SQLite

** The checksum is computed using 32-bit big-endian integers if the
** magic number in the first 4 bytes of the WAL is 0x377f0683 and it
** is computed using little-endian if the magic number is 0x377f0682.
** The checksum values are always stored in the frame header in a
** big-endian format regardless of which byte order is used to compute
** the checksum.  The checksum is computed by interpreting the input as
** an even number of unsigned 32-bit integers: x[0] through x[N].  The
** algorithm used for the checksum is as follows:
** 
**   for i from 0 to n-1 step 2:
**     s0 += x[i] + s1;
**     s1 += x[i+1] + s0;
**   endfor
**
** On a checkpoint, the WAL is first VFS.xSync-ed, then valid content of the
** WAL is transferred into the database, then the database is VFS.xSync-ed.
** The VFS.xSync operations serve as write barriers - all writes launched
** before the xSync must complete before any write that launches after the
** xSync begins.
**
** After each checkpoint, the salt-1 value is incremented and the salt-2
** value is randomized.  This prevents old and new frames in the WAL from
** being considered valid at the same time and being checkpointing together
** following a crash.

** HASHTABLE_NPAGE_ONE frames. The values of HASHTABLE_NPAGE_ONE and 
** HASHTABLE_NPAGE are selected so that together the wal-index header and
** first index block are the same size as all other index blocks in the
** wal-index.
**
** Each index block contains two sections, a page-mapping that contains the
** database page number associated with each wal frame, and a hash-table 
** that allows readers to query an index block for a specific page number.
** The page-mapping is an array of HASHTABLE_NPAGE (or HASHTABLE_NPAGE_ONE
** for the first index block) 32-bit page numbers. The first entry in the 
** first index-block contains the database page number corresponding to the
** first frame in the WAL file. The first entry in the second index block
** in the WAL file corresponds to the (HASHTABLE_NPAGE_ONE+1)th frame in
** the log, and so on.
**

  volatile u32 *aPgno;            /* Page number array for hash table */
  u32 iZero;                      /* frame == (aHash[x]+iZero) */
  int iLimit = 0;                 /* Zero values greater than this */
  int nByte;                      /* Number of bytes to zero in aPgno[] */
  int i;                          /* Used to iterate through aHash[] */

  assert( pWal->writeLock );
  testcase( pWal->hdr.mxFrame==HASHTABLE_NPAGE_ONE-1 );
  testcase( pWal->hdr.mxFrame==HASHTABLE_NPAGE_ONE );
  testcase( pWal->hdr.mxFrame==HASHTABLE_NPAGE_ONE+1 );

  if( pWal->hdr.mxFrame==0 ) return;

  /* Obtain pointers to the hash-table and page-number array containing 
  ** the entry that corresponds to frame pWal->hdr.mxFrame. It is guaranteed
  ** that the page said hash-table and array reside on is already mapped.
  */