Visual FoxPro

Large Persistent Hash Table Stored on Filesystem

See more Misc Examples

Demonstrates how to implement a large, persistent hash table that is stored on the filesystem and allows for quick retrieval using a hash key. Visual FoxPro

LOCAL lnSuccess
LOCAL loCache
LOCAL lcKey
LOCAL lcETag
LOCAL lcItemValue

lnSuccess = 0

* Let's say we would like to implement a persistent hash table with approximately 200,000 entries.
* We want something simple and straightforward.
* 
* The Chilkat Cache class is a solution that might fit.
* 
* Each hash table entry is a file.  Depending on the number of anticipated hash entries,
* the files can be contained in a single directory, or a collection of 256 directories,
* or a collection of 256x256 directories.

* There are 3 options:
* 
* Level 0: All cache files are in a single directory (the cache root).
* Level 1: Cache files are located in 256 sub-directories numbered 0 .. 255 directly under the cache root.
* Level 2: There are two levels of sub-directories under the cache root. 
*          The 1st level has 256 sub-directories numbered 0 .. 255 directly under the cache root. 
*          The 2nd level allows for up to 256 sub-directories (0..255) under each level-1 directory. 
*          Cache files are stored in the leaf directories. 

* For this example, given that we anticipate a larger number of hash entries, we choose a "level 2" cache.
loCache = CreateObject('Chilkat.Cache')

* We can also spread the cache among several root directories, but for this example we'll only use one root directory.
* Call AddRoot once for each root directory.
loCache.AddRoot("c:/temp/myCache")
loCache.Level = 2

* Add some key/values to the persisted hash table (i.e. the cache).
* The eTag is optional metadata.
lcKey = "apple"
lcETag = ""
lcItemValue = "macos"
lnSuccess = loCache.SaveTextNoExpire(lcKey,lcETag,lcItemValue)

* Add more items..
lnSuccess = loCache.SaveTextNoExpire("microsoft","","windows")
lnSuccess = loCache.SaveTextNoExpire("google","","android")

* Lookup items:
lcKey = "microsoft"
lcItemValue = loCache.FetchText(lcKey)
? lcKey + ": " + lcItemValue

lcKey = "apple"
lcItemValue = loCache.FetchText(lcKey)
? lcKey + ": " + lcItemValue

RELEASE loCache