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Microsoft: Visual FoxPro FAQ
Usefull Functions & Procedures
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How to obtain a guaranteed unique ID number by mgagnon
Posted: 31 Aug 02 (Edited 2 Nov 02)
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A GUID, or globally unique identifier, is a number that Microsoft guarantees is unique throughout the entire world. It is best used when you need a primary key that is absolutely, positively unique, and you have distributed systems in which data is entered.
For example, if you have a mobile sales force that uses laptop computers to enter sales orders and customers in the field, then uploads that data to a central system, simply assigning incremental integer values in the field will cause multiple laptops submitting identical key values. If, instead, the laptop programs generated a GUID, then you can be assured that even when the data is combined with data from other users from around the world each record's key (the GUID) will be unique.
Downside to using GUIDs: besides making an API call, GUIDs consume 36 bytes of storage - much larger than an integer. Therefore, they should only be used where you need to ensure unique values across a distributed system, or believe that someday you may be joining data from currently separate systems.
This code is based on a Microsoft article located at: http://support.microsoft.com/support/kb/articles/Q269/3/87.asp
article (c) 2000 Microsoft Corp.
* -- returns a Globally Unique IDentifier (GUID)
local cData1, cData2, cData3, cData4, cData5, cDataAll
private PGuid
store "" to cData1, cData2, cData3, cData4, cData5, cDataAll
* -- declare external function
DECLARE INTEGER CoCreateGuid IN OLE32.DLL STRING @pGuid
* -- Initialize the buffer that will hold the GUID with nulls
pGuid = REPLICATE(CHR(0),17)
* -- Call CoCreateGuid
IF CoCreateGuid(@pGuid) = 0 && success
* -- Store the first eight characters of the GUID in data1
cData1 = RIGHT(TRANSFORM(strtolong(LEFT(pGuid,4)),"@0"),8)
* -- Store the first group of four characters of the GUID in data2
cData2 = RIGHT(TRANSFORM(strtolong(SUBSTR(pGuid,5,2)),"@0"),4)
* -- Store the second group of four characters of the GUID in data3
cData3 = RIGHT(TRANSFORM(strtolong(SUBSTR(pGuid,7,2)),"@0"),4)
* -- Store the third group of four characters of the GUID in data4
cData4 = RIGHT(TRANSFORM(strtolong(SUBSTR(pGuid,9,1)),"@0"),2) + ;
RIGHT(TRANSFORM(strtolong(SUBSTR(pGuid,10,1)),"@0"),2)
* -- Initialize data5 to a null string
cData5 = ""
* -- Convert the final 12 characters of the GUID and store in data5
FOR nGuidLen = 1 TO 6
cData5=cData5+RIGHT(TRANSFORM(strtolong(SUBSTR(pGuid,10+nGuidLen,1))),2)
ENDFOR
* -- Check the length of data5. If less than 12, the final 12-len(data5)
* -- characters are '0'
IF LEN(cData5) < 12
cData5=cData5+REPLICATE("0",12-LEN(cData5))
ENDIF
* -- Assemble the GUID into a string
cDataAll = cData1+"-"+cData2+"-"+cData3+"-"+cData4+"-"+cData5
ENDIF
* -- Done with the call to CoCreateGuid, so clear the DLLs from memory
CLEAR DLLS
return cDataAll
*******************
FUNCTION strtolong
* -- Passed: 4-byte character string (lcLongstr) in low-high ASCII format
* -- Returns: long integer value
* -- Example:
* -- m.longstr = "1111"
* -- m.longval = strtolong(m.longstr)
PARAMETERS lcLongstr
LOCAL lnByte, lnRetval
lnRetval = 0
FOR lnByte = 0 TO 24 STEP 8
lnRetval = lnRetval + (ASC(lcLongstr) * (2^lnByte))
lcLongstr = RIGHT(lcLongstr, LEN(lcLongstr) - 1)
NEXT
RETURN lnRetval
Mike Gagnon
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