qubit-io 0.18.0

// =============================================================================
//    Copyright (c) 2026 Haixing Hu.
//
//    SPDX-License-Identifier: Apache-2.0
//
//    Licensed under the Apache License, Version 2.0.
// =============================================================================

use std::io::Error;
use std::io::ErrorKind;
use std::io::Result;

use super::validate_read_count;
use crate::StdRead;
use crate::util::SliceRange;

/// Minimal indexed input interface over items.
///
/// `Input` is intentionally smaller and lower-level than [`std::io::Read`]. It
/// only states that an implementor can read up to `count` items into
/// `output[index..index + count]`. The caller owns range validation so hot
/// paths can avoid repeated slicing and bounds checks.
///
/// # Coherence note
///
/// Every [`std::io::Read`] value automatically implements `Input<Item = u8>`
/// through the standard I/O integration. Because [`Input::Item`] is an
/// associated type rather than a trait parameter, a concrete type that
/// implements [`std::io::Read`] cannot also provide any other direct `Input`
/// implementation for the same type, including one with a different item type.
///
/// Use a wrapper/newtype when a type needs item-oriented input semantics that
/// differ from its byte-oriented [`std::io::Read`] implementation.
///
/// # Examples
///
/// Standard readers implement `Input<Item = u8>` automatically, so callers
/// can use the trait methods directly on them:
///
/// ```
/// use std::io::Cursor;
///
/// use qubit_io::Input;
///
/// let mut input = Cursor::new(b"data");
/// let mut output = [0; 4];
/// assert_eq!(Input::read(&mut input, &mut output).unwrap(), 4);
/// assert_eq!(&output, b"data");
/// ```
pub trait Input {
    /// The item type read from this input.
    type Item;

    /// Converts this byte input into a standard library reader.
    ///
    /// The conversion only wraps `self`; it performs no I/O and does not
    /// allocate. The wrapped input can be recovered with
    /// [`StdRead::into_inner`].
    ///
    /// # Examples
    ///
    /// ```
    /// use qubit_io::Input;
    /// use std::io::Read;
    ///
    /// let input = std::io::Cursor::new(b"hello");
    /// let mut reader = input.into_std_read();
    /// let mut bytes = Vec::new();
    /// reader.read_to_end(&mut bytes).unwrap();
    /// assert_eq!(bytes, b"hello");
    /// let input = reader.into_inner();
    /// assert_eq!(input.position(), 5);
    /// ```
    ///
    /// Non-byte inputs cannot be converted:
    ///
    /// ```compile_fail
    /// use qubit_io::Input;
    ///
    /// fn convert<I: Input<Item = char>>(input: I) {
    ///     let _reader = input.into_std_read();
    /// }
    /// ```
    #[must_use = "the returned reader is the converted input"]
    #[inline]
    fn into_std_read(self) -> StdRead<Self>
    where
        Self: Sized + Input<Item = u8>,
    {
        StdRead::new(self)
    }

    /// Returns whether this input already buffers items internally.
    ///
    /// # Returns
    ///
    /// `true` when callers should avoid wrapping this input in another generic
    /// item buffer automatically.
    #[inline(always)]
    #[must_use]
    fn is_buffered(&self) -> bool {
        false
    }

    /// Reads items into an indexed output range without checking the range.
    ///
    /// # Parameters
    ///
    /// * `output` - Destination storage.
    /// * `index` - Start index inside `output`.
    /// * `count` - Maximum number of items to read.
    ///
    /// # Returns
    ///
    /// The number of items written into `output[index..index + count]`. The
    /// value must be in `0..=count`.
    ///
    /// # Errors
    ///
    /// Returns the input error reported by the implementation.
    ///
    /// # Safety
    ///
    /// The caller must guarantee that `index..index + count` is a valid range
    /// inside `output` and that the addition does not overflow.
    unsafe fn read_unchecked(&mut self, output: &mut [Self::Item], index: usize, count: usize) -> Result<usize>;

    /// Reads items into the full output slice.
    ///
    /// # Parameters
    /// - `output`: Destination storage.
    ///
    /// # Returns
    /// The number of items read into `output`.
    ///
    /// # Errors
    ///
    /// Returns the input error reported by the implementation. Returns
    /// [`ErrorKind::InvalidData`] if the implementation reports reading more
    /// items than requested.
    #[inline(always)]
    fn read(&mut self, output: &mut [Self::Item]) -> Result<usize> {
        // SAFETY: The caller ensured the destination slice is valid.
        let read = unsafe { self.read_unchecked(output, 0, output.len()) }?;
        validate_read_count(read, output.len())?;
        Ok(read)
    }

    /// Reads items into an indexed output range until it is full or EOF is
    /// reached.
    ///
    /// This method retries interrupted reads and treats EOF as a successful
    /// partial result.
    ///
    /// # Parameters
    ///
    /// * `output` - Destination storage.
    /// * `index` - Start index inside `output`.
    /// * `count` - Maximum number of items to read.
    ///
    /// # Returns
    ///
    /// The number of items written into `output[index..index + count]`.
    ///
    /// # Errors
    ///
    /// Returns the first non-[`ErrorKind::Interrupted`] input error. Returns
    /// [`ErrorKind::InvalidData`] if the implementation reports more items than
    /// requested.
    ///
    /// # Panics
    ///
    /// Panics in debug builds if the requested output range does not fit.
    ///
    /// # Safety
    ///
    /// The caller must guarantee that `index..index + count` is a valid range
    /// inside `output` and that the addition does not overflow.
    unsafe fn read_fully_unchecked(&mut self, output: &mut [Self::Item], index: usize, count: usize) -> Result<usize> {
        debug_assert!(
            SliceRange::range_fits(output.len(), index, count),
            "unchecked read-fully range exceeds output buffer"
        );
        let mut total = 0;
        while total < count {
            let remaining = count - total;
            // SAFETY: The caller guarantees the original destination range is
            // valid; `total < count`, so this suffix remains inside it.
            match unsafe { self.read_unchecked(output, index + total, remaining) } {
                Ok(0) => break,
                Ok(read) => {
                    validate_read_count(read, remaining)?;
                    total += read;
                }
                Err(error) if error.kind() == ErrorKind::Interrupted => {}
                Err(error) => return Err(error),
            }
        }
        Ok(total)
    }

    /// Reads items into the full output slice until it is full or EOF is
    /// reached.
    ///
    /// # Parameters
    /// - `output`: Destination storage to fill as far as possible.
    ///
    /// # Returns
    /// The number of items written into `output`.
    ///
    /// # Errors
    /// Returns the first non-interrupted input error, or
    /// [`ErrorKind::InvalidData`] if the implementation reports an impossible
    /// item count.
    #[inline(always)]
    fn read_fully(&mut self, output: &mut [Self::Item]) -> Result<usize> {
        // SAFETY: The full output slice is a valid destination range.
        unsafe { self.read_fully_unchecked(output, 0, output.len()) }
    }

    /// Reads items until the full output slice is filled.
    ///
    /// # Parameters
    ///
    /// * `output` - Destination storage that must be filled completely.
    ///
    /// # Returns
    ///
    /// `Ok(())` after the destination has been filled completely.
    ///
    /// # Errors
    ///
    /// Returns [`ErrorKind::UnexpectedEof`] if the input ends before filling
    /// `output`. Returns the first non-interrupted input error, or
    /// [`ErrorKind::InvalidData`] if the implementation reports an impossible
    /// item count.
    #[inline]
    fn read_exactly(&mut self, output: &mut [Self::Item]) -> Result<()> {
        if self.read_fully(output)? == output.len() {
            Ok(())
        } else {
            Err(Error::new(
                ErrorKind::UnexpectedEof,
                "failed to fill whole input buffer",
            ))
        }
    }
}