====== Chess - Programming - de Bruijn Sequence - De Bruijn Sequence Generator ======

Compile the program, and start it with a parameter between 1 and 67108864 (2^26).

----

<code cpp>
debruijn 4061955
</code>

returns:

<code cpp>
const BitBoard magic = 0x022fdd63cc95386d; // The 4061955.

const unsigned int magictable[64] =
{
    0,  1,  2, 53,  3,  7, 54, 27,
    4, 38, 41,  8, 34, 55, 48, 28,
   62,  5, 39, 46, 44, 42, 22,  9,
   24, 35, 59, 56, 49, 18, 29, 11,
   63, 52,  6, 26, 37, 40, 33, 47,
   61, 45, 43, 21, 23, 58, 17, 10,
   51, 25, 36, 32, 60, 20, 57, 16,
   50, 31, 19, 15, 30, 14, 13, 12,
};


unsigned int bitScanForward (BitBoard b) 
{
  return magictable[((b&-b)*magic) >> 58];
}
</code>

----

===== Source =====

<code cpp>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

typedef unsigned __int64 U64; // long long

class CGenBitScan
{
public:
  //==========================================
  // constructor immediately starts the search
  //==========================================
  CGenBitScan(int match4nth) 
  {
    clock_t start, stop;
    m_dBCount = 0;
    m_Match4nth  = match4nth;
    initPow2();
    start = clock();
    m_Lock = pow2[32]; // Optimization to exclude 32, see remarks.
    try {findDeBruijn(0, 64-6, 0, 6);} catch(int){}
    stop = clock();
    printf("\n%.3f Seconds for %d De Bruijn Sequences found\n", (float)(stop - start) / CLOCKS_PER_SEC, m_dBCount);
  }

private:
  U64 pow2[64];    // single bits
  U64 m_Lock;      // locks each bit used
  int m_dBCount;   // counter
  int m_Match4nth; // to match

  //==========================================
  // on the fly initialization of pow2
  //==========================================
  void initPow2()  
  {
    pow2[0] = 1;
    for (int i=1; i < 64; i++)
       pow2[i] = 2*pow2[i-1];
  }

  //==========================================
  // print the bitscan routine and throw
  //==========================================
  void bitScanRoutineFound(U64 deBruijn) 
  {
    int index[64], i;
    for (i=0; i<64; i++) // init magic array.
      index[ (deBruijn<<i) >> (64-6) ] = i;
    printf("\nconst U64 magic = 0x%08x%08x; // the %d.\n\n",
          (int)(deBruijn>>32), (int)(deBruijn), m_dBCount);
    printf("const unsigned int magictable[64] =\n{\n");
    for (i=0; i<64; i++) 
    {
      if ( (i & 7) == 0 ) printf("\n  ");
      printf(" %2d,", index[i]);
    }
    
    printf("\n};\n\nunsigned int bitScanForward (U64 b) {\n");
    printf("    return magictable[((b&-b)*magic) >> 58];\n}\n");
    // throw 0; // unwind the stack until catched.
  }

  //============================================
  // recursive search
  //============================================
  void findDeBruijn(U64 seq, int depth, int vtx, int nz) 
  {
    if ((m_Lock & pow2[vtx]) == 0 && nz <= 32) // only if vertex is not locked.
    {
      if (depth == 0)  // depth zero, De Bruijn Sequence found, see remarks.
      {
        if (++m_dBCount == m_Match4nth)
          bitScanRoutineFound(seq);
      } 
      else 
      {
        m_Lock ^= pow2[vtx]; // set bit, lock the vertex to don't appear multiple.
        
        if (vtx == 31 && depth > 2) // optimization, see remarks.
        {
          findDeBruijn(seq | pow2[depth-1], depth-2, 62, nz+1);
        } 
        else 
        {
          findDeBruijn( seq, depth-1, (2*vtx)&63, nz+1); // even successor.
          findDeBruijn( seq | pow2[depth-1], depth-1, (2*vtx+1)&63, nz); // odd successor.
        }
        
        m_Lock ^= pow2[vtx]; // reset bit, unlock.
      }
    }
  }
};


int main(int argc, char* argv[])
{
 if (argc < 2)
   printf("usage: genBitScan 1 .. %d\n", 1<<26);
 else
   CGenBitScan(atoi(argv[1]));

 return 0;
}
</code>

<WRAP info>
**NOTE:** 

With **findDeBruijn(0, 64-6, 0, 6)**, starting with six leading zeros on its most significant top, the routine has 58 decrements to reach depth zero claiming a found De Bruijn Sequence.

  * The algorithm does not explicitly prove the uniqueness of six remaining indices which result from the up to five trailing hidden zeros with 100000B = 32 as least significant sub-sequence.
  * However, the constraints of the odd De Bruin sequence seem strong enough to make that test redundant, that is proving 64-6 unique keys seems sufficient.

As demonstrated in the [[https://www.chessprogramming.org/De_Bruijn_Sequence#BruijnGraphChessBoard|De Bruijn Graph on a Chess Board]], there are two branch-less series {32, 0, 1} and {31, 63, 62} respectively.

  * The recursive search routine performs some kind of pruning and reductions to take advantage of that.
  * 63 to follow 31 must not be locked and skipped for the 62 with an extra depth reduction.
  * 32 can not appear as index inside the 58 recursive tests in one path, and is therefore locked initially before the **findDeBruijn(0, 64-6, 0, 6)** call.

A small improvement was introducing an additional parameter for the number of binary zeros to check it not to become greater 32.

  * This makes also the depth > 0 condition for the even successors no longer necessary, to enumerate all 2^26 De Bruijn Sequences. 
</WRAP>

----

===== References =====

https://www.chessprogramming.org/De_Bruijn_Sequence_Generator

https://www.stmintz.com/ccc/index.php?id=339184