Interlisp.medley/library/Cash-File.tedit

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Lisp Library Modules, Medley Release 1.0, CASH-FILE
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Lisp Library Modules, Medley Release 1.0, CASH-FILE
CASH-FILE
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CASH-FILE
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CASH-FILE
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Cash-File is a front end to Hash-File which uses a hash table to cache accesses to hash files.  This can provide a significant performance improvement in applications which access a small number of keys repeatedly.  For example, the Where-Is library module uses this module to achieve acceptable interactive performance.
Cash-File(CASH-FILE NIL Cash-File NIL NIL 15) is similar to but not compatible with the LispUsers' module, HASHBUFFER.
All of the code for Cash-File is in a package called Cash-File.  Througout this document  Lisp symbols are printed as though in a package which uses the packages Cash-File, Hash-File, and Lisp.
Installation
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Load  CASH-FILE.DFASL and  HASH-FILE.DFASL  from the library.  
Functions
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The functional interface is designed to closely resemble that of Hash-File, which was in turn designed to resemble the Common Lisp hash table facility.
(make-cash-file(MAKE-CASH-FILE (Function) make-cash-file NIL NIL 15) file-name size cache-size)	[Function]
Creates and returns an empty cash file in file-name.  Size is passed as the size argument to make-hash-file, while cache-size is passed as the size argument to make-hash-table and determines the maximum number of entries to  be cached.
(get-cash-file(GET-CASH-FILE (Function) get-cash-file NIL NIL 15) key cash-file &optional default)	[Function]
Just like get-hash-file and gethash.  Retrieves the value stored under key in cash-file or default if there is none.  Also returns a second value which is true if a value was found for key. 
A setf method is also defined for get-cash-file .
(open-cash-file file-name cache-size &key direction)	[Function]
Open the existing hash file in file-name in direction (:input or :io).  Cache-size is passed as the size argument to make-hash-table and determines the maximum number of entries which will ever be cached.
(rem-cash-file key cash-file)	[Function]
Like rem-hash-file and remhash.  Deletes key from the hash file and the cache.  Returns true if and only if there was a value stored under key.
(cash-file-p object)	[Function]
Returns true if and only if object is a cash file.
(cash-file-p object)<29><> <20>r<EFBFBD>(typep object 'cash-file)
(cash-file-hash-file cash-file) 	[Function]
Returns the hash file object to which cash-file is a front end.
There are no cash file specific equivalents for close-hash-file, map-hash-file and hash-file-count.  For these use the hash file functions on the cash-file-hash-file . 
Implementation Notes
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A queue is maintained to enable cache deletion when the cache is full.  This queue is implemented as a list.  Each time a key is accessed, it is moved to the head of the queue.  The last element of the queue is deleted when a new key is accessed and the queue is full.
Limitations
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The cache time is not constant but grows linearly with the size of the cache.  For this reason, huge caches are not recommended. 
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