GeoUtils.cpp

#include <Core/Types.h>
#include <Functions/GeoUtils.h>

namespace
{

using namespace DB;

const char geohash_base32_encode_lookup_table[32] = {
    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
    'b', 'c', 'd', 'e', 'f', 'g', 'h', 'j', 'k', 'm',
    'n', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x',
    'y', 'z',
};

// TODO: this could be halved by excluding 128-255 range.
const UInt8 geohash_base32_decode_lookup_table[256] = {
    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
    0,   1,     2,    3,    4,    5,    6,    7,    8,    9,    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
    0xFF, 0xFF, 10,   11,   12,   13,   14,   15,   16,   0xFF, 17,   18,   0xFF, 19,   20,   0xFF,
    21,   22,   23,   24,   25,   26,   27,   28,   29,   30,   31,   0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
};

const size_t BITS_PER_SYMBOL = 5;
const size_t MAX_PRECISION = 12;
const size_t MAX_BITS = MAX_PRECISION * BITS_PER_SYMBOL * 1.5;

using Encoded = std::array<UInt8, MAX_BITS>;

enum CoordType
{
    LATITUDE,
    LONGITUDE,
};

inline UInt8 singleCoordBitsPrecision(UInt8 precision, CoordType type)
{
    // Single coordinate occupies only half of the total bits.
    const UInt8 bits = (precision * BITS_PER_SYMBOL) / 2;
    if (precision & 0x1 && type == LONGITUDE)
    {
        return bits + 1;
    }

    return bits;
}

inline Encoded encodeCoordinate(Float64 coord, Float64 min, Float64 max, UInt8 bits)
{
    Encoded result;
    result.fill(0);

    for (int i = 0; i < bits; ++i)
    {
        Float64 mid = (max + min) / 2;
        if (coord >= mid)
        {
            result[i] = 1;
            min = mid;
        }
        else
        {
            result[i] = 0;
            max = mid;
        }
    }

    return result;
}

inline Float64 decodeCoordinate(const Encoded & coord, Float64 min, Float64 max, UInt8 bits)
{
    Float64 mid = (max + min) / 2;
    for (int i = 0; i < bits; ++i)
    {
        const auto c = coord[i];
        if (c == 1)
        {
            min = mid;
        }
        else
        {
            max = mid;
        }

        mid = (max + min) / 2;
    }

    return mid;
}

inline Encoded merge(const Encoded & encodedLon, const Encoded & encodedLat, UInt8 precision)
{
    Encoded result;
    result.fill(0);

    const auto bits = (precision * BITS_PER_SYMBOL) / 2;
    UInt8 i = 0;
    for (; i < bits; ++i)
    {
        result[i * 2 + 0] = encodedLon[i];
        result[i * 2 + 1] = encodedLat[i];
    }
    // in case of even precision, add last bit of longitude
    if (precision & 0x1)
    {
        result[i * 2] = encodedLon[i];
    }

    return result;
}

inline std::tuple<Encoded, Encoded> split(const Encoded & combined, UInt8 precision)
{
    Encoded lat, lon;
    lat.fill(0);
    lon.fill(0);

    UInt8 i = 0;
    for (; i < precision * BITS_PER_SYMBOL - 1; i += 2)
    {
        // longitude is even bits
        lon[i/2] = combined[i];
        lat[i/2] = combined[i + 1];
    }
    // precision is even, read the last bit as lat.
    if (precision & 0x1)
    {
        lon[i/2] = combined[precision * BITS_PER_SYMBOL - 1];
    }

    return std::tie(lon, lat);
}

inline void base32Encode(const Encoded & binary, UInt8 precision, char * out)
{
    extern const char geohash_base32_encode_lookup_table[32];

    for (UInt8 i = 0; i < precision * BITS_PER_SYMBOL; i += 5)
    {
        UInt8 v = binary[i];
        v <<= 1;
        v |= binary[i + 1];
        v <<= 1;
        v |= binary[i + 2];
        v <<= 1;
        v |= binary[i + 3];
        v <<= 1;
        v |= binary[i + 4];

        assert(v < 32);

        *out = geohash_base32_encode_lookup_table[v];
        ++out;
    }
}

inline Encoded base32Decode(const char * encoded_string, size_t encoded_length)
{
    extern const UInt8 geohash_base32_decode_lookup_table[256];

    Encoded result;

    for (size_t i = 0; i < encoded_length; ++i)
    {
        const UInt8 c = static_cast<UInt8>(encoded_string[i]);
        const UInt8 decoded = geohash_base32_decode_lookup_table[c] & 0x1F;
        result[i * 5 + 4] = (decoded >> 0) & 0x01;
        result[i * 5 + 3] = (decoded >> 1) & 0x01;
        result[i * 5 + 2] = (decoded >> 2) & 0x01;
        result[i * 5 + 1] = (decoded >> 3) & 0x01;
        result[i * 5 + 0] = (decoded >> 4) & 0x01;
    }

    return result;
}

} // namespace

namespace DB
{

namespace GeoUtils
{

size_t geohashEncode(Float64 longitude, Float64 latitude, UInt8 precision, char *& out)
{
    if (precision == 0 || precision > MAX_PRECISION)
    {
        precision = MAX_PRECISION;
    }

    const Encoded combined = merge(
                encodeCoordinate(longitude, -180, 180, singleCoordBitsPrecision(precision, LONGITUDE)),
                encodeCoordinate(latitude, -90, 90, singleCoordBitsPrecision(precision, LATITUDE)),
                precision);

    base32Encode(combined, precision, out);

    return precision;
}

void geohashDecode(const char * encoded_string, size_t encoded_len, Float64 * longitude, Float64 * latitude)
{
    const UInt8 precision = std::min(encoded_len, MAX_PRECISION);
    if (precision == 0)
    {
        return;
    }

    Encoded lat_encoded, lon_encoded;
    std::tie(lon_encoded, lat_encoded) = split(base32Decode(encoded_string, precision), precision);

    *longitude = decodeCoordinate(lon_encoded, -180, 180, singleCoordBitsPrecision(precision, LONGITUDE));
    *latitude = decodeCoordinate(lat_encoded, -90, 90, singleCoordBitsPrecision(precision, LATITUDE));
}

} // namespace GeoUtils

} // namespace DB