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From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2008-07-12 11:48:09

Steven Ross wrote:
> Are there any more suggestions for the core algorithm?

Log2 can be done in O(log Nbits) steps, rather than O(Nbits).

I spent a while trying to understand this bit of code:

//Swap into place
//This dominates runtime, especially in the swap; std::sort calls are
the other main time-user
RandomAccessIter current = first;
for(unsigned u = 0; current != last; ++u) {
        for(current_bin = (bins + ((*current >> log_divisor) - div_min));
(current_bin->count > u);
                current_bin = bins + ((*current >> log_divisor) - div_min))
                        std::swap(*current, *(current_bin->current_position++));
        //Now that we've found the item belonging in this position, increment
the bucket count
        if(current_bin->current_position == current++)

You don't help understanding by changing the meaning of bin.count half
way through :-(. Anyway, I think you're doing more work than necessary
here. For each bin, the elements between the start of the bin and
bin.current_position will be correct and can be skipped over; this will
be on average half of the elements. This doesn't save any swaps but it
does save the shifting and some comparisons. Pseudo-code:

for each bin {
   for each element between this bin's current_position and its end {
     while (1) {
       determine the bin this element should be in
       if it's in the right bin { break }
       else {
         swap it with the element at the right bin's current_position
         increment that bin's current_position

Have I understood this correctly?


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