Constructing an odd-even mergesort sorting network

I was searching for a non-recursive odd-even mergesort implementation and found this one:

• Sedgewick R.

The resulting sorting network is not an odd-even-merge sort network. When one draws a diagram of the pairs, it can be seen that too many pairs are generated and others are missed.

So here is an image of a correct network with 32 inputs. A vertical line between 2 horizontal lines means compare value a[x] with a[y], if greater then swap the values in the array.

(clickable)

The code from the book translated into C:

#include 
#include 

void sort(int l, int r)
{ int n = r-l+1;

  for (int p=1; p0; k/=2)
      for (int j=k%p; j+k<n; j+=(k+k))
        for (int i=0; i<n-j-k; i++)
          if ((j+i)/(p+p) == (j+i+k)/(p+p))
              printf("%2i cmp %2i\n", l+j+i, l+j+i+k);
}
int main(char* argv, int args)
{ const int COUNT = 8;
  sort(0, COUNT-1);
}

See my StackOverflow question for more details and a diagram for the falsly generated network.

So I took the sorting network images for `length = 1

• Can it be improved? E.g. just 4 loops.

• It need not be very fast, because it's a hardware generation and check routine.

In general your code is quite hard to comprehend due to the bracing style and the single letter variable names. A bit more spaces and more meaningful names would do quite a bit for readability (I've kept your 2-indent spacing) and also remove the need for most of the comments:

#include 
#include 

int log2ceil(int value)
{
  int i;
  int cmp = 1;
  for (i = 0; cmp < value; i++)
  {
    cmp <<= 1;
  }
  return i;
}

void sort(int length)
{
  int groups = log2ceil(length);
  for (int group = 0; group < groups; group++)
  {
    int blocks = 1 << (groups - group - 1);
    for (int block = 0; block < blocks; block++)
    {
      for (int stage = 0; stage <= group; stage++)
      {
        int distance = 1 << (group - stage);
        int startPoint = (stage == 0) ? 0 : distance;
        for (int j = startPoint; j + distance < (2 << group); j += 2 * distance)
        {
          for (int i = 0; i < distance; i++)            // shift startpoints
          {
            int x = (block * (length / blocks)) + j + i;
            int y = x + distance;
            printf("%2i cmp %2i\n", x, y);
          }
        }
      }
    }
  }
}

int main(char* argv, int args)
{
  const int COUNT = 8;
  sort(COUNT);
}

StackExchange Code Review Q#114824, answer score: 3