// Copyright (c) 2013-2014 The Rust Project Developers.
// Copyright (c) 2015-2017 The rust-hex Developers.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//! Encoding and decoding hex strings.
//!
//! For most cases, you can simply use the `decode()`, `encode()` and
//! `encode_upper()` functions. If you need a bit more control, use the traits
//! `ToHex` and `FromHex` instead.

use std::error;
use std::fmt;

/// Encoding values as hex string.
///
/// This trait is implemented for all `T` which implement `AsRef<[u8]>`. This
/// includes `String`, `str`, `Vec<u8>` and `[u8]`.
///
/// *Note*: instead of using this trait, you might want to use `encode()`.
pub trait ToHex {
    /// Writes the hex string representing `self` into `w`. Lower case letters
    /// are used (e.g. `f9b4ca`).
    fn write_hex<W: fmt::Write>(&self, w: &mut W) -> fmt::Result;

    /// Writes the hex string representing `self` into `w`. Upper case letters
    /// are used (e.g. `F9B4CA`).
    fn write_hex_upper<W: fmt::Write>(&self, w: &mut W) -> fmt::Result;
}


impl<T: AsRef<[u8]>> ToHex for T {
    fn write_hex<W: fmt::Write>(&self, w: &mut W) -> fmt::Result {
        static CHARS: &'static [u8] = b"0123456789abcdef";

        for &byte in self.as_ref().iter() {
            w.write_char(CHARS[(byte >>  4) as usize].into())?;
            w.write_char(CHARS[(byte & 0xf) as usize].into())?;
        }

        Ok(())
    }

    fn write_hex_upper<W: fmt::Write>(&self, w: &mut W) -> fmt::Result {
        static CHARS: &'static [u8] = b"0123456789ABCDEF";

        for &byte in self.as_ref().iter() {
            w.write_char(CHARS[(byte >>  4) as usize].into())?;
            w.write_char(CHARS[(byte & 0xf) as usize].into())?;
        }

        Ok(())
    }
}

/// The error type for decoding a hex string into `Vec<u8>` or `[u8; N]`.
#[derive(Debug, Clone, Copy, PartialEq)]
pub enum FromHexError {
    /// An invalid character was found. Valid ones are: `0...9`, `a...f`
    /// or `A...F`.
    InvalidHexCharacter {
        c: char,
        index: usize,
    },

    /// A hex string's length needs to be even, as two digits correspond to
    /// one byte.
    OddLength,

    /// If the hex string is decoded into a fixed sized container, such as an
    /// array, the hex string's length * 2 has to match the container's
    /// length.
    InvalidStringLength,
}

impl error::Error for FromHexError {
    fn description(&self) -> &str {
        match *self {
            FromHexError::InvalidHexCharacter { .. } => "invalid character",
            FromHexError::OddLength => "odd number of digits",
            FromHexError::InvalidStringLength => "invalid string length",

        }
    }
}

impl fmt::Display for FromHexError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            FromHexError::InvalidHexCharacter { c, index } =>
                write!(f, "Invalid character '{}' at position {}", c, index),
            FromHexError::OddLength =>
                write!(f, "Odd number of digits"),
            FromHexError::InvalidStringLength =>
                write!(f, "Invalid string length"),
        }
    }
}

/// Types that can be decoded from a hex string.
///
/// This trait is implemented for `Vec<u8>` and small `u8`-arrays.
pub trait FromHex: Sized {
    type Error;

    /// Creates an instance of type `Self` from the given hex string, or fails
    /// with a custom error type.
    ///
    /// Both, upper and lower case characters are valid and can even be
    /// mixed (e.g. `f9b4ca`, `F9B4CA` and `f9B4Ca` are all valid strings).
    fn from_hex<T: AsRef<[u8]>>(hex: T) -> Result<Self, Self::Error>;
}

fn val(c: u8, idx: usize) -> Result<u8, FromHexError> {
    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(FromHexError::InvalidHexCharacter {
                c: c as char,
                index: idx,
            })
        }
    }
}

impl FromHex for Vec<u8> {
    type Error = FromHexError;

    fn from_hex<T: AsRef<[u8]>>(hex: T) -> Result<Self, Self::Error> {
        let hex = hex.as_ref();
        if hex.len() % 2 != 0 {
            return Err(FromHexError::OddLength);
        }

        hex.chunks(2).enumerate().map(|(i, pair)| {
            Ok(val(pair[0], 2 * i)? << 4 | val(pair[1], 2 * i + 1)?)
        }).collect()
    }
}

// Helper macro to implement the trait for a few fixed sized arrays. Once Rust
// has type level integers, this should be removed.
macro_rules! impl_from_hex_for_array {
    ($len:expr) => {
        impl FromHex for [u8; $len] {
            type Error = FromHexError;

            fn from_hex<T: AsRef<[u8]>>(hex: T) -> Result<Self, Self::Error> {
                let hex = hex.as_ref();
                if hex.len() % 2 != 0 {
                    return Err(FromHexError::OddLength);
                }
                if hex.len() / 2 != $len {
                    return Err(FromHexError::InvalidStringLength);
                }

                let mut out = [0; $len];
                for (i, byte) in out.iter_mut().enumerate() {
                    *byte = val(hex[2 * i], 2 * i)? << 4
                        | val(hex[2 * i + 1], 2 * i + 1)?;
                }

                Ok(out)
            }
        }
    }
}

impl_from_hex_for_array!(1);
impl_from_hex_for_array!(2);
impl_from_hex_for_array!(3);
impl_from_hex_for_array!(4);
impl_from_hex_for_array!(5);
impl_from_hex_for_array!(6);
impl_from_hex_for_array!(7);
impl_from_hex_for_array!(8);
impl_from_hex_for_array!(9);
impl_from_hex_for_array!(10);
impl_from_hex_for_array!(11);
impl_from_hex_for_array!(12);
impl_from_hex_for_array!(13);
impl_from_hex_for_array!(14);
impl_from_hex_for_array!(15);
impl_from_hex_for_array!(16);

// Throw in some multiples of 8 which may be useful.
impl_from_hex_for_array!(24);
impl_from_hex_for_array!(32);
impl_from_hex_for_array!(40);
impl_from_hex_for_array!(48);
impl_from_hex_for_array!(56);
impl_from_hex_for_array!(64);

/// Encodes `data` as hex string using lowercase characters.
///
/// Lowercase characters are used (e.g. `f9b4ca`). The resulting string's
/// length is always even, each byte in `data` is always encoded using two hex
/// digits. Thus, the resulting string contains exactly twice as many bytes as
/// the input data.
pub fn encode<T: AsRef<[u8]>>(data: T) -> String {
    let mut s = String::with_capacity(data.as_ref().len() * 2);

    // Writing to a string never errors, so we can unwrap here.
    data.write_hex(&mut s).unwrap();
    s
}

/// Encodes `data` as hex string using uppercase characters.
///
/// Apart from the characters' casing, this works exactly like `encode()`.
#[allow(dead_code)]
pub fn encode_upper<T: AsRef<[u8]>>(data: T) -> String {
    let mut s = String::with_capacity(data.as_ref().len() * 2);

    // Writing to a string never errors, so we can unwrap here.
    data.write_hex_upper(&mut s).unwrap();
    s
}

/// Decodes a hex string into raw bytes.
///
/// Both, upper and lower case characters are valid in the input string and can
/// even be mixed (e.g. `f9b4ca`, `F9B4CA` and `f9B4Ca` are all valid strings).
pub fn decode<T: AsRef<[u8]>>(data: T) -> Result<Vec<u8>, FromHexError> {
    FromHex::from_hex(data)
}


#[cfg(test)]
mod test {
    use super::{encode, decode, FromHex, FromHexError};

    #[test]
    fn test_encode() {
        assert_eq!(encode("foobar"), "666f6f626172");
    }

    #[test]
    fn test_decode() {
        assert_eq!(decode("666f6f626172"), Ok("foobar".to_owned().into_bytes()));
    }

    #[test]
    pub fn test_from_hex_okay_str() {
        assert_eq!(
            Vec::from_hex("666f6f626172").unwrap(),
            b"foobar"
        );
        assert_eq!(
            Vec::from_hex("666F6F626172").unwrap(),
            b"foobar"
        );
    }

    #[test]
    pub fn test_from_hex_okay_bytes() {
        assert_eq!(
            Vec::from_hex(b"666f6f626172").unwrap(),
            b"foobar"
        );
        assert_eq!(
            Vec::from_hex(b"666F6F626172").unwrap(),
            b"foobar"
        );
    }

    #[test]
    pub fn test_invalid_length() {
        assert_eq!(
            Vec::from_hex("1").unwrap_err(),
            FromHexError::OddLength
        );
        assert_eq!(
            Vec::from_hex("666f6f6261721").unwrap_err(),
            FromHexError::OddLength
        );
    }

    #[test]
    pub fn test_invalid_char() {
        assert_eq!(
            Vec::from_hex("66ag").unwrap_err(),
            FromHexError::InvalidHexCharacter {
                c: 'g',
                index: 3
            }
        );
    }

    #[test]
    pub fn test_empty() {
        assert_eq!(Vec::from_hex("").unwrap(), b"");
    }

    #[test]
    pub fn test_from_hex_whitespace() {
        assert_eq!(
            Vec::from_hex("666f 6f62617").unwrap_err(),
            FromHexError::InvalidHexCharacter {
                c: ' ',
                index: 4
            }
        );
    }

    #[test]
    pub fn test_from_hex_array() {
        assert_eq!(
            <[u8; 6] as FromHex>::from_hex("666f6f626172"),
            Ok([0x66, 0x6f, 0x6f, 0x62, 0x61, 0x72])
        );

        assert_eq!(
            <[u8; 5] as FromHex>::from_hex("666f6f626172"),
            Err(FromHexError::InvalidStringLength)
        );
    }
}