osrm-backend/include/engine/polyline_compressor.hpp

#ifndef POLYLINECOMPRESSOR_H_
#define POLYLINECOMPRESSOR_H_

#include "util/coordinate.hpp"

#include <boost/assert.hpp>
#include <algorithm>
#include <string>
#include <vector>

namespace osrm
{
namespace engine
{
namespace detail
 {
 constexpr double POLYLINE_DECODING_PRECISION = 1e5;
 constexpr double POLYLINE_TO_COORDINATE = COORDINATE_PRECISION / POLYLINE_DECODING_PRECISION;

 std::string encode(std::vector<int> &numbers);
 }
using CoordVectorForwardIter = std::vector<util::Coordinate>::const_iterator;
// Encodes geometry into polyline format.
// See: https://developers.google.com/maps/documentation/utilities/polylinealgorithm

template<unsigned POLYLINE_PRECISION=100000>
std::string encodePolyline(CoordVectorForwardIter begin, CoordVectorForwardIter end)
{
    double coordinate_to_polyline = POLYLINE_PRECISION / COORDINATE_PRECISION;
    auto size = std::distance(begin, end);
    if (size == 0)
    {
        return {};
    }

    std::vector<int> delta_numbers;
    BOOST_ASSERT(size > 0);
    delta_numbers.reserve((size - 1) * 2);
    int current_lat = 0;
    int current_lon = 0;
    std::for_each(
        begin, end, [&delta_numbers, &current_lat, &current_lon, coordinate_to_polyline](const util::Coordinate loc) {
            const int lat_diff =
                std::round(static_cast<int>(loc.lat) * coordinate_to_polyline) - current_lat;
            const int lon_diff =
                std::round(static_cast<int>(loc.lon) * coordinate_to_polyline) - current_lon;
            delta_numbers.emplace_back(lat_diff);
            delta_numbers.emplace_back(lon_diff);
            current_lat += lat_diff;
            current_lon += lon_diff;
        });
    return detail::encode(delta_numbers);
}

// Decodes geometry from polyline format
// See: https://developers.google.com/maps/documentation/utilities/polylinealgorithm
std::vector<util::Coordinate> decodePolyline(const std::string &polyline);
}
}

#endif /* POLYLINECOMPRESSOR_H_ */