/*

** 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: btreeInt.h,v 1.30 2008/08/01 20:10:08 drh Exp $

**

** 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 6 bytes  (4 bytes for the cell itself

** plus 2 bytes for the index to the cell in the page header).  Such

** small cells will be rare, but they are possible.

*/

#define MX_CELL(pBt) ((pBt->pageSize-8)/6)

/* 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 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 */

  u16 maskPage;        /* Mask for page offset */

  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 *db;       /* 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 another db */

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

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

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

  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 */

  sqlite3 *db;          /* Database connection currently using this Btree */

  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 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 */

  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 */

  BusyHandler busyHdr;  /* The busy handler for this btree */

#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 */

  Btree *pExclusive;    /* Btree with an EXCLUSIVE lock on the whole db */

#endif

  u8 *pTmpSpace;        /* BtShared.pageSize bytes of space for tmp use */

};

/*

** 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.db.

**

** 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 */

  struct KeyInfo *pKeyInfo; /* Argument passed to comparison function */

  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 atLast;                /* Cursor pointing to the last entry */

  u8 validNKey;             /* True if info.nKey is valid */

  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, restoreCursorPosition() 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 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.

*/