Rust check digit calculation for ISBN-13

isbn.rs

use crate::InvalidIsbn::{BadChecksum, TooLong, TooShort};
use std::str::FromStr;

// check digit calculation for ISBN-13

#[derive(Debug)]
struct Isbn {
    raw: String,
}

#[derive(Debug)]
enum InvalidIsbn {
    TooLong,
    TooShort,
    BadChecksum,
}

fn sum_string_functional(str: &str) -> u32 {
    str.chars().map(|c| c.to_digit(10).unwrap()).sum()
}

fn to_result(isbn: &str) -> Result<Isbn, InvalidIsbn> {
    let no_hyphens = isbn.replace("-", "");
    if no_hyphens.len() != 13 {
        return Err(if no_hyphens.len() < 13 {
            TooShort
        } else {
            TooLong
        });
    }

    let (first, checkdigit) = no_hyphens.split_at(12);

    let even_chars: String = first
        .chars()
        .enumerate()
        .filter(|(i, _)| i % 2 == 0)
        .map(|(_, c)| c)
        .collect();

    let even_sum = sum_string_functional(even_chars.as_str());

    let odd_chars: String = first
        .chars()
        .enumerate()
        .filter(|(i, _)| i % 2 != 0)
        .map(|(_, c)| c)
        .collect();

    let odd_sum = sum_string_functional(odd_chars.as_str()) * 3;

    let modulo = ((even_sum + odd_sum) % 10) as u8;

    let actual = (10 - modulo) % 10;
    let expected = checkdigit.chars().next().unwrap().to_digit(10).unwrap() as u8;

    if expected == actual {
        Ok(Isbn {
            raw: isbn.to_string(),
        })
    } else {
        Err(BadChecksum)
    }
}

// Remove hyphens from ISBN

impl FromStr for Isbn {
    type Err = InvalidIsbn;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        to_result(s)
    }
}

impl std::fmt::Display for Isbn {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        write!(f, "ISBN: {}", self.raw)
    }
}

fn main() {
    let isbn_obj: Isbn = "978-0-306-40615-7".parse().unwrap();
    println!("{}", isbn_obj);

    let isbn_bad: Result<Isbn, _> = "978-0-306-40615-2".parse();
    println!("{:?}", isbn_bad);

    let isbn_bad: Result<Isbn, _> = "978-0-305-7".parse();
    println!("{:?}", isbn_bad);

    let isbn_bad: Result<Isbn, _> = "978-0-306-40615-7-1234".parse();
    println!("{:?}", isbn_bad);
}

Output

ISBN: 978-0-306-40615-7
Err(BadChecksum)
Err(TooShort)
Err(TooLong)