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1 #!/usr/bin/env python2
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2 #
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3 # Copyright 2011-2016 Google Inc.
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4 #
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5 # Licensed under the Apache License, Version 2.0 (the "License");
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6 # you may not use this file except in compliance with the License.
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7 # You may obtain a copy of the License at
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8 #
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9 # http://www.apache.org/licenses/LICENSE-2.0
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10 #
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11 # Unless required by applicable law or agreed to in writing, software
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12 # distributed under the License is distributed on an "AS IS" BASIS,
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13 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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14 # See the License for the specific language governing permissions and
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15 # limitations under the License.
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16 #
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17 # vim: sw=2 sts=2 et
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18
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19 class CachingFuncSequence(object):
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20 def __init__(self, func, length, inc_func=None):
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21 """
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22 length: Length of this sequence.
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23 func: function(index)
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24 inc_func: function(index, value_of_previous)
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25 """
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26
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27 self.func = func
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28 self.inc_func = inc_func
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29 self.length = length
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30 self._cache = {}
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31
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32 def __getitem__(self, i):
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33 if i < 0:
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34 i += self.length
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35 if i < 0 or i >= self.length:
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36 raise IndexError()
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37
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38 v = self._cache.get(i)
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39 if v is not None:
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40 return v
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41
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42 if self.inc_func and i-1 in self._cache:
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43 v = self.inc_func(i, self._cache[i-1])
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44 else:
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45 v = self.func(i)
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46
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47 self._cache[i] = v
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48 return v
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49
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50 def __len__(self):
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51 return self.length
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52
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53 def __iter__(self):
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54 for i in range(self.length):
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55 yield self[i]
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