Page structure for the Heap File layer.

Introduction
In this assignment, you will implement the page structure for the Heap File layer. You will be given libraries for the lower layers (Buffer Manager and Disk Space Manager), and some driver routines (referenced at those just-mentioned links) to test the code.

Preliminary Work

Begin by reading the description of Heap Files in Section 9.5.1, and the description of page formats in Section 9.6. A HeapFile is seen as a collection of records. Internally, records are stored on a collection of HFPage objects.

You will be implementing the HFPage class (as described below). Read the description in the text of how variable length records can be stored on a slotted page, and follow this page organization.

## Compiling Your Code and Running the Tests

Download the zip file (cs560-proj1.tar.gz) for Project 1 and unzip it (e.g., tar xvfz cs560-proj1.tar.gz) into your working directory (~/). Move to ~/proj1/HFPage/src. If you make the project, it will create an executable named hfpage. Right now, it does not work; you will need to fill in the bodies of the HFPage class methods. The methods are defined (empty) in file hfpage.C.

An expected output of a correct implementation is available in expected_output in ~/proj1/HFPage/src.

## Design Overview and Implementation Details

The file hfpage.h contains the interfaces for the HFPage class. This class implements a "heap-file page" object which is MAX_SPACE size (1024 Bytes). Note that the protected data members of the page are given to you. All you should need to do is implement the public member functions. You should put all your code into the file hfpage.C.

A note of the slot directory: In the description in the text, the slot directory is located at the end of the page, and grows toward the beginning. This has confused students in the past, since it means that negative offsets into the slot directory have to be used, so the current definition of HFPage has the slot directory at the beginning of the page, after a few fixed member fields, and growing toward the end. This does mean, however, that you will need to write the code so the records themselves are placed beginning at the end of the page. Be very careful with your pointer arithmetic.

Also note that in order to add a record to a page, there has to be a room for the record itself in the data area (1000 Bytes per page after meta-data portion), and also room for a new slot in the data area (unless there happens to be a pre-allocated slot that's empty).

Please follow the Minibase Error Protocol. An example file (~/proj1/HFPage/src/ErrProc.sample) illustrates the use of the error protocol, covering much of what you need to know about the protocol. You can look at ~/HFPage/include/new_error.h for more details.

## The Methods to be Implemented

Public methods  

```void HFPage::init(PageId pageNo)```
This member function is used to initialize a new heap file page with page number pageNo. It should set the following data members to reasonable defaults: ```nextPage, PrevPage, slotCnt, curPage, usedPtr, freeSpace```. You will find the definitions of these data members in hfpage.h. The nextPage and prevPage data members are used for keeping track of pages in a HeapFile. A good default unknown value for a PageId is INVALID_PAGE, as defined in page.h. Note that usedPtr is an offset into the data array, not a pointer.

```PageId HFPage::getPrevPage()```   
This member function should return the page id stored in the prevPage data member.

```void HFPage::setPrevPage(PageId pageNo)```  
This member function sets the prevPage data member.

```PageId HFPage::getNextPage()```   
This member function should return the page id stored in the nextPage data member.

```void HFPage::setNextPage(PageId pageNo)```   
This member function sets the nextPage data member.

```Status HFPage::insertRecord(char* recPtr, int reclen, RID& rid)```   
This member function should add a new record to the page. It returns OK if everything went OK, and DONE if sufficient space does not exist on the page for the new record. If it returns OK, it should set rid to be the RID of the new record (otherwise it can leave rid untouched.) Please note in the parameter list recPtr is a char pointer and RID& denotes passed by reference. The Status enumerated type is defined in new_error.h if you're curious about it. You may want to look that file over and handle errors in a more informative manner than suggested here. The RID struct is defined to be:

```
Struct RID {
    PageID pageNo;
    int        slotNo;
    int operator == (const RID rid) const
    { return (pageNo == rid.pageNo) && (slotNo == rid.slotNo); };

    int operator != (const RID rid) const
    { return (pageNo != rid.pageNo) || (slotNo != rid.slotNo); };

    friend ostream& operator << (ostream& out, const struct RID rid);
  };
 ```
  
In C++, struct are aggregate data types built using elements of other types. The pageNo identifies a physical page number (something that the buffer manager and the DB layers understand) in the file. The slotNo specifies an entry in the slot array on the page.

```Status HFPage::deleteRecord(const RID& rid)```    
This member function deletes the record with RID rid from the page, compacting the hole created in the data area. Compacting the hole, in turn, requires that all the offsets (in the slot array) of all records after the hole be adjusted by the size of the hole, because you are moving these records to "fill" the hole. You should leave a "hole" in the slot array for the slot which pointed to the deleted record to make sure that the rids of the remaining records do not change. The slot array should be compacted only if the record corresponding to the last slot is being deleted. When compacting the slot array, only deleted entries are removed from the end of the slot array. It returns OK if everything goes OK, or FAIL otherwise. (what could go wrong here?)

```Status HFPage::firstRecord(RID& firstRid)```  
This routine should set firstRid to be the rid of the "first" record on the page. The order in which you return records from a page is entirely up to you. If you find a first record, return OK, else return DONE.

```Status HFPage::nextRecord(RID curRid, RID& nextRid)```  
Given a valid current RID, curRid, this member function stores the next RID on the page in the nextRid variable. Again, the order of your return records is up to you, but do make sure you return each record exactly once if someone continually calls nextRecord! Don't worry about changes to the page between successive calls (e.g. records inserted to or deleted from the page). If you find a next RID, return OK, else return DONE. In case of an error, return FAIL.

```Status HFPage::getRecord(RID rid, char * recPtr, int& recLen)```  
Given a rid, this routine copies the associated record into the memory address *recPtr. You may assume that the memory pointed by *recPtr has been allocated by the caller. RecLen is set to the number of bytes that the record occupies. If all goes well, return OK, else return FAIL.

```Status HFPage::returnRecord(RID rid, char*& recPtr, int& recLen)``` 
This routine is very similar to HFPage::getRecord, except in this case you do not copy the record into a caller-provided pointer, but instead you set the caller's recPtr to point directly to the record on the page. Again, return either OK or FAIL.
DONE is a special code for non-errors that are nonetheless not "OK": it generally means "finished" or "not found." FAIL is for errors that happen outside the bounds of a subsystem.

```int HFPage::available_space(void)``` 
This routine should return the amount of space available for a new record that is left on the page. For instance, if all slots are full and there are 100 bytes of free space on the page, this method should return (100 - sizeof(slot_t)) bytes. This accounts for the fact that sizeof(slot_t) bytes must be reserved for a new slot and cannot be used by a new record. Keep this in mind for how freeSpace variable works.
  
```bool HFPage::empty(void)``` 
Returns true if the page has no records in it, and false otherwise.