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#
# Licensed to the Apache Software Foundation (ASF) under one or more
# contributor license agreements. See the NOTICE file distributed with
# this work for additional information regarding copyright ownership.
# The ASF licenses this file to You under the Apache License, Version 2.0
# (the "License"); you may not use this file except in compliance with
# the License. You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#
import sys
import random
import math
class RDDSamplerBase(object):
def __init__(self, withReplacement, seed=None):
self._seed = seed if seed is not None else random.randint(0, sys.maxint)
self._withReplacement = withReplacement
self._random = None
def initRandomGenerator(self, split):
self._random = random.Random(self._seed ^ split)
# mixing because the initial seeds are close to each other
for _ in xrange(10):
self._random.randint(0, 1)
def getUniformSample(self):
return self._random.random()
def getPoissonSample(self, mean):
# Using Knuth's algorithm described in
# http://en.wikipedia.org/wiki/Poisson_distribution
if mean < 20.0:
# one exp and k+1 random calls
l = math.exp(-mean)
p = self._random.random()
k = 0
while p > l:
k += 1
p *= self._random.random()
else:
# switch to the log domain, k+1 expovariate (random + log) calls
p = self._random.expovariate(mean)
k = 0
while p < 1.0:
k += 1
p += self._random.expovariate(mean)
return k
def func(self, split, iterator):
raise NotImplementedError
class RDDSampler(RDDSamplerBase):
def __init__(self, withReplacement, fraction, seed=None):
RDDSamplerBase.__init__(self, withReplacement, seed)
self._fraction = fraction
def func(self, split, iterator):
self.initRandomGenerator(split)
if self._withReplacement:
for obj in iterator:
# For large datasets, the expected number of occurrences of each element in
# a sample with replacement is Poisson(frac). We use that to get a count for
# each element.
count = self.getPoissonSample(self._fraction)
for _ in range(0, count):
yield obj
else:
for obj in iterator:
if self.getUniformSample() < self._fraction:
yield obj
class RDDRangeSampler(RDDSamplerBase):
def __init__(self, lowerBound, upperBound, seed=None):
RDDSamplerBase.__init__(self, False, seed)
self._lowerBound = lowerBound
self._upperBound = upperBound
def func(self, split, iterator):
self.initRandomGenerator(split)
for obj in iterator:
if self._lowerBound <= self.getUniformSample() < self._upperBound:
yield obj
class RDDStratifiedSampler(RDDSamplerBase):
def __init__(self, withReplacement, fractions, seed=None):
RDDSamplerBase.__init__(self, withReplacement, seed)
self._fractions = fractions
def func(self, split, iterator):
self.initRandomGenerator(split)
if self._withReplacement:
for key, val in iterator:
# For large datasets, the expected number of occurrences of each element in
# a sample with replacement is Poisson(frac). We use that to get a count for
# each element.
count = self.getPoissonSample(self._fractions[key])
for _ in range(0, count):
yield key, val
else:
for key, val in iterator:
if self.getUniformSample() < self._fractions[key]:
yield key, val
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