qubit-codec-misc 0.8.2

// =============================================================================
//    Copyright (c) 2026 Haixing Hu.
//
//    SPDX-License-Identifier: Apache-2.0
//
//    Licensed under the Apache License, Version 2.0.
// =============================================================================
//! Low-level hexadecimal byte codec.

use core::num::NonZeroUsize;

use qubit_codec::Codec;
use qubit_codec::DecodeFailure;
use qubit_utils::nonzero;

use crate::MiscCodecError;
use crate::hex::hex_digit;
use crate::hex::hex_value;
use crate::hex::invalid_hex_digit;
use crate::misc_error::map_misc_decode_failure_with_consumed;

/// Encodes and decodes one byte as two ASCII hexadecimal units.
///
/// `HexByteCodec` is the low-level [`Codec`] implementation for streaming or
/// generic codec call sites. It does not understand whole-string prefixes,
/// per-byte prefixes, separators, or whitespace. Use [`crate::HexCodec`] for
/// owned byte-slice helpers with those formatting options.
///
/// For a complete safe streaming adapter example, see the [English README](https://github.com/qubit-ltd/rs-codec-misc/blob/main/README.md#safe-streaming-hex-byte-adapter)
/// and [Chinese README](https://github.com/qubit-ltd/rs-codec-misc/blob/main/README.zh_CN.md#安全的-hex-字节流式-adapter).
///
/// # Examples
///
/// ```
/// use qubit_codec::Codec;
/// use qubit_codec_misc::HexByteCodec;
///
/// let mut codec = HexByteCodec::upper();
/// let mut output = [0_u8; 2];
/// let written = unsafe { Codec::encode(&mut codec, &0xaf, &mut output, 0) }?;
///
/// assert_eq!(2, written);
/// assert_eq!(b"AF", &output);
/// # Ok::<(), qubit_codec_misc::MiscCodecError>(())
/// ```
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
pub struct HexByteCodec {
    /// Controls whether encoded hexadecimal digits use uppercase letters.
    uppercase: bool,
}

impl HexByteCodec {
    /// Creates a lowercase single-byte hexadecimal codec.
    #[must_use]
    #[inline]
    pub const fn new() -> Self {
        Self { uppercase: false }
    }

    /// Creates an uppercase single-byte hexadecimal codec.
    #[must_use]
    #[inline]
    pub const fn upper() -> Self {
        Self { uppercase: true }
    }

    /// Returns whether this byte codec emits uppercase hexadecimal digits.
    #[must_use]
    #[inline]
    pub const fn is_uppercase(self) -> bool {
        self.uppercase
    }

    /// Sets whether encoded digits should be uppercase.
    #[must_use]
    #[inline]
    pub const fn with_uppercase(mut self, uppercase: bool) -> Self {
        self.uppercase = uppercase;
        self
    }
}

impl Codec for HexByteCodec {
    type Value = u8;
    type Unit = u8;
    type DecodeError = MiscCodecError;
    type EncodeError = MiscCodecError;

    const MIN_UNITS_PER_VALUE: usize = 2;
    const MAX_ENCODE_UNITS_PER_VALUE: usize = 2;
    const MAX_DECODE_UNITS_PER_VALUE: usize = 2;

    /// Decodes one byte from two ASCII hexadecimal digits.
    ///
    /// The digits are read from `input` starting at `input_index`. Invalid
    /// digits are reported with their position in the input.
    ///
    /// # Parameters
    ///
    /// - `input`: The byte units containing the hexadecimal digits.
    /// - `input_index`: The index of the first digit.
    ///
    /// # Returns
    ///
    /// The decoded byte and the nonzero number of input units consumed (two).
    ///
    /// # Errors
    ///
    /// Returns a decode failure when either input unit is not an ASCII
    /// hexadecimal digit. The failure identifies the invalid digit and its
    /// input position.
    ///
    /// # Safety
    /// `input` must contain at least two readable units starting at
    /// `input_index`.
    #[inline]
    unsafe fn decode(
        &mut self,
        input: &[u8],
        input_index: usize,
    ) -> Result<(u8, NonZeroUsize), DecodeFailure<Self::DecodeError>> {
        debug_assert!(input_index + 2 <= input.len());

        let high_char = char::from(input[input_index]);
        let low_char = char::from(input[input_index + 1]);
        let high = hex_value(high_char)
            .ok_or_else(|| invalid_hex_digit(input_index, high_char))
            .map_err(|error| map_misc_decode_failure_with_consumed(error, nonzero(2)))?;
        let low = hex_value(low_char)
            .ok_or_else(|| invalid_hex_digit(input_index + 1, low_char))
            .map_err(|error| map_misc_decode_failure_with_consumed(error, nonzero(2)))?;
        Ok(((high << 4) | low, nonzero(2)))
    }

    /// Encodes one byte as two ASCII hexadecimal digits.
    ///
    /// # Parameters
    ///
    /// - `value`: The byte to encode.
    /// - `output`: The buffer that receives the encoded digits.
    /// - `output_index`: The index of the first output unit to write.
    ///
    /// # Returns
    ///
    /// The number of output units written (two).
    ///
    /// # Errors
    ///
    /// This implementation does not return an encoding error; the result is
    /// retained to satisfy the [`Codec`] interface.
    ///
    /// # Safety
    /// `output` must contain at least two writable units starting at
    /// `output_index`.
    #[inline]
    unsafe fn encode(
        &mut self,
        value: &u8,
        output: &mut [u8],
        output_index: usize,
    ) -> Result<usize, Self::EncodeError> {
        debug_assert!(output_index + 2 <= output.len());

        output[output_index] = hex_digit(*value >> 4, self.uppercase) as u8;
        output[output_index + 1] = hex_digit(*value & 0x0f, self.uppercase) as u8;
        Ok(2)
    }
}