condorcet.py

#!/usr/bin/python
import numpy as np

# Schulze Condorcet voting method:
# http://en.wikipedia.org/wiki/Schulze_method
def schulze(votes):
    """Input:  An array of votes. In each row are the candidates' rankings. The
               index of each entry in the row corresponds to a single candidate.
               Only the relative rankings of candidates matter.
               Lower numbers are better."""
    
    voter_count, candidate_count = votes.shape
    
    preferences = np.zeros((candidate_count, candidate_count))
    for vote in votes:
        for i, candidate1 in enumerate(vote):
            for j, candidate2 in enumerate(vote):
                if i <= j:
                    continue
                if candidate1 > candidate2: # c2 preferred to c1
                    preferences[j, i] += 1
                elif candidate2 > candidate1:
                    preferences[i, j] += 1
    
    strengths = np.zeros((candidate_count, candidate_count))
    for (i, j), voter_count in np.ndenumerate(preferences):
        if i == j:
            continue
        strengths[i, j] = voter_count if voter_count > preferences[j, i] else 0
    
    for (i, j), voter_count in np.ndenumerate(preferences):
        if i == j:
            continue
        for k in range(candidate_count):
            if i != k and j != k:
                strengths[j, k] = max(strengths[j, k],
                                      min(strengths[j, i],
                                          strengths[i, k]))
    
    # Condorcet winners are those candidates who are preferred to all others.
    winners = []
    for i, row in enumerate(strengths):
        column = strengths[:,i]
        if (row >= column).all():
            winners.append(i)
    return winners

with open('votes.txt', 'r') as f:
    rankings = []
    candidates = 'ABCDEFGHIJ'
    for vote in f:
        ranking = []
        for candidate in candidates:
            ranking.append(vote.index(candidate))
        rankings.append(ranking)
    print(rankings)
print("Winner:", candidates[schulze(np.array(rankings))[0]])