qubit-io 0.19.4

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

//! Adapter from a Qubit synchronous output to the standard library writer.

use std::io::IoSlice;
use std::io::Result;
use std::io::Seek;
use std::io::SeekFrom;
use std::io::Write;

use crate::ByteOutput;
use crate::Output;
use crate::traits::validate_write_count;

/// Adapts an [`Output`] of bytes to [`std::io::Write`].
///
/// Vectored writes forward the inner [`Output::write_vectored`] override when
/// their combined capacity is nonzero; actual scatter/gather behavior depends
/// on that implementation and the platform. Constructing the adapter does not
/// allocate. Exactly one descriptor uses stack storage; multiple descriptors
/// may allocate temporary descriptor storage, and the inner output may allocate
/// independently. Length overflow returns `InvalidInput` and descriptor
/// allocation failure returns `OutOfMemory` before the inner call. A count
/// larger than the total capacity returns `InvalidData` after the inner call;
/// its side effects are not undone. Short progress and inner errors are
/// returned as reported, and interrupted writes are not retried.
///
/// A wrapped tee reports successful primary progress even if its mirror branch
/// fails. Check `flush` after finite writes; a later non-empty write and flush
/// return the latched error.
///
/// # Type Parameters
///
/// - `O`: Output that accepts bytes.
///
/// # Examples
///
/// ```
/// use qubit_io::StdWrite;
/// use std::io::Write;
///
/// let mut writer = StdWrite::new(std::io::Cursor::new(Vec::new()));
/// writer.write_all(b"hello").expect("writing should succeed");
/// assert_eq!(writer.into_inner().into_inner(), b"hello");
/// ```
///
/// A write-only output implements `Write` without providing seek capability:
///
/// ```
/// use qubit_io::{Output, StdWrite};
/// use std::io::{Result, Write};
///
/// struct WriteOnly;
/// impl Output for WriteOnly {
///     type Item = u8;
///     unsafe fn write_unchecked(&mut self, _: &[u8], _: usize, count: usize) -> Result<usize> {
///         Ok(count)
///     }
///     fn flush(&mut self) -> Result<()> {
///         Ok(())
///     }
/// }
/// fn require_write<T: Write>(_: &T) {}
/// require_write(&StdWrite::new(WriteOnly));
/// ```
///
/// It does not provide `Seek` unless its output is seekable:
///
/// ```compile_fail,E0277
/// use qubit_io::{Output, StdWrite};
/// use std::io::{Result, Seek};
///
/// struct WriteOnly;
/// impl Output for WriteOnly {
///     type Item = u8;
///     unsafe fn write_unchecked(&mut self, _: &[u8], _: usize, count: usize) -> Result<usize> {
///         Ok(count)
///     }
///     fn flush(&mut self) -> Result<()> {
///         Ok(())
///     }
/// }
/// fn require_seek<T: Seek>() {}
/// require_seek::<StdWrite<WriteOnly>>();
/// ```
#[must_use]
#[repr(transparent)]
pub struct StdWrite<O> {
    /// Wrapped byte output.
    inner: O,
}

impl<O> StdWrite<O> {
    /// Creates a standard writer around `inner`.
    ///
    /// # Parameters
    ///
    /// - `inner`: Byte output to expose through [`std::io::Write`].
    ///
    /// # Returns
    ///
    /// A writer adapter that owns `inner` without performing I/O.
    #[inline]
    pub const fn new(inner: O) -> Self {
        Self { inner }
    }

    /// Returns a shared reference to the wrapped output.
    ///
    /// # Returns
    ///
    /// A reference tied to the borrow of this adapter.
    #[must_use = "inspect the wrapped output"]
    #[inline]
    pub const fn get_ref(&self) -> &O {
        &self.inner
    }

    /// Returns mutable access to the wrapped output.
    ///
    /// # Returns
    ///
    /// A mutable reference tied to the mutable borrow of this adapter.
    #[must_use = "use the mutable access to the wrapped output"]
    #[inline]
    pub fn get_mut(&mut self) -> &mut O {
        &mut self.inner
    }

    /// Unwraps and returns the wrapped output.
    ///
    /// # Returns
    ///
    /// The original output, with ownership transferred to the caller.
    #[inline]
    pub fn into_inner(self) -> O {
        self.inner
    }
}

impl<O> Write for StdWrite<O>
where
    O: ByteOutput,
{
    /// Writes a prefix of `input` through the wrapped byte output.
    ///
    /// # Parameters
    ///
    /// - `input`: Bytes offered to the wrapped output.
    ///
    /// # Returns
    ///
    /// The number of bytes accepted, which may be less than `input.len()`.
    ///
    /// # Errors
    ///
    /// Returns the error reported by the wrapped output. If it reports more
    /// bytes than `input` contains, returns `InvalidData` after the inner call;
    /// validation does not undo side effects. Interrupted writes are returned
    /// without retrying.
    #[inline]
    fn write(&mut self, input: &[u8]) -> Result<usize> {
        let written = Output::write(&mut self.inner, input)?;
        validate_write_count(written, input.len())?;
        Ok(written)
    }

    /// Writes from the supplied buffers in order, forwarding the inner
    /// vectored override.
    ///
    /// # Parameters
    ///
    /// - `buffers`: Byte descriptors whose contents are offered in order.
    ///
    /// # Returns
    ///
    /// The number of bytes accepted, which may be less than their combined
    /// length.
    ///
    /// # Errors
    ///
    /// Returns `InvalidInput` if the combined length overflows, `OutOfMemory`
    /// if descriptor preparation fails, or an error from the wrapped output.
    /// If the output reports more bytes than the combined capacity, returns
    /// `InvalidData` after the inner call; its side effects are not undone.
    #[inline]
    fn write_vectored(&mut self, buffers: &[IoSlice<'_>]) -> Result<usize> {
        let capacity = crate::util::requested_items(buffers.iter().map(|buffer| buffer.len()))?;
        if capacity == 0 {
            return Ok(0);
        }
        let count = if let [buffer] = buffers {
            let slices = [&buffer[..]];
            Output::write_vectored(&mut self.inner, &slices)?
        } else {
            let slices = crate::util::byte_slices(buffers)?;
            Output::write_vectored(&mut self.inner, &slices)?
        };
        crate::traits::validate_write_count(count, capacity)?;
        Ok(count)
    }

    /// Flushes the wrapped byte output.
    ///
    /// # Errors
    ///
    /// Returns the error reported by the wrapped output.
    #[inline]
    fn flush(&mut self) -> Result<()> {
        Output::flush(&mut self.inner)
    }
}

impl<O> Seek for StdWrite<O>
where
    O: ByteOutput + crate::Seekable<Unit = u8>,
{
    /// Changes the wrapped output's position.
    ///
    /// # Parameters
    ///
    /// - `position`: Positioning operation to apply to the wrapped output.
    ///
    /// # Returns
    ///
    /// The resulting position in bytes.
    ///
    /// # Errors
    ///
    /// Returns the error reported by the wrapped seekable output.
    #[inline]
    fn seek(&mut self, position: SeekFrom) -> Result<u64> {
        crate::Seekable::seek_to(&mut self.inner, position)
    }
}