In-Place Radix Sort

Question

This is a long text. Please bear with me. Boiled down, the question is: Is there a workable in-place radix sort algorithm?

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Preliminary

Answer 1

If your data set is so big, then I would think that a disk-based buffer approach would be best:

sort(List elements, int prefix)
    if (elements.Count < THRESHOLD)
         return InMemoryRadixSort(elements, prefix)
    else
         return DiskBackedRadixSort(elements, prefix)

DiskBackedRadixSort(elements, prefix)
    DiskBackedBuffer[] buckets
    foreach (element in elements)
        buckets[element.MSB(prefix)].Add(element);

    List ret
    foreach (bucket in buckets)
        ret.Add(sort(bucket, prefix + 1))

    return ret

I would also experiment grouping into a larger number of buckets, for instance, if your string was:

GATTACA

the first MSB call would return the bucket for GATT (256 total buckets), that way you make fewer branches of the disk based buffer. This may or may not improve performance, so experiment with it.