String CRC

[Walter VanDerwall]

Hello

Im looking for an .obj to get a checksum value of a string.
I want to $link it to my main and just feed it a string and get a return checksum value for that string.

Im using PB/DOS 3.5

I have looked through the forums and found a few samples but unfortunately im not skilled enough to understand them.
I was calling an .obj in QB with success, but it doesnt work in PB.

Thanks for any help
[email protected]


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[This message has been edited by Walter VanDerwall (edited May 24, 2005).]

[Donald Darden]

Exactly what are you trying to do? A CRC (Cycle Redundancy Check)
is designed to ensure that a sequence of bytes (words, or double-
words) is not accidently modified or corrupted. You start with a
CRC, and if you later create another CRC and it does not match the
original, you know that some minor error or change has occurred.

I say "monor", because there are chance changes that can occur that
will cause the two CRCs to still match, though the contents are
different. So the larger the CRC value, the less change that the
error will go undetected. For this reason CRC32 is generally more
dependable than CRC16.

Unless you need to use a standard CRC generator, it is very easy
to roll your own. Start with a variable set to zero, then as you
read each byte, word, or dword from as string, rotate the contents
of the variable left or right by one and XOR it with the byte,
word, or dword you just read. The XOR operator has the unique
property of not destroying or losing any bits in the first or
second parameters, merely flipping bits according to the content
of both. The variable mentioned receives each change, and is your
CRC value when it has processed the whole string.

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Old Navy Chief, Systems Engineer, Systems Analyst, now semi-retired

[Walter VanDerwall]

In the past in QuickBasic I made a call to a checksum.obj file. (its a Probas7 communication routine)
that calculated a checksum value for a string.

Call checksum(a$, num%)
a$=string to calculate
num%=return value

That .obj, when called in PowerBasic, does not work.
So Im looking for a .obj or some code that will do the same thing but in PB.

Thanks again

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[This message has been edited by Walter VanDerwall (edited May 24, 2005).]

[Donald Darden]

Simple approach to doing a CRC on a string. Note that this is
just one method designed to generate a CRC. There are more
sophisticated methods used elsewhere. But if this is just for
your use, you can make it work any way that you want.

FUNCTION StrCRC(SomeString AS STRING) AS DWORD
LOCAL Temp as DWORD, a AS DWORD
a = (LEN(SomeString) - 1) MOD 4 + 1
CASE 2
Temp = ASC(SomeString)
CASE 3
Temp = CVWRD(SomeString)
CASE 4
Temp = CVWRD(SomeString,2)
ROTATE LEFT Temp, 8
Temp = Temp XOR ASC(SomeString)
END SELECT
FOR a = a TO LEN(SomeString) STEP 4
ROTATE LEFT Temp, 1
Temp = Temp XOR CVDWD(SomeString,a)
NEXT
FUNCTION = Temp
END FUNCTION

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Old Navy Chief, Systems Engineer, Systems Analyst, now semi-retired

[Michael Mattias]

> was calling an .obj in QB with success, but it doesnt work in PB.

Strings are totally incompatible between QB and PB. No "obj" file designed for one will work with the other if it uses a string parameter. (And no doubt other parameters, but I don't know enough about those to say for sure).