在上一篇文章中提出用geohash解决匹配点的性能问题，该算法有个缺陷也就是如果在边界上的时候，该点附近的点可能就跨界了，也就是通过该点的geohash值无法找到对应的点。例如：

图中蓝色点为需要绑路映射的点
下方的红点为绑路计算之后的点
导致偏移的原因是
随州街上的点数据 在geohashcode分区时都属于蓝点上方的分区
在数据库中查询出来的结果显示 和蓝色点在一个分区的路网数据位于下图

所以找到的数据均位于下方的道路上 导致偏移问题
为了解决这个问题提出扩散hashcode的代码查找附近范围的geohashcode代码
代码如下
所以找到的数据均位于下方的道路上 导致偏移问题
为了解决这个问题提出扩散hashcode的代码查找附近范围的geohashcode代码
代码如下

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"""A simple GeoHash implementation."""`

# Forward and reverse base 32 map
BASESEQUENCE = '0123456789bcdefghjkmnpqrstuvwxyz'
BASE32MAP = dict((k, count) for count, k in enumerate(BASESEQUENCE))
BASE32MAPR = dict((count, k) for count, k in enumerate(BASESEQUENCE))

def _bits_to_float(bits, lower=-90.0, middle=0.0, upper=90.0):
    """Convert GeoHash bits to a float."""
    for i in bits:
        if i:
            lower = middle
        else:
            upper = middle
        middle = (upper + lower) / 2
    return middle

def _float_to_bits(value, lower=-90.0, middle=0.0, upper=90.0, length=15):
    """Convert a float to a list of GeoHash bits."""
    ret = []
    for i in range(length):
        if value >= middle:
            lower = middle
            ret.append(1)
        else:
            upper = middle
            ret.append(0)
        middle = (upper + lower) / 2
    return ret

def _geohash_to_bits(value):
    """Convert a GeoHash to a list of GeoHash bits."""
    b = map(BASE32MAP.get, value)
    ret = []
    for i in b:
        out = []
        for z in range(5):
            out.append(i & 0b1)
            i = i >> 1
        ret += out[::-1]
    return ret

def _bits_to_geohash(value):
    """Convert a list of GeoHash bits to a GeoHash."""
    ret = []
    # Get 5 bits at a time
    for i in (value[i:i + 5] for i in range(0, len(value), 5)):
        # Convert binary to integer
        # Note: reverse here, the slice above doesn't work quite right in reverse.
        total = sum([(bit * 2 ** count) for count, bit in enumerate(i[::-1])])
        ret.append(BASE32MAPR[total])
    # Join the string and return
    return "".join(ret)

# Public
def decode(value):
    """Decode a geohash. Returns a (lon,lat) pair."""
    assert value, "Invalid geohash: %s" % value
    # Get the GeoHash bits
    bits = _geohash_to_bits(value)
    # Unzip the GeoHash bits.
    lon = bits[0::2]
    lat = bits[1::2]
    # Convert to lat/lon
    return (
        _bits_to_float(lon, lower=-180.0, upper=180.0),
        _bits_to_float(lat)
    )

def encode(lonlat, length=12):
    """Encode a (lon,lat) pair to a GeoHash."""
    assert len(lonlat) == 2, "Invalid lon/lat: %s" % lonlat
    # Half the length for each component.
    length /= 2
    lon = _float_to_bits(lonlat[0], lower=-180.0, upper=180.0, length=length * 5)
    lat = _float_to_bits(lonlat[1], lower=-90.0, upper=90.0, length=length * 5)
    # Zip the GeoHash bits.
    ret = []
    for a, b in zip(lon, lat):
        ret.append(a)
        ret.append(b)
    return _bits_to_geohash(ret)

def adjacent(geohash, direction):
    """Return the adjacent geohash for a given direction."""
    # Based on an MIT licensed implementation by Chris Veness from:
    #   http://www.movable-type.co.uk/scripts/geohash.html
    assert direction in 'nsew', "Invalid direction: %s" % direction
    assert geohash, "Invalid geohash: %s" % geohash
    neighbor = {
        'n': ['p0r21436x8zb9dcf5h7kjnmqesgutwvy', 'bc01fg45238967deuvhjyznpkmstqrwx'],
        's': ['14365h7k9dcfesgujnmqp0r2twvyx8zb', '238967debc01fg45kmstqrwxuvhjyznp'],
        'e': ['bc01fg45238967deuvhjyznpkmstqrwx', 'p0r21436x8zb9dcf5h7kjnmqesgutwvy'],
        'w': ['238967debc01fg45kmstqrwxuvhjyznp', '14365h7k9dcfesgujnmqp0r2twvyx8zb']
    }
    border = {
        'n': ['prxz', 'bcfguvyz'],
        's': ['028b', '0145hjnp'],
        'e': ['bcfguvyz', 'prxz'],
        'w': ['0145hjnp', '028b']
    }
    last = geohash[-1]
    parent = geohash[0:-1]
    t = len(geohash) % 2
    # Check for edge cases
    if (last in border[direction][t]) and (parent):
        parent = adjacent(parent, direction)
    return parent + BASESEQUENCE[neighbor[direction][t].index(last)]

def neighbors(geohash):
    """Return all neighboring geohashes."""
    return {
        'n': adjacent(geohash, 'n'),
        'ne': adjacent(adjacent(geohash, 'n'), 'e'),
        'e': adjacent(geohash, 'e'),
        'se': adjacent(adjacent(geohash, 's'), 'e'),
        's': adjacent(geohash, 's'),
        'sw': adjacent(adjacent(geohash, 's'), 'w'),
        'w': adjacent(geohash, 'w'),
        'nw': adjacent(adjacent(geohash, 'n'), 'w'),
        'c': geohash
    }

def neighborsfit(centroid, points):
    centroid = encode(centroid)
    points = map(encode, points)
    for i in range(1, len(centroid)):
        g = centroid[0:i]
        n = set(neighbors(g).values())
        unbounded = [point for point in points if (point[0:i] not in n)]
        if unbounded:
            break
    return g[0:-1]

名称栏目：解决geohash编码跨区域问题
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