/// <summary>
/// Converts a set of candidates and ballots to a graph of wins and ties.
/// </summary>
/// <param name="ballots">Ranked ballots in the election.</param>
public PairwiseGraph(BallotSet ballots)
{
// Initialize candidate graph
HashSet <Candidate> candidates = (ballots as IEnumerable <CountedBallot>).SelectMany(x => x.Votes.Select(y => y.Candidate)).ToHashSet();
foreach (Candidate c in candidates)
{
nodes[c] = new Dictionary <Candidate, decimal>();
foreach (Candidate d in candidates.Except(new[] { c }))
{
nodes[c][d] = 0.0m;
}
}
void BuildGraph()
{
List <CountedBallot> bList = ballots.ToList();
int threadCount = Environment.ProcessorCount;
PairwiseGraph[] subsets = new PairwiseGraph[threadCount];
PairwiseGraph CountSubsets(int start, int end)
{
PairwiseGraph g = new PairwiseGraph();
foreach (Candidate c in candidates)
{
g.nodes[c] = new Dictionary <Candidate, decimal>();
foreach (Candidate d in candidates.Except(new[] { c }))
{
g.nodes[c][d] = 0.0m;
}
}
// Iterate each ballot and count who wins and who ties.
// This can support tied ranks and each ballot is O(SUM(1..n)) and o(n).
//foreach (CountedBallot b in ballots)
for (int i = start; i <= end; i++)
{
CountedBallot b = bList[i];
HashSet <Candidate> ranked = b.Votes.Select(x => x.Candidate).ToHashSet();
HashSet <Candidate> unranked = candidates.Except(ranked).ToHashSet();
// Iterate to compare each pair.
Stack <Vote> votes = new Stack <Vote>(b.Votes);
while (votes.Count > 0)
{
Vote v = votes.Pop();
foreach (Vote u in votes)
{
// Who is ranked first? No action if a tie.
if (v.Beats(u))
{
g.nodes[v.Candidate][u.Candidate] += b.Count;
}
else if (u.Beats(v))
{
g.nodes[u.Candidate][v.Candidate] += b.Count;
}
}
// Defeat all unranked candidates
foreach (Candidate c in unranked)
{
g.nodes[v.Candidate][c] += b.Count;
}
}
}
return(g);
}
// First divide all the processes up for background run
Parallel.For(0, threadCount, (i, state) =>
subsets[i] = CountSubsets(bList.Count() * i / threadCount, (bList.Count() * (i + 1) / threadCount) - 1));
// Add them all together
foreach (PairwiseGraph g in subsets)
{
AddGraph(g);
}
}
BuildGraph();
}