HistogramBase.GetValueAtPercentile — Double-Loop When Flat Array Index Is Available

[leecampbell-codeagent]

File: HistogramBase.cs

The percentile lookup iterates nested i / j loops over BucketCount × SubBucketCount. Since CountsArrayIndex(bucketIndex, subBucketIndex) maps this to a flat array, and the counts array is already flat in LongHistogram, the entire scan can be done as a single flat loop:

public long GetValueAtPercentile(double percentile)
{
    var countAtPercentile = Math.Max(
        (long)((Math.Min(percentile, 100.0) / 100.0 * TotalCount) + 0.5), 1);
long runningCount = 0;
for (int i = 0; i < CountsArrayLength; i++)
{
    runningCount += GetCountAtIndex(i);
    if (runningCount >= countAtPercentile)
        return HighestEquivalentValue(ValueFromIndex(i));
}
throw new ArgumentOutOfRangeException(...);

}

The nested loop computes GetCountAt(i, j) → GetCountAtIndex(CountsArrayIndex(i, j)) anyway — the flat version eliminates the CountsArrayIndex call per iteration and improves cache locality by accessing memory sequentially.