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memory.cpp

/* ScummVM - Graphic Adventure Engine
 *
 * ScummVM is the legal property of its developers, whose names
 * are too numerous to list here. Please refer to the COPYRIGHT
 * file distributed with this source distribution.
 *
 * Additional copyright for this file:
 * Copyright (C) 1994-1998 Revolution Software Ltd.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 *
 * $URL: https://scummvm.svn.sourceforge.net/svnroot/scummvm/scummvm/tags/release-0-11-1/engines/sword2/memory.cpp $
 * $Id: memory.cpp 30944 2008-02-23 22:50:18Z sev $
 */

// The new memory manager, now only used by the resource manager. The original
// one would allocated a 12 MB memory pool at startup, which may have been
// appropriate for the original Playstation version but didn't work very well
// with our PocketPC version.
//
// There is one thing that prevents us from replacing the whole memory manager
// with the standard memory allocation functions: Broken Sword 2 absolutely,
// positively needs to be able to encode pointers as 32-bit integers. The
// original engine did this simply by casting between pointers and integers,
// but as far as I know that's not a very portable thing to do.
//
// If it had only used pointers as opcode parameters it would have been
// possible, albeit messy, to extend the stack data type. However, there is
// code in walker.cpp that obviously violates that assumption, and there are
// probably other cases as well.
//
// Instead, we take advantage of the fact that the original memory manager
// could only handle up to 999 blocks of memory. That means we can encode a
// pointer as a 10-bit id and a 22-bit offset into the block. Judging by early
// testing, both should be plenty.
//
// The number zero is used to represent the NULL pointer.


#include "sword2/sword2.h"
#include "sword2/memory.h"

namespace Sword2 {

MemoryManager::MemoryManager(Sword2Engine *vm) : _vm(vm) {
      // The id stack contains all the possible ids for the memory blocks.
      // We use this to ensure that no two blocks ever have the same id.

      // The memory blocks are stored in an array, indexed on the block's
      // id. This means that given a block id we can find the pointer with a
      // simple array lookup.

      // The memory block index is an array of pointers to the memory block
      // array, sorted on the memory block's pointer. This means that given
      // a pointer into a memory block we can find its id with binary
      // searching.
      //
      // A balanced tree might have been more efficient - the index has to
      // be re-sorted every time a block is allocated or freed - but such
      // beasts are tricky to implement. Anyway, it wouldn't have made
      // encoding or decoding pointers any faster, and these are by far the
      // most common operations.

      _idStack = (int16 *)malloc(MAX_MEMORY_BLOCKS * sizeof(int16));
      _memBlocks = (MemBlock *)malloc(MAX_MEMORY_BLOCKS * sizeof(MemBlock));
      _memBlockIndex = (MemBlock **)malloc(MAX_MEMORY_BLOCKS * sizeof(MemBlock *));

      _totAlloc = 0;
      _numBlocks = 0;

      for (int i = 0; i < MAX_MEMORY_BLOCKS; i++) {
            _idStack[i] = MAX_MEMORY_BLOCKS - i - 1;
            _memBlocks[i].ptr = NULL;
            _memBlockIndex[i] = NULL;
      }

      _idStackPtr = MAX_MEMORY_BLOCKS;
}

MemoryManager::~MemoryManager() {
      for (int i = 0; i < MAX_MEMORY_BLOCKS; i++)
            free(_memBlocks[i].ptr);
      free(_memBlocks);
      free(_memBlockIndex);
      free(_idStack);
}

int32 MemoryManager::encodePtr(byte *ptr) {
      if (ptr == NULL)
            return 0;

      int idx = findPointerInIndex(ptr);

      assert(idx != -1);

      uint32 id = _memBlockIndex[idx]->id;
      uint32 offset = ptr - _memBlocks[id].ptr;

      assert(id < 0x03ff);
      assert(offset <= 0x003fffff);
      assert(offset < _memBlocks[id].size);

      return ((id + 1) << 22) | (ptr - _memBlocks[id].ptr);
}

byte *MemoryManager::decodePtr(int32 n) {
      if (n == 0)
            return NULL;

      uint32 id = ((n & 0xffc00000) >> 22) - 1;
      uint32 offset = n & 0x003fffff;

      assert(_memBlocks[id].ptr);
      assert(offset < _memBlocks[id].size);

      return _memBlocks[id].ptr + offset;
}

int16 MemoryManager::findExactPointerInIndex(byte *ptr) {
      int left = 0;
      int right = _numBlocks - 1;

      while (right >= left) {
            int n = (left + right) / 2;

            if (_memBlockIndex[n]->ptr == ptr)
                  return n;

            if (_memBlockIndex[n]->ptr > ptr)
                  right = n - 1;
            else
                  left = n + 1;
      }

      return -1;
}

int16 MemoryManager::findPointerInIndex(byte *ptr) {
      int left = 0;
      int right = _numBlocks - 1;

      while (right >= left) {
            int n = (left + right) / 2;

            if (_memBlockIndex[n]->ptr <= ptr && _memBlockIndex[n]->ptr + _memBlockIndex[n]->size > ptr)
                  return n;

            if (_memBlockIndex[n]->ptr > ptr)
                  right = n - 1;
            else
                  left = n + 1;
      }

      return -1;
}

int16 MemoryManager::findInsertionPointInIndex(byte *ptr) {
      if (_numBlocks == 0)
            return 0;

      int left = 0;
      int right = _numBlocks - 1;
      int n = 0;

      while (right >= left) {
            n = (left + right) / 2;

            if (_memBlockIndex[n]->ptr == ptr)
                  return -1;

            if (_memBlockIndex[n]->ptr > ptr)
                  right = n - 1;
            else
                  left = n + 1;
      }

      if (_memBlockIndex[n]->ptr < ptr)
            n++;

      return n;
}

byte *MemoryManager::memAlloc(uint32 size, int16 uid) {
      assert(_idStackPtr > 0);

      // Get the new block's id from the stack.
      int16 id = _idStack[--_idStackPtr];

      // Allocate the new memory block
      byte *ptr = (byte *)malloc(size);

      assert(ptr);

      _memBlocks[id].id = id;
      _memBlocks[id].uid = uid;
      _memBlocks[id].ptr = ptr;
      _memBlocks[id].size = size;

      // Update the memory block index.
      int16 idx = findInsertionPointInIndex(ptr);

      assert(idx != -1);

      for (int i = _numBlocks; i > idx; i--)
            _memBlockIndex[i] = _memBlockIndex[i - 1];

      _memBlockIndex[idx] = &_memBlocks[id];
      _numBlocks++;
      _totAlloc += size;

      return _memBlocks[id].ptr;
}

void MemoryManager::memFree(byte *ptr) {
      int16 idx = findExactPointerInIndex(ptr);

      if (idx == -1) {
            warning("Freeing non-allocated pointer %p", ptr);
            return;
      }

      // Put back the id on the stack
      _idStack[_idStackPtr++] = _memBlockIndex[idx]->id;

      // Release the memory block
      free(_memBlockIndex[idx]->ptr);
      _memBlockIndex[idx]->ptr = NULL;

      _totAlloc -= _memBlockIndex[idx]->size;

      // Remove the memory block from the index
      _numBlocks--;

      for (int i = idx; i < _numBlocks; i++)
            _memBlockIndex[i] = _memBlockIndex[i + 1];
}

} // End of namespace Sword2

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