Feature/encodedpolyline (#24)

* Add `GeoJSON.LineString(encodedPolyline: String)`

* Update GHA, test newer Swifts

Author: nighthawk

.github/workflows/swift.yml

@@ -10,7 +10,7 @@ jobs:
   macos:
     runs-on: macos-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Build
         run: swift build
       - name: Test
@@ -21,12 +21,12 @@ jobs:
 
     strategy:
       matrix:
-        swift: ["5.9.1", "5.7.3"]
+        swift: ["6.0", "5.10", "5.9", "5.8"]
 
     container:
       image: swift:${{ matrix.swift }}
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Build
         run: swift build
       - name: Test

Sources/GeoJSONKitTurf/GeoJSON+LineString+DecodePolyline.swift

@@ -0,0 +1,63 @@
+//
+//  GeoJSON+LineString+DecodePolylineString.swift
+//
+//  Created by Adrian Schoenig on 18/2/17.
+//
+//
+import Foundation
+
+import GeoJSONKit
+
+extension GeoJSON.LineString {
+  public init(encodedPolyline: String) {
+    let bytes = encodedPolyline.utf8CString
+    let length = bytes.count - 1 // ignore 0 at end
+    var idx = 0
+    
+    var array: [GeoJSON.Position] = []
+    
+    var latitude = 0.0
+    var longitude = 0.0
+    while idx < length {
+      var byte = 0
+      var res = 0
+      var shift = 0
+      
+      repeat {
+        if idx > length {
+          break
+        }
+        byte = Int(bytes[idx]) - 63
+        idx += 1
+        res |= (byte & 0x1F) << shift
+        shift += 5
+      } while byte >= 0x20
+      
+      let deltaLat = ((res & 1) != 0 ? ~(res >> 1) : (res >> 1));
+      latitude += Double(deltaLat)
+      
+      shift = 0
+      res = 0
+      
+      repeat {
+        if idx > length {
+          break
+        }
+        byte = Int(bytes[idx]) - 0x3F
+        idx += 1
+        res |= (byte & 0x1F) << shift
+        shift += 5
+      } while byte >= 0x20
+      
+      let deltaLon = ((res & 1) != 0 ? ~(res >> 1) : (res >> 1));
+      longitude += Double(deltaLon)
+      
+      let finalLat = latitude * 1E-5
+      let finalLon = longitude * 1E-5
+      let coordinate = GeoJSON.Position(latitude: finalLat, longitude: finalLon)
+      array.append(coordinate)
+    }
+    
+    self.init(positions: array)
+  }
+}