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## org.exist.storage.index
Class FreeList

    java.lang.Object
      org.exist.storage.index.FreeList

public class FreeList
extends java.lang.Object

Manages a list of pages containing unused sections. Class `BFile` stores all data in variable length records. As records may grow or shrink, the database has to keep track of the amount of free space currently available in pages. Class `BFile` will always check if FreeList has a page that can be filled before creating a new page. FreeList implements a linked list of `FreeSpace` objects. Each object in the list describes a page and the unused space in this page.

Author:
  wolf
See Also:
  `FreeList`

Field Summary
`static int` | `MAX_FREE_LIST_LEN`

Constructor Summary
`FreeList()`

Method Summary
`void` | `add(FreeSpace free)` Append a new `FreeSpace` object to the list, describing the amount of free space available on a page.
`FreeSpace` | `find(int requiredSize)` Try to find a page that has at least requiredSize bytes available.
`int` | `read(byte[] buf, int offset)` Read the list from a `RandomAccessFile`.
`void` | `remove(FreeSpace node)` Remove a record from the list.
`FreeSpace` | `retrieve(long pageNum)` Retrieve the record stored for the given page number.
`java.lang.String` | `toString()`
`int` | `write(byte[] buf, int offset)` Write the list to a `RandomAccessFile`.

Methods inherited from class java.lang.Object
`equals, getClass, hashCode, notify, notifyAll, wait, wait, wait`

Field Detail

### MAX_FREE_LIST_LEN

    public static final int MAX_FREE_LIST_LEN

See Also:
  Constant Field Values

Constructor Detail

### FreeList

    public FreeList()

Method Detail

### add

    public void add(FreeSpace free)

  Append a new `FreeSpace` object to the list, describing the amount of free space available on a page.

  Parameters:
    `free` -

### remove

    public void remove(FreeSpace node)

  Remove a record from the list.

  Parameters:
    `node` -

### retrieve

    public FreeSpace retrieve(long pageNum)

  Retrieve the record stored for the given page number.

  Parameters:
    `pageNum` -

### find

    public FreeSpace find(int requiredSize)

  Try to find a page that has at least requiredSize bytes available. This method selects the page with the smallest possible space. This guarantees that all pages will be filled before creating a new page.

  Parameters:
    `requiredSize` -

### toString

    public java.lang.String toString()

### read

    public int read(byte[] buf,
                    int offset)
             throws java.io.IOException

  Read the list from a `RandomAccessFile`.

  Parameters:
    `buf` -
    `offset` -
  Throws:
    `java.io.IOException`

### write

    public int write(byte[] buf,
                     int offset)
              throws java.io.IOException

  Write the list to a `RandomAccessFile`. As the list is written to the file header, its maximum length has to be restricted. The method will thus only store `MAX_FREE_LIST_LEN` entries and throw away the rest. Usually, this should not happen very often, so it is ok to waste some space.

  Parameters:
    `buf` -
    `offset` -
  Throws:
    `java.io.IOException`

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Copyright (C) Wolfgang Meier. All rights reserved.