[btree] / btree2t.c

// btree version 2t
// 04 FEB 2014

// author: karl malbrain, malbrain@cal.berkeley.edu

/*
This work, including the source code, documentation
and related data, is placed into the public domain.

The orginal author is Karl Malbrain.

THIS SOFTWARE IS PROVIDED AS-IS WITHOUT WARRANTY
OF ANY KIND, NOT EVEN THE IMPLIED WARRANTY OF
MERCHANTABILITY. THE AUTHOR OF THIS SOFTWARE,
ASSUMES _NO_ RESPONSIBILITY FOR ANY CONSEQUENCE
RESULTING FROM THE USE, MODIFICATION, OR
REDISTRIBUTION OF THIS SOFTWARE.
*/

// Please see the project home page for documentation
// code.google.com/p/high-concurrency-btree

#define _FILE_OFFSET_BITS 64
#define _LARGEFILE64_SOURCE

#ifdef linux
#define _GNU_SOURCE
#endif

#ifdef unix
#include <unistd.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <errno.h>
#else
#define WIN32_LEAN_AND_MEAN
#include <windows.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <fcntl.h>
#endif

#include <memory.h>
#include <string.h>

typedef unsigned long long      uid;

#ifndef unix
typedef unsigned long long      off64_t;
typedef unsigned short          ushort;
typedef unsigned int            uint;
#endif

#define BT_ro 0x6f72    // ro
#define BT_rw 0x7772    // rw
#define BT_fl 0x6c66    // fl

#define BT_maxbits              24                                      // maximum page size in bits
#define BT_minbits              9                                       // minimum page size in bits
#define BT_minpage              (1 << BT_minbits)       // minimum page size

/*
There are five lock types for each node in three independent sets: 
1. (set 1) AccessIntent: Sharable. Going to Read the node. Incompatible with NodeDelete. 
2. (set 1) NodeDelete: Exclusive. About to release the node. Incompatible with AccessIntent. 
3. (set 2) ReadLock: Sharable. Read the node. Incompatible with WriteLock. 
4. (set 2) WriteLock: Exclusive. Modify the node. Incompatible with ReadLock and other WriteLocks. 
5. (set 3) ParentModification: Exclusive. Change the node's parent keys. Incompatible with another ParentModification. 
*/

typedef enum{
        BtLockAccess,
        BtLockDelete,
        BtLockRead,
        BtLockWrite,
        BtLockParent
}BtLock;

//      Define the length of the page and key pointers

#define BtId 6

//      Page key slot definition.

//      If BT_maxbits is 15 or less, you can save 2 bytes
//      for each key stored by making the first two uints
//      into ushorts.  You can also save 4 bytes by removing
//      the tod field from the key.

//      Keys are marked dead, but remain on the page until
//      cleanup is called.

typedef struct {
        uint off:BT_maxbits;            // page offset for key start
        uint dead:1;                            // set for deleted key
        uint tod;                                       // time-stamp for key
        unsigned char id[BtId];         // id associated with key
} BtSlot;

//      The key structure occupies space at the upper end of
//      each page.  It's a length byte followed by the value
//      bytes.

typedef struct {
        unsigned char len;
        unsigned char key[0];
} *BtKey;

//      The first part of an index page.
//      It is immediately followed
//      by the BtSlot array of keys.

typedef struct BtPage_ {
        uint cnt;                                       // count of keys in page
        uint act;                                       // count of active keys
        uint min;                                       // next key offset
        unsigned char bits;                     // page size in bits
        unsigned char lvl:6;            // level of page
        unsigned char dirty:1;          // page is dirty
        unsigned char posted:1;         // page fence is posted
        unsigned char right[BtId];      // page number to right
        unsigned char fence[256];       // page fence key
} *BtPage;

//  The loadpage interface object

typedef struct {
        uid page_no;
        BtPage page;
} BtPageSet;

//      The memory mapping hash table entry

typedef struct {
        BtPage page;            // mapped page pointer
        uid  page_no;           // mapped page number
        void *lruprev;          // least recently used previous cache block
        void *lrunext;          // lru next cache block
        void *hashprev;         // previous cache block for the same hash idx
        void *hashnext;         // next cache block for the same hash idx
#ifndef unix
        HANDLE hmap;
#endif
}BtHash;

//      The object structure for Btree access

typedef struct _BtDb {
        uint page_size;         // each page size       
        uint page_bits;         // each page size in bits       
        uint seg_bits;          // segment size in pages in bits
        uid page_no;            // current page number  
        uid cursor_page;        // current cursor page number   
        int  err;
        uint mode;                      // read-write mode
        uint mapped_io;         // use memory mapping
        BtPage temp;            // temporary frame buffer (memory mapped/file IO)
        BtPage parent;          // current page's parent node (memory mapped/file IO)
        BtPage alloc;           // frame for alloc page  (memory mapped/file IO)
        BtPage cursor;          // cached frame for start/next (never mapped)
        BtPage frame;           // spare frame for the page split (never mapped)
        BtPage zero;            // zeroes frame buffer (never mapped)
        BtPage page;            // temporary page (memory mapped/file IO)
#ifdef unix
        int idx;
#else
        HANDLE idx;
#endif
        unsigned char *mem;     // frame, cursor, page memory buffer
        int nodecnt;            // highest page cache segment in use
        int nodemax;            // highest page cache segment allocated
        int hashmask;           // number of pages in segments - 1
        int hashsize;           // size of hash table
        BtHash *lrufirst;       // lru list head
        BtHash *lrulast;        // lru list tail
        ushort *cache;          // hash table for cached segments
        BtHash nodes[1];        // segment cache follows
        int posted;                     // last loadpage found posted key
        int found;                      // last insert/delete found key
} BtDb;

typedef enum {
BTERR_ok = 0,
BTERR_struct,
BTERR_ovflw,
BTERR_lock,
BTERR_map,
BTERR_wrt,
BTERR_hash
} BTERR;

// B-Tree functions
extern void bt_close (BtDb *bt);
extern BtDb *bt_open (char *name, uint mode, uint bits, uint cacheblk, uint pgblk);
extern BTERR  bt_insertkey (BtDb *bt, unsigned char *key, uint len, uint lvl, uid id, uint tod);
extern BTERR  bt_deletekey (BtDb *bt, unsigned char *key, uint len);
extern uid bt_findkey    (BtDb *bt, unsigned char *key, uint len);
extern uint bt_startkey  (BtDb *bt, unsigned char *key, uint len);
extern uint bt_nextkey   (BtDb *bt, uint slot);

//      Internal functions

BTERR bt_removepage (BtDb *bt, uid page_no, uint lvl, unsigned char *pagefence);

//  Helper functions to return slot values

extern BtKey bt_key (BtDb *bt, uint slot);
extern uid bt_uid (BtDb *bt, uint slot);
extern uint bt_tod (BtDb *bt, uint slot);

//  BTree page number constants
#define ALLOC_page              0
#define ROOT_page               1

//      Number of levels to create in a new BTree

#define MIN_lvl                 2

//  The page is allocated from low and hi ends.
//  The key offsets and row-id's are allocated
//  from the bottom, while the text of the key
//  is allocated from the top.  When the two
//  areas meet, the page is split into two.

//  A key consists of a length byte, two bytes of
//  index number (0 - 65534), and up to 253 bytes
//  of key value.  Duplicate keys are discarded.
//  Associated with each key is a 48 bit row-id.

//  The b-tree root is always located at page 1.
//      The first leaf page of level zero is always
//      located on page 2.

//      The b-tree pages are linked with right
//      pointers to facilitate enumerators,
//      and provide for concurrency.

//      When to root page fills, it is split in two and
//      the tree height is raised by a new root at page
//      one with two keys.

//      Deleted keys are marked with a dead bit until
//      page cleanup

//  Groups of pages from the btree are optionally
//  cached with memory mapping. A hash table is used to keep
//  track of the cached pages.  This behaviour is controlled
//  by the number of cache blocks parameter and pages per block
//      given to bt_open.

//  To achieve maximum concurrency one page is locked at a time
//  as the tree is traversed to find leaf key in question. The right
//  page numbers are used in cases where the page is being split,
//      or consolidated.

//  Page 0 is dedicated to lock for new page extensions,
//      and chains empty pages together for reuse.

//      Parent locks are obtained to prevent resplitting or deleting a node
//      before its fence is posted into its upper level.

//      Empty nodes are chained together through the ALLOC page and reused.

//      A special open mode of BT_fl is provided to safely access files on
//      WIN32 networks. WIN32 network operations should not use memory mapping.
//      This WIN32 mode sets FILE_FLAG_NOBUFFERING and FILE_FLAG_WRITETHROUGH
//      to prevent local caching of network file contents.

//      Access macros to address slot and key values from the page.
//      Page slots use 1 based indexing.

#define slotptr(page, slot) (((BtSlot *)(page+1)) + (slot-1))
#define keyptr(page, slot) ((BtKey)((unsigned char*)(page) + slotptr(page, slot)->off))

void bt_putid(unsigned char *dest, uid id)
{
int i = BtId;

        while( i-- )
                dest[i] = (unsigned char)id, id >>= 8;
}

uid bt_getid(unsigned char *src)
{
uid id = 0;
int i;

        for( i = 0; i < BtId; i++ )
                id <<= 8, id |= *src++; 

        return id;
}

// place write, read, or parent lock on requested page_no.

BTERR bt_lockpage(BtDb *bt, uid page_no, BtLock mode)
{
off64_t off = page_no << bt->page_bits;
#ifdef unix
int flag = PROT_READ | ( bt->mode == BT_ro ? 0 : PROT_WRITE );
struct flock lock[1];
#else
uint flags = 0, len;
OVERLAPPED ovl[1];
#endif

        if( mode == BtLockRead || mode == BtLockWrite )
                off +=  1 * sizeof(*bt->page);  // use second segment

        if( mode == BtLockParent )
                off +=  2 * sizeof(*bt->page);  // use third segment

#ifdef unix
        memset (lock, 0, sizeof(lock));

        lock->l_start = off;
        lock->l_type = (mode == BtLockDelete || mode == BtLockWrite || mode == BtLockParent) ? F_WRLCK : F_RDLCK;
        lock->l_len = sizeof(*bt->page);
        lock->l_whence = 0;

        if( fcntl (bt->idx, F_SETLKW, lock) < 0 )
                return bt->err = BTERR_lock;

        return 0;
#else
        memset (ovl, 0, sizeof(ovl));
        ovl->OffsetHigh = (uint)(off >> 32);
        ovl->Offset = (uint)off;
        len = sizeof(*bt->page);

        //      use large offsets to
        //      simulate advisory locking

        ovl->OffsetHigh |= 0x80000000;

        if( mode == BtLockDelete || mode == BtLockWrite || mode == BtLockParent )
                flags |= LOCKFILE_EXCLUSIVE_LOCK;

        if( LockFileEx (bt->idx, flags, 0, len, 0L, ovl) )
                return bt->err = 0;

        return bt->err = BTERR_lock;
#endif 
}

// remove write, read, or parent lock on requested page_no.

BTERR bt_unlockpage(BtDb *bt, uid page_no, BtLock mode)
{
off64_t off = page_no << bt->page_bits;
#ifdef unix
struct flock lock[1];
#else
OVERLAPPED ovl[1];
uint len;
#endif

        if( mode == BtLockRead || mode == BtLockWrite )
                off +=  1 * sizeof(*bt->page);  // use second segment

        if( mode == BtLockParent )
                off +=  2 * sizeof(*bt->page);  // use third segment

#ifdef unix
        memset (lock, 0, sizeof(lock));

        lock->l_start = off;
        lock->l_type = F_UNLCK;
        lock->l_len = sizeof(*bt->page);
        lock->l_whence = 0;

        if( fcntl (bt->idx, F_SETLK, lock) < 0 )
                return bt->err = BTERR_lock;
#else
        memset (ovl, 0, sizeof(ovl));
        ovl->OffsetHigh = (uint)(off >> 32);
        ovl->Offset = (uint)off;
        len = sizeof(*bt->page);

        //      use large offsets to
        //      simulate advisory locking

        ovl->OffsetHigh |= 0x80000000;

        if( !UnlockFileEx (bt->idx, 0, len, 0, ovl) )
                return GetLastError(), bt->err = BTERR_lock;
#endif

        return bt->err = 0;
}

//      close and release memory

void bt_close (BtDb *bt)
{
BtHash *hash;
#ifdef unix
        // release mapped pages

        if( hash = bt->lrufirst )
                do munmap (hash->page, (bt->hashmask+1) << bt->page_bits);
                while(hash = hash->lrunext);

        if ( bt->mem )
                free (bt->mem);
        close (bt->idx);
        free (bt->cache);
        free (bt);
#else
        if( hash = bt->lrufirst )
          do
          {
                FlushViewOfFile(hash->page, 0);
                UnmapViewOfFile(hash->page);
                CloseHandle(hash->hmap);
          } while(hash = hash->lrunext);

        if ( bt->mem)
                VirtualFree (bt->mem, 0, MEM_RELEASE);
        FlushFileBuffers(bt->idx);
        CloseHandle(bt->idx);
        GlobalFree (bt->cache);
        GlobalFree (bt);
#endif
}

//  open/create new btree
//      call with file_name, BT_openmode, bits in page size (e.g. 16),
//              size of mapped page cache (e.g. 8192) or zero for no mapping.

BtDb *bt_open (char *name, uint mode, uint bits, uint nodemax, uint pgblk)
{
uint lvl, attr, cacheblk, last;
BtLock lockmode = BtLockWrite;
BtPage alloc;
off64_t size;
uint amt[1];
BtKey key;
BtDb* bt;

#ifndef unix
SYSTEM_INFO sysinfo[1];
#endif

#ifdef unix
        bt = malloc (sizeof(BtDb) + nodemax * sizeof(BtHash));
        memset (bt, 0, sizeof(BtDb));

        switch (mode & 0x7fff)
        {
        case BT_fl:
        case BT_rw:
                bt->idx = open ((char*)name, O_RDWR | O_CREAT, 0666);
                break;

        case BT_ro:
        default:
                bt->idx = open ((char*)name, O_RDONLY);
                lockmode = BtLockRead;
                break;
        }
        if( bt->idx == -1 )
                return free(bt), NULL;
        
        if( nodemax )
                cacheblk = 4096;        // page size for unix
        else
                cacheblk = 0;

#else
        bt = GlobalAlloc (GMEM_FIXED|GMEM_ZEROINIT, sizeof(BtDb) + nodemax * sizeof(BtHash));
        attr = FILE_ATTRIBUTE_NORMAL;
        switch (mode & 0x7fff)
        {
        case BT_fl:
                attr |= FILE_FLAG_WRITE_THROUGH | FILE_FLAG_NO_BUFFERING;

        case BT_rw:
                bt->idx = CreateFile(name, GENERIC_READ| GENERIC_WRITE, FILE_SHARE_READ|FILE_SHARE_WRITE, NULL, OPEN_ALWAYS, attr, NULL);
                break;

        case BT_ro:
        default:
                bt->idx = CreateFile(name, GENERIC_READ, FILE_SHARE_READ|FILE_SHARE_WRITE, NULL, OPEN_EXISTING, attr, NULL);
                lockmode = BtLockRead;
                break;
        }
        if( bt->idx == INVALID_HANDLE_VALUE )
                return GlobalFree(bt), NULL;

        // normalize cacheblk to multiple of sysinfo->dwAllocationGranularity
        GetSystemInfo(sysinfo);

        if( nodemax )
                cacheblk = sysinfo->dwAllocationGranularity;
        else
                cacheblk = 0;
#endif

        // determine sanity of page size

        if( bits > BT_maxbits )
                bits = BT_maxbits;
        else if( bits < BT_minbits )
                bits = BT_minbits;

        if ( bt_lockpage(bt, ALLOC_page, lockmode) )
                return bt_close (bt), NULL;

#ifdef unix
        *amt = 0;

        // read minimum page size to get root info

        if( size = lseek (bt->idx, 0L, 2) ) {
                alloc = malloc (BT_minpage);
                pread(bt->idx, alloc, BT_minpage, 0);
                bits = alloc->bits;
                free (alloc);
        } else if( mode == BT_ro )
                return bt_close (bt), NULL;
#else
        size = GetFileSize(bt->idx, amt);

        if( size || *amt ) {
                alloc = VirtualAlloc(NULL, BT_minpage, MEM_COMMIT, PAGE_READWRITE);
                if( !ReadFile(bt->idx, (char *)alloc, BT_minpage, amt, NULL) )
                        return bt_close (bt), NULL;
                bits = alloc->bits;
                VirtualFree (alloc, 0, MEM_RELEASE);
        } else if( mode == BT_ro )
                return bt_close (bt), NULL;
#endif

        bt->page_size = 1 << bits;
        bt->page_bits = bits;

        bt->nodemax = nodemax;
        bt->mode = mode;

        // setup cache mapping

        if( cacheblk ) {
                if( cacheblk < bt->page_size )
                        cacheblk = bt->page_size;

                bt->hashsize = nodemax / 8;
                bt->hashmask = (cacheblk >> bits) - 1;
                bt->mapped_io = 1;
        }

        //      requested number of pages per memmap segment

        if( cacheblk )
          if( (1 << pgblk) > bt->hashmask )
                bt->hashmask = (1 << pgblk) - 1;

        bt->seg_bits = 0;

        while( (1 << bt->seg_bits) <= bt->hashmask )
                bt->seg_bits++;

#ifdef unix
        bt->mem = malloc (7 *bt->page_size);
        bt->cache = calloc (bt->hashsize, sizeof(ushort));
#else
        bt->mem = VirtualAlloc(NULL, 7 * bt->page_size, MEM_COMMIT, PAGE_READWRITE);
        bt->cache = GlobalAlloc (GMEM_FIXED|GMEM_ZEROINIT, bt->hashsize * sizeof(ushort));
#endif
        bt->frame = (BtPage)bt->mem;
        bt->cursor = (BtPage)(bt->mem + bt->page_size);
        bt->page = (BtPage)(bt->mem + 2 * bt->page_size);
        bt->alloc = (BtPage)(bt->mem + 3 * bt->page_size);
        bt->temp = (BtPage)(bt->mem + 4 * bt->page_size);
        bt->parent = (BtPage)(bt->mem + 5 * bt->page_size);
        bt->zero = (BtPage)(bt->mem + 6 * bt->page_size);

        if( size || *amt ) {
                if ( bt_unlockpage(bt, ALLOC_page, lockmode) )
                        return bt_close (bt), NULL;

                return bt;
        }

        // initializes an empty b-tree with root page and page of leaves

        memset (bt->alloc, 0, bt->page_size);
        bt_putid(bt->alloc->right, MIN_lvl+1);
        bt->alloc->bits = bt->page_bits;

#ifdef unix
        if( write (bt->idx, bt->alloc, bt->page_size) < bt->page_size )
                return bt_close (bt), NULL;
#else
        if( !WriteFile (bt->idx, (char *)bt->alloc, bt->page_size, amt, NULL) )
                return bt_close (bt), NULL;

        if( *amt < bt->page_size )
                return bt_close (bt), NULL;
#endif

        memset (bt->frame, 0, bt->page_size);
        bt->frame->bits = bt->page_bits;
        bt->frame->posted = 1;

        for( lvl=MIN_lvl; lvl--; ) {
                slotptr(bt->frame, 1)->off = bt->page_size - 3;
                bt_putid(slotptr(bt->frame, 1)->id, lvl ? MIN_lvl - lvl + 1 : 0);               // next(lower) page number
                key = keyptr(bt->frame, 1);
                key->len = 2;                   // create stopper key
                key->key[0] = 0xff;
                key->key[1] = 0xff;
                bt->frame->fence[0] = 2;
                bt->frame->fence[1] = 0xff;
                bt->frame->fence[2] = 0xff;
                bt->frame->min = bt->page_size - 3;
                bt->frame->lvl = lvl;
                bt->frame->cnt = 1;
                bt->frame->act = 1;
#ifdef unix
                if( write (bt->idx, bt->frame, bt->page_size) < bt->page_size )
                        return bt_close (bt), NULL;
#else
                if( !WriteFile (bt->idx, (char *)bt->frame, bt->page_size, amt, NULL) )
                        return bt_close (bt), NULL;

                if( *amt < bt->page_size )
                        return bt_close (bt), NULL;
#endif
        }

        // create empty page area by writing last page of first
        // cache area (other pages are zeroed by O/S)

        if( bt->mapped_io && bt->hashmask ) {
                memset(bt->frame, 0, bt->page_size);
                last = bt->hashmask;

                while( last < MIN_lvl + 1 )
                        last += bt->hashmask + 1;
#ifdef unix
                pwrite(bt->idx, bt->frame, bt->page_size, last << bt->page_bits);
#else
                SetFilePointer (bt->idx, last << bt->page_bits, NULL, FILE_BEGIN);
                if( !WriteFile (bt->idx, (char *)bt->frame, bt->page_size, amt, NULL) )
                        return bt_close (bt), NULL;
                if( *amt < bt->page_size )
                        return bt_close (bt), NULL;
#endif
        }

        if( bt_unlockpage(bt, ALLOC_page, lockmode) )
                return bt_close (bt), NULL;

        return bt;
}

//  compare two keys, returning > 0, = 0, or < 0
//  as the comparison value

int keycmp (BtKey key1, unsigned char *key2, uint len2)
{
uint len1 = key1->len;
int ans;

        if( ans = memcmp (key1->key, key2, len1 > len2 ? len2 : len1) )
                return ans;

        if( len1 > len2 )
                return 1;
        if( len1 < len2 )
                return -1;

        return 0;
}

//  Update current page of btree by writing file contents
//      or flushing mapped area to disk.

BTERR bt_update (BtDb *bt, BtPage page, uid page_no)
{
off64_t off = page_no << bt->page_bits;

#ifdef unix
    if ( !bt->mapped_io )
         if ( pwrite(bt->idx, page, bt->page_size, off) != bt->page_size )
                 return bt->err = BTERR_wrt;
#else
uint amt[1];
        if ( !bt->mapped_io )
        {
                SetFilePointer (bt->idx, (long)off, (long*)(&off)+1, FILE_BEGIN);
                if( !WriteFile (bt->idx, (char *)page, bt->page_size, amt, NULL) )
                        return GetLastError(), bt->err = BTERR_wrt;

                if( *amt < bt->page_size )
                        return GetLastError(), bt->err = BTERR_wrt;
        } 
        else if ( bt->mode == BT_fl ) {
                        FlushViewOfFile(page, bt->page_size);
                        FlushFileBuffers(bt->idx);
        }
#endif
        return 0;
}

// find page in cache 

BtHash *bt_findhash(BtDb *bt, uid page_no)
{
BtHash *hash;
uint idx;

        // compute cache block first page and hash idx 

        page_no &= ~bt->hashmask;
        idx = (uint)(page_no >> bt->seg_bits) % bt->hashsize;

        if( bt->cache[idx] ) 
                hash = bt->nodes + bt->cache[idx];
        else
                return NULL;

        do if( hash->page_no == page_no )
                 break;
        while(hash = hash->hashnext );

        return hash;
}

// add page cache entry to hash index

void bt_linkhash(BtDb *bt, BtHash *node, uid page_no)
{
uint idx = (uint)(page_no >> bt->seg_bits) % bt->hashsize;
BtHash *hash;

        if( bt->cache[idx] ) {
                node->hashnext = hash = bt->nodes + bt->cache[idx];
                hash->hashprev = node;
        }

        node->hashprev = NULL;
        bt->cache[idx] = (ushort)(node - bt->nodes);
}

// remove cache entry from hash table

void bt_unlinkhash(BtDb *bt, BtHash *node)
{
uint idx = (uint)(node->page_no >> bt->seg_bits) % bt->hashsize;
BtHash *hash;

        // unlink node
        if( hash = node->hashprev )
                hash->hashnext = node->hashnext;
        else if( hash = node->hashnext )
                bt->cache[idx] = (ushort)(hash - bt->nodes);
        else
                bt->cache[idx] = 0;

        if( hash = node->hashnext )
                hash->hashprev = node->hashprev;
}

// add cache page to lru chain and map pages

BtPage bt_linklru(BtDb *bt, BtHash *hash, uid page_no)
{
int flag;
off64_t off = (page_no & ~bt->hashmask) << bt->page_bits;
off64_t limit = off + ((bt->hashmask+1) << bt->page_bits);
BtHash *node;

        memset(hash, 0, sizeof(BtHash));
        hash->page_no = (page_no & ~bt->hashmask);
        bt_linkhash(bt, hash, page_no);

        if( node = hash->lrunext = bt->lrufirst )
                node->lruprev = hash;
        else
                bt->lrulast = hash;

        bt->lrufirst = hash;

#ifdef unix
        flag = PROT_READ | ( bt->mode == BT_ro ? 0 : PROT_WRITE );
        hash->page = (BtPage)mmap (0, (bt->hashmask+1) << bt->page_bits, flag, MAP_SHARED, bt->idx, off);
        if( hash->page == MAP_FAILED )
                return bt->err = BTERR_map, (BtPage)NULL;

#else
        flag = ( bt->mode == BT_ro ? PAGE_READONLY : PAGE_READWRITE );
        hash->hmap = CreateFileMapping(bt->idx, NULL, flag,     (DWORD)(limit >> 32), (DWORD)limit, NULL);
        if( !hash->hmap )
                return bt->err = BTERR_map, NULL;

        flag = ( bt->mode == BT_ro ? FILE_MAP_READ : FILE_MAP_WRITE );
        hash->page = MapViewOfFile(hash->hmap, flag, (DWORD)(off >> 32), (DWORD)off, (bt->hashmask+1) << bt->page_bits);
        if( !hash->page )
                return bt->err = BTERR_map, NULL;
#endif

        return (BtPage)((char*)hash->page + ((uint)(page_no & bt->hashmask) << bt->page_bits));
}

//      find or place requested page in page-cache
//      return memory address where page is located.

BtPage bt_hashpage(BtDb *bt, uid page_no)
{
BtHash *hash, *node, *next;
BtPage page;

        // find page in cache and move to top of lru list  

        if( hash = bt_findhash(bt, page_no) ) {
                page = (BtPage)((char*)hash->page + ((uint)(page_no & bt->hashmask) << bt->page_bits));
                // swap node in lru list
                if( node = hash->lruprev ) {
                        if( next = node->lrunext = hash->lrunext )
                                next->lruprev = node;
                        else
                                bt->lrulast = node;

                        if( next = hash->lrunext = bt->lrufirst )
                                next->lruprev = hash;
                        else
                                return bt->err = BTERR_hash, (BtPage)NULL;

                        hash->lruprev = NULL;
                        bt->lrufirst = hash;
                }
                return page;
        }

        // map pages and add to cache entry

        if( bt->nodecnt < bt->nodemax ) {
                hash = bt->nodes + ++bt->nodecnt;
                return bt_linklru(bt, hash, page_no);
        }

        // hash table is already full, replace last lru entry from the cache

        if( hash = bt->lrulast ) {
                // unlink from lru list
                if( node = bt->lrulast = hash->lruprev )
                        node->lrunext = NULL;
                else
                        return bt->err = BTERR_hash, (BtPage)NULL;

#ifdef unix
                munmap (hash->page, (bt->hashmask+1) << bt->page_bits);
#else
                FlushViewOfFile(hash->page, 0);
                UnmapViewOfFile(hash->page);
                CloseHandle(hash->hmap);
#endif
                // unlink from hash table

                bt_unlinkhash(bt, hash);

                // map and add to cache

                return bt_linklru(bt, hash, page_no);
        }

        return bt->err = BTERR_hash, (BtPage)NULL;
}

//  map a btree page onto current page

BTERR bt_mappage (BtDb *bt, BtPage *page, uid page_no)
{
off64_t off = page_no << bt->page_bits;
#ifndef unix
int amt[1];
#endif
        
        if( bt->mapped_io ) {
                bt->err = 0;
                *page = bt_hashpage(bt, page_no);
                return bt->err;
        }
#ifdef unix
        if ( pread(bt->idx, *page, bt->page_size, off) < bt->page_size )
                return bt->err = BTERR_map;
#else
        SetFilePointer (bt->idx, (long)off, (long*)(&off)+1, FILE_BEGIN);

        if( !ReadFile(bt->idx, *page, bt->page_size, amt, NULL) )
                return bt->err = BTERR_map;

        if( *amt <  bt->page_size )
                return bt->err = BTERR_map;
#endif
        return 0;
}

//      allocate a new page and write page into it

uid bt_newpage(BtDb *bt, BtPage page)
{
uid new_page;
char *pmap;
int reuse;

        // lock page zero

        if ( bt_lockpage(bt, ALLOC_page, BtLockWrite) )
                return 0;

        if( bt_mappage (bt, &bt->alloc, ALLOC_page) )
                return 0;

        // use empty chain first
        // else allocate empty page

        if( new_page = bt_getid(bt->alloc[1].right) ) {
                if( bt_mappage (bt, &bt->temp, new_page) )
                        return 0;       // don't unlock on error
                memcpy(bt->alloc[1].right, bt->temp->right, BtId);
                reuse = 1;
        } else {
                new_page = bt_getid(bt->alloc->right);
                bt_putid(bt->alloc->right, new_page+1);
                reuse = 0;
        }

        if( bt_update(bt, bt->alloc, ALLOC_page) )
                return 0;       // don't unlock on error

        if( !bt->mapped_io ) {
                if( bt_update(bt, page, new_page) )
                        return 0;       //don't unlock on error

                // unlock page zero 

                if ( bt_unlockpage(bt, ALLOC_page, BtLockWrite) )
                        return 0;

                return new_page;
        }

#ifdef unix
        if ( pwrite(bt->idx, page, bt->page_size, new_page << bt->page_bits) < bt->page_size )
                return bt->err = BTERR_wrt, 0;

        // if writing first page of hash block, zero last page in the block

        if ( !reuse && bt->hashmask > 0 && (new_page & bt->hashmask) == 0 )
        {
                // use temp buffer to write zeros
                memset(bt->zero, 0, bt->page_size);
                if ( pwrite(bt->idx,bt->zero, bt->page_size, (new_page | bt->hashmask) << bt->page_bits) < bt->page_size )
                        return bt->err = BTERR_wrt, 0;
        }
#else
        //      bring new page into page-cache and copy page.
        //      this will extend the file into the new pages.

        if( !(pmap = (char*)bt_hashpage(bt, new_page & ~bt->hashmask)) )
                return 0;

        memcpy(pmap+((new_page & bt->hashmask) << bt->page_bits), page, bt->page_size);
#endif

        // unlock page zero 

        if ( bt_unlockpage(bt, ALLOC_page, BtLockWrite) )
                return 0;

        return new_page;
}

//  find slot in page for given key at a given level
//      return 0 if beyond fence value

int bt_findslot (BtPageSet *set, unsigned char *key, uint len)
{
uint diff, higher = set->page->cnt, low = 1, slot;

        //      is page being deleted?  if so,
        //      tell caller to follow right link

        if( !set->page->act )
                return 0;

        //      make stopper key an infinite fence value

        if( bt_getid (set->page->right) )
                higher++;

        //      low is the next candidate, higher is already
        //      tested as .ge. the given key, loop ends when they meet

        while( diff = higher - low ) {
                slot = low + ( diff >> 1 );
                if( keycmp (keyptr(set->page, slot), key, len) < 0 )
                        low = slot + 1;
                else
                        higher = slot;
        }

        if( higher <= set->page->cnt )
                return higher;

        //      if leaf page, compare against fence value

        //      return zero if key is on right link page
        //      or return slot beyond last key

        if( set->page->lvl || keycmp ((BtKey)set->page->fence, key, len) < 0 )
                return 0;

        return higher;
}

//  find and load page at given level for given key
//      leave page rd or wr locked as requested

int bt_loadpage (BtDb *bt, BtPageSet *set, unsigned char *key, uint len, uint lvl, uint lock)
{
uid page_no = ROOT_page, prevpage = 0;
uint drill = 0xff, slot;
uint mode, prevmode;
int posted = 1;

  //  start at root of btree and drill down

  do {
        // determine lock mode of drill level
        mode = (lock == BtLockWrite) && (drill == lvl) ? BtLockWrite : BtLockRead; 

        set->page_no = page_no;

        // obtain access lock using lock chaining

        if( page_no > ROOT_page )
          if( bt_lockpage(bt, page_no, BtLockAccess) )
                return 0;                                                                       

        if( prevpage )
          if( bt_unlockpage(bt, prevpage, prevmode) )
                return 0;

        // obtain read lock using lock chaining

        if( bt_lockpage(bt, page_no, mode) )
                return 0;                                                                       

        if( page_no > ROOT_page )
          if( bt_unlockpage(bt, page_no, BtLockAccess) )
                return 0;                                                                       

        //      map/obtain page contents

        if( bt_mappage (bt, &set->page, page_no) )
                return 0;

        // re-read and re-lock root after determining actual level of root

        if( set->page->lvl != drill) {
                if ( page_no != ROOT_page )
                        return bt->err = BTERR_struct, 0;
                        
                drill = set->page->lvl;

                if( lock == BtLockWrite && drill == lvl )
                  if( bt_unlockpage(bt, page_no, mode) )
                        return 0;
                  else
                        continue;
        }

        prevpage = page_no;
        prevmode = mode;

        //  find key on page at this level
        //  and descend to requested level

        if( slot = bt_findslot (set, key, len) ) {
          if( drill == lvl )
                return bt->posted = posted, slot;

          if( slot > set->page->cnt )
                return bt->err = BTERR_struct, 0;

        //  if drilling down, find next active key

          while( slotptr(set->page, slot)->dead )
                if( slot++ < set->page->cnt )
                        continue;
                else
                        return bt->err = BTERR_struct, 0;

          page_no = bt_getid(slotptr(set->page, slot)->id);
          posted = 1;
          drill--;
          continue;
        }

        //  or slide right into next page
        //  (slide left from deleted page)

        page_no = bt_getid(set->page->right);
        posted = 0;

  } while( page_no );

  // return error on end of right chain

  bt->err = BTERR_struct;
  return 0;     // return error
}

// drill down fixing fence values for left sibling tree
//      starting with current left page in bt->temp
//  call with bt->temp write locked
//      return with all pages unlocked.

BTERR bt_fixfences (BtDb *bt, uid page_no, unsigned char *newfence)
{
unsigned char oldfence[256];
uid prevpage = 0;
int chk;

  memcpy (oldfence, bt->temp->fence, 256);

  //  start at left page of btree and drill down
  //  to update their fence values

  do {
        // obtain access lock using lock chaining

        if( prevpage ) {
          if( bt_lockpage(bt, page_no, BtLockAccess) )
                return 0;                                                                       

          if( bt_unlockpage(bt, prevpage, BtLockWrite) )
                return 0;

          // obtain parent/fence key maintenance lock

          if( bt_lockpage(bt, page_no, BtLockParent) )
                return 0;                                                                       

          // obtain write lock using lock chaining

          if( bt_lockpage(bt, page_no, BtLockWrite) )
                return 0;                                                                       

          if( bt_unlockpage(bt, page_no, BtLockAccess) )
                return 0;                                                                       

          //  map/obtain page contents

          if( bt_mappage (bt, &bt->temp, page_no) )
                return 0;
        }

        chk = keycmp ((BtKey)bt->temp->fence, oldfence + 1, *oldfence);
    prevpage = page_no;

        if( chk < 0 ) {
                page_no = bt_getid (bt->temp->right);
                continue;
        }

        if( chk > 0 )
                return bt->err = BTERR_struct;

        memcpy (bt->temp->fence, newfence, 256);

        if( bt_update (bt, bt->temp, page_no) )
                return bt->err;

        //  return when we reach a leaf page

        if( !bt->temp->lvl ) {
                if( bt_unlockpage (bt, page_no, BtLockWrite) )
                        return bt->err;
                return bt_unlockpage (bt, page_no, BtLockParent);
        }

        page_no = bt_getid(slotptr(bt->temp, bt->temp->cnt)->id);

  } while( page_no );

  // return error on end of right chain

  bt->err = BTERR_struct;
  return 0;     // return error
}

//      return page to free list
//      page must be delete & write locked

BTERR bt_freepage (BtDb *bt, BtPage page, uid page_no)
{
        //      lock & map allocation page

        if( bt_lockpage (bt, ALLOC_page, BtLockWrite) )
                return bt->err;

        if( bt_mappage (bt, &bt->alloc, ALLOC_page) )
                return bt->err;

        //      store chain in second right
        bt_putid(page->right, bt_getid(bt->alloc[1].right));
        bt_putid(bt->alloc[1].right, page_no);

        if( bt_update(bt, bt->alloc, ALLOC_page) )
                return bt->err;
        if( bt_update(bt, page, page_no) )
                return bt->err;

        // unlock page zero 

        if( bt_unlockpage(bt, ALLOC_page, BtLockWrite) )
                return bt->err;

        //  remove write lock on deleted node

        if( bt_unlockpage(bt, page_no, BtLockWrite) )
                return bt->err;

        return bt_unlockpage (bt, page_no, BtLockDelete);
}

//      remove the root level by promoting its only child

BTERR bt_removeroot (BtDb *bt, BtPage root, BtPage child, uid page_no)
{
uid next = 0;

  do {
        if( next ) {
          if( bt_lockpage (bt, next, BtLockDelete) )
                return bt->err;
          if( bt_lockpage (bt, next, BtLockWrite) )
                return bt->err;

          if( bt_mappage (bt, &child, next) )
                return bt->err;

          page_no = next;
        }

        memcpy (root, child, bt->page_size);
        next = bt_getid (slotptr(child, child->cnt)->id);

        if( bt_freepage (bt, child, page_no) )
                return bt->err;
  } while( root->lvl > 1 && root->cnt == 1 );

  if( bt_update (bt, root, ROOT_page) )
        return bt->err;

  return bt_unlockpage (bt, ROOT_page, BtLockWrite);
}

//  pull right page over ourselves in simple merge

BTERR bt_mergeright (BtDb *bt, uid page_no, BtPageSet *parent, uint slot)
{
uid right;
uint idx;

        // find our right neighbor
        //      right must exist because the stopper prevents
        //      the rightmost page from deleting

        for( idx = slot; idx++ < parent->page->cnt; )
          if( !slotptr(parent->page, idx)->dead )
                break;

        right = bt_getid (slotptr (parent->page, idx)->id);

        if( right != bt_getid (bt->page->right) )
                return bt->err = BTERR_struct;

        if( bt_lockpage (bt, right, BtLockDelete) )
                return bt->err;

        if( bt_lockpage (bt, right, BtLockWrite) )
                return bt->err;

        if( bt_mappage (bt, &bt->temp, right) )
                return bt->err;

        memcpy (bt->page, bt->temp, bt->page_size);

        if( bt_update(bt, bt->page, page_no) )
                return bt->err;

        //  install ourselves as child page
        //      and delete ourselves from parent

        bt_putid (slotptr(parent->page, idx)->id, page_no);
        slotptr(parent->page, slot)->dead = 1;
        parent->page->act--;

        //      collapse any empty slots

        while( idx = parent->page->cnt - 1 )
          if( slotptr(parent->page, idx)->dead ) {
                *slotptr(parent->page, idx) = *slotptr(parent->page, idx + 1);
                memset (slotptr(parent->page, parent->page->cnt--), 0, sizeof(BtSlot));
          } else
                  break;

        if( bt_freepage (bt, bt->temp, right) )
                return bt->err;

        //      do we need to remove a btree level?
        //      (leave the first page of leaves alone)

        if( parent->page_no == ROOT_page && parent->page->cnt == 1 )
          if( bt->page->lvl )
                return bt_removeroot (bt, parent->page, bt->page, page_no);

        if( bt_update (bt, parent->page, parent->page_no) )
                return bt->err;

        if( bt_unlockpage (bt, parent->page_no, BtLockWrite) )
                return bt->err;

        if( bt_unlockpage (bt, page_no, BtLockWrite) )
                return bt->err;

        if( bt_unlockpage (bt, page_no, BtLockDelete) )
                return bt->err;

        return 0;
}

//      remove both child and parent from the btree
//      from the fence position in the parent

BTERR bt_removeparent (BtDb *bt, uid page_no, BtPageSet *parent, uint lvl)
{
unsigned char rightfence[256], pagefence[256];
uid right, ppage_no;

        right = bt_getid (bt->page->right);

        if( bt_lockpage (bt, right, BtLockParent) )
                return bt->err;

        if( bt_lockpage (bt, right, BtLockAccess) )
                return bt->err;

        if( bt_lockpage (bt, right, BtLockWrite) )
                return bt->err;

        if( bt_unlockpage (bt, right, BtLockAccess) )
                return bt->err;

        if( bt_mappage (bt, &bt->temp, right) )
                return bt->err;

        //      save right page fence value and
        //      parent fence value

        memcpy (rightfence, bt->temp->fence, 256);
        memcpy (pagefence, bt->parent->fence, 256);
        ppage_no = parent->page_no;

        //  pull right sibling over ourselves and unlock

        memcpy (bt->page, bt->temp, bt->page_size);

        if( bt_update(bt, bt->page, page_no) )
                return bt->err;

        if( bt_unlockpage (bt, page_no, BtLockDelete) )
                return bt->err;

        if( bt_unlockpage (bt, page_no, BtLockWrite) )
                return bt->err;

        //  install ourselves into right link from old right page

        bt->temp->act = 0;              // tell bt_findslot to go right (left)
        bt_putid (bt->temp->right, page_no);

        if( bt_update (bt, bt->temp, right) )
                return bt->err;

        if( bt_unlockpage (bt, right, BtLockWrite) )
                return bt->err;

        //      remove our slot from our parent
        //      clear act to signal bt_findslot to move right

        slotptr(parent->page, parent->page->cnt)->dead = 1;
        parent->page->act = 0;

        if( bt_update (bt, parent->page, ppage_no) )
                return bt->err;
        if( bt_unlockpage (bt, ppage_no, BtLockWrite) )
                return bt->err;

        //      redirect right page pointer in its parent to our left
        //      and free the right page

        if( bt_insertkey (bt, rightfence+1, *rightfence, lvl, page_no, time(NULL)) )
                return bt->err;

        if( bt_removepage (bt, ppage_no, lvl, pagefence) )
                return bt->err;

        if( bt_unlockpage (bt, right, BtLockParent) )
                return bt->err;

        //      wait for others to drain away

        if( bt_lockpage (bt, right, BtLockDelete) )
                return bt->err;

        if( bt_lockpage (bt, right, BtLockWrite) )
                return bt->err;

        if( bt_mappage (bt, &bt->temp, right) )
                return bt->err;

        return bt_freepage (bt, bt->temp, right);
}

//      remove page from btree
//      call with page unlocked
//      returns with page on free list

BTERR bt_removepage (BtDb *bt, uid page_no, uint lvl, unsigned char *pagefence)
{
BtPageSet parent[1];
uint slot, idx;
BtKey ptr;
uid left;

        parent->page = bt->parent;

        //      load and lock our parent

retry:
        if( !(slot = bt_loadpage (bt, parent, pagefence+1, *pagefence, lvl+1, BtLockWrite)) )
                return bt->err;

        //      wait until we are posted in our parent

        if( !bt->posted ) {
                if( bt_unlockpage (bt, parent->page_no, BtLockWrite) )
                        return bt->err;
#ifdef unix
                sched_yield();
#else
                SwitchToThread();
#endif
                goto retry;
        }

        //      wait for others to finish
        //      and obtain final WriteLock

        if( bt_lockpage (bt, page_no, BtLockDelete) )
                return bt->err;

        if( bt_lockpage (bt, page_no, BtLockWrite) )
                return bt->err;

        if( bt_mappage (bt, &bt->page, page_no) )
                return bt->err;

        //  was page re-established?

        if( bt->page->act ) {
                if( bt_unlockpage (bt, page_no, BtLockWrite) )
                        return bt->err;
                if( bt_unlockpage (bt, page_no, BtLockDelete) )
                        return bt->err;

                return bt_unlockpage (bt, parent->page_no, BtLockWrite);
        }
                
        //      can we do a simple merge entirely
        //      between siblings on the parent page?

        if( slot < parent->page->cnt )
                return bt_mergeright(bt, page_no, parent, slot);

        //  find our left neighbor in our parent page

        for( idx = slot; --idx; )
          if( !slotptr(parent->page, idx)->dead )
                break;

        //      if no left neighbor, delete ourselves and our parent

        if( !idx )
                return bt_removeparent (bt, page_no, parent, lvl+1);

        // lock and map our left neighbor's page

        left = bt_getid (slotptr(parent->page, idx)->id);

        //      wait our turn on fence key maintenance

        if( bt_lockpage(bt, left, BtLockParent) )
                return bt->err;

        if( bt_lockpage(bt, left, BtLockAccess) )
                return bt->err;

        if( bt_lockpage(bt, left, BtLockWrite) )
                return bt->err;

        if( bt_unlockpage(bt, left, BtLockAccess) )
                return bt->err;

        if( bt_mappage (bt, &bt->temp, left) )
                return bt->err;

        //  wait until sibling is in our parent

        if( bt_getid (bt->temp->right) != page_no ) {
                if( bt_unlockpage (bt, parent->page_no, BtLockWrite) )
                        return bt->err;
                if( bt_unlockpage (bt, left, BtLockWrite) )
                        return bt->err;
                if( bt_unlockpage (bt, left, BtLockParent) )
                        return bt->err;
                if( bt_unlockpage (bt, page_no, BtLockWrite) )
                        return bt->err;
                if( bt_unlockpage (bt, page_no, BtLockDelete) )
                        return bt->err;
#ifdef linux
                sched_yield();
#else
                SwitchToThread();
#endif
                goto retry;
        }

        //      delete our left sibling from parent

        slotptr(parent->page,idx)->dead = 1;
        parent->page->dirty = 1;
        parent->page->act--;

        //      redirect our parent slot to our left sibling

        bt_putid (slotptr(parent->page, slot)->id, left);

        //      update left page with our fence key

        memcpy (bt->temp->right, bt->page->right, BtId);

        //      collapse dead slots from parent

        while( idx = parent->page->cnt - 1 )
          if( slotptr(parent->page, idx)->dead ) {
                *slotptr(parent->page, idx) = *slotptr(parent->page, parent->page->cnt);
                memset (slotptr(parent->page, parent->page->cnt--), 0, sizeof(BtSlot));
          } else
                  break;

        //  update & unlock parent page

        if( bt_update (bt, parent->page, parent->page_no) )
                return bt->err;

        if( bt_unlockpage (bt, parent->page_no, BtLockWrite) )
                return bt->err;

        //      go down the left node's fence keys to the leaf level
        //      and update the fence keys in each page

        if( bt_fixfences (bt, left, pagefence) )
                return bt->err;

        //  free our original page

        if( bt_mappage (bt, &bt->temp, page_no) )
                return bt->err;

        return bt_freepage (bt, bt->temp,page_no);
}

//  find and delete key on page by marking delete flag bit
//  when page becomes empty, delete it

BTERR bt_deletekey (BtDb *bt, unsigned char *key, uint len)
{
unsigned char pagefence[256];
uint slot, found, act, idx;
BtPageSet set[1];
BtKey ptr;

        set->page = bt->page;

        if( slot = bt_loadpage (bt, set, key, len, 0, BtLockWrite) )
                ptr = keyptr(set->page, slot);
        else
                return bt->err;

        // if key is found delete it, otherwise ignore request

        if( found = slot <= set->page->cnt )
          if( found = !keycmp (ptr, key, len) )
                if( found = slotptr(set->page, slot)->dead == 0 ) {
                  slotptr(set->page,slot)->dead = 1;
                  set->page->dirty = 1;
                  set->page->act--;

                  //    collapse empty slots

                  while( idx = set->page->cnt - 1 )
                        if( slotptr(set->page, idx)->dead ) {
                          *slotptr(set->page, idx) = *slotptr(set->page, idx + 1);
                          memset (slotptr(set->page, set->page->cnt--), 0, sizeof(BtSlot));
                        } else
                                break;

                  if( bt_update(bt, set->page, set->page_no) )
                        return bt->err;
                }

        // delete page when empty

        memcpy (pagefence, set->page->fence, 256);
        act = set->page->act;

        if( bt_unlockpage(bt, set->page_no, BtLockWrite) )
                return bt->err;

        if( !act )
          if( bt_removepage (bt, set->page_no, 0, pagefence) )
                return bt->err;

        bt->found = found;
        return 0;
}

//      find key in leaf level and return row-id

uid bt_findkey (BtDb *bt, unsigned char *key, uint len)
{
BtPageSet set[1];
uint  slot;
uid id = 0;
BtKey ptr;

        set->page = bt->page;

        if( slot = bt_loadpage (bt, set, key, len, 0, BtLockRead) )
                ptr = keyptr(set->page, slot);
        else
                return 0;

        // if key exists, return row-id
        //      otherwise return 0

        if( slot <= set->page->cnt )
          if( !keycmp (ptr, key, len) )
                id = bt_getid(slotptr(set->page,slot)->id);

        if ( bt_unlockpage(bt, set->page_no, BtLockRead) )
                return 0;

        return id;
}

//      check page for space available,
//      clean if necessary and return
//      0 - page needs splitting
//      >0 - new slot value
 
uint bt_cleanpage(BtDb *bt, BtPage page, uint amt, uint slot)
{
uint nxt = bt->page_size, off;
uint cnt = 0, idx = 0;
uint max = page->cnt;
uint newslot = max;
BtKey key;

        if( page->min >= (max+1) * sizeof(BtSlot) + sizeof(*page) + amt + 1 )
                return slot;

        //      skip cleanup if nothing to reclaim

        if( !page->dirty )
                return 0;

        memcpy (bt->frame, page, bt->page_size);

        // skip page info and set rest of page to zero

        memset (page+1, 0, bt->page_size - sizeof(*page));
        page->dirty = 0;
        page->act = 0;

        while( cnt++ < max ) {
                if( cnt == slot )
                        newslot = idx + 1;
                if( slotptr(bt->frame,cnt)->dead )
                        continue;

                // copy key
                if( !page->lvl || cnt < max ) {
                        key = keyptr(bt->frame, cnt);
                        off = nxt -= key->len + 1;
                        memcpy ((unsigned char *)page + nxt, key, key->len + 1);
                } else
                        off = offsetof(struct BtPage_, fence);

                // copy slot
                memcpy(slotptr(page, ++idx)->id, slotptr(bt->frame, cnt)->id, BtId);
                slotptr(page, idx)->tod = slotptr(bt->frame, cnt)->tod;
                slotptr(page, idx)->off = off;
                page->act++;
        }
        page->min = nxt;
        page->cnt = idx;

        if( page->min >= (idx+1) * sizeof(BtSlot) + sizeof(*page) + amt + 1 )
                return newslot;

        return 0;
}

// split the root and raise the height of the btree

BTERR bt_splitroot(BtDb *bt, BtPageSet *root, uid page_no2)
{
unsigned char leftkey[256];
uint nxt = bt->page_size;
uid new_page;

        //  Obtain an empty page to use, and copy the current
        //  root contents into it, e.g. lower keys

        memcpy (leftkey, root->page->fence, 256);
        root->page->posted = 1;

        if( !(new_page = bt_newpage(bt, root->page)) )
                return bt->err;

        // preserve the page info at the bottom
        // of higher keys and set rest to zero

        memset(root->page+1, 0, bt->page_size - sizeof(*root->page));
        memset(root->page->fence, 0, 256);
        root->page->fence[0] = 2;
        root->page->fence[1] = 0xff;
        root->page->fence[2] = 0xff;

        // insert new page fence key on newroot page

        nxt -= *leftkey + 1;
        memcpy ((unsigned char *)root->page + nxt, leftkey, *leftkey + 1);
        bt_putid(slotptr(root->page, 1)->id, new_page);
        slotptr(root->page, 1)->off = nxt;
        
        // insert stopper key on newroot page
        // and increase the root height

        bt_putid(slotptr(root->page, 2)->id, page_no2);
        slotptr(root->page, 2)->off = offsetof(struct BtPage_, fence);

        bt_putid(root->page->right, 0);
        root->page->min = nxt;          // reset lowest used offset and key count
        root->page->cnt = 2;
        root->page->act = 2;
        root->page->lvl++;

        // update and release root

        if( bt_update(bt, root->page, root->page_no) )
                return bt->err;

        return bt_unlockpage(bt, root->page_no, BtLockWrite);
}

//  split already locked full node
//      return unlocked.

BTERR bt_splitpage (BtDb *bt, BtPageSet *set)
{
uint cnt = 0, idx = 0, max, nxt = bt->page_size, off;
uid right, page_no = set->page_no;
unsigned char fencekey[256];
uint lvl = set->page->lvl;
BtKey key;

        //  split higher half of keys to bt->frame

        memset (bt->frame, 0, bt->page_size);
        max = set->page->cnt;
        cnt = max / 2;
        idx = 0;

        while( cnt++ < max ) {
                if( !lvl || cnt < max ) {
                        key = keyptr(set->page, cnt);
                        off = nxt -= key->len + 1;
                        memcpy ((unsigned char *)bt->frame + nxt, key, key->len + 1);
                } else
                        off = offsetof(struct BtPage_, fence);

                memcpy(slotptr(bt->frame,++idx)->id, slotptr(set->page,cnt)->id, BtId);
                slotptr(bt->frame, idx)->tod = slotptr(set->page, cnt)->tod;
                slotptr(bt->frame, idx)->off = off;
                bt->frame->act++;
        }

        if( page_no == ROOT_page )
                bt->frame->posted = 1;

        memcpy (bt->frame->fence, set->page->fence, 256);
        bt->frame->bits = bt->page_bits;
        bt->frame->min = nxt;
        bt->frame->cnt = idx;
        bt->frame->lvl = lvl;

        // link right node

        if( page_no > ROOT_page )
                memcpy (bt->frame->right, set->page->right, BtId);

        //      get new free page and write higher keys to it.

        if( !(right = bt_newpage(bt, bt->frame)) )
                return bt->err;

        //      update lower keys to continue in old page

        memcpy (bt->frame, set->page, bt->page_size);
        memset (set->page+1, 0, bt->page_size - sizeof(*set->page));
        nxt = bt->page_size;
        set->page->posted = 0;
        set->page->dirty = 0;
        set->page->act = 0;
        cnt = 0;
        idx = 0;

        //  assemble page of smaller keys

        while( cnt++ < max / 2 ) {
                key = keyptr(bt->frame, cnt);

                if( !lvl || cnt < max / 2 ) {
                        off = nxt -= key->len + 1;
                        memcpy ((unsigned char *)set->page + nxt, key, key->len + 1);
                } else 
                        off = offsetof(struct BtPage_, fence);

                memcpy(slotptr(set->page,++idx)->id, slotptr(bt->frame,cnt)->id, BtId);
                slotptr(set->page, idx)->tod = slotptr(bt->frame, cnt)->tod;
                slotptr(set->page, idx)->off = off;
                set->page->act++;
        }

        // install fence key for smaller key page

        memset(set->page->fence, 0, 256);
        memcpy(set->page->fence, key, key->len + 1);

        bt_putid(set->page->right, right);
        set->page->min = nxt;
        set->page->cnt = idx;

        // if current page is the root page, split it

        if( page_no == ROOT_page )
                return bt_splitroot (bt, set, right);

        // insert new fences in parent page

        while( 1 ) {
                memcpy (fencekey, set->page->fence, 256);
                right = bt_getid (set->page->right);
                set->page->posted = 1;

                if( bt_update (bt, set->page, page_no) )
                        return bt->err; 

                if( bt_unlockpage (bt, page_no, BtLockWrite) )
                        return bt->err;

                if( bt_insertkey (bt, fencekey+1, *fencekey, lvl+1, page_no, time(NULL)) )
                        return bt->err;

                if( bt_unlockpage (bt, page_no, BtLockParent) )
                        return bt->err;

                if( !(page_no = right) )
                        break;

                // obtain fence modification/installation lock

                if( bt_lockpage (bt, page_no, BtLockParent) )
                        return bt->err;

                if( bt_lockpage (bt, page_no, BtLockAccess) )
                        return bt->err;

                if( bt_lockpage (bt, page_no, BtLockWrite) )
                        return bt->err;

                if( bt_unlockpage (bt, page_no, BtLockAccess) )
                        return bt->err;

                if( bt_mappage (bt, &set->page, page_no) )
                        return bt->err;

                if( set->page->posted ) {
                        if( bt_unlockpage (bt, page_no, BtLockWrite) )
                                return bt->err;
                        return bt_unlockpage (bt, page_no, BtLockParent);
                }

        }

        return 0;
}

//  Insert new key into the btree at requested level.

BTERR bt_insertkey (BtDb *bt, unsigned char *key, uint len, uint lvl, uid id, uint tod)
{
BtPageSet set[1];
uint slot, idx;
BtKey ptr;

  set->page = bt->page;

  while( 1 ) {
        if( slot = bt_loadpage (bt, set, key, len, lvl, BtLockWrite) )
                ptr = keyptr(set->page, slot);
        else
        {
                if ( !bt->err )
                        bt->err = BTERR_ovflw;
                return bt->err;
        }

        // if key already exists, update id and return

        if( slot <= set->page->cnt )
          if( !keycmp (ptr, key, len) ) {
                if( slotptr(set->page, slot)->dead )
                        set->page->act++;

                slotptr(set->page, slot)->dead = 0;
                slotptr(set->page, slot)->tod = tod;
                bt_putid(slotptr(set->page,slot)->id, id);

                if ( bt_update(bt, set->page, set->page_no) )
                        return bt->err;

                return bt_unlockpage(bt, set->page_no, BtLockWrite);
          }

        // check if page has enough space

        if( slot = bt_cleanpage (bt, set->page, len, slot) )
                break;

        if( bt_splitpage (bt, set) )
                return bt->err;
  }

  // calculate next available slot and copy key into page

  set->page->min -= len + 1; // reset lowest used offset
  ((unsigned char *)set->page)[set->page->min] = len;
  memcpy ((unsigned char *)set->page + set->page->min +1, key, len );

  for( idx = slot; idx <= set->page->cnt; idx++ )
        if( slotptr(set->page, idx)->dead )
                break;

  // now insert key into array before slot

  if( idx > set->page->cnt )
        set->page->cnt++;

  set->page->act++;

  while( idx > slot )
        *slotptr(set->page, idx) = *slotptr(set->page, idx -1), idx--;

  bt_putid(slotptr(set->page,slot)->id, id);
  slotptr(set->page, slot)->off = set->page->min;
  slotptr(set->page, slot)->tod = tod;
  slotptr(set->page, slot)->dead = 0;

  if( bt_update(bt, set->page, set->page_no) )
        return bt->err;

  return bt_unlockpage(bt, set->page_no, BtLockWrite);
}

//  cache page of keys into cursor and return starting slot for given key

uint bt_startkey (BtDb *bt, unsigned char *key, uint len)
{
BtPageSet set[1];
uint slot;

        set->page = bt->page;

        // cache page for retrieval
        if( slot = bt_loadpage (bt, set, key, len, 0, BtLockRead) )
                memcpy (bt->cursor, set->page, bt->page_size);
        bt->cursor_page = set->page_no;
        if ( bt_unlockpage(bt, set->page_no, BtLockRead) )
                return 0;

        return slot;
}

//  return next slot for cursor page
//  or slide cursor right into next page

uint bt_nextkey (BtDb *bt, uint slot)
{
BtPageSet set[1];
uid right;

  do {
        right = bt_getid(bt->cursor->right);
        while( slot++ < bt->cursor->cnt )
          if( slotptr(bt->cursor,slot)->dead )
                continue;
          else if( right || (slot < bt->cursor->cnt)) // skip infinite stopper
                return slot;
          else
                break;

        if( !right )
                break;

        bt->cursor_page = right;
        set->page = bt->page;

    if( bt_lockpage(bt, right, BtLockRead) )
                return 0;

        if( bt_mappage (bt, &set->page, right) )
                break;

        memcpy (bt->cursor, set->page, bt->page_size);

        if( bt_unlockpage(bt, right, BtLockRead) )
                return 0;

        slot = 0;
  } while( 1 );

  return bt->err = 0;
}

BtKey bt_key(BtDb *bt, uint slot)
{
        return keyptr(bt->cursor, slot);
}

uid bt_uid(BtDb *bt, uint slot)
{
        return bt_getid(slotptr(bt->cursor,slot)->id);
}

uint bt_tod(BtDb *bt, uint slot)
{
        return slotptr(bt->cursor,slot)->tod;
}


#ifdef STANDALONE
//  standalone program to index file of keys
//  then list them onto std-out

int main (int argc, char **argv)
{
uint slot, line = 0, off = 0, found = 0;
int dead, ch, cnt = 0, bits = 12;
unsigned char key[256];
clock_t done, start;
uint pgblk = 0;
time_t tod[1];
uint scan = 0;
uint len = 0;
uint map = 0;
BtKey ptr;
BtDb *bt;
FILE *in;

        if( argc < 4 ) {
                fprintf (stderr, "Usage: %s idx_file src_file Read/Write/Scan/Delete/Find [page_bits mapped_pool_segments pages_per_segment start_line_number]\n", argv[0]);
                fprintf (stderr, "  page_bits: size of btree page in bits\n");
                fprintf (stderr, "  mapped_pool_segments: size of buffer pool in segments\n");
                fprintf (stderr, "  pages_per_segment: size of buffer pool segment in pages in bits\n");
                exit(0);
        }

        start = clock();
        time(tod);

        if( argc > 4 )
                bits = atoi(argv[4]);

        if( argc > 5 )
                map = atoi(argv[5]);

        if( map > 65536 )
                fprintf (stderr, "Warning: buffer_pool > 65536 segments\n");

        if( map && map < 8 )
                fprintf (stderr, "Buffer_pool too small\n");

        if( argc > 6 )
                pgblk = atoi(argv[6]);

        if( bits + pgblk > 30 )
                fprintf (stderr, "Warning: very large buffer pool segment size\n");

        if( argc > 7 )
                off = atoi(argv[7]);

        bt = bt_open ((argv[1]), BT_rw, bits, map, pgblk);

        if( !bt ) {
                fprintf(stderr, "Index Open Error %s\n", argv[1]);
                exit (1);
        }

        switch(argv[3][0]| 0x20)
        {
        case 'w':
                fprintf(stderr, "started indexing for %s\n", argv[2]);
                if( argc > 2 && (in = fopen (argv[2], "rb")) )
                  while( ch = getc(in), ch != EOF )
                        if( ch == '\n' )
                        {
                          if( off )
                                sprintf((char *)key+len, "%.9d", line + off), len += 9;

                          if( bt_insertkey (bt, key, len, 0, ++line, *tod) )
                                fprintf(stderr, "Error %d Line: %d\n", bt->err, line), exit(0);
                          len = 0;
                        }
                        else if( len < 245 )
                                key[len++] = ch;
                fprintf(stderr, "finished adding keys, %d \n", line);
                break;

        case 'd':
                fprintf(stderr, "started deleting keys for %s\n", argv[2]);
                if( argc > 2 && (in = fopen (argv[2], "rb")) )
                  while( ch = getc(in), ch != EOF )
                        if( ch == '\n' )
                        {
                          if( off )
                                sprintf((char *)key+len, "%.9d", line + off), len += 9;
                          line++;
                          if( bt_deletekey (bt, key, len) )
                                fprintf(stderr, "Error %d Line: %d\n", bt->err, line), exit(0);
                          len = 0;
                        }
                        else if( len < 245 )
                                key[len++] = ch;
                fprintf(stderr, "finished deleting keys, %d \n", line);
                break;

        case 'f':
                fprintf(stderr, "started finding keys for %s\n", argv[2]);
                if( argc > 2 && (in = fopen (argv[2], "rb")) )
                  while( ch = getc(in), ch != EOF )
                        if( ch == '\n' )
                        {
                          if( off )
                                sprintf((char *)key+len, "%.9d", line + off), len += 9;
                          line++;
                          if( bt_findkey (bt, key, len) )
                                found++;
                          else if( bt->err )
                                fprintf(stderr, "Error %d Syserr %d Line: %d\n", bt->err, errno, line), exit(0);
                          len = 0;
                        }
                        else if( len < 245 )
                                key[len++] = ch;
                fprintf(stderr, "finished search of %d keys, found %d\n", line, found);
                break;

        case 's':
                scan++;
                break;

        }

        done = clock();
        fprintf(stderr, " Time to complete: %.2f seconds\n", (float)(done - start) / CLOCKS_PER_SEC);

        dead = cnt = 0;
        len = key[0] = 0;

        fprintf(stderr, "started reading\n");

        if( slot = bt_startkey (bt, key, len) )
          slot--;
        else
          fprintf(stderr, "Error %d in StartKey. Syserror: %d\n", bt->err, errno), exit(0);

        while( slot = bt_nextkey (bt, slot) )
          if( cnt++, scan ) {
                        ptr = bt_key(bt, slot);
                        fwrite (ptr->key, ptr->len, 1, stdout);
                        fputc ('\n', stdout);
          }

        fprintf(stderr, " Total keys read %d\n", cnt);
        return 0;
}

#endif  //STANDALONE