords(coord);
+      }
 
-        // Largest LineString to be placed in the first position of the coordinates array
-        if (orderCoords) {
-          var area = _calculateArea(bbox(LineString(coordinates: coord)));
-          if (area > largestArea) {
-            multiCoords.insert(0, coord);
-            largestArea = area;
-          } else {
-            multiCoords.add(coord);
-          }
+      // Largest LineString to be placed in the first position of the coordinates array
+      if (orderCoords) {
+        var area = _calculateArea(bbox(LineString(coordinates: coord)));
+        if (area > largestArea) {
+          multiCoords.insert(0, coord);
+          largestArea = area;
         } else {
           multiCoords.add(coord);
         }
-      },
-    );
+      } else {
+        multiCoords.add(coord);
+      }
+    }
     return Feature(
         geometry: Polygon(coordinates: multiCoords), properties: properties);
   } else {

lib/src/polygon_smooth.dart

@@ -0,0 +1,152 @@
+import 'package:turf/meta.dart';
+import 'package:turf/turf.dart';
+
+///
+/// Smooths a [Polygon], [MultiPolygon], also inside [Feature]s, [FeatureCollection]s, or [GeometryCollection]. Based on [Chaikin's algorithm](http://graphics.cs.ucdavis.edu/education/CAGDNotes/Chaikins-Algorithm/Chaikins-Algorithm.html).
+/// Warning: may create degenerate polygons.
+/// The optional parameter [iterations] is the number of times to smooth the polygon. A higher value means a smoother polygon.
+/// The functions returns a [FeatureCollection] of [Polygon]s and [MultiPolygon]s.
+///
+/// ```dart
+/// var polygon = Polygon(coordinates: [
+///     [
+///         Position(11, 0),
+///         Position(22, 4),
+///         Position(31, 0),
+///         Position(31, 11),
+///         Position(21, 15),
+///         Position(11, 11),
+///         Position(11, 0),
+///     ]
+/// ]);
+///
+/// var smoothed = polygonSmooth(polygon, iterations: 3);
+/// ```
+FeatureCollection polygonSmooth(GeoJSONObject inputPolys,
+    {int iterations = 1}) {
+  var outPolys = <Feature>[];
+
+  geomEach(inputPolys, (
+    GeometryType? geom,
+    int? geomIndex,
+    Map<String, dynamic>? featureProperties,
+    BBox? featureBBox,
+    dynamic featureId,
+  ) {
+    var outCoords;
+    var poly;
+    var tempOutput;
+
+    switch (geom?.type) {
+      case GeoJSONObjectType.polygon:
+        outCoords = <List<Position>>[[]];
+        for (var i = 0; i < iterations; i++) {
+          tempOutput = <List<Position>>[[]];
+          poly = geom;
+          if (i > 0) poly = Polygon(coordinates: outCoords);
+          _processPolygon(poly, tempOutput);
+          outCoords = List<List<Position>>.of(tempOutput);
+        }
+        outPolys.add(Feature(
+            geometry: Polygon(coordinates: outCoords),
+            properties: featureProperties));
+        break;
+      case GeoJSONObjectType.multiPolygon:
+        outCoords = [
+          [<Position>[]]
+        ];
+        for (var y = 0; y < iterations; y++) {
+          tempOutput = <List<List<Position>>>[
+            [[]]
+          ];
+          poly = geom;
+          if (y > 0) poly = MultiPolygon(coordinates: outCoords);
+          _processMultiPolygon(poly, tempOutput);
+          outCoords = List<List<List<Position>>>.of(tempOutput);
+        }
+        outPolys.add(Feature(
+            geometry: MultiPolygon(coordinates: outCoords),
+            properties: featureProperties));
+        break;
+      default:
+        throw Exception("geometry is invalid, must be Polygon or MultiPolygon");
+    }
+  });
+  return FeatureCollection(features: outPolys);
+}
+
+_processPolygon(Polygon poly, List<List<Position>> tempOutput) {
+  var prevGeomIndex = 0;
+  var subtractCoordIndex = 0;
+
+  coordEach(poly, (currentCoord, coordIndex, featureIndex, multiFeatureIndex,
+      geometryIndex) {
+    if (geometryIndex! > prevGeomIndex) {
+      prevGeomIndex = geometryIndex;
+      subtractCoordIndex = coordIndex!;
+      tempOutput.add([]);
+    }
+    var realCoordIndex = coordIndex! - subtractCoordIndex;
+    var p1 = poly.coordinates[geometryIndex][realCoordIndex + 1];
+    var p0x = currentCoord!.lng;
+    var p0y = currentCoord.lat;
+    var p1x = p1.lng;
+    var p1y = p1.lat;
+    tempOutput[geometryIndex].add(Position(
+      0.75 * p0x + 0.25 * p1x,
+      0.75 * p0y + 0.25 * p1y,
+    ));
+    tempOutput[geometryIndex].add(Position(
+      0.25 * p0x + 0.75 * p1x,
+      0.25 * p0y + 0.75 * p1y,
+    ));
+  }, true);
+  for (var ring in tempOutput) {
+    ring.add(ring[0]);
+  }
+}
+
+_processMultiPolygon(poly, List<List<List<Position>>> tempOutput) {
+  var prevGeomIndex = 0;
+  var subtractCoordIndex = 0;
+  var prevMultiIndex = 0;
+
+  coordEach(poly, (currentCoord, coordIndex, featureIndex, multiFeatureIndex,
+      geometryIndex) {
+    if (multiFeatureIndex! > prevMultiIndex) {
+      prevMultiIndex = multiFeatureIndex;
+      subtractCoordIndex = coordIndex!;
+      tempOutput.add([[]]);
+    }
+    if (geometryIndex! > prevGeomIndex) {
+      prevGeomIndex = geometryIndex;
+      subtractCoordIndex = coordIndex!;
+      tempOutput[multiFeatureIndex].add([]);
+    }
+    var realCoordIndex = coordIndex! - subtractCoordIndex;
+    if (realCoordIndex + 1 ==
+        poly.coordinates[multiFeatureIndex][geometryIndex].length) {
+      return;
+    }
+    var p1 =
+        poly.coordinates[multiFeatureIndex][geometryIndex][realCoordIndex + 1];
+    var p0x = currentCoord!.lng;
+    var p0y = currentCoord.lat;
+    var p1x = p1.lng;
+    var p1y = p1.lat;
+    tempOutput[multiFeatureIndex][geometryIndex].add(Position(
+      0.75 * p0x + 0.25 * p1x,
+      0.75 * p0y + 0.25 * p1y,
+    ));
+    tempOutput[multiFeatureIndex][geometryIndex].add(Position(
+      0.25 * p0x + 0.75 * p1x,
+      0.25 * p0y + 0.75 * p1y,
+    ));
+  }, true);
+
+  for (var poly in tempOutput) {
+    for (var ring in poly) {
+      ring.add(ring[0]);
+    }
+  }
+}

lib/src/polygon_to_line.dart

@@ -48,7 +48,9 @@ FeatureCollection _multiPolygonToLine(MultiPolygon geom,
   properties = properties ?? <String, dynamic>{};
 
   var lines = <Feature>[];
-  coords.forEach((coord) => {lines.add(_coordsToLine(coord, properties))});
+  for (var coord in coords) {
+    lines.add(_coordsToLine(coord, properties));
+  }
   return FeatureCollection(features: lines);
 }