Remove duplicated crc32 algorithm

[martinlindhe]

There's a crc32 implementation in src/crc.rs

A duplicate also lives in tests/check_testimages.rs

This issue is for reusing the code from src/crc.rs and deleting the duplicated code in tests/check_testimages.rs

[martinlindhe]

As mentioned in #128, another option could be to instead depend on a external crate for crc32. Any opinions?

[HeroicKatora]

I'd only depend on an external crate if it doesn't have dependencies or if it offers direct benefits.

The crc crate seems a bit to immature for me, has too wide of a scope and for some reason generates its tables a build time?

Another criterion could be performance. crc32fast offers –according to its own benchmark–six times the throughput at 1499 MB/s and up to 7314 MB/s on sse platforms. That could be worth a great deal of tradeoff, for example in additional dependencies and would lower our maintenance burder. Edit: Its interface looks also much like our own, which is also close to the standard Hasher interface except with u32 output.

[martinlindhe]

I'll make a PR using crc32fast and we can take it from there

[martinlindhe]

I have a crc32fast branch working, but it currently depends on srijs/rust-crc32fast#8 in order to work.

I'm awaiting follow up on that PR before I can go ahead and open a PR for this.
Meanwhile my branch lives in https://github.com/martinlindhe/image-png/tree/crc32fast

[HeroicKatora]

I think State implements Clone, so self.current_chunk.0.clone().finalize() could work as well.

[martinlindhe]

Thanks, I'll have a look at that

[martinlindhe]

I think State implements Clone,

It does not. I added Clone to State & U32Value but same issue.

so self.current_chunk.0.finalize() could work as well.

This is what the code's already doing and is the issue resolved by srijs/rust-crc32fast#8

[HeroicKatora]

Oops, that should have been self.current_chunk.0.clone().finalize(). Corrected it above. But rust-crc32fast does implement it for State. I'm a bit confused, why is that not accessible?

[martinlindhe]

Just me being confused too :-) I was looking at this State: https://github.com/image-rs/image-png/blob/master/src/decoder/stream.rs#L29

[martinlindhe]

Oops, that should have been self.current_chunk.0.clone().finalize(). Corrected it above.

Yep, this works! Rebasing etc