How to write CRC/checksum code?

I'm coding an application, and I'm looking for an algorithm that can tell me if a file has changed, without only using the file's timestamp. I was thinking of using something like a checksum.

This would be used for files < 20 MB in size, and on these files, it should be able to operate in a few seconds, preferably under 1s for files under 1 MB.

My idea was to store the last modified date, the checksum, and the file length for each file as it was discovered, and later, to use these as a test to determine if the file was modified or replaced; if all 3 match, the file is considered unchanged; if not, the file has been altered.

I just need a fast algorithm to generate some kind of checksum. Any resources?

Don't know how fast this is, but some files I have worked with of similar size used this algorithm (pseudocode)-

checksum = 0
previousByte = 0
for (i = 0 to filesize - 1)

checksum = checksum + fileByte[i] + (i / previousByte + 1)

previousByte = fileByte[i]

Hmm, I've been looking, but one issue is that most of them, including the only "official" CRC code I've found, end up with a word value (16 bits) or smaller.

I'd prefer something that would come up with a longer CRC/checksum val, like 32 or 64 bits.

Doing a quick search turns up -

http://www.createwindow.com/programming/crc32

I don't know whether this meets your speed requirements, and if you've been looking around you may have already found this out for yourself.

enjoy!

http://www.funet.fi/pub/crypt/hash/crc/

Thanks, these should work out!