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// =============================================================================
// Copyright (c) 2026 Haixing Hu.
//
// SPDX-License-Identifier: Apache-2.0
//
// Licensed under the Apache License, Version 2.0.
// =============================================================================
use Result;
use SeekFrom;
/// Minimal seek interface measured in stream items.
///
/// Unlike [`std::io::Seek`], which measures positions and offsets in bytes,
/// `Seekable` measures them in units of [`Self::Unit`]. For byte streams,
/// set `Unit = u8`; offsets passed through [`SeekFrom`] then count units
/// rather than bytes.
///
/// The return value of [`Seekable::seek_to`] is the new absolute position from
/// the start of the stream, in items.
///
/// # Byte- and item-oriented seeking
///
/// [`Seekable::seek_to`] expresses positions in `Self::Unit`, while
/// [`std::io::Seek::seek`] always expresses positions in bytes. In generic code
/// where both traits are implemented for the same value, fully qualified syntax
/// can make the selected unit semantics explicit:
///
/// ```
/// use std::io::{
/// Result,
/// Seek,
/// SeekFrom,
/// };
///
/// use qubit_io::Seekable;
///
/// fn seek_items<T>(stream: &mut T, position: SeekFrom) -> Result<u64>
/// where
/// T: Seekable + Seek,
/// {
/// <T as Seekable>::seek_to(stream, position)
/// }
///
/// fn seek_bytes<T>(stream: &mut T, position: SeekFrom) -> Result<u64>
/// where
/// T: Seekable + Seek,
/// {
/// Seek::seek(stream, position)
/// }
/// ```
///
/// # Coherence note
///
/// The standard I/O integration maps [`std::io::Seek`] to `Unit = u8` for
/// binary compatibility. If a concrete type already implements `Seek`, it
/// already has an implicit `Seekable<Unit = u8>` impl from this blanket, so
/// another `Seekable` impl with the same `(Self, Unit)` pair would be a
/// coherence conflict.
///
/// For example, this is rejected by the compiler:
///
/// ```rust,compile_fail,E0119
/// use std::io::{Result, Seek, SeekFrom};
///
/// use qubit_io::Seekable;
///
/// struct LegacyStream;
///
/// impl Seek for LegacyStream {
/// fn seek(&mut self, _pos: SeekFrom) -> Result<u64> {
/// Ok(0)
/// }
/// }
///
/// impl Seekable for LegacyStream {
/// type Unit = u8;
/// fn seek_to(&mut self, _pos: SeekFrom) -> Result<u64> {
/// Ok(0)
/// }
/// }
/// ```
///
/// ```text
/// error[E0119]: conflicting implementations of trait `Seekable`
/// for type `LegacyStream`
/// ```
///
/// The stable workaround is to keep byte-positioned seeking on the original
/// type and introduce a wrapper/newtype when another item interpretation is
/// needed: implement `Seekable` for the wrapper with a different `Unit`, and
/// keep `std::io::Seek`/byte semantics on the original type.