VBScript

Large Persistent Hash Table Stored on Filesystem

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

Dim fso, outFile
Set fso = CreateObject("Scripting.FileSystemObject")
'Create a Unicode (utf-16) output text file.
Set outFile = fso.CreateTextFile("output.txt", True, True)

success = 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.
set cache = 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.
cache.AddRoot "c:/temp/myCache"
cache.Level = 2

' Add some key/values to the persisted hash table (i.e. the cache).
' The eTag is optional metadata.
key = "apple"
eTag = ""
itemValue = "macos"
success = cache.SaveTextNoExpire(key,eTag,itemValue)

' Add more items..
success = cache.SaveTextNoExpire("microsoft","","windows")
success = cache.SaveTextNoExpire("google","","android")

' Lookup items:
key = "microsoft"
itemValue = cache.FetchText(key)
outFile.WriteLine(key & ": " & itemValue)

key = "apple"
itemValue = cache.FetchText(key)
outFile.WriteLine(key & ": " & itemValue)

outFile.Close