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GeoHash 算法

2021-08-30  微信公众号  听枫逐日
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对于很多初学者来说，“附近的人”或者类似功能，在技术实现上还有点摸不着头脑。本文将简要的为你讲解“附近的人”的基本理论原理，并以redis的GEO系列地理位置操作指令为例，理论联系实际地为你讲解它们是如何被高效实现的。

 

阅读提示：本文适合有一定Redis使用经验和经纬度知识的服务器后端开发人员阅读。

 

经纬度常识

 

 

GeoHash 算法

 


GeoHash 算法

 

 

什么是geohash

 

 

GeoHash 算法

 

 

GeoHash 算法

 

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GeoHash 算法

 

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GeoHash 算法

 

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GeoHash 算法

 

 

以经纬度值：（116.389550， 39.928167）进行算法说明，对纬度39.928167进行逼近编码 (地球纬度区间是[-90,90]）

 

  1. 区间[-90,90]进行二分为[-90,0),[0,90]，称为左右区间，可以确定39.928167属于右区间[0,90]，给标记为1
  2. 接着将区间[0,90]进行二分为 [0,45),[45,90]，可以确定39.928167属于左区间 [0,45)，给标记为0
  3. 递归上述过程39.928167总是属于某个区间[a,b]。随着每次迭代区间[a,b]总在缩小，并越来越逼近39.928167
  4. 如果给定的纬度x（39.928167）属于左区间，则记录0，如果属于右区间则记录1，序列的长度跟给定的区间划分次数有关，如下图

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GeoHash 算法

 

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GeoHash 算法

 

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GeoHash 算法

 

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GeoHash 算法

 

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GeoHash 原理

 

 

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GeoHash 算法

 

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GeoHash 算法

 

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对照

 

GeoHash 算法

 

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GeoHash 算法

 


GeoHash 算法

 


GeoHash 算法

 

附近的人，附近的加油站如何实现

 

它需要做以下两件事情：

 

1）在使用“附近的人”功能前提交自己的地理位置；

2）根据“我”的地理位置，计算出别人跟我的距离；

3）将第2步中计算出的距离由近及远，进行排序。

 

具体在产品技术上的实现原理和技术难点

 

1）现在移动端（IOS、Android等），通过系统的API很容易抓到用户当前的位置（即经纬度数据）；

2）根据第1步中的经纬度数据，很容易计算出两个点之间的距离

3）对第2步中的计算结果排序就更简单了。

 

技术难点

 

1）如何高效地进行两点距离的计算

2）如何高效地进行地理围栏的圈定

 public class GeoHash {
public static final double MINLAT = -90;
public static final double MAXLAT = 90;
public static final double MINLNG = -180;
public static final double MAXLNG = 180;

private static int numbits = 3 * 5; //经纬度单独编码长度

private static double minLat;
private static double minLng;

private 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' };

//定义编码映射关系
final static HashMap<Character, Integer> lookup = new HashMap<Character, Integer>();
//初始化编码映射内容
static {
    int i = 0;
    for (char c : digits)
        lookup.put(c, i++);
}

public GeoHash(){
    setMinLatLng();
}

public 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');
    }
    String code = base32(Long.parseLong(buffer.toString(), 2));
    //Log.i("okunu", "encode  lat = " + lat + "  lng = " + lon + "  code = " + code);
    return code;
}

public ArrayList<String> getArroundGeoHash(double lat, double lon){
    //Log.i("okunu", "getArroundGeoHash  lat = " + lat + "  lng = " + lon);
    ArrayList<String> list = new ArrayList<>();
    double uplat = lat + minLat;
    double downLat = lat - minLat;

    double leftlng = lon - minLng;
    double rightLng = lon + minLng;

    String leftUp = encode(uplat, leftlng);
    list.add(leftUp);

    String leftMid = encode(lat, leftlng);
    list.add(leftMid);

    String leftDown = encode(downLat, leftlng);
    list.add(leftDown);

    String midUp = encode(uplat, lon);
    list.add(midUp);

    String midMid = encode(lat, lon);
    list.add(midMid);

    String midDown = encode(downLat, lon);
    list.add(midDown);

    String rightUp = encode(uplat, rightLng);
    list.add(rightUp);

    String rightMid = encode(lat, rightLng);
    list.add(rightMid);

    String rightDown = encode(downLat, rightLng);
    list.add(rightDown);

    //Log.i("okunu", "getArroundGeoHash list = " + list.toString());
    return list;
}

//根据经纬度和范围，获取对应的二进制
private 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;
}

//将经纬度合并后的二进制进行指定的32位编码
private 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));
}

private void setMinLatLng() {
    minLat = MAXLAT - MINLAT;
    for (int i = 0; i < numbits; i++) {
        minLat /= 2.0;
    }
    minLng = MAXLNG - MINLNG;
    for (int i = 0; i < numbits; i++) {
        minLng /= 2.0;
    }
}

//根据二进制和范围解码
private 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;
}

//对编码后的字符串解码
public 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();

    //偶数位，经度
    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);
    }

    //奇数位，纬度
    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 s = geohash.encode(40.222012, 116.248283);
//        System.out.println(s);
    geohash.getArroundGeoHash(40.222012, 116.248283);
//        double[] geo = geohash.decode(s);
//        System.out.println(geo[0]+" "+geo[1]);
}
}
xxxxxxxxxxbr  public class GeoHash {brpublic static final double MINLAT = -90;brpublic static final double MAXLAT = 90;brpublic static final double MINLNG = -180;brpublic static final double MAXLNG = 180;brbrprivate static int numbits = 3 * 5; //经纬度单独编码长度brbrprivate static double minLat;brprivate static double minLng;brbrprivate final static char[] digits = { '0', '1', '2', '3', '4', '5', '6', '7', '8',br        '9', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'j', 'k', 'm', 'n', 'p',br        'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z' };brbr//定义编码映射关系brfinal static HashMap<Character, Integer> lookup = new HashMap<Character, Integer>();br//初始化编码映射内容brstatic {br    int i = 0;br    for (char c : digits)br        lookup.put(c, i++);br}brbrpublic GeoHash(){br    setMinLatLng();br}brbrpublic String encode(double lat, double lon) {br    BitSet latbits = getBits(lat, -90, 90);br    BitSet lonbits = getBits(lon, -180, 180);br    StringBuilder buffer = new StringBuilder();br    for (int i = 0; i < numbits; i++) {br        buffer.append( (lonbits.get(i))?'1':'0');br        buffer.append( (latbits.get(i))?'1':'0');br    }br    String code = base32(Long.parseLong(buffer.toString(), 2));br    //Log.i("okunu", "encode  lat = " + lat + "  lng = " + lon + "  code = " + code);br    return code;br}brbrpublic ArrayList<String> getArroundGeoHash(double lat, double lon){br    //Log.i("okunu", "getArroundGeoHash  lat = " + lat + "  lng = " + lon);br    ArrayList<String> list = new ArrayList<>();br    double uplat = lat + minLat;br    double downLat = lat - minLat;brbr    double leftlng = lon - minLng;br    double rightLng = lon + minLng;brbr    String leftUp = encode(uplat, leftlng);br    list.add(leftUp);brbr    String leftMid = encode(lat, leftlng);br    list.add(leftMid);brbr    String leftDown = encode(downLat, leftlng);br    list.add(leftDown);brbr    String midUp = encode(uplat, lon);br    list.add(midUp);brbr    String midMid = encode(lat, lon);br    list.add(midMid);brbr    String midDown = encode(downLat, lon);br    list.add(midDown);brbr    String rightUp = encode(uplat, rightLng);br    list.add(rightUp);brbr    String rightMid = encode(lat, rightLng);br    list.add(rightMid);brbr    String rightDown = encode(downLat, rightLng);br    list.add(rightDown);brbr    //Log.i("okunu", "getArroundGeoHash list = " + list.toString());br    return list;br}brbr//根据经纬度和范围，获取对应的二进制brprivate BitSet getBits(double lat, double floor, double ceiling) {br    BitSet buffer = new BitSet(numbits);br    for (int i = 0; i < numbits; i++) {br        double mid = (floor + ceiling) / 2;br        if (lat >= mid) {br            buffer.set(i);br            floor = mid;br        } else {br            ceiling = mid;br        }br    }br    return buffer;br}brbr//将经纬度合并后的二进制进行指定的32位编码brprivate String base32(long i) {br    char[] buf = new char[65];br    int charPos = 64;br    boolean negative = (i < 0);br    if (!negative){br        i = -i;br    }br    while (i <= -32) {br        buf[charPos--] = digits[(int) (-(i % 32))];br        i /= 32;br    }br    buf[charPos] = digits[(int) (-i)];br    if (negative){br        buf[--charPos] = '-';br    }br    return new String(buf, charPos, (65 - charPos));br}brbrprivate void setMinLatLng() {br    minLat = MAXLAT - MINLAT;br    for (int i = 0; i < numbits; i++) {br        minLat /= 2.0;br    }br    minLng = MAXLNG - MINLNG;br    for (int i = 0; i < numbits; i++) {br        minLng /= 2.0;br    }br}brbr//根据二进制和范围解码brprivate double decode(BitSet bs, double floor, double ceiling) {br    double mid = 0;br    for (int i=0; i<bs.length(); i++) {br        mid = (floor + ceiling) / 2;br        if (bs.get(i))br            floor = mid;br        elsebr            ceiling = mid;br    }br    return mid;br}brbr//对编码后的字符串解码brpublic double[] decode(String geohash) {br    StringBuilder buffer = new StringBuilder();br    for (char c : geohash.toCharArray()) {br        int i = lookup.get(c) + 32;br        buffer.append( Integer.toString(i, 2).substring(1) );br    }brbr    BitSet lonset = new BitSet();br    BitSet latset = new BitSet();brbr    //偶数位，经度br    int j =0;br    for (int i=0; i< numbits*2;i+=2) {br        boolean isSet = false;br        if ( i < buffer.length() )br            isSet = buffer.charAt(i) == '1';br        lonset.set(j++, isSet);br    }brbr    //奇数位，纬度br    j=0;br    for (int i=1; i< numbits*2;i+=2) {br        boolean isSet = false;br        if ( i < buffer.length() )br            isSet = buffer.charAt(i) == '1';br        latset.set(j++, isSet);br    }brbr    double lon = decode(lonset, -180, 180);br    double lat = decode(latset, -90, 90);brbr    return new double[] {lat, lon};br}brbrpublic static void main(String[] args)  throws Exception{br    GeoHash geohash = new GeoHash();br//        String s = geohash.encode(40.222012, 116.248283);br//        System.out.println(s);br    geohash.getArroundGeoHash(40.222012, 116.248283);br//        double[] geo = geohash.decode(s);br//        System.out.println(geo[0]+" "+geo[1]);br}b核心算法获取任意两点距离

核心算法获取任意两点距离

/**

 * 计算地球上任意两点(经纬度)距离    

 *     

 * @param long1 第一点经度    

 * @param lat1 第一点纬度    

 * @param long2 第二点经度    

 * @param lat2 第二点纬度    

 * @return 返回距离 单位：米

 */

public static double Distance(double long1, double lat1, double long2, double lat2)

{

    double a, b, R;

    R = 6378137; // 地球半径        

    lat1 = lat1 * Math.PI / 180.0;

    lat2 = lat2 * Math.PI / 180.0;

    a = lat1 - lat2;

    b = (long1 - long2) * Math.PI / 180.0;

    double d;

    double sa2, sb2;

    sa2 = Math.sin(a / 2.0);

    sb2 = Math.sin(b / 2.0);

    d = 2* R * Math.asin(Math.sqrt(sa2 * sa2 + Math.cos(lat1) * Math.cos(lat2) * sb2 * sb2));

    return d;

}
xxxxxxxxxxbr /**brbr * 计算地球上任意两点(经纬度)距离    brbr *     brbr * @param long1 第一点经度    brbr * @param lat1 第一点纬度    brbr * @param long2 第二点经度    brbr * @param lat2 第二点纬度    brbr * @return 返回距离 单位：米brbr */brbrpublic static double Distance(double long1, double lat1, double long2, double lat2)brbr{brbr    double a, b, R;brbr    R = 6378137; // 地球半径        brbr    lat1 = lat1 * Math.PI / 180.0;brbr    lat2 = lat2 * Math.PI / 180.0;brbr    a = lat1 - lat2;brbr    b = (long1 - long2) * Math.PI / 180.0;brbr    double d;brbr    double sa2, sb2;brbr    sa2 = Math.sin(a / 2.0);brbr    sb2 = Math.sin(b / 2.0);brbr    d = 2* R * Math.asin(Math.sqrt(sa2 * sa2 + Math.cos(lat1) * Math.cos(lat2) * sb2 * sb2));brbr    return d;brbr}

 

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