/*

 ** 2004 April 6

 **

 ** The author disclaims copyright to this source code.  In place of

 ** a legal notice, here is a blessing:

 **

 **    May you do good and not evil.

 **    May you find forgiveness for yourself and forgive others.

 **    May you share freely, never taking more than you give.

 **

 *************************************************************************

 ** $Id$

 **

 ** This file implements a external (disk-based) database using BTrees.

 ** For a detailed discussion of BTrees, refer to

 **

 **     Donald E. Knuth, THE ART OF COMPUTER PROGRAMMING, Volume 3:

 **     "Sorting And Searching", pages 473-480. Addison-Wesley

 **     Publishing Company, Reading, Massachusetts.

 **

 ** The basic idea is that each page of the file contains N database

 ** entries and N+1 pointers to subpages.

 **

 **   ----------------------------------------------------------------

 **   |  Ptr(0) | Key(0) | Ptr(1) | Key(1) | ... | Key(N-1) | Ptr(N) |

 **   ----------------------------------------------------------------

 **

 ** All of the keys on the page that Ptr(0) points to have values less

 ** than Key(0).  All of the keys on page Ptr(1) and its subpages have

 ** values greater than Key(0) and less than Key(1).  All of the keys

 ** on Ptr(N) and its subpages have values greater than Key(N-1).  And

 ** so forth.

 **

 ** Finding a particular key requires reading O(log(M)) pages from the 

 ** disk where M is the number of entries in the tree.

 **

 ** In this implementation, a single file can hold one or more separate 

 ** BTrees.  Each BTree is identified by the index of its root page.  The

 ** key and data for any entry are combined to form the "payload".  A

 ** fixed amount of payload can be carried directly on the database

 ** page.  If the payload is larger than the preset amount then surplus

 ** bytes are stored on overflow pages.  The payload for an entry

 ** and the preceding pointer are combined to form a "Cell".  Each 

 ** page has a small header which contains the Ptr(N) pointer and other

 ** information such as the size of key and data.

 **

 ** FORMAT DETAILS

 **

 ** The file is divided into pages.  The first page is called page 1,

 ** the second is page 2, and so forth.  A page number of zero indicates

 ** "no such page".  The page size can be anything between 512 and 65536.

 ** Each page can be either a btree page, a freelist page or an overflow

 ** page.

 **

 ** The first page is always a btree page.  The first 100 bytes of the first

 ** page contain a special header (the "file header") that describes the file.

 ** The format of the file header is as follows:

 **

 **   OFFSET   SIZE    DESCRIPTION

 **      0      16     Header string: "SQLite format 3\000"

 **     16       2     Page size in bytes.  

 **     18       1     File format write version

 **     19       1     File format read version

 **     20       1     Bytes of unused space at the end of each page

 **     21       1     Max embedded payload fraction

 **     22       1     Min embedded payload fraction

 **     23       1     Min leaf payload fraction

 **     24       4     File change counter

 **     28       4     Reserved for future use

 **     32       4     First freelist page

 **     36       4     Number of freelist pages in the file

 **     40      60     15 4-byte meta values passed to higher layers

 **

 ** All of the integer values are big-endian (most significant byte first).

 **

 ** The file change counter is incremented when the database is changed

 ** This counter allows other processes to know when the file has changed

 ** and thus when they need to flush their cache.

 **

 ** The max embedded payload fraction is the amount of the total usable

 ** space in a page that can be consumed by a single cell for standard

 ** B-tree (non-LEAFDATA) tables.  A value of 255 means 100%.  The default

 ** is to limit the maximum cell size so that at least 4 cells will fit

 ** on one page.  Thus the default max embedded payload fraction is 64.

 **

 ** If the payload for a cell is larger than the max payload, then extra

 ** payload is spilled to overflow pages.  Once an overflow page is allocated,

 ** as many bytes as possible are moved into the overflow pages without letting

 ** the cell size drop below the min embedded payload fraction.

 **

 ** The min leaf payload fraction is like the min embedded payload fraction

 ** except that it applies to leaf nodes in a LEAFDATA tree.  The maximum

 ** payload fraction for a LEAFDATA tree is always 100% (or 255) and it

 ** not specified in the header.

 **

 ** Each btree pages is divided into three sections:  The header, the

 ** cell pointer array, and the cell content area.  Page 1 also has a 100-byte

 ** file header that occurs before the page header.

 **

 **      |----------------|

 **      | file header    |   100 bytes.  Page 1 only.

 **      |----------------|

 **      | page header    |   8 bytes for leaves.  12 bytes for interior nodes

 **      |----------------|

 **      | cell pointer   |   |  2 bytes per cell.  Sorted order.

 **      | array          |   |  Grows downward

 **      |                |   v

 **      |----------------|

 **      | unallocated    |

 **      | space          |

 **      |----------------|   ^  Grows upwards

 **      | cell content   |   |  Arbitrary order interspersed with freeblocks.

 **      | area           |   |  and free space fragments.

 **      |----------------|

 **

 ** The page headers looks like this:

 **

 **   OFFSET   SIZE     DESCRIPTION

 **      0       1      Flags. 1: intkey, 2: zerodata, 4: leafdata, 8: leaf

 **      1       2      byte offset to the first freeblock

 **      3       2      number of cells on this page

 **      5       2      first byte of the cell content area

 **      7       1      number of fragmented free bytes

 **      8       4      Right child (the Ptr(N) value).  Omitted on leaves.

 **

 ** The flags define the format of this btree page.  The leaf flag means that

 ** this page has no children.  The zerodata flag means that this page carries

 ** only keys and no data.  The intkey flag means that the key is a integer

 ** which is stored in the key size entry of the cell header rather than in

 ** the payload area.

 **

 ** The cell pointer array begins on the first byte after the page header.

 ** The cell pointer array contains zero or more 2-byte numbers which are

 ** offsets from the beginning of the page to the cell content in the cell

 ** content area.  The cell pointers occur in sorted order.  The system strives

 ** to keep free space after the last cell pointer so that new cells can

 ** be easily added without having to defragment the page.

 **

 ** Cell content is stored at the very end of the page and grows toward the

 ** beginning of the page.

 **

 ** Unused space within the cell content area is collected into a linked list of

 ** freeblocks.  Each freeblock is at least 4 bytes in size.  The byte offset

 ** to the first freeblock is given in the header.  Freeblocks occur in

 ** increasing order.  Because a freeblock must be at least 4 bytes in size,

 ** any group of 3 or fewer unused bytes in the cell content area cannot

 ** exist on the freeblock chain.  A group of 3 or fewer free bytes is called

 ** a fragment.  The total number of bytes in all fragments is recorded.

 ** in the page header at offset 7.

 **

 **    SIZE    DESCRIPTION

 **      2     Byte offset of the next freeblock

 **      2     Bytes in this freeblock

 **

 ** Cells are of variable length.  Cells are stored in the cell content area at

 ** the end of the page.  Pointers to the cells are in the cell pointer array

 ** that immediately follows the page header.  Cells is not necessarily

 ** contiguous or in order, but cell pointers are contiguous and in order.

 **

 ** Cell content makes use of variable length integers.  A variable

 ** length integer is 1 to 9 bytes where the lower 7 bits of each 

 ** byte are used.  The integer consists of all bytes that have bit 8 set and

 ** the first byte with bit 8 clear.  The most significant byte of the integer

 ** appears first.  A variable-length integer may not be more than 9 bytes long.

 ** As a special case, all 8 bytes of the 9th byte are used as data.  This

 ** allows a 64-bit integer to be encoded in 9 bytes.

 **

 **    0x00                      becomes  0x00000000

 **    0x7f                      becomes  0x0000007f

 **    0x81 0x00                 becomes  0x00000080

 **    0x82 0x00                 becomes  0x00000100

 **    0x80 0x7f                 becomes  0x0000007f

 **    0x8a 0x91 0xd1 0xac 0x78  becomes  0x12345678

 **    0x81 0x81 0x81 0x81 0x01  becomes  0x10204081

 **

 ** Variable length integers are used for rowids and to hold the number of

 ** bytes of key and data in a btree cell.

 **

 ** The content of a cell looks like this:

 **

 **    SIZE    DESCRIPTION

 **      4     Page number of the left child. Omitted if leaf flag is set.

 **     var    Number of bytes of data. Omitted if the zerodata flag is set.

 **     var    Number of bytes of key. Or the key itself if intkey flag is set.

 **      *     Payload

 **      4     First page of the overflow chain.  Omitted if no overflow

 **

 ** Overflow pages form a linked list.  Each page except the last is completely

 ** filled with data (pagesize - 4 bytes).  The last page can have as little

 ** as 1 byte of data.

 **

 **    SIZE    DESCRIPTION

 **      4     Page number of next overflow page

 **      *     Data

 **

 ** Freelist pages come in two subtypes: trunk pages and leaf pages.  The

 ** file header points to the first in a linked list of trunk page.  Each trunk

 ** page points to multiple leaf pages.  The content of a leaf page is

 ** unspecified.  A trunk page looks like this:

 **

 **    SIZE    DESCRIPTION

 **      4     Page number of next trunk page

 **      4     Number of leaf pointers on this page

 **      *     zero or more pages numbers of leaves

 */

 #include "sqliteInt.h"

 #include "pager.h"

 #include "btree.h"

 #include "os.h"

 #include <assert.h>

 /* Round up a number to the next larger multiple of 8.  This is used

 ** to force 8-byte alignment on 64-bit architectures.

 */

 #define ROUND8(x)   ((x+7)&~7)

 /* The following value is the maximum cell size assuming a maximum page

 ** size give above.

 */

 #define MX_CELL_SIZE(pBt)  (pBt->pageSize-8)

 /* The maximum number of cells on a single page of the database.  This

 ** assumes a minimum cell size of 3 bytes.  Such small cells will be

 ** exceedingly rare, but they are possible.

 */

 #define MX_CELL(pBt) ((pBt->pageSize-8)/3)

 /* Forward declarations */

 typedef struct MemPage MemPage;

 typedef struct BtLock BtLock;

 /*

 ** This is a magic string that appears at the beginning of every

 ** SQLite database in order to identify the file as a real database.

 **

 ** You can change this value at compile-time by specifying a

 ** -DSQLITE_FILE_HEADER="..." on the compiler command-line.  The

 ** header must be exactly 16 bytes including the zero-terminator so

 ** the string itself should be 15 characters long.  If you change

 ** the header, then your custom library will not be able to read 

 ** databases generated by the standard tools and the standard tools

 ** will not be able to read databases created by your custom library.

 */

 #ifndef SQLITE_FILE_HEADER /* 123456789 123456 */

 #  define SQLITE_FILE_HEADER "SQLite format 3"

 #endif

 /*

 ** Page type flags.  An ORed combination of these flags appear as the

 ** first byte of on-disk image of every BTree page.

 */

 #define PTF_INTKEY    0x01

 #define PTF_ZERODATA  0x02

 #define PTF_LEAFDATA  0x04

 #define PTF_LEAF      0x08

 /*

 ** As each page of the file is loaded into memory, an instance of the following

 ** structure is appended and initialized to zero.  This structure stores

 ** information about the page that is decoded from the raw file page.

 **

 ** The pParent field points back to the parent page.  This allows us to

 ** walk up the BTree from any leaf to the root.  Care must be taken to

 ** unref() the parent page pointer when this page is no longer referenced.

 ** The pageDestructor() routine handles that chore.

 **

 ** Access to all fields of this structure is controlled by the mutex

 ** stored in MemPage.pBt->mutex.

 */

 struct MemPage {

   u8 isInit;           /* True if previously initialized. MUST BE FIRST! */

   u8 idxShift;         /* True if Cell indices have changed */

   u8 nOverflow;        /* Number of overflow cell bodies in aCell[] */

   u8 intKey;           /* True if intkey flag is set */

   u8 leaf;             /* True if leaf flag is set */

   u8 zeroData;         /* True if table stores keys only */

   u8 leafData;         /* True if tables stores data on leaves only */

   u8 hasData;          /* True if this page stores data */

   u8 hdrOffset;        /* 100 for page 1.  0 otherwise */

   u8 childPtrSize;     /* 0 if leaf==1.  4 if leaf==0 */

   u16 maxLocal;        /* Copy of BtShared.maxLocal or BtShared.maxLeaf */

   u16 minLocal;        /* Copy of BtShared.minLocal or BtShared.minLeaf */

   u16 cellOffset;      /* Index in aData of first cell pointer */

   u16 idxParent;       /* Index in parent of this node */

   u16 nFree;           /* Number of free bytes on the page */

   u16 nCell;           /* Number of cells on this page, local and ovfl */

   struct _OvflCell {   /* Cells that will not fit on aData[] */

     u8 *pCell;          /* Pointers to the body of the overflow cell */

     u16 idx;            /* Insert this cell before idx-th non-overflow cell */

   } aOvfl[5];

   BtShared *pBt;       /* Pointer to BtShared that this page is part of */

   u8 *aData;           /* Pointer to disk image of the page data */

   DbPage *pDbPage;     /* Pager page handle */

   Pgno pgno;           /* Page number for this page */

   MemPage *pParent;    /* The parent of this page.  NULL for root */

 };

 /*

 ** The in-memory image of a disk page has the auxiliary information appended

 ** to the end.  EXTRA_SIZE is the number of bytes of space needed to hold

 ** that extra information.

 */

 #define EXTRA_SIZE sizeof(MemPage)

 /* A Btree handle

 **

 ** A database connection contains a pointer to an instance of

 ** this object for every database file that it has open.  This structure

 ** is opaque to the database connection.  The database connection cannot

 ** see the internals of this structure and only deals with pointers to

 ** this structure.

 **

 ** For some database files, the same underlying database cache might be 

 ** shared between multiple connections.  In that case, each contection

 ** has it own pointer to this object.  But each instance of this object

 ** points to the same BtShared object.  The database cache and the

 ** schema associated with the database file are all contained within

 ** the BtShared object.

 **

 ** All fields in this structure are accessed under sqlite3.mutex.

 ** The pBt pointer itself may not be changed while there exists cursors 

 ** in the referenced BtShared that point back to this Btree since those

 ** cursors have to do go through this Btree to find their BtShared and

 ** they often do so without holding sqlite3.mutex.

 */

 struct Btree {

   sqlite3 *pSqlite;  /* The database connection holding this btree */

   BtShared *pBt;     /* Sharable content of this btree */

   u8 inTrans;        /* TRANS_NONE, TRANS_READ or TRANS_WRITE */

   u8 sharable;       /* True if we can share pBt with other pSqlite */

   u8 locked;         /* True if pSqlite currently has pBt locked */

   int wantToLock;    /* Number of nested calls to sqlite3BtreeEnter() */

   Btree *pNext;      /* List of other sharable Btrees from the same pSqlite */

   Btree *pPrev;      /* Back pointer of the same list */

 };

 /*

 ** Btree.inTrans may take one of the following values.

 **

 ** If the shared-data extension is enabled, there may be multiple users

 ** of the Btree structure. At most one of these may open a write transaction,

 ** but any number may have active read transactions.

 */

 #define TRANS_NONE  0

 #define TRANS_READ  1

 #define TRANS_WRITE 2

 /*

 ** An instance of this object represents a single database file.

 ** 

 ** A single database file can be in use as the same time by two

 ** or more database connections.  When two or more connections are

 ** sharing the same database file, each connection has it own

 ** private Btree object for the file and each of those Btrees points

 ** to this one BtShared object.  BtShared.nRef is the number of

 ** connections currently sharing this database file.

 **

 ** Fields in this structure are accessed under the BtShared.mutex

 ** mutex, except for nRef and pNext which are accessed under the

 ** global SQLITE_MUTEX_STATIC_MASTER mutex.  The pPager field

 ** may not be modified once it is initially set as long as nRef>0.

 ** The pSchema field may be set once under BtShared.mutex and

 ** thereafter is unchanged as long as nRef>0.

 */

 struct BtShared {

   Pager *pPager;        /* The page cache */

   BtCursor *pCursor;    /* A list of all open cursors */

   MemPage *pPage1;      /* First page of the database */

   u8 inStmt;            /* True if we are in a statement subtransaction */

   u8 readOnly;          /* True if the underlying file is readonly */

   u8 maxEmbedFrac;      /* Maximum payload as % of total page size */

   u8 minEmbedFrac;      /* Minimum payload as % of total page size */

   u8 minLeafFrac;       /* Minimum leaf payload as % of total page size */

   u8 pageSizeFixed;     /* True if the page size can no longer be changed */

 #ifndef SQLITE_OMIT_AUTOVACUUM

   u8 autoVacuum;        /* True if auto-vacuum is enabled */

   u8 incrVacuum;        /* True if incr-vacuum is enabled */

   Pgno nTrunc;          /* Non-zero if the db will be truncated (incr vacuum) */

 #endif

   u16 pageSize;         /* Total number of bytes on a page */

   u16 usableSize;       /* Number of usable bytes on each page */

   int maxLocal;         /* Maximum local payload in non-LEAFDATA tables */

   int minLocal;         /* Minimum local payload in non-LEAFDATA tables */

   int maxLeaf;          /* Maximum local payload in a LEAFDATA table */

   int minLeaf;          /* Minimum local payload in a LEAFDATA table */

   BusyHandler *pBusyHandler;   /* Callback for when there is lock contention */

   u8 inTransaction;     /* Transaction state */

   int nTransaction;     /* Number of open transactions (read + write) */

   void *pSchema;        /* Pointer to space allocated by sqlite3BtreeSchema() */

   void (*xFreeSchema)(void*);  /* Destructor for BtShared.pSchema */

   sqlite3_mutex *mutex; /* Non-recursive mutex required to access this struct */

 #ifndef SQLITE_OMIT_SHARED_CACHE

   int nRef;             /* Number of references to this structure */

   BtShared *pNext;      /* Next on a list of sharable BtShared structs */

   BtLock *pLock;        /* List of locks held on this shared-btree struct */

 #endif

 };

 /*

 ** An instance of the following structure is used to hold information

 ** about a cell.  The parseCellPtr() function fills in this structure

 ** based on information extract from the raw disk page.

 */

 typedef struct CellInfo CellInfo;

 struct CellInfo {

   u8 *pCell;     /* Pointer to the start of cell content */

   i64 nKey;      /* The key for INTKEY tables, or number of bytes in key */

   u32 nData;     /* Number of bytes of data */

   u32 nPayload;  /* Total amount of payload */

   u16 nHeader;   /* Size of the cell content header in bytes */

   u16 nLocal;    /* Amount of payload held locally */

   u16 iOverflow; /* Offset to overflow page number.  Zero if no overflow */

   u16 nSize;     /* Size of the cell content on the main b-tree page */

 };

 /*

 ** A cursor is a pointer to a particular entry within a particular

 ** b-tree within a database file.

 **

 ** The entry is identified by its MemPage and the index in

 ** MemPage.aCell[] of the entry.

 **

 ** When a single database file can shared by two more database connections,

 ** but cursors cannot be shared.  Each cursor is associated with a

 ** particular database connection identified BtCursor.pBtree.pSqlite.

 **

 ** Fields in this structure are accessed under the BtShared.mutex

 ** found at self->pBt->mutex. 

 */

 struct BtCursor {

   Btree *pBtree;            /* The Btree to which this cursor belongs */

   BtShared *pBt;            /* The BtShared this cursor points to */

   BtCursor *pNext, *pPrev;  /* Forms a linked list of all cursors */

   int (*xCompare)(void*,int,const void*,int,const void*); /* Key comp func */

   void *pArg;               /* First arg to xCompare() */

   Pgno pgnoRoot;            /* The root page of this tree */

   MemPage *pPage;           /* Page that contains the entry */

   int idx;                  /* Index of the entry in pPage->aCell[] */

   CellInfo info;            /* A parse of the cell we are pointing at */

   u8 wrFlag;                /* True if writable */

   u8 eState;                /* One of the CURSOR_XXX constants (see below) */

   void *pKey;      /* Saved key that was cursor's last known position */

   i64 nKey;        /* Size of pKey, or last integer key */

   int skip;        /* (skip<0) -> Prev() is a no-op. (skip>0) -> Next() is */

 #ifndef SQLITE_OMIT_INCRBLOB

   u8 isIncrblobHandle;      /* True if this cursor is an incr. io handle */

   Pgno *aOverflow;          /* Cache of overflow page locations */

 #endif

 };

 /*

 ** Potential values for BtCursor.eState.

 **

 ** CURSOR_VALID:

 **   Cursor points to a valid entry. getPayload() etc. may be called.

 **

 ** CURSOR_INVALID:

 **   Cursor does not point to a valid entry. This can happen (for example) 

 **   because the table is empty or because BtreeCursorFirst() has not been

 **   called.

 **

 ** CURSOR_REQUIRESEEK:

 **   The table that this cursor was opened on still exists, but has been 

 **   modified since the cursor was last used. The cursor position is saved

 **   in variables BtCursor.pKey and BtCursor.nKey. When a cursor is in 

 **   this state, restoreOrClearCursorPosition() can be called to attempt to

 **   seek the cursor to the saved position.

 **

 ** CURSOR_FAULT:

 **   A unrecoverable error (an I/O error or a malloc failure) has occurred

 **   on a different connection that shares the BtShared cache with this

 **   cursor.  The error has left the cache in an inconsistent state.

 **   Do nothing else with this cursor.  Any attempt to use the cursor

 **   should return the error code stored in BtCursor.skip

 */

 #define CURSOR_INVALID           0

 #define CURSOR_VALID             1

 #define CURSOR_REQUIRESEEK       2

 #define CURSOR_FAULT             3

 /*

 ** The TRACE macro will print high-level status information about the

 ** btree operation when the global variable sqlite3_btree_trace is

 ** enabled.

 */

 #if SQLITE_TEST

 # define TRACE(X)   if( sqlite3_btree_trace ){ printf X; fflush(stdout); }

 #else

 # define TRACE(X)

 #endif

 /*

 ** Routines to read and write variable-length integers.  These used to

 ** be defined locally, but now we use the varint routines in the util.c

 ** file.

 */

 #define getVarint    sqlite3GetVarint

 #define getVarint32(A,B)  ((*B=*(A))<=0x7f?1:sqlite3GetVarint32(A,B))

 #define putVarint    sqlite3PutVarint

 /* The database page the PENDING_BYTE occupies. This page is never used.

 ** TODO: This macro is very similary to PAGER_MJ_PGNO() in pager.c. They

 ** should possibly be consolidated (presumably in pager.h).

 **

 ** If disk I/O is omitted (meaning that the database is stored purely

 ** in memory) then there is no pending byte.

 */

 #ifdef SQLITE_OMIT_DISKIO

 # define PENDING_BYTE_PAGE(pBt)  0x7fffffff

 #else

 # define PENDING_BYTE_PAGE(pBt) ((PENDING_BYTE/(pBt)->pageSize)+1)

 #endif

 /*

 ** A linked list of the following structures is stored at BtShared.pLock.

 ** Locks are added (or upgraded from READ_LOCK to WRITE_LOCK) when a cursor 

 ** is opened on the table with root page BtShared.iTable. Locks are removed

 ** from this list when a transaction is committed or rolled back, or when

 ** a btree handle is closed.

 */

 struct BtLock {

   Btree *pBtree;        /* Btree handle holding this lock */

   Pgno iTable;          /* Root page of table */

   u8 eLock;             /* READ_LOCK or WRITE_LOCK */

   BtLock *pNext;        /* Next in BtShared.pLock list */

 };

 /* Candidate values for BtLock.eLock */

 #define READ_LOCK     1

 #define WRITE_LOCK    2

 /*

 ** These macros define the location of the pointer-map entry for a 

 ** database page. The first argument to each is the number of usable

 ** bytes on each page of the database (often 1024). The second is the

 ** page number to look up in the pointer map.

 **

 ** PTRMAP_PAGENO returns the database page number of the pointer-map

 ** page that stores the required pointer. PTRMAP_PTROFFSET returns

 ** the offset of the requested map entry.

 **

 ** If the pgno argument passed to PTRMAP_PAGENO is a pointer-map page,

 ** then pgno is returned. So (pgno==PTRMAP_PAGENO(pgsz, pgno)) can be

 ** used to test if pgno is a pointer-map page. PTRMAP_ISPAGE implements

 ** this test.

 */

 #define PTRMAP_PAGENO(pBt, pgno) ptrmapPageno(pBt, pgno)

 #define PTRMAP_PTROFFSET(pBt, pgno) (5*(pgno-ptrmapPageno(pBt, pgno)-1))

 #define PTRMAP_ISPAGE(pBt, pgno) (PTRMAP_PAGENO((pBt),(pgno))==(pgno))

 /*

 ** The pointer map is a lookup table that identifies the parent page for

 ** each child page in the database file.  The parent page is the page that

 ** contains a pointer to the child.  Every page in the database contains

 ** 0 or 1 parent pages.  (In this context 'database page' refers

 ** to any page that is not part of the pointer map itself.)  Each pointer map

 ** entry consists of a single byte 'type' and a 4 byte parent page number.

 ** The PTRMAP_XXX identifiers below are the valid types.

 **

 ** The purpose of the pointer map is to facility moving pages from one

 ** position in the file to another as part of autovacuum.  When a page

 ** is moved, the pointer in its parent must be updated to point to the

 ** new location.  The pointer map is used to locate the parent page quickly.

 **

 ** PTRMAP_ROOTPAGE: The database page is a root-page. The page-number is not

 **                  used in this case.

 **

 ** PTRMAP_FREEPAGE: The database page is an unused (free) page. The page-number 

 **                  is not used in this case.

 **

 ** PTRMAP_OVERFLOW1: The database page is the first page in a list of 

 **                   overflow pages. The page number identifies the page that

 **                   contains the cell with a pointer to this overflow page.

 **

 ** PTRMAP_OVERFLOW2: The database page is the second or later page in a list of

 **                   overflow pages. The page-number identifies the previous

 **                   page in the overflow page list.

 **

 ** PTRMAP_BTREE: The database page is a non-root btree page. The page number

 **               identifies the parent page in the btree.

 */

 #define PTRMAP_ROOTPAGE 1

 #define PTRMAP_FREEPAGE 2

 #define PTRMAP_OVERFLOW1 3

 #define PTRMAP_OVERFLOW2 4

 #define PTRMAP_BTREE 5

 /* A bunch of assert() statements to check the transaction state variables

 ** of handle p (type Btree*) are internally consistent.

 */

 #define btreeIntegrity(p) \

   assert( p->pBt->inTransaction!=TRANS_NONE || p->pBt->nTransaction==0 ); \

   assert( p->pBt->inTransaction>=p->inTrans ); 

 /*

 ** The ISAUTOVACUUM macro is used within balance_nonroot() to determine

 ** if the database supports auto-vacuum or not. Because it is used

 ** within an expression that is an argument to another macro 

 ** (sqliteMallocRaw), it is not possible to use conditional compilation.

 ** So, this macro is defined instead.

 */

 #ifndef SQLITE_OMIT_AUTOVACUUM

 #define ISAUTOVACUUM (pBt->autoVacuum)

 #else

 #define ISAUTOVACUUM 0

 #endif

 /*

 ** This structure is passed around through all the sanity checking routines

 ** in order to keep track of some global state information.

 */

 typedef struct IntegrityCk IntegrityCk;

 struct IntegrityCk {

   BtShared *pBt;    /* The tree being checked out */

   Pager *pPager;    /* The associated pager.  Also accessible by pBt->pPager */

   int nPage;        /* Number of pages in the database */

   int *anRef;       /* Number of times each page is referenced */

   int mxErr;        /* Stop accumulating errors when this reaches zero */

   char *zErrMsg;    /* An error message.  NULL if no errors seen. */

   int nErr;         /* Number of messages written to zErrMsg so far */

 };

 /*

 ** Read or write a two- and four-byte big-endian integer values.

 */

 #define get2byte(x)   ((x)[0]<<8 | (x)[1])

 #define put2byte(p,v) ((p)[0] = (v)>>8, (p)[1] = (v))

 #define get4byte sqlite3Get4byte

 #define put4byte sqlite3Put4byte

 /*

 ** Internal routines that should be accessed by the btree layer only.

 */

 int sqlite3BtreeGetPage(BtShared*, Pgno, MemPage**, int);

 int sqlite3BtreeInitPage(MemPage *pPage, MemPage *pParent);

 void sqlite3BtreeParseCellPtr(MemPage*, u8*, CellInfo*);

 void sqlite3BtreeParseCell(MemPage*, int, CellInfo*);

 #ifdef SQLITE_TEST

 u8 *sqlite3BtreeFindCell(MemPage *pPage, int iCell);

 #endif

 int sqlite3BtreeRestoreOrClearCursorPosition(BtCursor *pCur);

 void sqlite3BtreeGetTempCursor(BtCursor *pCur, BtCursor *pTempCur);

 void sqlite3BtreeReleaseTempCursor(BtCursor *pCur);

 int sqlite3BtreeIsRootPage(MemPage *pPage);

 void sqlite3BtreeMoveToParent(BtCursor *pCur);