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# -*- coding: utf-8 -*-
"""
A simple consistent hashing implementation.
Originally from http://pypi.python.org/pypi/hash_ring by Amir Salihefendic.
"""
__license__ = "BSD"
import md5
import math
from bisect import bisect
class HashRing(object):
def __init__(self, nodes=None, weights=None):
"""`nodes` is a list of objects that have a proper __str__ representation.
`weights` is dictionary that sets weights to the nodes. The default
weight is that all nodes are equal.
"""
self.ring = dict()
self._sorted_keys = []
self.nodes = nodes
if not weights:
weights = {}
self.weights = weights
self._generate_circle()
def _generate_circle(self):
"""Generates the circle.
"""
total_weight = 0
for node in self.nodes:
total_weight += self.weights.get(node, 1)
for node in self.nodes:
weight = 1
if node in self.weights:
weight = self.weights.get(node)
factor = math.floor((40*len(self.nodes)*weight) / total_weight);
for j in xrange(0, int(factor)):
b_key = self._hash_digest( '%s-%s' % (node, j) )
for i in xrange(0, 3):
key = self._hash_val(b_key, lambda x: x+i*4)
self.ring[key] = node
self._sorted_keys.append(key)
self._sorted_keys.sort()
def get_node(self, string_key):
"""Given a string key a corresponding node in the hash ring is returned.
If the hash ring is empty, `None` is returned.
"""
pos = self.get_node_pos(string_key)
if pos is None:
return None
return self.ring[ self._sorted_keys[pos] ]
def get_node_pos(self, string_key):
"""Given a string key a corresponding node in the hash ring is returned
along with it's position in the ring.
If the hash ring is empty, (`None`, `None`) is returned.
"""
if not self.ring:
return None
key = self.gen_key(string_key)
nodes = self._sorted_keys
pos = bisect(nodes, key)
if pos == len(nodes):
return 0
else:
return pos
def iterate_nodes(self, string_key, distinct=True):
"""Given a string key it returns the nodes as a generator that can hold the key.
The generator iterates one time through the ring
starting at the correct position.
if `distinct` is set, then the nodes returned will be unique,
i.e. no virtual copies will be returned.
"""
if not self.ring:
yield None, None
returned_values = set()
def distinct_filter(value):
if str(value) not in returned_values:
returned_values.add(str(value))
return value
pos = self.get_node_pos(string_key)
for key in self._sorted_keys[pos:]:
val = distinct_filter(self.ring[key])
if val:
yield val
for i, key in enumerate(self._sorted_keys):
if i < pos:
val = distinct_filter(self.ring[key])
if val:
yield val
def gen_key(self, key):
"""Given a string key it returns a long value,
this long value represents a place on the hash ring.
md5 is currently used because it mixes well.
"""
b_key = self._hash_digest(key)
return self._hash_val(b_key, lambda x: x)
def _hash_val(self, b_key, entry_fn):
return (( b_key[entry_fn(3)] << 24)
|(b_key[entry_fn(2)] << 16)
|(b_key[entry_fn(1)] << 8)
| b_key[entry_fn(0)] )
def _hash_digest(self, key):
m = md5.new()
m.update(key)
return map(ord, m.digest())