GeoHash.java

package com.knowledgebooks.rdf.implementation;

/**
 * From: http://code.google.com/p/geospatialweb/
 * License: LGPL
 */

import java.util.BitSet;
import java.util.HashMap;

public class GeoHash {

  private static int numbits = 6 * 5;
  final static char[] digits = {'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'};

  final static HashMap<Character, Integer> lookup = new HashMap<Character, Integer>();

  static {
    int i = 0;
    for (char c : digits)
      lookup.put(c, i++);
  }

  public static void main(String[] args) {
    double[] latlon = new GeoHash().decode("dj248j248j24");
    System.out.println(latlon[0] + " " + latlon[1]);

    GeoHash e = new GeoHash();
    String s = e.encode(30, -90.0);
    System.out.println(s);
    latlon = e.decode(s);
    System.out.println(latlon[0] + ", " + latlon[1]);
  }

  public static double[] decode(String geohash) {
    StringBuilder buffer = new StringBuilder();
    for (char c : geohash.toCharArray()) {

      int i = lookup.get(c) + 32;
      buffer.append(Integer.toString(i, 2).substring(1));
    }

    BitSet lonset = new BitSet();
    BitSet latset = new BitSet();

    //even bits
    int j = 0;
    for (int i = 0; i < numbits * 2; i += 2) {
      boolean isSet = false;
      if (i < buffer.length())
        isSet = buffer.charAt(i) == '1';
      lonset.set(j++, isSet);
    }

    //odd bits
    j = 0;
    for (int i = 1; i < numbits * 2; i += 2) {
      boolean isSet = false;
      if (i < buffer.length())
        isSet = buffer.charAt(i) == '1';
      latset.set(j++, isSet);
    }

    double lon = decode(lonset, -180, 180);
    double lat = decode(latset, -90, 90);

    return new double[]{lat, lon};
  }

  private static double decode(BitSet bs, double floor, double ceiling) {
    double mid = 0;
    for (int i = 0; i < bs.length(); i++) {
      mid = (floor + ceiling) / 2;
      if (bs.get(i))
        floor = mid;
      else
        ceiling = mid;
    }
    return mid;
  }


  public static String encode(double lat, double lon) {
    BitSet latbits = getBits(lat, -90, 90);
    BitSet lonbits = getBits(lon, -180, 180);
    StringBuilder buffer = new StringBuilder();
    for (int i = 0; i < numbits; i++) {
      buffer.append((lonbits.get(i)) ? '1' : '0');
      buffer.append((latbits.get(i)) ? '1' : '0');
    }
    return base32(Long.parseLong(buffer.toString(), 2));
  }

  private static BitSet getBits(double lat, double floor, double ceiling) {
    BitSet buffer = new BitSet(numbits);
    for (int i = 0; i < numbits; i++) {
      double mid = (floor + ceiling) / 2;
      if (lat >= mid) {
        buffer.set(i);
        floor = mid;
      } else {
        ceiling = mid;
      }
    }
    return buffer;
  }

  public static String base32(long i) {
    char[] buf = new char[65];
    int charPos = 64;
    boolean negative = (i < 0);
    if (!negative)
      i = -i;
    while (i <= -32) {
      buf[charPos--] = digits[(int) (-(i % 32))];
      i /= 32;
    }
    buf[charPos] = digits[(int) (-i)];

    if (negative)
      buf[--charPos] = '-';
    return new String(buf, charPos, (65 - charPos));
  }
}