// btree version 2d

#define _FILE_OFFSET_BITS 64
#define _LARGEFILE64_SOURCE

#ifdef linux
#define _GNU_SOURCE
#endif

#ifdef unix
#include <unistd.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

#ifdef unix
typedef unsigned long long uid;
#else 
typedef unsigned __int64	uid;
typedef unsigned __int64	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_maxpage		32768	// maximum page size
#define BT_minpage		512		// minimum page size

#define BT_hashsize		512		// size of hash index for page cache
#define BT_hashmax		1024	// size of page cache
#define BT_hashprime	8191	// prime number for hashing

typedef enum{
	BtLockRead = 0,
	BtLockWrite
}BtLock;

typedef struct {
	unsigned char id[6];		// id associated with key
	ushort off:15;				// page offset for key start
	ushort dead:1;				// set for deleted key
	uint tod;					// time-stamp for key
} BtSlot;

typedef struct {
	unsigned char len;
	unsigned char key[1];
} *BtKey;

typedef struct {
	ushort cnt;					// count of keys in page
	unsigned char lvl:4;		// level of page
	unsigned char bits:4;		// page size in bits
	unsigned char filler;		// unused 
	unsigned char right[6];		// page number to right
	BtSlot keys[2];				// nxt avail/array of key offsets in page
} *BtPage;

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;

typedef struct _BtDb {
	int page_bits;		// each page size	
	uid page_no;		// current page number	
	int  err;
#ifdef unix
	int idx;
#else
	HANDLE idx;
	HANDLE hmap;
#endif
	unsigned char *mem;			// frame, cursor, page memory buffer
	ushort cache[BT_hashsize];	// hash index for cache
	BtHash nodes[BT_hashmax+1];	// page cache
	BtHash *lrufirst;			// lru list head
	BtHash *lrulast;			// lru list tail
	int nodecnt;				// highest number of cache block in use
	int hashmask;				// number of pages in cache block - 1

	uint mode;			// read-write mode
	BtPage alloc;		// frame buffer for alloc page ( page 0 )
	BtPage cursor;		// cached frame for start/next
	BtPage frame;		// spare frame for the page split
	BtPage page;		// current page
} 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);
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 uint 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);
extern BtKey bt_key (BtDb *bt, uint slot);
extern uid bt_uid (BtDb *bt, uint slot);
extern uint bt_tod (BtDb *bt, uint slot);

#define ALLOC_page		0
#define ROOT_page		1
#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.

//	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 leaf keys are marked with a dead bit, but
//	are not removed until the page is cleaned.
//	Pages are never deleted.

//  Under unix, 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 cache block size parameter 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.

//  Read locks are acquired only under Memory Mapping.
//  Page 0 is dedicated to lock for new page extensions.

#define keyptr(slot) ((BtKey)((char*)bt->page + bt->page->keys[slot].off))

void bt_putid(unsigned char *dest, uid id)
{
int i = 6;

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

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

	for( i = 0; i < 6; i++ )
		id <<= 8, id |= *src++; 

	return id;
}

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 = LOCKFILE_FAIL_IMMEDIATELY;
OVERLAPPED ovl[1];
uint cnt = 5000;
#endif

#ifndef linux
	if ( !bt->hashmask && mode != BtLockWrite )
		return 0;
#endif

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

	lock->l_start = off;
	lock->l_type = mode == BtLockWrite ? 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;

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

	do if( LockFileEx (bt->idx, flags, 0, sizeof(*bt->page), 0L, ovl) )
		return bt->err = 0;
	  else
		Sleep (1);
	while( cnt-- );

	return bt->err = BTERR_lock;
#endif 
}

// remove lock on first or second BtPage bytes
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];
#endif

#ifndef linux
	if ( !bt->hashmask && mode != BtLockWrite )
		return 0;
#endif

#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;
	if( !UnlockFileEx (bt->idx, 0, sizeof(*bt->page), 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);
#else
	if( hash = bt->lrufirst )
	  do
	  {
		UnmapViewOfFile(hash->page);
		CloseHandle(hash->hmap);
	  } while(hash = hash->lrunext);

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

//  open/create new btree
//	call with file_name, BT_openmode, bits in page size (e.g. 12),
//		size of map cache-frame (e.g. 65536) or zero for no mapping.

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

#ifdef unix
int flag;
#else
SYSTEM_INFO sysinfo[1];
int cnt = 5000;
#endif

#ifdef unix
	bt = malloc (sizeof(BtDb));
	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), bt=NULL;
	
#else
	bt = GlobalAlloc (GMEM_FIXED|GMEM_ZEROINIT, sizeof(BtDb));
	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( cacheblk )
		if( *amt = cacheblk % sysinfo->dwAllocationGranularity )
			cacheblk += sysinfo->dwAllocationGranularity - *amt;
#endif

	// determine sanity of page size

	if( bits > 15 )
		bits = 15;
	else if( bits < 9 )
		bits = 9;

	bt->page_bits = bits;
	bt->mode = mode;

	if ( mode == BT_ro )
	{
#ifdef unix
		*amt = 0;
		size = lseek (bt->idx, 0L, 2);
#else
		size = GetFileSize(bt->idx, amt);
#endif
		if ( !size && !*amt )
		{
			bt_close (bt);
			return NULL;
		}
	}

	if ( lockmode == BtLockWrite )
	  if ( bt_lockpage(bt, ALLOC_page, lockmode) )
	  {
		bt_close (bt);
		return NULL;
	  }

#ifdef unix
	bt->mem = malloc (4 *BT_maxpage);
	bt->frame = (BtPage)bt->mem;
	bt->cursor = (BtPage)(bt->mem + BT_maxpage);
	bt->page = (BtPage)(bt->mem + 2 * BT_maxpage);
	bt->alloc = (BtPage)(bt->mem + 3 * BT_maxpage);
	flag = PROT_READ | ( bt->mode == BT_ro ? 0 : PROT_WRITE );

	if( size = lseek (bt->idx, 0L, 2) ) {

		// read minimum page size to get root info
		pread(bt->idx, bt->frame, BT_minpage, 0);
		bt->page_bits = bt->frame->bits;

		if ( cacheblk && (cacheblk < (4U << bt->page_bits)) )
			cacheblk = 4U << bt->page_bits;
		if( cacheblk )
			bt->hashmask = (cacheblk >> bt->page_bits) - 1;

		size >>= bt->page_bits;

		// disallow mapping if file is not multiple of mapping

		if( size & bt->hashmask )
			bt->hashmask = 0;

		if ( lockmode == BtLockWrite )
		  if ( bt_unlockpage(bt, ALLOC_page, lockmode) )
		  {
			bt_close (bt);
			return NULL;
		  }
		return bt;
	}
#else
	bt->mem = VirtualAlloc(NULL, 4 * BT_maxpage, MEM_COMMIT, PAGE_READWRITE);

	bt->frame = (BtPage)bt->mem;
	bt->cursor = (BtPage)(bt->mem + BT_maxpage);
	bt->page = (BtPage)(bt->mem + 2 * BT_maxpage);
	bt->alloc = (BtPage)(bt->mem + 3 * BT_maxpage);

	size = GetFileSize(bt->idx, amt);

	if( size || *amt ) {
		// read minimum page size to get page 0 info
		do if( ReadFile(bt->idx, (char *)bt->frame, BT_minpage, amt, NULL) )
			break;
		  else
			Sleep (1);
		while( --cnt );

		if( !cnt || (*amt < BT_minpage) )
		{
			bt_close (bt);
			return NULL;
		}

		bt->page_bits = bt->frame->bits;

		if ( cacheblk && (cacheblk < (4U << bt->page_bits)) )
			cacheblk = 4U << bt->page_bits;

		if( cacheblk )
			bt->hashmask = (cacheblk >> bt->page_bits) - 1; 

		size >>= bt->page_bits;

		if( size & bt->hashmask )
			bt->hashmask = 0;

		if ( lockmode == BtLockWrite )
		  if ( bt_unlockpage(bt, ALLOC_page, lockmode) )
		  {
			bt_close (bt);
			return NULL;
		  }
		return bt;
	}
#endif

	// set page-cache mapping size in pages

	if ( cacheblk && (cacheblk < (4U << bt->page_bits)) )
		cacheblk = 4U << bt->page_bits;
	if( cacheblk )
		bt->hashmask = (cacheblk >> bt->page_bits) - 1; 

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

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

#ifdef unix
	if( write (bt->idx, bt->alloc, 1U << bt->page_bits) < (1U << bt->page_bits) )
		bt->err = BTERR_wrt;
#else
	WriteFile (bt->idx, (char *)bt->alloc, (1 << bt->page_bits), amt, NULL);
	if( *amt < (1U << bt->page_bits) )
		bt->err = BTERR_wrt;
#endif

	memset (bt->frame, 0, (1 << bt->page_bits));
	bt->frame->bits = bt->page_bits;
	for( lvl=MIN_lvl; lvl--; ) {
		bt_putid(bt->frame->keys[0].id, MIN_lvl - lvl);			// current page number
		bt->frame->keys[0].off = (1 << bt->page_bits) - 3;
		bt->frame->keys[1].off = (1 << bt->page_bits) - 3;
		bt_putid(bt->frame->keys[1].id, lvl ? MIN_lvl - lvl + 1 : 0);		// next(lower) page number
		bt->frame->lvl = lvl;
		bt->frame->cnt = 1;
		key = (BtKey)((unsigned char *)bt->frame + (1 << bt->page_bits) - 3);
		key->len = 2;			// create stopper key
		key->key[0] = 0xff;
		key->key[1] = 0xff;
#ifdef unix
		if( write (bt->idx, bt->frame, 1 << bt->page_bits) < (1 << bt->page_bits) )
			bt->err = BTERR_wrt;
#else
		WriteFile (bt->idx, (char *)bt->frame, (1 << bt->page_bits), amt, NULL);
		if( *amt < (1U << bt->page_bits) )
			bt->err = BTERR_wrt;
#endif
	}

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

	if( bt->hashmask ) {
		memset(bt->frame, 0, 1 << bt->page_bits);

#ifdef unix
		pwrite(bt->idx, bt->frame, 1 << bt->page_bits, bt->hashmask << bt->page_bits);
#else
		SetFilePointer (bt->idx, bt->hashmask << bt->page_bits, NULL, FILE_BEGIN);
		WriteFile (bt->idx, (char *)bt->frame, (1 << bt->page_bits), amt, NULL);
		if( *amt < (1U << bt->page_bits) )
			bt->err = BTERR_wrt;
#endif
	}

	bt_unlockpage(bt, ALLOC_page, lockmode);

	if( bt->err )
		return bt_close (bt), 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;
}

// place write or read lock on first or second BtPage bytes
// of requested page_no.

//  Update current page of btree
//	Under unix, the pages are optionally mapped directly into memory
//	and are updated immediately.

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

#ifdef unix
    if ( !bt->hashmask )
	 if ( pwrite(bt->idx, page, 1 << bt->page_bits, off) != (1 << bt->page_bits) )
		 return bt->err = BTERR_wrt;
#else
uint amt[1];
	if ( !bt->hashmask )
	{
		SetFilePointer (bt->idx, (long)off, (long*)(&off)+1, FILE_BEGIN);
		WriteFile (bt->idx, (char *)page, (1 << bt->page_bits), amt, NULL);
		
		if( *amt < (1U << bt->page_bits) )
			return GetLastError(), bt->err = BTERR_wrt;
	}
#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_hashprime % 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->hashmask) * BT_hashprime % 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->hashmask) * BT_hashprime % 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( (int)hash->page == -1 )
		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_hashmax ) {
		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
		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
//  and place a ro/wr lock on the page

BTERR bt_mappage (BtDb *bt, BtPage *page, uid page_no)
{
off64_t off = page_no << bt->page_bits;
#ifndef unix
int amt[1], cnt = 500;
#endif
	
	if( bt->hashmask ) {
		bt->err = 0;
		*page = bt_hashpage(bt, page_no);
		return bt->err;
	}

#ifdef unix
	if ( pread(bt->idx, *page, 1 << bt->page_bits, off) < (1 << bt->page_bits) )
		return bt->err = BTERR_map;
#else
	SetFilePointer (bt->idx, (long)off, (long*)(&off)+1, FILE_BEGIN);

	do if( ReadFile(bt->idx, *page, 1 << bt->page_bits, amt, NULL) )
		break;
	  else
		Sleep (1);
	while( --cnt );

	if( !cnt || (*amt <  (1 << bt->page_bits)) )
		return bt->err = BTERR_map;

#endif
	return 0;
}

//  find slot in page for given key at a given level

int bt_findslot (BtDb *bt, unsigned char *key, uint len)
{
uint diff, higher = (int)bt->page->cnt + 1, low = 1, slot;
uint good = 0;

	//	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(slot), key, len) < 0 )
			low = slot + 1;
		else
			higher = slot, good++;
	}

	//	return zero if last key < given key

	return good ? higher : 0;
}

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

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

	//  start at root of btree and drill down

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

		bt->page_no = page_no;
		if( bt_lockpage(bt, bt->page_no, mode) )
			return 0;

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

		// re-read and re-lock root after determining actual level of root
		if( bt->page->lvl != drill) {
			if ( bt->page_no != ROOT_page )
				return bt->err = BTERR_struct, 0;
			
			drill = bt->page->lvl;
			if( lock == BtLockWrite && drill == lvl )
			  if( bt_unlockpage(bt, page_no, mode) )
				return 0;
			  else
				continue;
		}

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

		if( slot = bt_findslot (bt, key, len) )
		  if( drill > lvl )
			page_no = (uint)bt_getid(bt->page->keys[slot].id), drill--;
		  else
			return slot;

		//  or slide right into next page

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

		//  release lock

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

		// break on end of right chain and return error
		if( !page_no )
			break;

	} while( 1 );

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

//  find and delete key on page by marking delete flag bit

BTERR bt_deletekey (BtDb *bt, unsigned char *key, uint len)
{
int dirty = 0;
BtKey nxt;
uint slot;

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

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

	if( !keycmp (nxt, key, len) )
		if( bt->page->keys[slot].dead == 0 )
			dirty = bt->page->keys[slot].dead = 1;

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

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

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

uint bt_findkey (BtDb *bt, unsigned char *key, uint len)
{
uint  slot;
BtKey nxt;

	if( slot = bt_loadpage (bt, key, len, 0, BtLockRead) )
		nxt = keyptr(slot);
	else
		return 0;

	// if key exists, return id

	if( bt->page->keys[slot].dead || memcmp (nxt->key, key, len) )
	  slot = 0;

	memcpy(bt->cursor, bt->page, (1 << bt->page_bits));
	if ( bt_unlockpage(bt, bt->page_no, BtLockRead) )
		return 0;

	return slot;
}

void bt_cleanpage(BtDb *bt)
{
uint off = (1 << bt->page_bits);
BtPage page = bt->page;
uint cnt = 0, idx = 0;
uint max = page->cnt;
BtKey key;

	memcpy (bt->frame, page, (1 << bt->page_bits));

	// skip page info and set rest of page to zero
	memset (page+1, 0, (1 << bt->page_bits) - sizeof(*page));

	// try cleaning up page first

	while( cnt++ < max ) {
		if( cnt < max && bt->frame->keys[cnt].dead )
			continue;
		key = (BtKey)((unsigned char *)bt->frame + bt->frame->keys[cnt].off);
		off -= key->len + 1;
		memcpy ((unsigned char *)page + off, key, key->len + 1);
		page->keys[++idx].dead = bt->frame->keys[cnt].dead;
		memcpy(page->keys[idx].id, bt->frame->keys[cnt].id, sizeof(bt->frame->keys->id));
		page->keys[idx].tod = bt->frame->keys[cnt].tod;
		page->keys[idx].off = off;
	}
	memcpy(page->keys[0].id,bt->frame->keys[0].id, sizeof(bt->frame->keys->id));
	page->keys[0].off = off;
	page->cnt = idx;
}

//	allocate a new page and write page into it

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

	// lock page zero and allocate empty page
	if ( bt_lockpage(bt, ALLOC_page, BtLockWrite) )
		return 0;

	if( bt_mappage (bt, &bt->alloc, ALLOC_page) )
		return 0;
	new_page = bt_getid(bt->alloc->right);
	bt_putid(bt->alloc->right, new_page+1);

	if( !bt->hashmask ) {
		bt_putid(page->keys[0].id, new_page);
		if( bt_update(bt, bt->alloc, ALLOC_page) )
			return 0;	// don't unlock on error
		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;
	}

	bt_putid(page->keys[0].id, new_page);
#ifdef unix
	if ( pwrite(bt->idx, page, 1 << bt->page_bits, new_page << bt->page_bits) < (1 << bt->page_bits) )
		return bt->err = BTERR_wrt, 0;

	// write last page in the block
	if ( (new_page & bt->hashmask) == 0 )
	{
		// use ALLOC_page buffer to write zeros
		memset((bt->mem + 3 * BT_maxpage), 0, 1 << bt->page_bits);
		if ( pwrite(bt->idx,bt->mem + 3 * BT_maxpage, 1 << bt->page_bits, (new_page | bt->hashmask) << bt->page_bits) < (1 << bt->page_bits) )
			return bt->err = BTERR_wrt, 0;
	}
#else
	//	bring end of file area into page-cache
	if( !(pmap = (char*)bt_hashpage(bt, new_page & ~bt->hashmask)) )
		return 0;
	memcpy(pmap+((new_page & bt->hashmask) << bt->page_bits), page, 1 << bt->page_bits);
	if ( (new_page & bt->hashmask) == 0 )
		memset(pmap+(bt->hashmask << bt->page_bits), 0, 1 << bt->page_bits);
#endif

	// unlock page zero 
	if ( bt_unlockpage(bt, ALLOC_page, BtLockWrite) )
		return 0;

	return new_page;
}

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

BTERR bt_splitroot(BtDb *bt,  unsigned char *newkey, unsigned char *oldkey, uid page_no2)
{
BtPage root = bt->page;
uid new_page;
uint off;

	bt_putid(root->right, page_no2);

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

	// preserve the page info and set rest to zero

	memset(root+1, 0, (1 << bt->page_bits) - sizeof(*root));

	// insert first key on newroot page

	off = (1 << bt->page_bits) - *newkey - 1;
	bt_putid(root->keys[1].id, new_page);
	root->keys[1].off = off ;
	((unsigned char *)root)[off] = *newkey;
	memcpy ((unsigned char *)root + off +1, newkey+1, *newkey);
	
	// insert second key on newroot page

	off -= *oldkey + 1;
	bt_putid(root->keys[2].id, page_no2);
	root->keys[2].off = off;
	((unsigned char *)root)[off] = *oldkey;
	memcpy ((unsigned char *)root + off +1, oldkey+1, *oldkey);

	root->keys[0].off = off;		// reset lowest used offset
	root->cnt = 2;
	root->lvl++;
	bt_putid(root->right, 0);

	// update and release root (bt->page)
	if( bt_update(bt, root, bt->page_no) )
		return bt->err;

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

//  repair/split full block

BTERR bt_splitpage (BtDb *bt, uint len)
{
unsigned char oldkey[256], lowerkey[256];
uint cnt = 0, idx = 0, max, off;
uid page_no = bt->page_no;
BtPage page = bt->page;
uint lvl = page->lvl;
uid new_page;
uint tod = 0;
BtKey key;

    if ( lvl == 0 )
	{
		bt_cleanpage(bt);
		if( page->keys[0].off >= page->cnt * sizeof(BtSlot) + sizeof(*page) + len + 1)
		{
			if ( bt_update(bt, bt->page, bt->page_no) )
				return bt->err;
			return bt_unlockpage(bt, page_no, BtLockWrite);
		}
	}

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

	tod = (uint)time(NULL);

	memset (bt->frame, 0, (1 << bt->page_bits));
	off = (1 << bt->page_bits);
	max = (int)page->cnt;
	cnt = max / 2;
	idx = 0;

	while( cnt++ < max ) {
		key = (BtKey)((char*)page + page->keys[cnt].off);
		off -= key->len + 1;
		memcpy ((unsigned char *)bt->frame + off, key, key->len + 1);
		memcpy(bt->frame->keys[++idx].id, page->keys[cnt].id, sizeof(page->keys->id));
		bt->frame->keys[idx].tod = page->keys[cnt].tod;
		bt->frame->keys[idx].off = off;
	}

	// remember existing page key for new page

	memcpy (oldkey, key, key->len + 1);

	bt->frame->keys[0].off = off;
	bt->frame->cnt = idx;
	bt->frame->lvl = lvl;

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

	// get new free page

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

	// update lower keys to continue in old page

	memcpy (bt->frame, page, (1 << bt->page_bits));
	memset (page+1, 0, (1 << bt->page_bits) - sizeof(*page));
	off = (1 << bt->page_bits);
	cnt = 0;
	idx = 0;

	//  assemble page of smaller keys

	while( cnt++ < max / 2 ) {
		key = (BtKey)((char*)bt->frame + bt->frame->keys[cnt].off);
		off -= key->len + 1;
		memcpy ((unsigned char *)page + off, key, key->len + 1);
		memcpy(page->keys[++idx].id, bt->frame->keys[cnt].id, sizeof(bt->frame->keys->id));
		page->keys[idx].tod = bt->frame->keys[cnt].tod;
		page->keys[idx].off = off;
	}

	bt_putid(page->keys[0].id, page_no);
	bt_putid(page->right, new_page);
	page->keys[0].off = off;
	page->cnt = idx;
	key = (BtKey)((char*)page + off);
	memcpy(lowerkey+1, key->key, key->len);
	*lowerkey = key->len;

	// split the root page, if current page is the root page
	if( page_no == ROOT_page )
		return bt_splitroot (bt, lowerkey, oldkey, new_page);

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

	// insert key for reformulated lower block

	if( bt_insertkey (bt, lowerkey+1, *lowerkey, lvl + 1, page_no, tod) )
		return bt->err;

	//  release wr lock on lower page

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

	// fix key for newly allocated higher block page

	return bt_insertkey (bt, oldkey+1, *oldkey, lvl + 1, new_page, tod);
}

//  insert new key into the btree at requested level
//  level zero pages are leaf pages

BTERR bt_insertkey (BtDb *bt, unsigned char *key, uint len, uint lvl, uid id, uint tod)
{
uint off, slot, idx;
BtPage page;
BtKey nxt;

  while( 1 ) {
	if( slot = bt_loadpage (bt, key, len, lvl, BtLockWrite) )
		nxt = keyptr(slot);
	else
	{
		if ( !bt->err )
			bt->err = BTERR_ovflw;
		return bt->err;
	}
	// if key already exists, update id

	page = bt->page;

	if( !keycmp (nxt, key, len) ) {
		page->keys[slot].dead = 0;
		bt_putid(page->keys[slot].id, id);
		page->keys[slot].tod = tod;
		if ( bt_update(bt, bt->page, bt->page_no) )
			return bt->err;
		return bt_unlockpage(bt, bt->page_no, BtLockWrite);
	}

	// check if page has enough space
	if( page->keys[0].off >= page->cnt * sizeof(BtSlot) + sizeof(*page) + len + 1)
		break;

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

  // calculate next available slot and copy key into page

  off = page->keys[0].off - len - 1;
  page->keys[0].off = off; // reset lowest used offset
  ((unsigned char *)page)[off] = len;
  memcpy ((unsigned char *)page + off +1, key, len );

  // now insert key into array before slot

  for( idx = (int)++page->cnt; idx > slot; idx-- )
	page->keys[idx] = page->keys[idx - 1];

  page->keys[slot].off = off;
  page->keys[slot].dead = 0;
  bt_putid(page->keys[slot].id, id);
  page->keys[slot].tod = tod;
  if ( bt_update(bt, bt->page, bt->page_no) )
	  return bt->err;

  return bt_unlockpage(bt, bt->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)
{
uint slot;

	// cache page for retrieval
	if( slot = bt_loadpage (bt, key, len, 0, BtLockRead) )
		memcpy (bt->cursor, bt->page, (1 << bt->page_bits));

	if ( bt_unlockpage(bt, bt->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)
{
off64_t right;

  do {
	right = bt_getid(bt->cursor->right);
	while( slot++ < bt->cursor->cnt )
	  if( bt->cursor->keys[slot].dead )
		continue;
	  else if( right || (slot < bt->cursor->cnt))
		return slot;
	  else
		break;

	if( !right )
		break;

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

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

	memcpy (bt->cursor, bt->page, (1 << bt->page_bits));
	if ( bt_unlockpage(bt, right, BtLockRead) )
		return 0;

	slot = 0;
  } while( 1 );

  return 0;
}

BtKey bt_key(BtDb *bt, uint slot)
{
	return (BtKey)((unsigned char*)bt->cursor + bt->cursor->keys[slot].off);
}

uid bt_uid(BtDb *bt, uint slot)
{
	return bt_getid(bt->cursor->keys[slot].id);
}

uint bt_tod(BtDb *bt, uint slot)
{
	return bt->cursor->keys[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;
unsigned char key[256];
int dead, ch, cnt = 0;
uint done, start;
int bits = 12;
uint scan = 0;
uint len = 0;
BtKey nxt;
BtDb *bt;
FILE *in;

	if( argc < 3 ) {
		printf ("Usage: %s index_file src_file read/write/scan [page_bits line_off]", argv[0]);
		exit(0);
	}

	start = (uint)time(NULL);

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

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

	//  use memory mapping in 64K chunks

	bt = bt_open ((argv[1]), BT_rw, bits, 65536);

	switch(argv[3][0]| 0x20)
	{
	case 'w':
		printf("started indexing for %s at: %ld\n", argv[2], start);
		if( argc > 2 && (in = fopen (argv[2], "rb")) )
		  while( ch = getc(in), ch != EOF )
			if( ch == '\n' )
			{
			  if( off )
		  		sprintf((char *)key+len, "%.8d", line + off), len += 8;

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

	case 'd':
		printf("started deleting keys for %s at: %ld\n", argv[2], start);
		if( argc > 2 && (in = fopen (argv[2], "rb")) )
		  while( ch = getc(in), ch != EOF )
			if( ch == '\n' )
			{
			  if( off )
		  		sprintf((char *)key+len, "%.8d", line + off), len += 8;
			  line++;
			  if( bt_deletekey (bt, key, len) )
				printf("Error %d Line: %d\n", bt->err, line), exit(0);
			  len = 0;
			}
			else if( len < 245 )
				key[len++] = ch;
		printf("finished deleting keys, %d \n", line);
		break;

	case 's':
		scan++;
		break;

	}

	dead = cnt = len = 0;

	printf("started reading, %d \n", line);

	if( slot = bt_startkey (bt, key, len) )
	  slot--;
	else
	  printf("Error %d in StartKey. Line: %d\n", bt->err, line), exit(0);

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

	done = (uint)time(NULL);
	printf("Done at : %ld \n Time to complete: %ld seconds\n", done, done - start);
	printf(" Total keys read %d \n", cnt);
}

#endif	//STANDALONE