Move intersection/2 to GeoJSON.LineString.intersection(with:) method

Adopted from mapbox/turf-swift#167

README.md

@@ -53,7 +53,7 @@ Turf.js | Turf-swift
 [turf-helpers#radiansToDegrees](https://github.com/Turfjs/turf/tree/master/packages/turf-helpers/#radiansToDegrees) | `GeoJSON.DegreesRadians.toDegrees()`
 [turf-helpers#convertLength](https://github.com/Turfjs/turf/tree/master/packages/turf-helpers#convertlength)<br>[turf-helpers#convertArea](https://github.com/Turfjs/turf/tree/master/packages/turf-helpers#convertarea) | `Measurement.converted(to:)`
 [turf-length](https://github.com/Turfjs/turf/tree/master/packages/turf-length/) | `GeoJSON.LineString.distance(from:to:)`
-[turf-line-intersect](https://github.com/Turfjs/turf/tree/master/packages/turf-line-intersect/) | `intersection(_:_:)`
+[turf-line-intersect](https://github.com/Turfjs/turf/tree/master/packages/turf-line-intersect/) | `GeoJSON.LineString.intersection(with:)`
 [turf-line-slice](https://github.com/Turfjs/turf/tree/master/packages/turf-line-slice/) | `GeoJSON.LineString.sliced(from:to:)`
 [turf-line-slice-along](https://github.com/Turfjs/turf/tree/master/packages/turf-line-slice-along/) | `GeoJSON.LineString.trimmed(from:distance:)`, `GeoJSON.LineString.trimmed(from:to:)` 
 [turf-midpoint](https://github.com/Turfjs/turf/blob/master/packages/turf-midpoint/index.js) | `mid(_:_:)`

Sources/GeoJSONKitTurf/Turf+LineString.swift

@@ -5,6 +5,13 @@ import GeoJSONKit
 extension GeoJSON.LineString {
   var coordinates: [GeoJSON.Position] { positions }
 
+  /**
+    Representation of current `LineString` as an array of `LineSegment`s.
+    */
+   var segments: [LineSegment] {
+     return zip(coordinates.dropLast(), coordinates.dropFirst()).map { LineSegment($0.0, $0.1) }
+   }
+
   /// Returns a new line string based on bezier transformation of the input line.
   ///
   /// ported from https://github.com/Turfjs/turf/blob/1ea264853e1be7469c8b7d2795651c9114a069aa/packages/turf-bezier-spline/index.ts
@@ -261,5 +268,24 @@ extension GeoJSON.LineString {
 
     return closestCoordinate
   }
+
+
+  /**
+   Returns all intersections with another `LineString`.
+
+   This function is roughly equivalent to the [turf-line-intersect](https://turfjs.org/docs/#lineIntersect) package of Turf.js ([source code](https://github.com/Turfjs/turf/tree/master/packages/turf-line-intersect/)). Order of found intersections is not determined.
+   */
+  public func intersections(with line: GeoJSON.LineString) -> Set<GeoJSON.Position> {
+    var intersections = Set<GeoJSON.Position>()
+    for segment1 in segments {
+      for segment2 in line.segments {
+        if let intersection = intersection(LineSegment(segment1.0, segment1.1),
+                                           LineSegment(segment2.0, segment2.1)) {
+          intersections.insert(intersection)
+        }
+      }
+    }
+    return intersections
+  }
 
 }

Sources/GeoJSONKitTurf/Turf.swift

@@ -6,12 +6,12 @@ let metersPerRadian: GeoJSON.Distance = 6_373_000.0
 // WGS84 equatorial radius as specified by the International Union of Geodesy and Geophysics
 let equatorialRadius: GeoJSON.Distance = 6_378_137
 
-public typealias LineSegment = (GeoJSON.Position, GeoJSON.Position)
+typealias LineSegment = (GeoJSON.Position, GeoJSON.Position)
 
 /**
  Returns the intersection of two line segments.
  */
-public func intersection(_ line1: LineSegment, _ line2: LineSegment) -> GeoJSON.Position? {
+func intersection(_ line1: LineSegment, _ line2: LineSegment) -> GeoJSON.Position? {
   // Ported from https://github.com/Turfjs/turf/blob/142e137ce0c758e2825a260ab32b24db0aa19439/packages/turf-point-on-line/index.js, in turn adapted from http://jsfiddle.net/justin_c_rounds/Gd2S2/light/
   let denominator = ((line2.1.latitude - line2.0.latitude) * (line1.1.longitude - line1.0.longitude))
   - ((line2.1.longitude - line2.0.longitude) * (line1.1.latitude - line1.0.latitude))
@@ -31,9 +31,9 @@ public func intersection(_ line1: LineSegment, _ line2: LineSegment) -> GeoJSON.
                                           longitude: line1.0.longitude + a * (line1.1.longitude - line1.0.longitude))
 
   /// True if line 1 is finite and line 2 is infinite.
-  let intersectsWithLine1 = a > 0 && a < 1
+  let intersectsWithLine1 = a >= 0 && a <= 1
   /// True if line 2 is finite and line 1 is infinite.
-  let intersectsWithLine2 = b > 0 && b < 1
+  let intersectsWithLine2 = b >= 0 && b <= 1
   return intersectsWithLine1 && intersectsWithLine2 ? intersection : nil
 }
 

Tests/GeoJSONKitTurfTests/LineStringTests.swift

@@ -392,4 +392,34 @@ class LineStringTests: XCTestCase {
     sliced = line1.trimmed(from: startDistance, to: stopDistance)
     XCTAssertNil(sliced, "should return nil")
   }
+
+  func testIntersections() {
+    let lineString = GeoJSON.LineString(positions: [.init(latitude: 2, longitude: 1),
+                                                    .init(latitude: 2, longitude: 5),
+                                                    .init(latitude: 2, longitude: 9)])
+
+    let intersectingLineString = GeoJSON.LineString(positions: [.init(latitude: 4, longitude: 1),
+                                                                .init(latitude: 0, longitude: 5),
+                                                                .init(latitude: 2, longitude: 9)])
+
+    var intersections = lineString.intersections(with: intersectingLineString)
+
+    XCTAssertEqual(intersections.sorted(by: { $0.longitude < $1.longitude }),
+                   [.init(latitude: 2, longitude: 3),
+                    .init(latitude: 2, longitude: 9)], "LineString intersections are not correct.")
+
+    let notIntersectingLineString = GeoJSON.LineString(positions: [.init(latitude: 1, longitude: 1),
+                                                                   .init(latitude: 1, longitude: 5),
+                                                                   .init(latitude: 1, longitude: 9)])
+
+    intersections = lineString.intersections(with: notIntersectingLineString)
+
+    XCTAssertTrue(intersections.isEmpty, "Found impossible LineString intersection(s).")
+
+    let emptyLineString = GeoJSON.LineString(positions: [])
+
+    intersections = lineString.intersections(with: emptyLineString)
+
+    XCTAssertTrue(intersections.isEmpty, "Found impossible LineString intersection(s).")
+  }
 }

Tests/GeoJSONKitTurfTests/TurfTests.swift

@@ -32,6 +32,13 @@ class TurfTests: XCTestCase {
     XCTAssertEqual(a, coord1)
   }
 
+  func testIntersectionOnEnd() {
+    let coord1 = GeoJSON.Position(latitude: 20, longitude: 20)
+    let a = intersection((GeoJSON.Position(latitude: 20, longitude: 20), GeoJSON.Position(latitude: 40, longitude: 40)),
+                         (GeoJSON.Position(latitude: 20, longitude: 20), GeoJSON.Position(latitude: 40, longitude: 20)))
+    XCTAssertEqual(a, coord1)
+  }
+
   func testDegreeesToRadians() {
     let degree: GeoJSON.Degrees = 100
     let a = degree.toRadians()