Csdrhrt

I'm working with a number of data lists that are keyed by document name. The document names, while very descriptive, are quite cumbersome if I need to view them on (up to 256 bytes is a lot of real estate) and I'd love to be able to create a smaller keyfield that's readily reproducible in case I need to do a VLOOKUP from another workseet or workbook.

I'm thinking a hash from the title that'd be unique and reproducible for each title would be most appropriate. Is there a function available, or am I looking at developing my own algorithm?

Any thoughts or ideas on this or another strategy?

You don't need to write your own function - others already did that for you.

For example I collected and compared five VBA hash functions on this stackoverflow answer

Personally I use this VBA function

• its called with =BASE64SHA1(A1) in Excel after you copied the macro to a VBA module
  


• requires .NET since it uses the library "Microsoft MSXML" (with late binding)

Public Function BASE64SHA1(ByVal sTextToHash As String)



    Dim asc As Object

    Dim enc As Object

    Dim TextToHash() As Byte

    Dim SharedSecretKey() As Byte

    Dim bytes() As Byte

    Const cutoff As Integer = 5



    Set asc = CreateObject("System.Text.UTF8Encoding")

    Set enc = CreateObject("System.Security.Cryptography.HMACSHA1")



    TextToHash = asc.GetBytes_4(sTextToHash)

    SharedSecretKey = asc.GetBytes_4(sTextToHash)

    enc.Key = SharedSecretKey



    bytes = enc.ComputeHash_2((TextToHash))

    BASE64SHA1 = EncodeBase64(bytes)

    BASE64SHA1 = Left(BASE64SHA1, cutoff)



    Set asc = Nothing

    Set enc = Nothing



End Function



Private Function EncodeBase64(ByRef arrData() As Byte) As String



    Dim objXML As Object

    Dim objNode As Object



    Set objXML = CreateObject("MSXML2.DOMDocument")

    Set objNode = objXML.createElement("b64")



    objNode.DataType = "bin.base64"

    objNode.nodeTypedValue = arrData

    EncodeBase64 = objNode.text



    Set objNode = Nothing

    Set objXML = Nothing



End Function

Customizing the hash length

• the hash initially is a 28 characters long unicode string (case sensitive + special chars)


• You customize the hash length with this line: Const cutoff As Integer = 5
  


• 4 digits hash = 36 collisions in 6895 lines = 0.5 % collision rate


• 5 digits hash = 0 collisions in 6895 lines = 0 % collision rate

There are also hash functions (all three CRC16 functions) which doesn't require .NET and doesn't use external libraries. But the hash is longer and produces more collisions.

You could also just download this example workbook and play around with all 5 hash implementations. As you see there is a good comparison on the first sheet

I don't care very much about collisions, but needed a weak pseudorandomizer of
rows based on a variable-length string field. Here's one insane solution that worked well:

=MOD(MOD(MOD(MOD(MOD(IF(LEN(Z2)>=1,CODE(MID(Z2,1,1))+10,31),1009)*IF(LEN(Z2)>=3,CODE(MID(Z2,3,1))+10,41),1009)*IF(LEN(Z2)>=5,CODE(MID(Z2,5,1))+10,59),1009)*IF(LEN(Z2)>=7,CODE(MID(Z2,7,1))+10,26),1009)*IF(LEN(Z2)>=9,CODE(MID(Z2,9,1))+10,53),1009)

Where Z2 is the cell containing the string you want to hash.

"MOD"s are there to prevent overflowing to scientific notation. 1009 is a prime, could use anything X so that X*255 < max_int_size. 10 is arbitrary; use anything. "Else" values are arbitrary (digits of pi here!); use anything. Location of characters (1,3,5,7,9) are arbitrary; use anything.

For a reasonably small list you can create a scrambler (poor man's hash function) using built-in Excel functions.

E.g.

 =CODE(A2)*LEN(A2) + CODE(MID(A2,$A$1,$B$1))*LEN(MID(A2,$A$1,$B$1))

Here A1 and B1 hold a random start letter and string length.

A little fiddling and checking and in most cases you can get a workable unique ID quite quickly.

How it Works: The formula uses the first letter of the string and a fixed letter taken from mid-string and uses LEN() as a 'fanning function' to reduce the chance of collisions.

CAVEAT: this is not a hash, but when you need to get something done quickly, and can inspect the results to see that there are no collisions, it works quite well.

Edit:
If your strings should have variable lengths (e.g. full names) but are pulled from a database record with fixed width fields, you will want to do it like this:

 =CODE(TRIM(C8))*LEN(TRIM(C8))

       +CODE(MID(TRIM(C8),$A$1,1))*LEN(MID(TRIM(C8),$A$1,$B$1))

so that the lengths are a meaningful scrambler.

You can try this. Run a Pseudo# on two columns:

=+IF(AND(ISBLANK(D3),ISBLANK(E3)),"",CODE(TRIM(D3&E3))*LEN(TRIM(D3&E3))+CODE(MID(TRIM(D3&E3),$A$1*LEN(D3&E3),1))INT(LEN(TRIM(D3&E3))$B$1))

Where A1 and B1 store random seeds entered manually: 0

To my knowledge there is no hash function build into Excel - you'd need to build one as a User Defined Function in VBA.

However, please note that for your purpose I don't think using a hash is required or really advantageous! VLOOKUP will work just as well on 256 bytes as it'll be on a smaller hash. Sure, it might be a tiny bit slower - bit that is for sure so small that it is immeasurable. And then adding the hash values is more effort for you - and for Excel...

I am using this which gives pretty good results with preventing clashing without needing to run a script each time. I needed a value between 0 - 1.

=ABS(COS((CODE(MID(A2,ROUNDUP(LEN(A2)/9,0),1))*(CODE(MID(A2,ROUNDUP(LEN(A2)/5,0),1))+100)/CODE(MID(A2,ROUNDUP(LEN(A2)/3,0),1))*(CODE(MID(A2,ROUNDUP(LEN(A2)*8/9,0),1))+25)/CODE(MID(A2,ROUNDUP(LEN(A2)*6/9,0),1))*(CODE(MID(A2,ROUNDUP(LEN(A2)*4/9,0),1))-25))/LEN(A2)+CODE(A2)))

It picks letters from across the string, takes the value of each of those letters, adds a value (to prevent same letters in different places giving same results), multiplies/divides each and runs a COS function over the total.