APP在保存轨迹时，如果只有较少的数据时，还能应对，如果面对上百到上千的轨迹点时，数据量还是相当大，我们不仅要考虑简单的存储方式，也要考虑存储时的磁盘空间占用情况。

此项问题，在Google地图开发中开到了他们常用的轨迹压缩算法 Polyline encoding algorith^[1]，将大量的点位坐标转换为字符串实现压缩，比如：

ml{yDk|dzRrg@pGn\\lEdU_NzXbAtNhCrU?tT?fTuCvM`@pPQbPr@hO{AfNtAjLuA|K?bIgBjM|DbEm@lEuAxGcA?G?G?gE?}|@

这个是已将25条经纬度进行压缩后的字符串，无论是传输到服务器还是本地存储，都有极大便利，在使用时，将坐标转换回来方可使用。

代码

已经在Playground中执行通过

struct Polyline {

    private static func encodeCoordinate(_ coordinate: Double) -> String {
        var coord = Int(round(coordinate * 1e5))
        coord <<= 1
        if coord < 0 {
            coord = ~coord
        }
        var encodedString = ""

        while coord >= 0x20 {
            let value = (0x20 | (coord & 0x1f)) + 63
            encodedString.append(Character(UnicodeScalar(value)!))
            coord >>= 5
        }
        coord += 63
        encodedString.append(Character(UnicodeScalar(coord)!))
        return encodedString
    }
    //编码
    public static func encodePolyline(_ coordinates:[(latitude: Double, longitude: Double)])->String{
        var encodedPolyline = ""
        var previousLatitude = 0.0
        var previousLongitude = 0.0

        for coordinate in coordinates {
            let latitudeChange = coordinate.latitude - previousLatitude
            let longitudeChange = coordinate.longitude - previousLongitude

            encodedPolyline += Polyline.encodeCoordinate(latitudeChange)
            encodedPolyline += Polyline.encodeCoordinate(longitudeChange)

            previousLatitude = coordinate.latitude
            previousLongitude = coordinate.longitude
        }
        return encodedPolyline
    }


    private static func decodeCoordinate(from encodedString: inout String) -> Double {
        var coordinate = 0
        var shift = 0
        var result = 0
        var continueDecoding = true

        while continueDecoding && !encodedString.isEmpty {
            let character = encodedString.removeFirst().unicodeScalars.first!.value - 63
            result = Int(character) & 0x1F
            coordinate |= (result << shift)
            shift += 5
            continueDecoding = (character >= 0x20)
        }

        if (coordinate & 1) != 0 {
            coordinate = ~(coordinate >> 1)
        } else {
            coordinate >>= 1
        }

        return Double(coordinate) * 1e-5
    }

    // 解码一条多段线
    public static func decodePolyline(_ encodedString: String) -> [(latitude: Double, longitude: Double)] {
        var encodedString = encodedString
        var coordinates: [(latitude: Double, longitude: Double)] = []
        var latitude = 0.0
        var longitude = 0.0

        while !encodedString.isEmpty {
            let latitudeChange = decodeCoordinate(from: &encodedString)
            let longitudeChange = decodeCoordinate(from: &encodedString)

            latitude += latitudeChange
            longitude += longitudeChange

            coordinates.append((latitude: latitude, longitude: longitude))
        }

        return coordinates
    }
}

使用

// 测试数据
let coordinates = [
  (latitude: 30.61975,longitude:104.06870),
  (latitude: 30.61325,longitude:104.06733),
  (latitude: 30.60853,longitude:104.06630),
  (latitude: 30.60498,longitude:104.06870),
  (latitude: 30.60084,longitude:104.06836),
  (latitude: 30.59833,longitude:104.06767),
  (latitude: 30.59471,longitude:104.06767),
  (latitude: 30.59124,longitude:104.06767),
  (latitude: 30.58784,longitude:104.06842),
  (latitude: 30.58548,longitude:104.06825),
  (latitude: 30.58267,longitude:104.06834),
  (latitude: 30.57993,longitude:104.06808),
  (latitude: 30.57732,longitude:104.06854),
  (latitude: 30.57488,longitude:104.06811),
  (latitude: 30.57274,longitude:104.06854),
  (latitude: 30.57067,longitude:104.06854),
  (latitude: 30.56905,longitude:104.06906),
  (latitude: 30.56675,longitude:104.06811),
  (latitude: 30.56577,longitude:104.06834),
  (latitude: 30.56474,longitude:104.06877),
  (latitude: 30.56333,longitude:104.06911),
  (latitude: 30.56333,longitude:104.06915),
  (latitude: 30.56333,longitude:104.06919),
  (latitude: 30.56333,longitude:104.07019),
  (latitude: 30.56333,longitude:104.08010),
]

let str = Polyline.encodePolyline(coordinates)
let v = Polyline.decodePolyline(str)

检查

利用Google的实用工具^[2]，对以上生成的数据进行检查

1. 编码多段线算法 ↩

2. Polyline decoder utility ↩