Dart Geohash Implementation

GeoHash.dart

String chars = "0123456789bcdefghjkmnpqrstuvwxyz";
const BITS_PER_BASE32_CHAR = 5;

main() {
  LatLng location = LatLng(22.5678, 72.1234);
  String geohash = encode(location, 8);
  print("Original: " + location.toString());
  print("GeoHash: " + geohash);
  print("Decoded: " + decodeGeoCode(geohash).toString());
}

String encode(LatLng position, int precision) {
  String geocode = "";
  List<double> longitude_range = [-180, 180];
  List<double> latitude_range = [-90, 90];

  // go through each character for precision
  for (int i = 0; i < precision; i++) {
    int hash = 0;

    // generate a base32 char
    for (int j = 0; j < BITS_PER_BASE32_CHAR; j++) {
      bool isEvenBit = ((i * BITS_PER_BASE32_CHAR) + j) % 2 == 0;
      double val = isEvenBit ? position.longitude : position.latitude;
      List<double> range = isEvenBit ? longitude_range : latitude_range;
      double mid = (range[0] + range[1]) / 2;

      if (val > mid) {
        hash = (hash << 1) + 1;
        range[0] = mid;
      } else {
        hash = hash << 1;
        range[1] = mid;
      }
    }

    geocode += base32Char(hash);
  }

  return geocode;
}

LatLng decodeGeoCode(String geocode) {
  String binaryCode = "";

  // Convert geocode into string of binary
  for (int i = 0; i < geocode.length; i++) {
    // get decimal of character
    int decimal = chars.indexOf(geocode[i]);

    // convert decimal to binary
    String binary = dec2bin(decimal).padLeft(5, '0');
    binaryCode += binary;
  }

  // parse longitude for geocode
  double longitude = parseLongitudeFromBinary(binaryCode);

  // parse latitude for geocode
  double latitude = parseLatitudeFromBinary(binaryCode);

  // Construct LatLng object & return it
  return LatLng(latitude, longitude);
}

double parseLongitudeFromBinary(String binaryString) {
  double max = 180;
  double min = -180;
  double mid = 0;

  for (int i = 0; i < binaryString.length; i += 2) {
    max = binaryString[i] == "0" ? mid : max;
    min = binaryString[i] == "0" ? min : mid;
    mid = (max + min) / 2;
  }

  return mid;
}

double parseLatitudeFromBinary(String binaryString) {
  double max = 90;
  double min = -90;
  double mid = 0;

  for (int i = 1; i < binaryString.length; i += 2) {
    max = binaryString[i] == "0" ? mid : max;
    min = binaryString[i] == "0" ? min : mid;
    mid = (max + min) / 2;
  }

  return mid;
}

String base32Char(int value) {
  return chars.substring(value, value + 1);
}

String dec2bin(int dec) {
  var bin = '';
  int positions = 0;
  while (dec > 0) {
    bin = (dec % 2 == 0 ? '0' : '1') + bin;
    dec ~/= 2;
    positions++;
  }
  return bin;
}

class LatLng {
  double latitude, longitude;
  LatLng(this.latitude, this.longitude){
    if(this.latitude > 90 || this.latitude < -90){
      throw Exception("Latitude out of range");
    }else if(this.longitude > 180 || this.longitude < -180){
      throw Exception("Longitude out of range");
    }
  }

  String toString() {
    return "${this.latitude}, ${this.longitude}";
  }
}