Hash of a string to be of specific length

If I truncate the 41-byte hash to 33, I'd probably (certainly!) lost the uniqueness.

What makes you think you've got uniqueness now? Yes, there's clearly a higher chance of collision when you're only playing with 33 bytes instead of 41, but you need to be fully aware that collisions are only ever unlikely, not impossible, for any situation where it makes sense to use a hash in the first place. If you're hashing more than 41 bytes of data, there are clearly more possible combinations than there are hashes available.

Now, whether you'd be better off truncating the SHA-1 hash or using a shorter hash such as MD5, I don't know. I think I'd be more generally confident when keeping the whole of a hash, but MD5 has known vulnerabilities which may or may not be a problem for your particular application.

The way hashes are calculated that's unfortunately not possible. To limit the hash length to 33 bytes, you will have to cut it. You could xor the first and last 33 bytes, as that might keep more of the information. But even with 33 bytes you don't have that big a chance of a collision.

md5: http://www.md5hashing.com/c++/

btw. md5 is 16 bytes, sha1 20 bytes and sha256 is 32 bytes, however as hexstrings, they all double in size. If you can store bytes, you can even use sha256.

There is no more chance of collision with substring(sha_hash, 0, 33) than with any other hash that is 33 bytes long, due to the way hash algorithms are designed (entropy is evenly spread out in the resulting string).