37 lines
1.2 KiB
Python
37 lines
1.2 KiB
Python
# pyRCV2: Preferential vote counting
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# Copyright © 2020–2021 Lee Yingtong Li (RunasSudo)
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#
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# This program is free software: you can redistribute it and/or modify
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# it under the terms of the GNU Affero General Public License as published by
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# the Free Software Foundation, either version 3 of the License, or
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# (at your option) any later version.
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#
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# This program is distributed in the hope that it will be useful,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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# GNU Affero General Public License for more details.
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#
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# You should have received a copy of the GNU Affero General Public License
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# along with this program. If not, see <https://www.gnu.org/licenses/>.
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import hashlib
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class SHARandom:
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MAX_VAL = 2 ** 256
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def __init__(self, seed):
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self.seed = seed
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self.ctr = 0
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def next(self, modulus):
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c = hashlib.sha256()
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c.update((self.seed + ',' + str(self.ctr)).encode('utf-8'))
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self.ctr += 1
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val = int.from_bytes(c.digest(), byteorder='big', signed=False)
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if val >= (SHARandom.MAX_VAL // modulus) * modulus:
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# Discard this value to avoid bias
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return self.next(modulus)
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return val % modulus
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