GeoHash解析及java实现

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

/**
 * 地理知识补充: Latitude(纬度)[-90, 90],Longitude(经度)[-180, 180]
 * 
 * @author FengKang 2014-10-03
 *
 */
public class Geohash {
    private static int numbits = 6 * 5; // 最大划分次数，也是生成二进制位的最大长度

    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‘ };

    // decode时进行解码之用
    final static HashMap<Character, Integer> lookup = new HashMap<>();
    static {
        int i = 0;
        for (char ch : digits) {
            lookup.put(ch, i++);
        }
    }

    /**
     * 在[floor, ceil]区间内对lat进行编码 区间的划分次数为numbits次
     * 
     * @param lat
     *            -待编码经/纬度
     * @param floor
     * @param ceil
     * @return 二进制编码
     */
    private BitSet getBits(double lat, double floor, double ceil) {
        BitSet buffer = new BitSet(numbits);
        for (int i = 0; i < numbits; ++i) {
            double mid = (floor + ceil) / 2;
            if (lat >= mid) {
                buffer.set(i);
                floor = mid;
            } else {
                ceil = mid;
            }
        }

        return buffer;
    }

    /**
     * 将long型i的二进制位中，每5位映射为一个digits[]中的编码 
     * 这里的实现方法是参照Long.java中toString的实现
     * 
     * @param i
     * @return
     */
    private static String base32(long i) {
        char[] buf = new char[65]; // buf[0]备用，若为负数则存储‘-‘
        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));
    }

    public String encode(double lat, double lon) {
        BitSet latbits = getBits(lat, -90, 90);
        BitSet lonbits = getBits(lon, -180, 180);

        /* ! 先经度后纬度交错组合编码 */
        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‘);
        }

        System.out.println(buffer.toString());
        // 将2进制字符串转换为10进制long型
        return base32(Long.parseLong(buffer.toString(), 2));
    }

    /**
     * 根据bs对区间[floor, ceil]进行划分 
     * 0表示位于区间[floor, (floor + ceil)/2]
     * 1表示位于区间[(floor + ceil)/2, ceil]
     * 
     * @param bs
     *            二进制序列
     * @param floor
     *            初始区间边界[floor, ceil]
     * @param ceil
     * @return 区间边界，最接近bs序列的区间边界
     */
    private double decode(BitSet bs, double floor, double ceil) {
        double mid = 0;
        for (int i = 0; i < bs.length(); ++i) {
            mid = (floor + ceil) / 2;
            if (bs.get(i)) {
                floor = mid; // 右区间
            } else {
                ceil = mid; // 左区间
            }
        }

        return mid;
    }

    public double[] decode(String geohash) {
        StringBuilder buffer = new StringBuilder();
        for (char ch : geohash.toCharArray()) {
            int i = lookup.get(ch) + 32; // 保证转换后的二进制字符串为5位
            // +32转换为1#####，然后从下标为1处进行截取，即#####
            buffer.append(Integer.toString(i, 2).substring(1));
        }
        // System.out.println(buffer);

        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 };
    }

    public static void main(String[] args) throws Exception {
        Geohash geoHash = new Geohash();
        String encodes = geoHash.encode(45, 125);
        System.out.println(encodes);

        double[] decodes = geoHash.decode(encodes);
        System.out.println(decodes[0]);
        System.out.println(decodes[1]);

        /**
         * 输出结果 111100101000000010101000000010101000000010101000000010101000
         * yb0bh2n0p058 45.0 124.9999999254942
         */
    }
}