/*

** 2007 October 14

**

** 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.

**

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

** This file contains the C functions that implement a memory

** allocation subsystem for use by SQLite. 

**

** This version of the memory allocation subsystem omits all

** use of malloc(). The SQLite user supplies a block of memory

** before calling sqlite3_initialize() from which allocations

** are made and returned by the xMalloc() and xRealloc() 

** implementations. Once sqlite3_initialize() has been called,

** the amount of memory available to SQLite is fixed and cannot

** be changed.

**

** This version of the memory allocation subsystem is included

** in the build only if SQLITE_ENABLE_MEMSYS3 is defined.

**

** $Id: mem3.c,v 1.20 2008/07/18 18:56:17 drh Exp $

*/

#include "sqliteInt.h"

/*

** This version of the memory allocator is only built into the library

** SQLITE_ENABLE_MEMSYS3 is defined. Defining this symbol does not

** mean that the library will use a memory-pool by default, just that

** it is available. The mempool allocator is activated by calling

** sqlite3_config().

*/

#ifdef SQLITE_ENABLE_MEMSYS3

/*

** Maximum size (in Mem3Blocks) of a "small" chunk.

*/

#define MX_SMALL 10

/*

** Number of freelist hash slots

*/

#define N_HASH  61

/*

** A memory allocation (also called a "chunk") consists of two or 

** more blocks where each block is 8 bytes.  The first 8 bytes are 

** a header that is not returned to the user.

**

** A chunk is two or more blocks that is either checked out or

** free.  The first block has format u.hdr.  u.hdr.size4x is 4 times the

** size of the allocation in blocks if the allocation is free.

** The u.hdr.size4x&1 bit is true if the chunk is checked out and

** false if the chunk is on the freelist.  The u.hdr.size4x&2 bit

** is true if the previous chunk is checked out and false if the

** previous chunk is free.  The u.hdr.prevSize field is the size of

** the previous chunk in blocks if the previous chunk is on the

** freelist. If the previous chunk is checked out, then

** u.hdr.prevSize can be part of the data for that chunk and should

** not be read or written.

**

** We often identify a chunk by its index in mem3.aPool[].  When

** this is done, the chunk index refers to the second block of

** the chunk.  In this way, the first chunk has an index of 1.

** A chunk index of 0 means "no such chunk" and is the equivalent

** of a NULL pointer.

**

** The second block of free chunks is of the form u.list.  The

** two fields form a double-linked list of chunks of related sizes.

** Pointers to the head of the list are stored in mem3.aiSmall[] 

** for smaller chunks and mem3.aiHash[] for larger chunks.

**

** The second block of a chunk is user data if the chunk is checked 

** out.  If a chunk is checked out, the user data may extend into

** the u.hdr.prevSize value of the following chunk.

*/

typedef struct Mem3Block Mem3Block;

struct Mem3Block {

  union {

    struct {

      u32 prevSize;   /* Size of previous chunk in Mem3Block elements */

      u32 size4x;     /* 4x the size of current chunk in Mem3Block elements */

    } hdr;

    struct {

      u32 next;       /* Index in mem3.aPool[] of next free chunk */

      u32 prev;       /* Index in mem3.aPool[] of previous free chunk */

    } list;

  } u;

};

/*

** All of the static variables used by this module are collected

** into a single structure named "mem3".  This is to keep the

** static variables organized and to reduce namespace pollution

** when this module is combined with other in the amalgamation.

*/

static struct {

  /*

  ** True if we are evaluating an out-of-memory callback.

  */

  int alarmBusy;

  /*

  ** Mutex to control access to the memory allocation subsystem.

  */

  sqlite3_mutex *mutex;

  /*

  ** The minimum amount of free space that we have seen.

  */

  u32 mnMaster;

  /*

  ** iMaster is the index of the master chunk.  Most new allocations

  ** occur off of this chunk.  szMaster is the size (in Mem3Blocks)

  ** of the current master.  iMaster is 0 if there is not master chunk.

  ** The master chunk is not in either the aiHash[] or aiSmall[].

  */

  u32 iMaster;

  u32 szMaster;

  /*

  ** Array of lists of free blocks according to the block size 

  ** for smaller chunks, or a hash on the block size for larger

  ** chunks.

  */

  u32 aiSmall[MX_SMALL-1];   /* For sizes 2 through MX_SMALL, inclusive */

  u32 aiHash[N_HASH];        /* For sizes MX_SMALL+1 and larger */

  /*

  ** Memory available for allocation. nPool is the size of the array

  ** (in Mem3Blocks) pointed to by aPool less 2.

  */

  u32 nPool;

  Mem3Block *aPool;

} mem3;

/*

** Unlink the chunk at mem3.aPool[i] from list it is currently

** on.  *pRoot is the list that i is a member of.

*/

static void memsys3UnlinkFromList(u32 i, u32 *pRoot){

  u32 next = mem3.aPool[i].u.list.next;

  u32 prev = mem3.aPool[i].u.list.prev;

  assert( sqlite3_mutex_held(mem3.mutex) );

  if( prev==0 ){

    *pRoot = next;

  }else{

    mem3.aPool[prev].u.list.next = next;

  }

  if( next ){

    mem3.aPool[next].u.list.prev = prev;

  }

  mem3.aPool[i].u.list.next = 0;

  mem3.aPool[i].u.list.prev = 0;

}

/*

** Unlink the chunk at index i from 

** whatever list is currently a member of.

*/

static void memsys3Unlink(u32 i){

  u32 size, hash;

  assert( sqlite3_mutex_held(mem3.mutex) );

  assert( (mem3.aPool[i-1].u.hdr.size4x & 1)==0 );

  assert( i>=1 );

  size = mem3.aPool[i-1].u.hdr.size4x/4;

  assert( size==mem3.aPool[i+size-1].u.hdr.prevSize );

  assert( size>=2 );

  if( size <= MX_SMALL ){

    memsys3UnlinkFromList(i, &mem3.aiSmall[size-2]);

  }else{

    hash = size % N_HASH;

    memsys3UnlinkFromList(i, &mem3.aiHash[hash]);

  }

}

/*

** Link the chunk at mem3.aPool[i] so that is on the list rooted

** at *pRoot.

*/

static void memsys3LinkIntoList(u32 i, u32 *pRoot){

  assert( sqlite3_mutex_held(mem3.mutex) );

  mem3.aPool[i].u.list.next = *pRoot;

  mem3.aPool[i].u.list.prev = 0;

  if( *pRoot ){

    mem3.aPool[*pRoot].u.list.prev = i;

  }

  *pRoot = i;

}

/*

** Link the chunk at index i into either the appropriate

** small chunk list, or into the large chunk hash table.

*/

static void memsys3Link(u32 i){

  u32 size, hash;

  assert( sqlite3_mutex_held(mem3.mutex) );

  assert( i>=1 );

  assert( (mem3.aPool[i-1].u.hdr.size4x & 1)==0 );

  size = mem3.aPool[i-1].u.hdr.size4x/4;

  assert( size==mem3.aPool[i+size-1].u.hdr.prevSize );

  assert( size>=2 );

  if( size <= MX_SMALL ){

    memsys3LinkIntoList(i, &mem3.aiSmall[size-2]);

  }else{

    hash = size % N_HASH;

    memsys3LinkIntoList(i, &mem3.aiHash[hash]);

  }

}

/*

** If the STATIC_MEM mutex is not already held, obtain it now. The mutex

** will already be held (obtained by code in malloc.c) if

** sqlite3Config.bMemStat is true.

*/

static void memsys3Enter(void){

  if( sqlite3Config.bMemstat==0 && mem3.mutex==0 ){

    mem3.mutex = sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_MEM);

  }

  sqlite3_mutex_enter(mem3.mutex);

}

static void memsys3Leave(void){

  sqlite3_mutex_leave(mem3.mutex);

}

/*

** Called when we are unable to satisfy an allocation of nBytes.

*/

static void memsys3OutOfMemory(int nByte){

  if( !mem3.alarmBusy ){

    mem3.alarmBusy = 1;

    assert( sqlite3_mutex_held(mem3.mutex) );

    sqlite3_mutex_leave(mem3.mutex);

    sqlite3_release_memory(nByte);

    sqlite3_mutex_enter(mem3.mutex);

    mem3.alarmBusy = 0;

  }

}

/*

** Chunk i is a free chunk that has been unlinked.  Adjust its 

** size parameters for check-out and return a pointer to the 

** user portion of the chunk.

*/

static void *memsys3Checkout(u32 i, int nBlock){

  u32 x;

  assert( sqlite3_mutex_held(mem3.mutex) );

  assert( i>=1 );

  assert( mem3.aPool[i-1].u.hdr.size4x/4==nBlock );

  assert( mem3.aPool[i+nBlock-1].u.hdr.prevSize==nBlock );

  x = mem3.aPool[i-1].u.hdr.size4x;

  mem3.aPool[i-1].u.hdr.size4x = nBlock*4 | 1 | (x&2);

  mem3.aPool[i+nBlock-1].u.hdr.prevSize = nBlock;

  mem3.aPool[i+nBlock-1].u.hdr.size4x |= 2;

  return &mem3.aPool[i];

}

/*

** Carve a piece off of the end of the mem3.iMaster free chunk.

** Return a pointer to the new allocation.  Or, if the master chunk

** is not large enough, return 0.

*/

static void *memsys3FromMaster(int nBlock){

  assert( sqlite3_mutex_held(mem3.mutex) );

  assert( mem3.szMaster>=nBlock );

  if( nBlock>=mem3.szMaster-1 ){

    /* Use the entire master */

    void *p = memsys3Checkout(mem3.iMaster, mem3.szMaster);

    mem3.iMaster = 0;

    mem3.szMaster = 0;

    mem3.mnMaster = 0;

    return p;

  }else{

    /* Split the master block.  Return the tail. */

    u32 newi, x;

    newi = mem3.iMaster + mem3.szMaster - nBlock;

    assert( newi > mem3.iMaster+1 );

    mem3.aPool[mem3.iMaster+mem3.szMaster-1].u.hdr.prevSize = nBlock;

    mem3.aPool[mem3.iMaster+mem3.szMaster-1].u.hdr.size4x |= 2;

    mem3.aPool[newi-1].u.hdr.size4x = nBlock*4 + 1;

    mem3.szMaster -= nBlock;

    mem3.aPool[newi-1].u.hdr.prevSize = mem3.szMaster;

    x = mem3.aPool[mem3.iMaster-1].u.hdr.size4x & 2;

    mem3.aPool[mem3.iMaster-1].u.hdr.size4x = mem3.szMaster*4 | x;

    if( mem3.szMaster < mem3.mnMaster ){

      mem3.mnMaster = mem3.szMaster;

    }

    return (void*)&mem3.aPool[newi];

  }

}

/*

** *pRoot is the head of a list of free chunks of the same size

** or same size hash.  In other words, *pRoot is an entry in either

** mem3.aiSmall[] or mem3.aiHash[].  

**

** This routine examines all entries on the given list and tries

** to coalesce each entries with adjacent free chunks.  

**

** If it sees a chunk that is larger than mem3.iMaster, it replaces 

** the current mem3.iMaster with the new larger chunk.  In order for

** this mem3.iMaster replacement to work, the master chunk must be

** linked into the hash tables.  That is not the normal state of

** affairs, of course.  The calling routine must link the master

** chunk before invoking this routine, then must unlink the (possibly

** changed) master chunk once this routine has finished.

*/

static void memsys3Merge(u32 *pRoot){

  u32 iNext, prev, size, i, x;

  assert( sqlite3_mutex_held(mem3.mutex) );

  for(i=*pRoot; i>0; i=iNext){

    iNext = mem3.aPool[i].u.list.next;

    size = mem3.aPool[i-1].u.hdr.size4x;

    assert( (size&1)==0 );

    if( (size&2)==0 ){

      memsys3UnlinkFromList(i, pRoot);

      assert( i > mem3.aPool[i-1].u.hdr.prevSize );

      prev = i - mem3.aPool[i-1].u.hdr.prevSize;

      if( prev==iNext ){

        iNext = mem3.aPool[prev].u.list.next;

      }

      memsys3Unlink(prev);

      size = i + size/4 - prev;

      x = mem3.aPool[prev-1].u.hdr.size4x & 2;

      mem3.aPool[prev-1].u.hdr.size4x = size*4 | x;

      mem3.aPool[prev+size-1].u.hdr.prevSize = size;

      memsys3Link(prev);

      i = prev;

    }else{

      size /= 4;

    }

    if( size>mem3.szMaster ){

      mem3.iMaster = i;

      mem3.szMaster = size;

    }

  }

}

/*

** Return a block of memory of at least nBytes in size.

** Return NULL if unable.

**

** This function assumes that the necessary mutexes, if any, are

** already held by the caller. Hence "Unsafe".

*/

static void *memsys3MallocUnsafe(int nByte){

  u32 i;

  int nBlock;

  int toFree;

  assert( sqlite3_mutex_held(mem3.mutex) );

  assert( sizeof(Mem3Block)==8 );

  if( nByte<=12 ){

    nBlock = 2;

  }else{

    nBlock = (nByte + 11)/8;

  }

  assert( nBlock>=2 );

  /* STEP 1:

  ** Look for an entry of the correct size in either the small

  ** chunk table or in the large chunk hash table.  This is

  ** successful most of the time (about 9 times out of 10).

  */

  if( nBlock <= MX_SMALL ){

    i = mem3.aiSmall[nBlock-2];

    if( i>0 ){

      memsys3UnlinkFromList(i, &mem3.aiSmall[nBlock-2]);

      return memsys3Checkout(i, nBlock);

    }

  }else{

    int hash = nBlock % N_HASH;

    for(i=mem3.aiHash[hash]; i>0; i=mem3.aPool[i].u.list.next){

      if( mem3.aPool[i-1].u.hdr.size4x/4==nBlock ){

        memsys3UnlinkFromList(i, &mem3.aiHash[hash]);

        return memsys3Checkout(i, nBlock);

      }

    }

  }

  /* STEP 2:

  ** Try to satisfy the allocation by carving a piece off of the end

  ** of the master chunk.  This step usually works if step 1 fails.

  */

  if( mem3.szMaster>=nBlock ){

    return memsys3FromMaster(nBlock);

  }

  /* STEP 3:  

  ** Loop through the entire memory pool.  Coalesce adjacent free

  ** chunks.  Recompute the master chunk as the largest free chunk.

  ** Then try again to satisfy the allocation by carving a piece off

  ** of the end of the master chunk.  This step happens very

  ** rarely (we hope!)

  */

  for(toFree=nBlock*16; toFree<(mem3.nPool*16); toFree *= 2){

    memsys3OutOfMemory(toFree);

    if( mem3.iMaster ){

      memsys3Link(mem3.iMaster);

      mem3.iMaster = 0;

      mem3.szMaster = 0;

    }

    for(i=0; i<N_HASH; i++){

      memsys3Merge(&mem3.aiHash[i]);

    }

    for(i=0; i<MX_SMALL-1; i++){

      memsys3Merge(&mem3.aiSmall[i]);

    }

    if( mem3.szMaster ){

      memsys3Unlink(mem3.iMaster);

      if( mem3.szMaster>=nBlock ){

        return memsys3FromMaster(nBlock);

      }

    }

  }

  /* If none of the above worked, then we fail. */

  return 0;

}

/*

** Free an outstanding memory allocation.

**

** This function assumes that the necessary mutexes, if any, are

** already held by the caller. Hence "Unsafe".

*/

void memsys3FreeUnsafe(void *pOld){

  Mem3Block *p = (Mem3Block*)pOld;

  int i;

  u32 size, x;

  assert( sqlite3_mutex_held(mem3.mutex) );

  assert( p>mem3.aPool && p<&mem3.aPool[mem3.nPool] );

  i = p - mem3.aPool;

  assert( (mem3.aPool[i-1].u.hdr.size4x&1)==1 );

  size = mem3.aPool[i-1].u.hdr.size4x/4;

  assert( i+size<=mem3.nPool+1 );

  mem3.aPool[i-1].u.hdr.size4x &= ~1;

  mem3.aPool[i+size-1].u.hdr.prevSize = size;

  mem3.aPool[i+size-1].u.hdr.size4x &= ~2;

  memsys3Link(i);

  /* Try to expand the master using the newly freed chunk */

  if( mem3.iMaster ){

    while( (mem3.aPool[mem3.iMaster-1].u.hdr.size4x&2)==0 ){

      size = mem3.aPool[mem3.iMaster-1].u.hdr.prevSize;

      mem3.iMaster -= size;

      mem3.szMaster += size;

      memsys3Unlink(mem3.iMaster);

      x = mem3.aPool[mem3.iMaster-1].u.hdr.size4x & 2;

      mem3.aPool[mem3.iMaster-1].u.hdr.size4x = mem3.szMaster*4 | x;

      mem3.aPool[mem3.iMaster+mem3.szMaster-1].u.hdr.prevSize = mem3.szMaster;

    }

    x = mem3.aPool[mem3.iMaster-1].u.hdr.size4x & 2;

    while( (mem3.aPool[mem3.iMaster+mem3.szMaster-1].u.hdr.size4x&1)==0 ){

      memsys3Unlink(mem3.iMaster+mem3.szMaster);

      mem3.szMaster += mem3.aPool[mem3.iMaster+mem3.szMaster-1].u.hdr.size4x/4;

      mem3.aPool[mem3.iMaster-1].u.hdr.size4x = mem3.szMaster*4 | x;

      mem3.aPool[mem3.iMaster+mem3.szMaster-1].u.hdr.prevSize = mem3.szMaster;

    }

  }

}

/*

** Return the size of an outstanding allocation, in bytes.  The

** size returned omits the 8-byte header overhead.  This only

** works for chunks that are currently checked out.

*/

static int memsys3Size(void *p){

  Mem3Block *pBlock;

  if( p==0 ) return 0;

  pBlock = (Mem3Block*)p;

  assert( (pBlock[-1].u.hdr.size4x&1)!=0 );

  return (pBlock[-1].u.hdr.size4x&~3)*2 - 4;

}

/*

** Round up a request size to the next valid allocation size.

*/

static int memsys3Roundup(int n){

  if( n<=12 ){

    return 12;

  }else{

    return ((n+11)&~7) - 4;

  }

}

/*

** Allocate nBytes of memory.

*/

static void *memsys3Malloc(int nBytes){

  sqlite3_int64 *p;

  assert( nBytes>0 );          /* malloc.c filters out 0 byte requests */

  memsys3Enter();

  p = memsys3MallocUnsafe(nBytes);

  memsys3Leave();

  return (void*)p; 

}

/*

** Free memory.

*/

void memsys3Free(void *pPrior){

  assert( pPrior );

  memsys3Enter();

  memsys3FreeUnsafe(pPrior);

  memsys3Leave();

}

/*

** Change the size of an existing memory allocation

*/

void *memsys3Realloc(void *pPrior, int nBytes){

  int nOld;

  void *p;

  if( pPrior==0 ){

    return sqlite3_malloc(nBytes);

  }

  if( nBytes<=0 ){

    sqlite3_free(pPrior);

    return 0;