// SPDX-License-Identifier: Apache-2.0
//
// Taken from bitcoin_hashes crate
// Written in 2018 by Andrew Poelstra <apoelstra@wpsoftware.net>
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License. You may obtain a copy of
// the License at <http://www.apache.org/licenses/LICENSE-2.0>
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
// License for the specific language governing permissions and limitations under
// the License.

//! # Hex encoding and decoding

#[cfg(feature = "alloc")]
use alloc::{format, string::String, vec::Vec};
use core::{fmt, str};

/// Hex decoding error
#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub enum Error {
    /// non-hexadecimal character
    InvalidChar(u8),
    /// purported hex string had odd length
    OddLengthString(usize),
    /// tried to parse fixed-length hash from a string with the wrong type
    /// (expected, got)
    InvalidLength(usize, usize),
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            Error::InvalidChar(ch) => write!(f, "invalid hex character {ch}"),
            Error::OddLengthString(ell) => write!(f, "odd hex string length {ell}"),
            Error::InvalidLength(ell, ell2) => {
                write!(f, "bad hex string length {ell2} (expected {ell})")
            }
        }
    }
}

#[cfg(feature = "std")]
impl std::error::Error for Error {}

/// Trait for objects that can be serialized as hex strings
#[cfg(any(test, feature = "std", feature = "alloc"))]
pub trait ToHex {
    /// Hex representation of the object
    fn to_hex(&self) -> String;
}

/// Trait for objects that can be deserialized from hex strings
pub trait FromHex: Sized {
    /// Produce an object from a byte iterator
    fn from_byte_iter<I>(iter: I) -> Result<Self, Error>
    where I: Iterator<Item = Result<u8, Error>> + ExactSizeIterator + DoubleEndedIterator;

    /// Produce an object from a hex string
    fn from_hex(s: &str) -> Result<Self, Error> { Self::from_byte_iter(HexIterator::new(s)?) }
}

#[cfg(any(test, feature = "std", feature = "alloc"))]
impl<T: fmt::LowerHex> ToHex for T {
    /// Outputs the hash in hexadecimal form
    fn to_hex(&self) -> String { format!("{self:x}") }
}

/// Iterator over a hex-encoded string slice which decodes hex and yields bytes.
pub struct HexIterator<'a> {
    /// The `Bytes` iterator whose next two bytes will be decoded to yield
    /// the next byte.
    iter: str::Bytes<'a>,
}

impl<'a> HexIterator<'a> {
    /// Constructs a new `HexIterator` from a string slice. If the string is of
    /// odd length it returns an error.
    pub fn new(s: &'a str) -> Result<HexIterator<'a>, Error> {
        if s.len() % 2 != 0 {
            Err(Error::OddLengthString(s.len()))
        } else {
            Ok(HexIterator { iter: s.bytes() })
        }
    }
}

fn chars_to_hex(hi: u8, lo: u8) -> Result<u8, Error> {
    let hih = (hi as char).to_digit(16).ok_or(Error::InvalidChar(hi))?;
    let loh = (lo as char).to_digit(16).ok_or(Error::InvalidChar(lo))?;

    let ret = (hih << 4) + loh;
    Ok(ret as u8)
}

impl<'a> Iterator for HexIterator<'a> {
    type Item = Result<u8, Error>;

    fn next(&mut self) -> Option<Result<u8, Error>> {
        let hi = self.iter.next()?;
        let lo = self.iter.next().unwrap();
        Some(chars_to_hex(hi, lo))
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let (min, max) = self.iter.size_hint();
        (min / 2, max.map(|x| x / 2))
    }
}

impl<'a> DoubleEndedIterator for HexIterator<'a> {
    fn next_back(&mut self) -> Option<Result<u8, Error>> {
        let lo = self.iter.next_back()?;
        let hi = self.iter.next_back().unwrap();
        Some(chars_to_hex(hi, lo))
    }
}

impl<'a> ExactSizeIterator for HexIterator<'a> {}

/// Output hex into an object implementing `fmt::Write`, which is usually more
/// efficient than going through a `String` using `ToHex`.
pub fn format_hex(data: &[u8], f: &mut fmt::Formatter) -> fmt::Result {
    let prec = f.precision().unwrap_or(2 * data.len());
    let width = f.width().unwrap_or(2 * data.len());
    for _ in (2 * data.len())..width {
        f.write_str("0")?;
    }
    for ch in data.iter().take(prec / 2) {
        write!(f, "{:02x}", *ch)?;
    }
    if prec < 2 * data.len() && prec % 2 == 1 {
        write!(f, "{:x}", data[prec / 2] / 16)?;
    }
    Ok(())
}

/// Output hex in reverse order; used for Sha256dHash whose standard hex
/// encoding has the bytes reversed.
pub fn format_hex_reverse(data: &[u8], f: &mut fmt::Formatter) -> fmt::Result {
    let prec = f.precision().unwrap_or(2 * data.len());
    let width = f.width().unwrap_or(2 * data.len());
    for _ in (2 * data.len())..width {
        f.write_str("0")?;
    }
    for ch in data.iter().rev().take(prec / 2) {
        write!(f, "{:02x}", *ch)?;
    }
    if prec < 2 * data.len() && prec % 2 == 1 {
        write!(f, "{:x}", data[data.len() - 1 - prec / 2] / 16)?;
    }
    Ok(())
}

#[cfg(any(test, feature = "std", feature = "alloc"))]
impl ToHex for [u8] {
    fn to_hex(&self) -> String {
        use core::fmt::Write;
        let mut ret = String::with_capacity(2 * self.len());
        for ch in self {
            write!(ret, "{ch:02x}").expect("writing to string");
        }
        ret
    }
}

#[cfg(any(test, feature = "std", feature = "alloc"))]
impl FromHex for Vec<u8> {
    fn from_byte_iter<I>(iter: I) -> Result<Self, Error>
    where I: Iterator<Item = Result<u8, Error>> + ExactSizeIterator + DoubleEndedIterator {
        iter.collect()
    }
}

macro_rules! impl_fromhex_array {
    ($len:expr) => {
        impl FromHex for [u8; $len] {
            fn from_byte_iter<I>(iter: I) -> Result<Self, Error>
            where I: Iterator<Item = Result<u8, Error>> + ExactSizeIterator + DoubleEndedIterator
            {
                if iter.len() == $len {
                    let mut ret = [0; $len];
                    for (n, byte) in iter.enumerate() {
                        ret[n] = byte?;
                    }
                    Ok(ret)
                } else {
                    Err(Error::InvalidLength(2 * $len, 2 * iter.len()))
                }
            }
        }
    };
}

impl_fromhex_array!(2);
impl_fromhex_array!(4);
impl_fromhex_array!(6);
impl_fromhex_array!(8);
impl_fromhex_array!(10);
impl_fromhex_array!(12);
impl_fromhex_array!(14);
impl_fromhex_array!(16);
impl_fromhex_array!(20);
impl_fromhex_array!(24);
impl_fromhex_array!(28);
impl_fromhex_array!(32);
impl_fromhex_array!(33);
impl_fromhex_array!(64);
impl_fromhex_array!(65);
impl_fromhex_array!(128);
impl_fromhex_array!(256);
impl_fromhex_array!(384);
impl_fromhex_array!(512);

#[cfg(test)]
mod tests {
    use core::fmt;

    use super::*;

    #[test]
    fn hex_roundtrip() {
        let expected = "0123456789abcdef";
        let expected_up = "0123456789ABCDEF";

        let parse: Vec<u8> = FromHex::from_hex(expected).expect("parse lowercase string");
        assert_eq!(parse, vec![0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef]);
        let ser = parse.to_hex();
        assert_eq!(ser, expected);

        let parse: Vec<u8> = FromHex::from_hex(expected_up).expect("parse uppercase string");
        assert_eq!(parse, vec![0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef]);
        let ser = parse.to_hex();
        assert_eq!(ser, expected);

        let parse: [u8; 8] = FromHex::from_hex(expected_up).expect("parse uppercase string");
        assert_eq!(parse, [0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef]);
        let ser = parse.to_hex();
        assert_eq!(ser, expected);
    }

    #[test]
    fn hex_truncate() {
        struct HexBytes(Vec<u8>);
        impl fmt::LowerHex for HexBytes {
            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { format_hex(&self.0, f) }
        }

        let bytes = HexBytes(vec![1u8, 2, 3, 4, 5, 6, 7, 8, 9, 10]);

        assert_eq!(format!("{:x}", bytes), "0102030405060708090a");

        for i in 0..20 {
            assert_eq!(format!("{:.prec$x}", bytes, prec = i), &"0102030405060708090a"[0..i]);
        }

        assert_eq!(format!("{:25x}", bytes), "000000102030405060708090a");
        assert_eq!(format!("{:26x}", bytes), "0000000102030405060708090a");
    }

    #[test]
    fn hex_truncate_rev() {
        struct HexBytes(Vec<u8>);
        impl fmt::LowerHex for HexBytes {
            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { format_hex_reverse(&self.0, f) }
        }

        let bytes = HexBytes(vec![1u8, 2, 3, 4, 5, 6, 7, 8, 9, 10]);

        assert_eq!(format!("{:x}", bytes), "0a090807060504030201");

        for i in 0..20 {
            assert_eq!(format!("{:.prec$x}", bytes, prec = i), &"0a090807060504030201"[0..i]);
        }

        assert_eq!(format!("{:25x}", bytes), "000000a090807060504030201");
        assert_eq!(format!("{:26x}", bytes), "0000000a090807060504030201");
    }

    #[test]
    fn hex_error() {
        let oddlen = "0123456789abcdef0";
        let badchar1 = "Z123456789abcdef";
        let badchar2 = "012Y456789abcdeb";
        let badchar3 = "«23456789abcdef";

        assert_eq!(Vec::<u8>::from_hex(oddlen), Err(Error::OddLengthString(17)));
        assert_eq!(<[u8; 4]>::from_hex(oddlen), Err(Error::OddLengthString(17)));
        assert_eq!(<[u8; 8]>::from_hex(oddlen), Err(Error::OddLengthString(17)));
        assert_eq!(Vec::<u8>::from_hex(badchar1), Err(Error::InvalidChar(b'Z')));
        assert_eq!(Vec::<u8>::from_hex(badchar2), Err(Error::InvalidChar(b'Y')));
        assert_eq!(Vec::<u8>::from_hex(badchar3), Err(Error::InvalidChar(194)));
    }
}
