#sha-rust

About

Implementation of the sha1 algorithm in rust - my first rust program :)

This was a way to learn more about sha1 and to make a real program using rust. Sha1 uses lots of bit twiddling and does just enough input, output, looping and memory allocation to be a good first program.

Usage

Send data to stdin and the sha program will print the sha1 hash on std out:

$ echo -n "What is the sha1 hash of this string?" | ./sha
4810253f49ccefe8804a3ba76efe0acc8a4ca9fe

Notes

I learned a lot about rust writing this, and there are a few things I really love:

• Built in testing with #[test], this made developing the code and finding bugs so easy
• Built in bit maniplation is great. Operators like ^, & and | work just like you would expect, but there are also things like overflowing_add, and rotate_left built in to primative integer types that make twiddling easy.

There are also several things about rust that I struggled with:

• str vs String
• Slices and their sizes. What is checked at compile time? Are slice range lengths?
• overflow_add returns a tuple, which makes it awkward to chain together
• How many allocations am I really doing? I'm still not clear on what ends up on the stack vs what is dynamically allocated with a vec. I ended up refactoring the program at the end so that there is basically one big allocation at the beginning to store the incoming message and I think that is the only alloc, but I'm not really sure.
• Macros - I don't yet have a sense for when I should expect something to be a macro vs expecting it to be in a library. Does concat! only work wtih static strings? Why are some thing macros instead of library functions?
• Weird to me that sometimes mut applies to the binding, and sometimes to the type. Not always intuitive to me where to put the 'mut' keyword. I think mut never applies to the type, but goes along with it in function signatures.(?)
• I though slices and ranges would work like in python. I enjoy having these features in a lower level language, but didn't find the usage as intuitive.

Performance

Performance of this code is interesting. It seems to be a lot faster than shasum distributed with OSX on smaller files, but slower on larger files (larger than 150MB).

$ du -h ~/Downloads/sintel.mp4 
123M	/Users/nathan/Downloads/sintel.mp4
$ time cat ~/Downloads/sintel.mp4 | ./target/release/sha 
50f1c70eeabbe3445d11a1c602c7851648156c2d

real	0m1.325s
user	0m1.216s
sys	0m0.220s
$ time cat ~/Downloads/sintel.mp4 | shasum
50f1c70eeabbe3445d11a1c602c7851648156c2d  -

real	0m0.746s
user	0m0.695s
sys	0m0.181s

$ time echo -n "What is my sha1 message digest?" | ./target/release/sha 
3255d4afb6e4fcd2392d1da6b8105f8cb2ca3dda

real	0m0.010s
user	0m0.003s
sys	0m0.006s
$ time echo -n "What is my sha1 message digest?" | shasum
3255d4afb6e4fcd2392d1da6b8105f8cb2ca3dda  -

real	0m0.056s
user	0m0.041s
sys	0m0.012s