samplestats.cpp

struct SampleStatistics {
    u64 count;
    double sum;
    double sum_of_squares;
    double mean;
    double standard_deviation;
    double min;
    double p25;
    double median;
    double p75;
    double max;
};

static void
CalculateSampleStatistics(double * values, u64 count, SampleStatistics * result) {
    std::sort(values, values + count);

    result->count = count;
    result->min = values[0];
    result->max = values[count - 1];
    result->median = values[count / 2]
                   + values[count / 2 + (~count & 1)]
                   / 2.0;

    u64 half_count = count / 2;
    result->p25 = values[half_count / 2]
                + values[half_count + (~half_count & 1)]
                / 2.0;
    result->p75 = values[half_count + (count & 1) + half_count / 2]
                + values[half_count + (count & 1) + half_count / 2 + (~half_count & 1)]
                / 2.0;

    for (u64 i = 0; i < count; ++i) {
        double x = values[i];
        result->sum += x;
        result->sum_of_squares += x * x;
    }

    result->mean = result->sum / count;
    result->standard_deviation = sqrt((result->sum_of_squares - (result->sum*result->sum)/result->count) / (result->count - 1));
}