use std::fmt::Write;

#[derive(Debug)]
pub enum DecodeError {
    InvalidCharacter(u8),
    InvalidCharacterCount(usize),
}

impl std::fmt::Display for DecodeError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::InvalidCharacter(c) => {
                write!(f, "Invalid character: {c} not in [0123456789ABCDEF]")
            }
            Self::InvalidCharacterCount(n) => {
                write!(f, "Invalid character count: {n} % 2 != 0")
            }
        }
    }
}

impl std::error::Error for DecodeError {}

pub fn encode(input: &[u8]) -> String {
    let mut s = String::new();
    for byte in input {
        write!(s, "{byte:02x}").unwrap();
    }
    s
}

fn val(c: u8) -> Result<u8, DecodeError> {
    match c {
        b'A'..=b'F' => Ok(c - b'A' + 10),
        b'a'..=b'f' => Ok(c - b'a' + 10),
        b'0'..=b'9' => Ok(c - b'0'),
        _ => Err(DecodeError::InvalidCharacter(c)),
    }
}

pub fn decode(input: &str) -> Result<Vec<u8>, DecodeError> {
    let len = input.len();
    if len % 2 != 0 {
        return Err(DecodeError::InvalidCharacterCount(len));
    }

    input
        .as_bytes()
        .chunks_exact(2)
        .map(|pair| Ok(val(pair[0])? << 4 | val(pair[1])?))
        .collect()
}