//! Zero-dependency hex encoding and decoding.

use std::fmt::Write;

/// The type could not be decoded.
#[derive(Debug)]
pub enum DecodeError {
    /// An invalid character was encountered.
    InvalidCharacter(u8),

    /// The amount of characters was invalid. Hex strings must be in pairs of two.
    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 {}

/// Encode a given input as a hex string.
///
/// # Examples
/// ```rust
/// let data = b"hello world!";
/// let result = smex::encode(&data);
/// assert_eq!(result, "68656c6c6f20776f726c6421");
/// ```
pub fn encode(input: impl AsRef<[u8]>) -> String {
    let input = input.as_ref();
    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)),
    }
}

/// Attempt to decode a string as hex.
///
/// # Errors
/// The function may error if a non-hex character is encountered or if the character count is not
/// evenly divisible by two.
///
/// # Examples
/// ```rust
/// let data = b"hello world!";
/// let encoded = smex::encode(&data);
/// let decoded = smex::decode(&encoded).unwrap();
/// assert_eq!(data.as_slice(), decoded.as_slice());
/// ```
///
/// The function may return an error if the given input is not valid hex.
///
/// ```rust,should_panic
/// let data = b"hello world!";
/// let mut encoded = smex::encode(&data);
/// encoded.push('G');
/// let decoded = smex::decode(&encoded).unwrap();
/// assert_eq!(data.as_slice(), decoded.as_slice());
/// ```
pub fn decode(input: impl AsRef<str>) -> Result<Vec<u8>, DecodeError> {
    let input = input.as_ref();
    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()
}