pyRCV2/tests/test_meekm.py

#   pyRCV2: Preferential vote counting
#   Copyright © 2020–2021  Lee Yingtong Li (RunasSudo)
#
#   This program is free software: you can redistribute it and/or modify
#   it under the terms of the GNU Affero General Public License as published by
#   the Free Software Foundation, either version 3 of the License, or
#   (at your option) any later version.
#
#   This program is distributed in the hope that it will be useful,
#   but WITHOUT ANY WARRANTY; without even the implied warranty of
#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#   GNU Affero General Public License for more details.
#
#   You should have received a copy of the GNU Affero General Public License
#   along with this program.  If not, see <https://www.gnu.org/licenses/>.

import pytest

import pyRCV2.blt
import pyRCV2.numbers
from pyRCV2.method.base_stv import STVException
from pyRCV2.method.meek import MeekSTVCounter
from pyRCV2.model import CandidateState, CountCompleted
from pyRCV2.ties import TiesBackwards

def test_meekm():
	"""Compare count of meekm.blt with model result produced by Hill et al. reference implementation"""
	
	pyRCV2.numbers.set_numclass(pyRCV2.numbers.FixedGuarded)
	pyRCV2.numbers.set_dps(4)
	
	with open('tests/data/meekm.blt', 'r') as f:
		election = pyRCV2.blt.readBLT(f.read())
	
	assert len(election.candidates) == 4
	
	cand_a = next(c for c in election.candidates if c.name == 'Adam')
	cand_b = next(c for c in election.candidates if c.name == 'Basil')
	cand_c = next(c for c in election.candidates if c.name == 'Charlotte')
	cand_d = next(c for c in election.candidates if c.name == 'Donald')
	
	assert election.withdrawn == [cand_b]
	
	counter = MeekSTVCounter(election, {
		'quota': 'droop_exact',
		'quota_criterion': 'gt',
		'quota_mode': 'progressive',
		'ties': []
	})
	
	# Stage 1
	result = counter.reset()
	assert result.comment == 'First preferences'
	assert result.total == pyRCV2.numbers.Num('13')
	assert result.quota == pyRCV2.numbers.Num('4')
	
	assert len(result.candidates.impl) == 4
	assert result.candidates[cand_a].state == CandidateState.ELECTED
	assert result.candidates[cand_b].state == CandidateState.WITHDRAWN
	assert result.candidates[cand_c].state == CandidateState.HOPEFUL
	assert result.candidates[cand_d].state == CandidateState.HOPEFUL
	
	assert result.candidates[cand_a].votes == pyRCV2.numbers.Num('7')
	assert result.candidates[cand_c].votes == pyRCV2.numbers.Num('1')
	assert result.candidates[cand_d].votes == pyRCV2.numbers.Num('4')
	assert result.exhausted.votes == pyRCV2.numbers.Num('1')
	
	# Stage 2
	result = counter.step()
	assert result.total == pyRCV2.numbers.Num('13')
	assert result.quota == pyRCV2.numbers.Num('4')
	
	assert result.candidates[cand_a].state == CandidateState.ELECTED
	assert result.candidates[cand_b].state == CandidateState.WITHDRAWN
	assert result.candidates[cand_c].state == CandidateState.HOPEFUL
	assert result.candidates[cand_d].state == CandidateState.HOPEFUL
	
	assert result.candidates[cand_a].votes == pyRCV2.numbers.Num('4')
	assert result.candidates[cand_c].votes == pyRCV2.numbers.Num('4')
	assert result.candidates[cand_d].votes == pyRCV2.numbers.Num('4')
	assert result.exhausted.votes == pyRCV2.numbers.Num('1')
	
	# Stage 3
	# FIXME: This fails in Javascript (rounding errors?)
	with pytest.raises(STVException) as exc_info:
		result = counter.step()
	assert exc_info.value.message == 'Unable to resolve tie'
	
	# Retry further stages
	counter.options['ties'] = [TiesBackwards(counter)]
	result = counter.step()
	while not isinstance(result, CountCompleted):
		result = counter.step()
	
	assert result.candidates[cand_a].state == CandidateState.ELECTED
	assert result.candidates[cand_b].state == CandidateState.WITHDRAWN
	assert result.candidates[cand_c].state == CandidateState.EXCLUDED # Charlotte loses the tie if broken backwards
	assert result.candidates[cand_d].state == CandidateState.ELECTED