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Implementation of a B-Tree Database Class

, 29 Jun 2006
An article and source code regarding the implmentation of B-Trees in C++.
/***************************************************************************
 TreeNode.cpp  -  Source file that contains the implementation of the
                  TreeNode class.

 begin     : April 2004
 copyright : (C) 2004 by Phil Cairns
 email     : philcairns@hotmail.com

 This code may be used in compiled form in any way you desire (including
 commercial use). The code may be redistributed unmodified by any means
 providing it is not sold for profit without the authors written consent,
 and providing that this notice and the authors name and all copyright
 notices remains intact.

 This software is provided "as is" without express or implied warranty. Use
 it at your own risk!
 ***************************************************************************/

#include "stdafx.h"
#include "treenode.h"

namespace Database
{
	// Constructor initialises everything to its default value.
	// Not that we assume to start with that the node is a leaf,
	// and it is not loaded from the disk.
	TreeNode::TreeNode()
		: childNo((size_t)-1)
		, objCount(0)
		, isLeaf(true)
		, loaded(false)
		, fpos(-1)
	{
	}

	// To delete a TreeNode, we must unload each of its children.
	// This must be done, or we won't end up cleaning up all of
	// the loaded children, thus introducing memory leaks.
	TreeNode::~TreeNode()
	{
		TREENODEVECTOR::iterator tnvit = children.begin();
		while (tnvit != children.end())
		{
			if ((TreeNode*)(*tnvit) != 0)
			{
				(*tnvit)->unload();
			}
			++tnvit;
		}
	}

	// Read a node from the disk
	bool TreeNode::read(FILE* f, size_t recSize)
	{
		// Bug out if we don't have a good file.
		if (!f)
		{
			return false;
		}

		// get to the right location
		if (0 != fseek(f, fpos, SEEK_SET))
		{
			return false;
		}

		// read the leaf flag and the object count
		byte leafFlag = 0;
		if (1 != fread(&leafFlag, sizeof(byte), 1, f))
		{
			return false;
		}
		if (1 != fread(&objCount, sizeof(size_t), 1, f))
		{
			return false;
		}
		isLeaf = (leafFlag == 1);

		// read the contents
		objects.resize(objCount);
		byte* pBuf = new byte[recSize];
		for (size_t ctr = 0; ctr < objCount; ctr++)
		{
			if (1 != fread(pBuf, recSize, 1, f))
			{
				delete[] pBuf;	// thanks AdamWynne
				return false;
			}
			DbObjPtr pObj = new DbObj(pBuf, recSize);
			objects[ctr] = pObj;
		}
		delete[] pBuf;	// thanks AdamWynne

		// read the addresses of the child pages
		if (objCount > 0)
		{
			long* childAddresses = new long[objCount + 1];
			long* thisChild = childAddresses;
			memset(childAddresses, 0xff, sizeof(long) * (objCount + 1));
			if (objCount + 1 != fread(childAddresses, sizeof(long), objCount + 1, f))
			{
				delete[] childAddresses;	// thanks AdamWynne
				return false;
			}
			children.resize(objCount + 1);
			for (size_t ctr = 0; ctr <= objCount; ctr++)
			{
				TreeNodePtr newNode = new TreeNode;
				newNode->fpos = *thisChild++;
				children[ctr] = newNode;
				newNode->childNo = ctr;
			}
			delete[] childAddresses;	// thanks AdamWynne
		}
		loaded = true;
		return true;
	}

	// Write a node to the disk
	bool TreeNode::write(FILE* f)
	{
		// If we're not loaded, we haven't been changed,
		// so we can say that the flush was successful.
		if (!loaded)
		{
			return true;
		}

		// Can't read without a good file ...
		if (!f)
		{
			return false;
		}

		// get to the right location
		if (0 != fseek(f, fpos, SEEK_SET))
		{
			return false;
		}

		// write the leaf flag and the object count
		byte leafFlag = isLeaf ? 1 : 0;
		if (1 != fwrite(&leafFlag, sizeof(byte), 1, f))
		{
			return false;
		}
		if (1 != fwrite(&objCount, sizeof(size_t), 1, f))
		{
			return false;
		}

		// write the contents
		DBOBJVECTOR::iterator dovit = objects.begin();
		while (dovit != objects.end())
		{
			DbObj* pObj = (DbObj*)(*dovit);
			if (1 != fwrite(pObj->getData(), pObj->getSize(), 1, f))
			{
				return false;
			}
			++dovit;
		}

		// write the addresses of the child pages
		if (objCount > 0 && !isLeaf)
		{
			long* childAddresses = new long[objCount + 1];
			long* thisChild = childAddresses;
			memset(childAddresses, 0xff, sizeof(long) * (objCount + 1));
			TREENODEVECTOR::iterator tnvit = children.begin();
			while (tnvit != children.end())
			{
				if ((TreeNode*)(*tnvit) != 0)
				{
					*thisChild = (*tnvit)->fpos;
				}
				++thisChild;
				++tnvit;
			}
			size_t longsWritten = fwrite(childAddresses, sizeof(long), objCount + 1, f);
			delete[] childAddresses;
			if (objCount + 1 != longsWritten)
			{
				return false;
			}
		}
		return true;
	}

	// Load a child node from the disk. This requires that we
	// have the filepos already in place.
	TreeNodePtr TreeNode::loadChild(size_t childNo, FILE* f, size_t recSize)
	{
		TreeNodePtr child = children[childNo];
		if ((TreeNode*)child == 0)
		{
			child = new TreeNode;
			children[childNo] = child;
		}
		if (!child->loaded)
		{
			child->read(f, recSize);
			child->parent = this;
		}
		return child;
	}

	// Unload a child. This means that we get rid of all
	// children in the children vector.
	void TreeNode::unload()
	{
		if (loaded)
		{
			// Clear out all of the objects
			DBOBJVECTOR::iterator dovit = objects.begin();
			while (dovit != objects.end())
			{
				*dovit = (DbObj*)0;
				++dovit;
			}
			objects.resize(0);

			// Clear out all of the children
			TREENODEVECTOR::iterator tnvit = children.begin();
			while (!isLeaf && tnvit != children.end())
			{
				(*tnvit)->unload();
				*tnvit = (TreeNode*)0;
				++tnvit;
			}
			children.resize(0);

			// Empty the parent node and indicate that the
			// node is no longer loaded.
			parent = (TreeNode*)0;
			loaded = false;
		}
	}

	// Delete a child from a given node.
	bool TreeNode::delFromLeaf(size_t objNo)
	{
		bool ret = isLeaf;
		if (ret)
		{
			objects[objNo] = (DbObj*)0;
			for (size_t ctr = objNo + 1; ctr < objCount; ctr++)
			{
				objects[ctr - 1] = objects[ctr];
			}
			setCount(objCount - 1);
		}
		return ret;
	}
	
	// Find the position of the object in a node. If the key is at pos
	// the function returns (pos, ECP_INTHIS). If the key is in a child to
	// the left of pos, the function returns (pos, ECP_INLEFT). If the node
	// is an internal node, the function returns (objCount, ECP_INRIGHT).
	// Otherwise, the function returns ((size_t)-1, false).
	// The main assumption here is that we won't be searching for a key
	// in this node unless it (a) is not in the tree, or (b) it is in the
	// subtree rooted at this node.
	OBJECTPOS TreeNode::findPos(const DbObjPtr& key, compareFn cfn)
	{
		OBJECTPOS ret((size_t)-1, ECP_NONE);
		DBOBJVECTOR::iterator dovit = objects.begin();
		size_t ctr = 0;
		while (dovit < objects.end())
		{
			int compVal = cfn(key, *dovit);
			if (compVal == 0)
			{
				return OBJECTPOS(ctr, ECP_INTHIS);
			}
			else if (compVal < 0)
			{
				if (isLeaf)
				{
					return ret;
				}
				else
				{
					return OBJECTPOS(ctr, ECP_INLEFT);
				}
			}
			++dovit, ++ctr;
		}
		if (!isLeaf)
		{
			return OBJECTPOS(ctr - 1, ECP_INRIGHT);
		}
		return ret;
	}
}

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