Add method of LineString.trimmed(from:to:) (#166)

README.md

@@ -89,7 +89,7 @@ Turf.js | Turf for Swift
 [turf-length#length](https://turfjs.org/docs/#length) | `LineString.distance(from:to:)`
 [turf-line-intersect#lineIntersect](https://turfjs.org/docs/#lineIntersect) | `intersection(_:_:)`
 [turf-line-slice#lineSlice](https://turfjs.org/docs/#lineSlice) | `LineString.sliced(from:to:)`
-[turf-line-slice-along#lineSliceAlong](https://turfjs.org/docs/#lineSliceAlong) | `LineString.trimmed(from:distance:)`
+[turf-line-slice-along#lineSliceAlong](https://turfjs.org/docs/#lineSliceAlong) | `LineString.trimmed(from:to:)`
 [turf-midpoint#midpoint](https://turfjs.org/docs/#midpoint) | `mid(_:_:)`
 [turf-nearest-point-on-line#nearestPointOnLine](https://turfjs.org/docs/#nearestPointOnLine) | `LineString.closestCoordinate(to:)`
 [turf-polygon-to-line#polygonToLine](https://turfjs.org/docs/#polygonToLine) | `LineString(_:)`<br>`MultiLineString(_:)`

Sources/Turf/Geometries/LineString.swift

@@ -78,9 +78,65 @@ extension LineString {
     }
 
     /**
-     Returns the portion of the line string that begins at the given coordinate and extends the given distance along the line string.
+     Returns the portion of the line string that begins at the given start distance and extends the given stop distance along the line string.
 
-     This method is roughly equivalent to the [turf-line-slice-along](https://turfjs.org/docs/#lineSliceAlong) package of Turf.js ([source code](https://github.com/Turfjs/turf/tree/master/packages/turf-line-slice-along/)), except that it accepts a starting position instead of a starting distance along the line string.
+     This method is equivalent to the [turf-line-slice-along](https://turfjs.org/docs/#lineSliceAlong) package of Turf.js ([source code](https://github.com/Turfjs/turf/tree/master/packages/turf-line-slice-along/)).
+     */
+    public func trimmed(from startDistance: LocationDistance, to stopDistance: LocationDistance) -> LineString? {
+        // The method is porting from https://github.com/Turfjs/turf/blob/5375941072b90d489389db22b43bfe809d5e451e/packages/turf-line-slice-along/index.js
+        guard startDistance >= 0.0 && stopDistance >= startDistance else { return nil }
+        let coordinates = self.coordinates
+        var traveled: LocationDistance = 0
+        var slice = [LocationCoordinate2D]()
+
+        for i in 0..<coordinates.endIndex {
+            if startDistance >= traveled && i == coordinates.endIndex - 1 {
+                break
+            } else if traveled > startDistance && slice.isEmpty {
+                let overshoot = startDistance - traveled
+                if overshoot == 0.0 {
+                    slice.append(coordinates[i])
+                    return LineString(slice)
+                }
+                let direction = coordinates[i].direction(to: coordinates[i - 1]) - 180
+                let interpolated = coordinates[i].coordinate(at: overshoot, facing: direction)
+                slice.append(interpolated)
+            }
+
+            if traveled >= stopDistance {
+                let overshoot = stopDistance - traveled
+                if overshoot == 0.0 {
+                    slice.append(coordinates[i])
+                    return LineString(slice)
+                }
+                let direction = coordinates[i].direction(to: coordinates[i - 1]) - 180
+                let interpolated = coordinates[i].coordinate(at: overshoot, facing: direction)
+                slice.append(interpolated)
+                return LineString(slice)
+            }
+
+            if traveled >= startDistance {
+                slice.append(coordinates[i])
+            }
+
+            if i == coordinates.count - 1 {
+                return LineString(slice)
+            }
+
+            traveled += distance(from: coordinates[i], to: coordinates[i + 1]) ?? 0.0
+        }
+
+        if traveled < startDistance { return nil }
+
+        if let last = coordinates.last {
+            return LineString([last, last])
+        }
+
+        return nil
+    }
+
+    /**
+     Returns the portion of the line string that begins at the given coordinate and extends the given distance along the line string.
      */
     public func trimmed(from coordinate: LocationCoordinate2D, distance: LocationDistance) -> LineString? {
         let startVertex = closestCoordinate(to: coordinate)

Tests/TurfTests/LineStringTests.swift

@@ -350,4 +350,64 @@ class LineStringTests: XCTestCase {
         XCTAssertEqual(slicedCoordinates?.count, 2, "no duplicated coords")
         XCTAssertNotEqual(slicedCoordinates?.first, slicedCoordinates?.last, "vertical slice should not collapse to first coordinate")
     }
+
+    func testTrimmed() {
+        // https://github.com/Turfjs/turf/blob/5375941072b90d489389db22b43bfe809d5e451e/packages/turf-line-slice-along/test.js
+
+        // turf-line-slice-along -- line1
+        let coordinates = [
+            [113.99414062499999, 22.350075806124867],
+            [116.76269531249999, 23.241346102386135],
+            [117.7734375, 24.367113562651276],
+            [118.828125, 25.20494115356912],
+            [119.794921875, 26.78484736105119],
+            [120.80566406250001, 28.110748760633534],
+            [121.59667968749999, 29.49698759653577],
+            [121.59667968749999, 31.12819929911196],
+            [120.84960937499999, 32.84267363195431],
+            [119.83886718750001, 34.125447565116126],
+            [118.69628906249999, 35.31736632923788],
+            [121.4208984375, 36.80928470205937],
+            [122.82714843749999, 37.37015718405753]
+          ]
+        let line1 = LineString(coordinates.map{
+            CLLocationCoordinate2D(latitude: $0.last!, longitude: $0.first!)
+        })
+
+        var startDistance = 804672.0
+        var stopDistance = 1207008.0
+
+        var startPoint = line1.coordinateFromStart(distance: startDistance)
+        var endPoint = line1.coordinateFromStart(distance: stopDistance)
+        var sliced = line1.trimmed(from: startDistance, to: stopDistance)
+        XCTAssertNotNil(sliced, "should return valid lineString")
+        XCTAssertEqual(sliced!.coordinates.first!, startPoint)
+        XCTAssertEqual(sliced!.coordinates.last!, endPoint)
+
+        stopDistance = 2414016.0
+        endPoint = line1.coordinateFromStart(distance: stopDistance)
+        sliced = line1.trimmed(from: startDistance, to: stopDistance)
+        XCTAssertNotNil(sliced, "should return valid lineString")
+        XCTAssertEqual(sliced!.coordinates.first!, startPoint)
+        XCTAssertEqual(sliced!.coordinates.last!, endPoint)
+
+        startDistance = 8046720
+        stopDistance = 12874752.0
+        sliced = line1.trimmed(from: startDistance, to: stopDistance)
+        XCTAssertNil(sliced, "should return nil")
+
+        startDistance = line1.distance()!
+        stopDistance = startDistance + 100.0
+        startPoint = line1.coordinateFromStart(distance: startDistance)
+        endPoint = line1.coordinateFromStart(distance: stopDistance)
+        sliced = line1.trimmed(from: startDistance, to: stopDistance)
+        XCTAssertNotNil(sliced, "should return valid lineString")
+        XCTAssertEqual(sliced!.coordinates.first!, startPoint)
+        XCTAssertEqual(sliced!.coordinates.last!, endPoint)
+
+        startDistance = -0.376
+        stopDistance = 543.0
+        sliced = line1.trimmed(from: startDistance, to: stopDistance)
+        XCTAssertNil(sliced, "should return nil")
+    }
 }