Base37 encoder/decoder written in Rust

base37.rs

const VALID_NAME_CHARS: [char; 37] = [
    '_', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i',
    'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's',
    't', 'u', 'v', 'w', 'x', 'y', 'z', '0', '1', '2',
    '3', '4', '5', '6', '7', '8', '9'
];

pub fn encode_base37<T: AsRef<str>>(input: T) -> u64 {
    let mut combined = input.as_ref().chars().fold(0u64, |acc, c| {
        let acc = acc.wrapping_mul(37);
        match c {
            'A'..='Z' => acc + (c as u64 - 65) + 1,
            'a'..='z' => acc + (c as u64 - 97) + 1,
            '0'..='9' => acc + (c as u64 - 48) + 27,
            _ => acc
        }
    });

    while combined % 37 == 0 && combined != 0 {
        combined = combined.wrapping_div(37);
    }
    combined
}

pub fn decode_base37(mut input: u64) -> anyhow::Result<String> {
    if input == 0 || input >= 0x5B5B57F8A98A5DD1 || input % 37 == 0 {
        return Err(anyhow::anyhow!("invalid name"));
    }

    let mut result = ['\0'; 12];
    let mut index = 0;
    while input != 0 {
        let local_input = input;
        input /= 37;
        index += 1;
        result[11 - index] = (VALID_NAME_CHARS[(local_input - input * 37) as usize]);
    }
    Ok(result.iter().filter(|c| **c != '\0').collect::<String>())
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    pub fn test_encode_base37() {
        assert_eq!(encode_base37("smrkn"), 36292611);
        assert_eq!(encode_base37("csh"), 4818);
    }

    #[test]
    pub fn test_decode_base37() {
        assert_eq!(decode_base37(36292611).unwrap(), String::from("smrkn"));
        assert_eq!(decode_base37(4818).unwrap(), String::from("csh"));
    }
}