Const parsing of &str -> usize in Rust

LICENSE

MIT License

Copyright (c) 2021 DutchGhost

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

parse.rs

#[derive(Clone, Copy, Debug)]
enum ParseIntError {
    InvalidDigit,
}

const fn parse_byte(b: u8, pow10: usize) -> Result<usize, ParseIntError> {
    let r = b.wrapping_sub(48);

    if r > 9 {
        Err(ParseIntError::InvalidDigit)
    } else {
        Ok((r as usize) * pow10)
    }
}

pub(crate) const POW10: [usize; 20] = {
    let mut array = [0; 20];
    let mut current = 1;
    
    let mut index = 20;
    
    loop {
        index -= 1;
        array[index] = current;        
        
        if index == 0 { break }

        current *= 10;
        
    }
    
    array
    
};

const fn parse(b: &str) -> Result<usize, ParseIntError> {
    let bytes = b.as_bytes();
    
    let mut result: usize = 0;
    
    let len = bytes.len();
    
    // Start at the correct index of the table,
    // (skip the power's that are too large)
    let mut index_const_table = POW10.len().wrapping_sub(len);
    let mut index = 0;
    
    while index < b.len() {
        let a = bytes[index];
        let p = POW10[index_const_table];
        
        let r = match parse_byte(a, p) {
            Err(e) => return Err(e),
            Ok(d) => d,
        };
        
        result = result.wrapping_add(r);
        
        index += 1;
        index_const_table += 1;
    }
    
    Ok(result)
}

const fn unwarp(opt: Result<usize, ParseIntError>) -> usize {
    match opt {
        Ok(t) => t,
        _ => loop {}
    }
}

fn main() {
    const P: usize = unwarp(parse("9999912"));

    println!("{:?}", P);
}

Thank you again! I did these changes to use this in 32-bit, which I donate to use under the same license as stated here (they are trivial anyway), for simplicity, if necessary. https://github.com/bluss/matrixmultiply/pull/64/files I guess it could be made more optimal for 32-bit but didn't need to look into it.

Thank you again! I did these changes to use this in 32-bit, which I donate to use under the same license as stated here (they are trivial anyway), for simplicity, if necessary. https://github.com/bluss/matrixmultiply/pull/64/files I guess it could be made more optimal for 32-bit but didn't need to look into it.

Thanks! Happy this code is usefull.
As for performance, perhaps something could be made faster. The algorithm used is invented by Andrei Alexandrescu, in one of his fastware talks.

https://youtu.be/o4-CwDo2zpg (on 00:40:00)

I like those talks. For now, parsing at compile time feels free, performance cost is 0 at runtime, which is the best part, and for my use case I'm at most parsing usize 12 times, which I assume has a negligible cost.