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FreeBlockList.java
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FreeBlockList.java
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// -------------------------------------------------------------------------
/**
* A class that represents free blocks in a memory pool.
* The blocks are arranged in decreasing order according to size
* in a doubly linked list. Each block knows its size and index in memory;
* blocks can be used or freed.
*
* @author Joshua Rush
* @author Benjamin Roble
* @version Aug 29, 2011
*/
public class FreeBlockList
{
private Node head;
private Node tail;
/**
* Create a new FreeblockList with one node representing a free
* block the size of the memory pool.
* @param memorySize the initial size of the freeblockList in bytes.
*/
public FreeBlockList(int memorySize)
{
Node node = new Node(memorySize, 0, tail, head);
head.next = node;
tail.prev = node;
}
/**
* Uses up a block of the indicated size.
* @param size the size of the block of memory being used.
* @return the position in memory where the used block will start
*/
public int use(int size)
{
Node currentNode = tail.prev;
while (currentNode.size < size)
{
currentNode = currentNode.prev;
if (currentNode == head)
{
System.out.println("not enough free space exists");
return -1;
}
}
remove(currentNode);
if (currentNode.size > size)
{
sort(new Node(currentNode.size - size, currentNode.index + size));
}
return currentNode.index;
}
/**
* A node of the indicated size and position is added to the
* freelist. Merge adjacent nodes if necessary.
* @param size the size in bytes of the new node being added
* @param position the position in memory where the new node starts
*/
public void free(int size, int position)
{
Node newNode = new Node(size, position);
newNode = merge(newNode);
sort(newNode);
}
/**
* Sort the new node into the correct position in the linked list.
* @param node the node to be sorted into the list
*/
private void sort(Node node)
{
Node currentNode = head.next;
Node newNode = node;
while (newNode.next == null)
{
if (currentNode == tail || currentNode.size < newNode.size ||
(currentNode.size == newNode.size &&
newNode.index < currentNode.index))
{
insertBefore(newNode, currentNode);
}
currentNode = currentNode.next;
}
}
/**
* Insert the new node before the target node in the linked list.
* @param newNode the node to be inserted into the list
* @param beforeNode the node the new node will be added before
*/
private void insertBefore(Node newNode, Node beforeNode)
{
beforeNode.prev.next = newNode;
newNode.next = beforeNode;
newNode.prev = beforeNode.prev;
beforeNode.prev = newNode;
}
/**
* Merge any adjacent nodes to the node being added to the list
* and return the merged node.
* @param newNode the node being added to the list
* @return the new Node after merging with any adjacent nodes
*/
private Node merge(Node newNode)
{
Node currentNode = head.next;
Node mergeNode = newNode;
while (currentNode != tail)
{
if (currentNode.index + currentNode.size == mergeNode.index)
{
remove(currentNode);
mergeNode.size += currentNode.size;
mergeNode.index = currentNode.index;
}
else if (mergeNode.index + mergeNode.size == currentNode.index)
{
remove(currentNode);
mergeNode.size += currentNode.size;
}
currentNode = currentNode.next;
}
return mergeNode;
}
private void remove(Node node)
{
node.prev.next = node.next;
node.next.prev = node.prev;
}
/**
* A basic node in a doubly linked list, storing a single data value and
* references to both the next node and the previous node in the linked
* chain.
*/
private static class Node
{
//~ Instance/static variables .........................................
private int size;
private int index;
private Node next;
private Node prev;
// ----------------------------------------------------------
/**
* Creates a node with null next and previous pointers.
* @param size the size of memory this node represent
* @param index the starting position of this block of memory
*/
public Node(int size, int index)
{
this(size, index, null, null);
}
/**
* Creates a node with a link to the specified node.
* @param size the size of free space the node represents
* @param index the position the block starts at
* @param next the node to follow this one in the list
* @param prev the node to precede this one in the list
*/
public Node(int size, int index, Node next, Node prev)
{
setSize(size);
setIndex(index);
setNext(next);
setPrev(prev);
}
//~ Public methods ....................................................
// ----------------------------------------------------------
/**
* Set the size value stored in this node.
* @param value the new size value to set
*/
public void setSize(int value)
{
size = value;
}
/**
* Set the index value stored in this node.
* @param value the new index value to set
*/
public void setIndex(int value)
{
index = value;
}
/**
* Set the value of this node's next pointer.
* @param value the node is to point to as the next one in the circle
*/
public void setNext(Node value)
{
next = value;
}
/**
* Set the value of this node's previous pointer.
* @param value the node is to point to as the previous one in
* the circle
*/
public void setPrev(Node value)
{
prev = value;
}
}
}