#42: Encode/decode polylines

README.md

@@ -16,7 +16,7 @@ This package requires Swift 5.9 or higher (at least Xcode 13), and compiles on i
 
 ```swift
 dependencies: [
-    .package(url: "https://github.com/Outdooractive/gis-tools", from: "1.0.0"),
+    .package(url: "https://github.com/Outdooractive/gis-tools", from: "1.3.0"),
 ],
 targets: [
     .target(name: "MyTarget", dependencies: [
@@ -35,6 +35,7 @@ targets: [
 - Spatial search with a R-tree
 - Includes many spatial algorithms, and more to come
 - Has a helper for working with x/y/z map tiles (center/bounding box/resolution/…)
+- Can encode/decode Polylines
 
 ## Usage
 
@@ -778,6 +779,14 @@ Also, not directly related to map tiles:
 let mpp = MapTile.metersPerPixel(at: 15.0, latitude: 45.0)
 ```
 
+# Polylines
+Provides an encoder/decoder for Polylines.
+
+```swift
+let polyline = [Coordinate3D(latitude: 47.56, longitude: 10.22)].encodePolyline()
+let coordinates = polyline.decodePolyline()
+```
+
 # Algorithms
 Hint: Most algorithms are optimized for EPSG:4326. Using other projections will have a performance penalty due to added projections.
 

Sources/GISTools/Extensions/DataExtensions.swift

@@ -18,6 +18,10 @@ extension Data {
         self.init(bytes)
     }
 
+    var asUTF8EncodedString: String? {
+        String(data: self, encoding: .utf8)
+    }
+
     /// The data, or nil if it is empty
     var nilIfEmpty: Data? {
         guard !isEmpty else { return nil }

Sources/GISTools/Extensions/DoubleExtensions.swift

@@ -74,4 +74,8 @@ extension Double {
         self = rounded(precision: precision)
     }
 
+    var toInt: Int {
+        Int(self)
+    }
+
 }

Sources/GISTools/Extensions/StringExtensions.swift

@@ -34,4 +34,8 @@ extension String {
         return self
     }
 
+    var asUTF8EncodedData: Data? {
+        self.data(using: .utf8)
+    }
+
 }

Sources/GISTools/GISTool.swift

@@ -23,4 +23,7 @@ public enum GISTool {
     /// The length in pixels of a map tile.
     public static let tileSideLength: Double = 256.0
 
+    /// The default precision for encoding/decoding Polylines.
+    public static let defaultPolylinePrecision: Double = 1e5
+
 }

Sources/GISTools/GeoJson/Polyline.swift

@@ -0,0 +1,172 @@
+#if !os(Linux)
+    import CoreLocation
+#endif
+import Foundation
+
+extension [Coordinate3D] {
+
+    /// Encodes the coordinates to a Polyline with the given precision.
+    public func encodePolyline(precision: Double = GISTool.defaultPolylinePrecision) -> String {
+        Polyline.encode(coordinates: self, precision: precision)
+    }
+
+}
+
+extension String {
+
+    /// Decodes a Polyline to coordinates with the given precision (must match the encoding precision).
+    public func decodePolyline(precision: Double = GISTool.defaultPolylinePrecision) -> [Coordinate3D]? {
+        Polyline.decode(polyline: self, precision: precision)
+    }
+
+}
+
+// Algorithm: https://developers.google.com/maps/documentation/utilities/polylinealgorithm
+enum Polyline {
+
+    /// Encodes the coordinates to a Polyline with the given precision.
+    public static func encode(
+        coordinates: [Coordinate3D],
+        precision: Double = GISTool.defaultPolylinePrecision)
+        -> String
+    {
+        var previousIntLatitude = 0,
+            previousIntLongitude = 0
+
+        var result = ""
+
+        for coordinate in coordinates {
+            let intLatitude = (coordinate.latitude * precision).rounded().toInt
+            let intLongitude = (coordinate.longitude * precision).rounded().toInt
+
+            result += encodeInt(intLatitude - previousIntLatitude)
+            result += encodeInt(intLongitude - previousIntLongitude)
+
+            previousIntLatitude = intLatitude
+            previousIntLongitude = intLongitude
+        }
+
+        return result
+    }
+
+    /// Decodes a Polyline to coordinates with the given precision (must match the encoding precision).
+    public static func decode(
+        polyline: String,
+        precision: Double = GISTool.defaultPolylinePrecision)
+        -> [Coordinate3D]?
+    {
+        guard let data = polyline.asUTF8EncodedData else { return nil }
+
+        let length = data.count
+        return data.withUnsafeBytes({ buffer -> [Coordinate3D]? in
+            var coordinates: [Coordinate3D] = []
+
+            var position = 0
+            var latitude = 0.0
+            var longitude = 0.0
+
+            while position < length {
+                guard
+                    let currentLatitude = decodeValue(
+                        buffer: buffer,
+                        position: &position,
+                        length: length,
+                        precision: precision),
+                    let currentLongitude = decodeValue(
+                        buffer: buffer,
+                        position: &position,
+                        length: length,
+                        precision: precision)
+                else { return nil }
+
+                latitude += currentLatitude
+                longitude += currentLongitude
+
+                coordinates.append(Coordinate3D(latitude: latitude, longitude: longitude))
+            }
+
+            return coordinates
+        })
+    }
+
+    // MARK: - Private
+
+    private static let firstBitBitmask = 0b0000_0001
+    private static let fiveBitsBitmask = 0b0001_1111
+    private static let sixthBitBitmask = 0b0010_0000
+    private static let base64BaseValue = 63
+
+    private static func encodeInt(_ value: Int) -> String {
+        var value = value
+        if value < 0 {
+            value = value << 1
+            value = ~value
+        }
+        else {
+            value = value << 1
+        }
+
+        var result = ""
+        var fiveBitChunk = 0
+
+        repeat {
+            fiveBitChunk = value & fiveBitsBitmask
+
+            if value >= sixthBitBitmask {
+                fiveBitChunk |= sixthBitBitmask
+            }
+
+            value = value >> 5
+            fiveBitChunk += base64BaseValue
+
+            result += String(UnicodeScalar(fiveBitChunk)!)
+        }
+        while value != 0
+
+        return result
+    }
+
+    private static func decodeValue(
+        buffer: UnsafeRawBufferPointer,
+        position: inout Int,
+        length: Int,
+        precision: Double)
+        -> Double?
+    {
+        guard position < length else { return nil }
+
+        var value = 0
+        var scalar = 0
+        var components = 0
+        var fiveBitChunk = 0
+
+        repeat {
+            scalar = Int(buffer[position]) - base64BaseValue
+            fiveBitChunk = scalar & fiveBitsBitmask
+
+            value |= (fiveBitChunk << (5 * components))
+
+            position += 1
+            components += 1
+        }
+        while (scalar & sixthBitBitmask) == sixthBitBitmask
+            && position < length
+            && components < 6
+
+        if components == 6,
+           (scalar & sixthBitBitmask) == sixthBitBitmask
+        {
+            return nil
+        }
+
+        if (value & firstBitBitmask) == firstBitBitmask {
+            value = ~(value >> 1)
+        }
+        else {
+            value = value >> 1
+        }
+
+        return Double(value) / precision
+    }
+
+}

Tests/GISToolsTests/GeoJson/PolylineTests.swift

@@ -0,0 +1,30 @@
+@testable import GISTools
+import XCTest
+
+final class PolylineTests: XCTestCase {
+
+    let coordinates: [Coordinate3D] = [
+        .init(latitude: 38.5, longitude: -120.2),
+        .init(latitude: 40.7, longitude: -120.95),
+        .init(latitude: 43.252, longitude: -126.453),
+    ]
+    let polylines: [String] = [
+        "_p~iF~ps|U",
+        "_flwFn`faV",
+        "_t~fGfzxbW",
+    ]
+    let encodedPolyline = "_p~iF~ps|U_ulLnnqC_mqNvxq`@"
+
+    func testEncodePolyline() throws {
+        for (i, coordinate) in coordinates.enumerated() {
+            XCTAssertEqual(Polyline.encode(coordinates: [coordinate]), polylines[i])
+        }
+
+        XCTAssertEqual(coordinates.encodePolyline(), encodedPolyline)
+    }
+
+    func testDecodePolyline() throws {
+        XCTAssertEqual(encodedPolyline.decodePolyline(), coordinates)
+    }
+
+}