Implement greatCircle

lib/great_circle.dart

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+library turf_great_circle;
+
+export 'src/great_circle.dart'; 

lib/src/great_circle.dart

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+import 'dart:math' as math;
+import 'package:turf/turf.dart';
+import 'helpers.dart';
+
+/// Calculates the great circle route between two points on a sphere 
+/// 
+/// Useful link: https://en.wikipedia.org/wiki/Great-circle_distance
+
+List<List<double>> greatCircle(
+  dynamic start,
+  dynamic end,
+  { 
+    Map <String, dynamic> properties = const {},
+    int npoints = 100, // npoints = number of intermediate points less one (e.g if you want 5 intermediate points, set npoints = 6)
+    int offset = 10
+  }) {
+    if (start.length != 2 || end.length != 2) {
+      /// Coordinate checking 
+      throw ArgumentError("Both start and end coordinates should have two values - a latitude and longitude");
+    }
+
+    // If start and end points are the same, 
+    if (start[0] == end[0] && start[1] == end[1]) {
+      return List.generate(npoints, (_) => [start[0], start[1]]);
+    }
+    // Coordinate checking for valid values
+    if (start[0] < -90) {
+      throw ArgumentError("Starting latitude (vertical) coordinate is less than -90. This is not a valid coordinate.");
+    } 
+
+    if (start[0] > 90) {
+      throw ArgumentError("Starting latitude (vertical) coordinate is greater than 90. This is not a valid coordinate.");
+    }
+
+    if (start[1] < -180) {
+      throw ArgumentError('Starting longitude (horizontal) coordinate is less than -180. This is not a valid coordinate.');
+    }
+
+    if (start[1] > 180) {
+      throw ArgumentError('Starting longitude (horizontal) coordinate is greater than 180. This is not a valid coordinate.');
+    }
+
+    if (end[0] < -90) {
+      throw ArgumentError("Ending latitude (vertical) coordinate is less than -90. This is not a valid coordinate.");
+    } 
+
+    if (end[0] > 90) {
+      throw ArgumentError("Ending latitude (vertical) coordinate is greater than 90. This is not a valid coordinate.");
+    }
+
+    if (end[1] < -180) {
+      throw ArgumentError('Ending longitude (horizontal) coordinate is less than -180. This is not a valid coordinate.');
+    }
+
+    if (end[1] > 180) {
+      throw ArgumentError('Ending longitude (horizontal) coordinate is greater than 180. This is not a valid coordinate.');
+    }
+
+    List<List<double>> line = [];
+
+    num lat1 = degreesToRadians(start[0]);
+    num lng1 = degreesToRadians(start[1]);
+    num lat2 = degreesToRadians(end[0]);
+    num lng2 = degreesToRadians(end[1]);
+
+    // Harvesine formula 
+    for (int i = 0; i <= npoints; i++) {
+    double f = i / npoints;
+    double delta = 2 *
+        math.asin(math.sqrt(math.pow(math.sin((lat2 - lat1) / 2), 2) +
+            math.cos(lat1) * math.cos(lat2) * math.pow(math.sin((lng2 - lng1) / 2), 2)));
+    double A = math.sin((1 - f) * delta) / math.sin(delta);
+    double B = math.sin(f * delta) / math.sin(delta);
+    double x = A * math.cos(lat1) * math.cos(lng1) + B * math.cos(lat2) * math.cos(lng2);
+    double y = A * math.cos(lat1) * math.sin(lng1) + B * math.cos(lat2) * math.sin(lng2);
+    double z = A * math.sin(lat1) + B * math.sin(lat2);
+
+    double lat = math.atan2(z, math.sqrt(x * x + y * y));
+    double lng = math.atan2(y, x);
+
+    List<double> point = [double.parse(radiansToDegrees(lat).toStringAsFixed(2)), double.parse(radiansToDegrees(lng).toStringAsFixed(2))]; 
+    line.add(point);
+    }
+    /// Check for multilinestring if path crosses anti-meridian
+    bool crossAntiMeridian = (start[0] - end[0]).abs() > 180;
+
+    /// If it crossed antimeridian, we need to split our lines
+    if (crossAntiMeridian) {
+      List<List<double>> multiLine = [];
+      List<List<double>> currentLine = [];
+
+      for (var point in line) {
+        if ((point[0] - line[0][0]).abs() > 180) {
+          multiLine.addAll(currentLine);
+          currentLine = [];
+        }
+        currentLine.add(point);
+      }
+      multiLine.addAll(currentLine);
+      return multiLine;
+    }
+    return line;
+}

test/components/great_circle_test.dart

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+import 'package:turf/great_circle.dart';
+import 'package:test/test.dart';
+
+void main() {
+  //First test - simple coordinates
+
+  List<double> start = [-90, 0];
+  List<double> end = [-80,0];
+
+  List<List<double>> resultsFirstTest1 = [[-90.0, 0.0], [-88.0,0.0], [-86.0,0.0], [-84.0, 0.0], [-82.0, 0.0],[-80.0, 0.0]];
+  List<List<double>> resultsFirstTest2 = [[-90.0, 0.0], [-89.0,0.0], [-88.0,0.0], [-87.0, 0.0], [-86.0, 0.0],[-85.0,0.0], [-84.0,0.0], [-83.0, 0.0], [-82.0, 0.0],[-81.0, 0.0], [-80.0, 0.0]];
+
+  test('Great circle simple tests:', () {
+    expect(greatCircle(start, end, npoints: 5), resultsFirstTest1);
+    expect(greatCircle(start, end, npoints: 10), resultsFirstTest2);
+  });
+
+  // Second test - intermediate coordiantes (non-straight lines)
+  List<double> start2 = [48, -122];
+  List<double> end2 = [39, -77];
+
+  List<List<double>> resultsSecondTest1 = [[48.0, -122.0], [45.75, -97.73], [39.0, -77.0]];
+  List<List<double>> resultsSecondTest2 = [[48.0, -122.0], [47.52, -109.61], [45.75, -97.73], [42.85, -86.80], [39.0, -77.0]];
+
+
+  test('Great circle intermediate tests:', () {
+    expect(greatCircle(start2, end2, npoints: 2), resultsSecondTest1);
+    expect(greatCircle(start2, end2, npoints: 4), resultsSecondTest2);
+  });
+
+  // Third test - complex coordinates (crossing anti-meridian)
+
+  List<double> start3 = [-21, 143];
+  List<double> end3 = [41, -140];
+
+  List<List<double>> resultsThirdTest1 = [[-21.0, 143.0], [12.65, 176.68], [41, -140]];
+  List<List<double>> resultsThirdTest2 = [[-21.0, 143.0], [-4.36, 160.22], [12.65, 176.68], [28.52, -164.56], [41, -140]];
+  test('Great circle complex tests:', () {
+    expect(greatCircle(start3, end3, npoints: 2), resultsThirdTest1);
+    expect(greatCircle(start3, end3, npoints: 4), resultsThirdTest2);
+  });
+}