int count = trees.Count;
var sim = new TriangularMatrix<double>(count);
var partitioner = Partitioner.Create(0, sim.Length);
var locker = new object();
var calculated = 0;
Parallel.ForEach(partitioner, new ParallelOptions { MaxDegreeOfParallelism = 4 }, (range, loop) =>
{
    var calc = new SymbolicExpressionTreeBottomUpSimilarityCalculator();
    int startRow, endRow, startCol, endCol;

    sim.GetMatrixCoordinates(range.Item1, out startRow, out startCol);
    sim.GetMatrixCoordinates(range.Item2 - 1, out endRow, out endCol);

    for (int i = startRow; i <= endRow; ++i) {
        var start = i == startRow ? startCol : 0;
        var end = i == endRow ? endCol : i;

        for (int j = start; j <= end; ++j) {
            var s = i == j ? 1 : calc.CalculateSimilarity(trees[i], trees[j]);
            sim[i, j] = 1 - s;
        }
    }
    lock (locker) {
        calculated += range.Item2 - range.Item1;
        Console.WriteLine("{0} / {1} calculated", calculated, sim.Length);
    }
});