A Practical Introduction to Data Structures and Algorithm Analysis

296 Chap. 8 File Processing and External Sortingtents must then issue a readBlock request to read the data from disk into thebuffer, and then a getDataPointer request to gain direct access to the buffer’sdata contents./** Improved ADT for buffer pools using the buffer-passingstyle. Most user functionality is in the buffer class,not the buffer pool itself. *//** A single buffer in the buffer pool */public interface BufferADT {/** Read the associated block from disk (if necessary) andreturn a pointer to the data */public byte[] readBlock();}/** Return a pointer to the buffer’s data array(without reading from disk) */public byte[] getDataPointer();/** Flag the buffer’s contents as having changed, so thatflushing the block will write it back to disk */public void markDirty();/** Release the block’s access to this buffer. Furtheraccesses to this buffer are illegal. */public void releaseBuffer();/** The bufferpool */public interface BufferPoolADT {}/** Relate a block to a buffer, returning a pointer to abuffer object */Buffer acquireBuffer(int block);Clearly, the buffer-passing approach places more obligations on the user of thebuffer pool. These obligations include knowing the size of a block, not corruptingthe buffer pool’s storage space, and informing the buffer pool both when a blockhas been modified and when it is no longer needed. So many obligations make thisapproach prone to error. An advantage is that there is no need to do an extra copystep when getting information from the user to the buffer. If the size of the recordsstored is small, this is not an important consideration. If the size of the records islarge (especially if the record size and the buffer size are the same, as typically is thecase when implementing B-trees, see Section 10.5), then this efficiency issue mightbecome important. Note however that the in-memory copy time will always be farless than the time required to write the contents of a buffer to disk. For applications