//! Utilities for producing and flexibly parsing strings containing hexadecimal binary data.

/// Utility for parsing hex binary data from strings.
pub enum HexParser {
    /// "Liberal" parsing simply ignores characters that are not (case-insensitive) hex digits.
    Liberal,
    /// "Whitespace allowed" parsing ignores whitespace, but fails a parse on anything other
    /// than hex or whitespace.
    WhitespaceAllowed,
    /// "Strict" parsing accepts only (case-insensitive) hex digits; no whitespace, no other
    /// characters.
    Strict,
}

/// Utility for formatting binary data as hex.
pub enum HexFormatter {
    /// Produces LF-separated lines with a maximum of `usize` hex digits in each line.
    Lines(usize),
    /// Simply packs hex digits in as tightly as possible.
    Packed,
}

/// Convert a number 0..15 to a hex digit [char].
///
/// # Panics
///
/// Panics if given `v` outside the range 0..15 inclusive.
///
pub fn hexdigit(v: u8) -> char {
    char::from_digit(v as u32, 16).expect("hexadecimal digit value")
}

impl HexParser {
    /// Decode `s` according to the given rules for `self`; see [HexParser].
    /// If the parse fails, yield `None`.
    pub fn decode(&self, s: &str) -> Option<Vec<u8>> {
        let mut result = Vec::new();
        let mut buf: u8 = 0;
        let mut buf_full = false;
        for c in s.chars() {
            match c.to_digit(16) {
                None => match self {
                    HexParser::Liberal => (),
                    HexParser::WhitespaceAllowed => {
                        if !c.is_whitespace() {
                            return None;
                        }
                    }
                    HexParser::Strict => return None,
                },
                Some(nibble) => {
                    if buf_full {
                        result.push(buf << 4 | (nibble as u8));
                        buf_full = false;
                    } else {
                        buf = nibble as u8;
                        buf_full = true;
                    }
                }
            }
        }
        if buf_full {
            None // odd number of hexits
        } else {
            Some(result)
        }
    }
}

impl HexFormatter {
    /// Encode `bs` according to the given rules for `self; see [HexFormatter].
    pub fn encode(&self, bs: &[u8]) -> String {
        match self {
            HexFormatter::Lines(max_line_length) => {
                let mut lines = Vec::new();
                let mut line = String::new();
                for b in bs {
                    if line.len() + 2 > *max_line_length {
                        lines.push(std::mem::take(&mut line));
                    }
                    line.push(hexdigit(b >> 4));
                    line.push(hexdigit(b & 15));
                }
                lines.push(std::mem::take(&mut line));
                lines.join("\n")
            }
            HexFormatter::Packed => {
                let mut result = String::new();
                for b in bs {
                    result.push(hexdigit(b >> 4));
                    result.push(hexdigit(b & 15));
                }
                result
            }
        }
    }
}

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

    #[test]
    fn test_decode_packed() {
        let s = "01ab00ff";
        assert_eq!(HexParser::Strict.decode(s), Some(vec![1, 171, 0, 255]));
        assert_eq!(
            HexParser::WhitespaceAllowed.decode(s),
            Some(vec![1, 171, 0, 255])
        );
        assert_eq!(HexParser::Liberal.decode(s), Some(vec![1, 171, 0, 255]));
    }

    #[test]
    fn test_decode_whitespace() {
        let s = "01ab  00ff";
        assert_eq!(HexParser::Strict.decode(s), None);
        assert_eq!(
            HexParser::WhitespaceAllowed.decode(s),
            Some(vec![1, 171, 0, 255])
        );
        assert_eq!(HexParser::Liberal.decode(s), Some(vec![1, 171, 0, 255]));
    }

    #[test]
    fn test_decode_liberal() {
        let s = "01ab zz 00ff";
        assert_eq!(HexParser::Strict.decode(s), None);
        assert_eq!(HexParser::WhitespaceAllowed.decode(s), None);
        assert_eq!(HexParser::Liberal.decode(s), Some(vec![1, 171, 0, 255]));
    }

    #[test]
    fn test_encode_lines() {
        assert_eq!(
            HexFormatter::Lines(10).encode(&vec![0x5a; 11]),
            "5a5a5a5a5a\n5a5a5a5a5a\n5a"
        );
        assert_eq!(
            HexFormatter::Lines(10).encode(&vec![0x5a; 10]),
            "5a5a5a5a5a\n5a5a5a5a5a"
        );
        assert_eq!(
            HexFormatter::Lines(10).encode(&vec![0x5a; 9]),
            "5a5a5a5a5a\n5a5a5a5a"
        );
        assert_eq!(
            HexFormatter::Lines(9).encode(&vec![0x5a; 11]),
            "5a5a5a5a\n5a5a5a5a\n5a5a5a"
        );
        assert_eq!(
            HexFormatter::Lines(9).encode(&vec![0x5a; 10]),
            "5a5a5a5a\n5a5a5a5a\n5a5a"
        );
        assert_eq!(
            HexFormatter::Lines(9).encode(&vec![0x5a; 9]),
            "5a5a5a5a\n5a5a5a5a\n5a"
        );
        assert_eq!(
            HexFormatter::Lines(8).encode(&vec![0x5a; 11]),
            "5a5a5a5a\n5a5a5a5a\n5a5a5a"
        );
        assert_eq!(
            HexFormatter::Lines(8).encode(&vec![0x5a; 10]),
            "5a5a5a5a\n5a5a5a5a\n5a5a"
        );
        assert_eq!(
            HexFormatter::Lines(8).encode(&vec![0x5a; 9]),
            "5a5a5a5a\n5a5a5a5a\n5a"
        );
    }

    #[test]
    fn test_encode_packed() {
        assert_eq!(
            HexFormatter::Packed.encode(&vec![0x5a; 11]),
            "5a5a5a5a5a5a5a5a5a5a5a"
        );
        assert_eq!(
            HexFormatter::Packed.encode(&vec![0x5a; 10]),
            "5a5a5a5a5a5a5a5a5a5a"
        );
        assert_eq!(
            HexFormatter::Packed.encode(&vec![0x5a; 9]),
            "5a5a5a5a5a5a5a5a5a"
        );
    }
}