qubit_codec/transcode/engine/

transcode_encode_engine.rs

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

use core::num::NonZeroUsize;

use super::super::internal::encode_attempt::EncodeAttempt;
use super::super::internal::encode_state::EncodeState;
use super::super::internal::lifecycle_guard::LifecycleGuard;
use super::EncodeOutcome;
use super::EncodeUnencodableAction;
use super::TranscodeEncodeHooks;
use crate::CapacityError;
use crate::Codec;
use crate::TranscodeEncodeError;
use crate::TranscodeEncodeErrorOf;
use crate::TranscodeEncoder;
use crate::TranscodeFailure;
use crate::TranscodeProgress;
use crate::Transcoder;
use crate::codec::assert_unit_bounds;

/// Reusable buffered encoding engine for codec-backed encoders.
///
/// The engine owns the low-level codec and hook object. It keeps the common
/// buffered encoding loop private: input-index validation, output-capacity
/// checks, input consumption, output progress, and [`crate::TranscodeStatus`]
/// reporting.
///
/// Use this type to build a streaming encoder over a one-value [`Codec`]. The
/// engine does not allocate output. It encodes values accepted by
/// [`Codec::can_encode_value`] directly through the codec. When a value is
/// outside the codec's encodable domain, the engine asks hooks whether to
/// reject, skip, or replace it. If the current output buffer is too small for
/// the selected value, the engine returns
/// [`crate::TranscodeStatus::NeedOutput`] without consuming that input; the
/// caller can provide a larger or fresh output buffer and resume with the
/// returned input index.
///
/// For the common strict policy that simply wraps codec errors, use
/// [`crate::CodecTranscodeEncoder`]. Use `TranscodeEncodeEngine` directly when
/// the encode policy needs custom planning, replacement, skipped values, or
/// finish-time output.
///
/// # Examples
///
/// ```rust
/// use core::convert::Infallible;
/// use core::num::NonZeroUsize;
///
/// use qubit_codec::Codec;
/// use qubit_codec::DecodeFailure;
/// use qubit_codec::TranscodeEncodeErrorOf;
/// use qubit_codec::TranscodeStatus;
/// use qubit_codec::engine::EncodeContext;
/// use qubit_codec::engine::EncodeUnencodableAction;
/// use qubit_codec::engine::TranscodeEncodeEngine;
/// use qubit_codec::engine::TranscodeEncodeHooks;
///
/// #[derive(Clone, Copy)]
/// struct ByteCodec;
///
/// impl Codec for ByteCodec {
///     type Value = u8;
///     type Unit = u8;
///     type DecodeError = Infallible;
///     type EncodeError = Infallible;
///
///     const MIN_UNITS_PER_VALUE: usize = 1;
///     const MAX_ENCODE_UNITS_PER_VALUE: usize = 1;
///     const MAX_DECODE_UNITS_PER_VALUE: usize = 1;
///
///     unsafe fn decode(
///         &mut self,
///         input: &[u8],
///         index: usize,
///     ) -> Result<(u8, NonZeroUsize), DecodeFailure<Self::DecodeError>> {
///         Ok((input[index], NonZeroUsize::MIN))
///     }
///
///     unsafe fn encode(
///         &mut self,
///         value: &u8,
///         output: &mut [u8],
///         index: usize,
///     ) -> Result<usize, Self::EncodeError> {
///         output[index] = *value;
///         Ok(1)
///     }
/// }
///
/// struct StrictHooks;
///
/// impl TranscodeEncodeHooks<ByteCodec> for StrictHooks {
///     fn handle_unencodable_encode(
///         &mut self,
///         _codec: &mut ByteCodec,
///         _context: &EncodeContext<'_, u8>,
///     ) -> Result<EncodeUnencodableAction<u8>, TranscodeEncodeErrorOf<ByteCodec>> {
///         unreachable!("ByteCodec accepts every u8")
///     }
/// }
///
/// let mut engine = TranscodeEncodeEngine::new(ByteCodec, StrictHooks);
/// let input = [1_u8, 2, 3];
/// let mut output = [0_u8; 2];
/// let mut reset_output = [];
/// engine.reset(&mut reset_output, 0)?;
///
/// let progress = engine.transcode(&input, 0, &mut output, 0)?;
/// match progress.status() {
///     TranscodeStatus::Complete => unreachable!("output is intentionally short"),
///     TranscodeStatus::NeedOutput { .. } => {
///         assert_eq!(2, progress.written());
///         assert_eq!([1, 2], output);
///         // Write out `output[..progress.written()]`, then resume at
///         // `progress.read()` with fresh output capacity.
///     }
///     TranscodeStatus::NeedInput { .. } => unreachable!("encoders do not read encoded input"),
/// }
/// # Ok::<(), TranscodeEncodeErrorOf<ByteCodec>>(())
/// ```
///
/// # Type Parameters
///
/// - `C`: Low-level codec used by the engine.
/// - `H`: Policy hook object used by the engine.
#[derive(Debug)]
pub struct TranscodeEncodeEngine<C, H> {
    /// Owned codec carrying the encode-side stream state.
    codec: C,
    /// Owned policy hooks for unsupported values and lifecycle output.
    hooks: H,
    /// Guard for the `reset → transcode* → finish` lifecycle in every build
    /// profile.
    lifecycle: LifecycleGuard,
}

impl<C, H> TranscodeEncodeEngine<C, H>
where
    C: Codec,
    H: TranscodeEncodeHooks<C>,
{
    /// Creates a buffered encoder engine.
    ///
    /// # Parameters
    ///
    /// - `codec`: Low-level codec used for one-value encoding.
    /// - `hooks`: Policy hooks used for planning and writing values.
    ///
    /// # Returns
    ///
    /// Returns a buffered encoder engine.
    ///
    /// # Compile-Time Checks
    ///
    /// Fails to compile when the supplied codec declares a zero decode unit
    /// bound or when [`Codec::MIN_UNITS_PER_VALUE`] exceeds
    /// [`Codec::MAX_DECODE_UNITS_PER_VALUE`]. The encode bound may be zero for
    /// a fully buffered codec.
    #[inline]
    #[must_use]
    pub fn new(codec: C, hooks: H) -> Self {
        assert_unit_bounds::<C>();
        Self {
            codec,
            hooks,
            lifecycle: LifecycleGuard::new(),
        }
    }

    /// Returns the wrapped low-level codec.
    ///
    /// # Returns
    ///
    /// Returns a shared reference to the codec owned by this engine.
    #[inline]
    #[must_use]
    pub const fn codec(&self) -> &C {
        &self.codec
    }

    /// Returns the wrapped low-level codec mutably.
    ///
    /// # Returns
    ///
    /// Returns a mutable reference to the codec owned by this engine.
    #[inline]
    #[must_use]
    pub fn codec_mut(&mut self) -> &mut C {
        &mut self.codec
    }

    /// Returns the encode hooks used by this engine.
    ///
    /// # Returns
    ///
    /// Returns a shared reference to the hook object owned by this engine.
    #[inline]
    #[must_use]
    pub const fn hooks(&self) -> &H {
        &self.hooks
    }

    /// Returns the encode hooks mutably.
    ///
    /// # Returns
    ///
    /// Returns a mutable reference to the hook object owned by this engine.
    #[inline]
    #[must_use]
    pub fn hooks_mut(&mut self) -> &mut H {
        &mut self.hooks
    }

    /// Consumes the engine and returns its codec and hooks.
    ///
    /// Any lifecycle state owned by the engine is discarded.
    ///
    /// # Returns
    ///
    /// Returns the wrapped codec followed by the encode hooks.
    #[inline]
    #[must_use]
    pub fn into_parts(self) -> (C, H) {
        let Self { codec, hooks, .. } = self;
        (codec, hooks)
    }

    /// Gets a conservative upper bound for output units needed for
    /// `input_len` values.
    ///
    /// This bound covers only the streaming encode phase. It is delegated to
    /// [`TranscodeEncodeHooks::max_transcode_output_len`], so it includes hook
    /// policy and is valid for every reachable transient codec and hook state.
    /// Downstream encoders must use this engine-level API for capacity planning
    /// instead of recomputing the bound from [`Codec`] constants.
    ///
    /// # Parameters
    ///
    /// - `input_len`: Number of input values the caller plans to encode.
    ///
    /// # Returns
    ///
    /// Returns a conservative upper bound for output units.
    ///
    /// # Errors
    ///
    /// Returns a capacity error when hook planning overflows.
    #[inline]
    pub fn max_transcode_output_len(&self, input_len: usize) -> Result<usize, CapacityError> {
        self.hooks.max_transcode_output_len(&self.codec, input_len)
    }

    /// Gets the global maximum output units emitted by stream reset.
    ///
    /// # Returns
    ///
    /// Returns the codec's reset-output upper bound. This query never fails.
    #[inline]
    pub fn max_reset_output_len(&self) -> Result<usize, CapacityError> {
        Ok(C::MAX_ENCODE_RESET_UNITS)
    }

    /// Gets the global maximum output units emitted by finishing codec and hook
    /// state.
    ///
    /// Returns the sum of [`Codec::MAX_ENCODE_FINISH_UNITS`] and the
    /// hook-provided final-output bound. The codec finish portion covers units
    /// written by [`Codec::encode_finish`]; hook implementations must not
    /// include that portion in
    /// [`TranscodeEncodeHooks::max_finish_output_len`]. Both component bounds
    /// cover every reachable transient state.
    ///
    /// # Returns
    ///
    /// Returns the combined codec-finish and hook-finish output bound.
    ///
    /// # Errors
    ///
    /// Returns [`CapacityError::OutputLengthOverflow`] when the sum overflows.
    #[inline]
    pub fn max_finish_output_len(&self) -> Result<usize, CapacityError> {
        C::MAX_ENCODE_FINISH_UNITS
            .checked_add(self.hooks.max_finish_output_len(&self.codec))
            .ok_or(CapacityError::OutputLengthOverflow)
    }

    /// Gets the maximum output units needed by a complete one-shot encode
    /// stream.
    ///
    /// The returned bound covers reset output, the streaming encode phase for
    /// `input_len` values, and finish output. Its components are global across
    /// transient state. Higher-level complete encode helpers should use this
    /// engine-level bound instead of recomputing capacity from [`Codec`]
    /// constants, because hook policy may change streaming or finish output.
    ///
    /// # Parameters
    ///
    /// - `input_len`: Number of input values in the complete stream.
    ///
    /// # Returns
    ///
    /// Returns the complete-stream output bound.
    ///
    /// # Errors
    ///
    /// Returns a capacity error if a component query or their sum overflows.
    #[inline]
    pub fn max_total_output_len(&self, input_len: usize) -> Result<usize, CapacityError> {
        let reset = self.max_reset_output_len()?;
        let transcode = self.max_transcode_output_len(input_len)?;
        let finish = self.max_finish_output_len()?;
        reset
            .checked_add(transcode)
            .and_then(|len| len.checked_add(finish))
            .ok_or(CapacityError::OutputLengthOverflow)
    }

    /// Resets codec encode state, hook-owned state, and stream-start output.
    ///
    /// # Parameters
    ///
    /// - `output`: Complete output unit slice visible to the encoder.
    /// - `output_index`: Absolute output unit index where writing starts.
    ///
    /// # Returns
    ///
    /// Returns the number of reset units written.
    ///
    /// # Errors
    ///
    /// Returns framework errors when the caller provides invalid or
    /// insufficient output capacity. Returns domain errors when codec reset or
    /// hook reset handling fails. Capacity and index failures occur before any
    /// reset state is changed. Once reset execution starts, an error poisons
    /// the engine until a later reset succeeds.
    ///
    /// # Panics
    ///
    /// Panics if the codec reports reset output beyond its declared bound.
    pub fn reset(&mut self, output: &mut [C::Unit], output_index: usize) -> Result<usize, TranscodeEncodeErrorOf<C>> {
        let required = self.max_reset_output_len()?;
        TranscodeFailure::ensure_output_capacity(output.len(), output_index, required)?;
        self.lifecycle.on_reset_start();
        self.hooks.reset_hooks(&mut self.codec);
        let written = unsafe {
            // SAFETY: The capacity check above reserves the codec's declared
            // reset-output bound at `output_index`.
            self.codec.encode_reset(output, output_index)
        }
        .map_err(TranscodeEncodeError::domain_reset)?;
        assert!(written <= required, "Codec::encode_reset wrote beyond its reset bound",);
        self.lifecycle.on_reset_success();
        Ok(written)
    }

    /// Encodes values into a caller-provided output buffer.
    ///
    /// The engine stops before consuming the next input value when the current
    /// output buffer does not satisfy that value's planned capacity bound.
    ///
    /// # Parameters
    ///
    /// - `input`: Complete input value slice visible to the encoder.
    /// - `input_index`: Absolute input value index where encoding starts.
    /// - `output`: Complete output unit slice visible to the encoder.
    /// - `output_index`: Absolute output unit index where writing starts.
    ///
    /// # Returns
    ///
    /// Returns progress describing input values consumed, output units written,
    /// and why encoding stopped.
    ///
    /// # Errors
    ///
    /// Returns framework errors when `input_index` is outside `input` or
    /// `output_index` is outside `output`, and domain errors when codec or
    /// hook processing fails or rejects a value. Returns
    /// [`TranscodeFailure::TranscodeBeforeReset`] when the engine has not
    /// completed its first reset. Returns
    /// [`TranscodeFailure::TranscodeAfterFinish`] when the logical stream was
    /// already finished and has not been reset, or
    /// [`TranscodeFailure::LifecyclePoisoned`] when an earlier reset or finish
    /// failed after execution started.
    ///
    /// # Panics
    ///
    /// Panics if the codec violates its declared encode length or hooks
    /// return an unencodable replacement.
    pub fn transcode(
        &mut self,
        input: &[C::Value],
        input_index: usize,
        output: &mut [C::Unit],
        output_index: usize,
    ) -> Result<TranscodeProgress, TranscodeEncodeErrorOf<C>> {
        self.lifecycle.on_transcode()?;
        TranscodeFailure::ensure_transcode_indices(input.len(), input_index, output.len(), output_index)?;
        let mut state = EncodeState::new(input, input_index, output, output_index);

        while state.has_input() {
            // SAFETY: The loop condition proves that the current input cursor
            // points at an available value.
            let attempt = unsafe { state.context_unchecked() };
            let outcome = self.encode_one(attempt)?;
            if let Some(progress) = state.apply_encode_outcome(outcome) {
                return Ok(progress);
            }
        }

        Ok(state.complete_progress())
    }

    /// Finishes codec and hook-owned output after EOF.
    ///
    /// Finalization first finishes encode-side codec state through
    /// [`Codec::encode_finish`], then lets hook implementations finish their
    /// own retained state. The caller must provide enough output capacity for
    /// [`TranscodeEncodeEngine::max_finish_output_len`], which includes both
    /// the codec finish bound and the hook-owned finish bound.
    ///
    /// # Parameters
    ///
    /// - `output`: Complete output unit slice visible to the encoder.
    /// - `output_index`: Absolute output unit index where writing starts.
    ///
    /// # Returns
    ///
    /// Returns the number of units written by finalization.
    ///
    /// # Errors
    ///
    /// Returns framework errors when the caller provides invalid or
    /// insufficient output capacity. Returns domain errors when codec finish or
    /// hook finalization fails. Returns
    /// [`TranscodeFailure::FinishBeforeReset`] when the engine has not
    /// completed its first reset. Returns
    /// [`TranscodeFailure::FinishAfterFinish`] when the logical stream was
    /// already finished and has not been reset, or
    /// [`TranscodeFailure::LifecyclePoisoned`] when an earlier reset or finish
    /// failed after execution started. Capacity and index failures occur before
    /// finish execution and remain retryable; later failures poison the engine
    /// until reset succeeds.
    ///
    /// # Panics
    ///
    /// Panics when the codec finish or finish hooks write beyond their
    /// independently declared output bounds.
    pub fn finish(&mut self, output: &mut [C::Unit], output_index: usize) -> Result<usize, TranscodeEncodeErrorOf<C>> {
        self.lifecycle.on_finish_attempt()?;
        let hook_required = self.hooks.max_finish_output_len(&self.codec);
        let required = C::MAX_ENCODE_FINISH_UNITS
            .checked_add(hook_required)
            .ok_or(CapacityError::OutputLengthOverflow)?;
        TranscodeFailure::ensure_output_capacity(output.len(), output_index, required)?;
        self.lifecycle.on_finish_start();
        let finished = unsafe {
            // SAFETY: The capacity check above reserves the codec's declared
            // finish-output bound at `output_index`.
            self.codec.encode_finish(output, output_index)
        }
        .map_err(TranscodeEncodeError::domain_finish)?;
        assert!(
            finished <= C::MAX_ENCODE_FINISH_UNITS,
            "Codec::encode_finish wrote beyond its finish bound",
        );
        let written = self
            .hooks
            .finish_hooks(&mut self.codec, output, output_index + finished)?;
        assert!(
            written <= hook_required,
            "TranscodeEncodeEngine hook wrote beyond its finish bound",
        );
        self.lifecycle.on_finish_success();
        Ok(finished + written)
    }

    /// Runs a complete one-shot `reset -> transcode -> finish` encode stream.
    ///
    /// The complete input is supplied as `input`, and output starts at index
    /// `0` in `output`. Callers that need subranges should slice their
    /// buffers before calling this method. Downstream one-shot encoder
    /// helpers should call this engine method instead of reproducing the
    /// reset, transcode, and finish sequence themselves.
    ///
    /// # Parameters
    ///
    /// - `input`: Complete input values.
    /// - `output`: Output unit slice for the whole encoded stream.
    ///
    /// # Returns
    ///
    /// Returns the number of output units written.
    ///
    /// # Errors
    ///
    /// Returns framework errors for insufficient output or capacity overflow,
    /// and domain errors from reset, encode, or finish.
    ///
    /// # Panics
    ///
    /// Panics if codec or hook implementations violate the output or
    /// replacement contracts of [`Self::reset`], [`Self::transcode`], or
    /// [`Self::finish`].
    #[inline]
    pub fn transcode_complete_into(
        &mut self,
        input: &[C::Value],
        output: &mut [C::Unit],
    ) -> Result<usize, TranscodeEncodeErrorOf<C>> {
        <Self as Transcoder>::transcode_complete_into(self, input, output)
    }

    /// Encodes one value through the codec and unencodable-value hooks.
    ///
    /// This is the single entry point for `TranscodeConvertEngine` to drive
    /// the encode side without accessing `codec` and `hooks` directly.
    ///
    /// # Parameters
    ///
    /// - `attempt`: Current value, absolute indices, and writable output
    ///   buffer.
    ///
    /// # Returns
    ///
    /// Returns whether the value was consumed or needs more output capacity.
    ///
    /// For an unencodable value, this checks capacity for the codec's maximum
    /// value width before invoking policy hooks. A shortage returns
    /// `NeedOutput` without changing hook state; this conservative requirement
    /// also applies when the hook would skip or reject. Errors after a hook
    /// runs do not roll back mutations made by that hook.
    ///
    /// # Errors
    ///
    /// Returns an engine-domain error when the codec fails or when the hook
    /// rejects an unencodable value.
    ///
    /// # Panics
    ///
    /// Panics if the codec violates its encode length contract or hooks
    /// supply an unencodable replacement.
    pub(in crate::transcode) fn encode_one(
        &mut self,
        attempt: EncodeAttempt<'_, C::Value, C::Unit>,
    ) -> Result<EncodeOutcome, TranscodeEncodeErrorOf<C>> {
        let encodable = {
            let context = attempt.context();
            self.codec.can_encode_value(context.input_value())
        };
        if encodable {
            return self.encode_encodable_value(attempt);
        }
        let required = C::MAX_ENCODE_UNITS_PER_VALUE;
        if attempt.available_output() < required {
            return Ok(EncodeOutcome::need_output(
                NonZeroUsize::new(required).expect("required output is non-zero when capacity is insufficient"),
            ));
        }
        let action = {
            let context = attempt.context();
            self.hooks.handle_unencodable_encode(&mut self.codec, &context)?
        };
        self.apply_unencodable_action(action, attempt)
    }

    /// Encodes an encodable input value.
    ///
    /// # Parameters
    ///
    /// - `attempt`: Current value, absolute indices, and writable output
    ///   buffer.
    ///
    /// # Returns
    ///
    /// Returns the encode outcome for the current value.
    ///
    /// # Errors
    ///
    /// Returns a codec error when encoding fails.
    ///
    /// # Panics
    ///
    /// Panics when [`Codec::encode_len`] exceeds
    /// [`Codec::MAX_ENCODE_UNITS_PER_VALUE`] or [`Codec::encode`] writes a
    /// different number of units than planned.
    fn encode_encodable_value(
        &mut self,
        attempt: EncodeAttempt<'_, C::Value, C::Unit>,
    ) -> Result<EncodeOutcome, TranscodeEncodeErrorOf<C>> {
        let required = self.codec.encode_len(attempt.value());
        assert!(
            required <= C::MAX_ENCODE_UNITS_PER_VALUE,
            "Codec::encode_len exceeded Codec::MAX_ENCODE_UNITS_PER_VALUE",
        );
        if attempt.available_output() < required {
            return Ok(EncodeOutcome::need_output(
                NonZeroUsize::new(required).expect("required output is non-zero when capacity is insufficient"),
            ));
        }
        let (value, input_index, output, output_index) = attempt.into_parts();
        let written = unsafe {
            // SAFETY: The capacity check above reserves the exact value width.
            self.codec.encode(value, output, output_index)
        }
        .map_err(|error| TranscodeEncodeError::domain_main(error, input_index))?;
        assert!(
            written == required,
            "Codec::encode wrote a different length than Codec::encode_len",
        );
        Ok(EncodeOutcome::consumed(written))
    }

    /// Applies the hook-selected unencodable-value action.
    ///
    /// # Parameters
    ///
    /// - `action`: Policy action selected by the encode hooks.
    /// - `attempt`: Rejected value, absolute indices, and writable output
    ///   buffer.
    ///
    /// # Returns
    ///
    /// Returns the encode outcome for the current input value.
    ///
    /// # Errors
    ///
    /// Returns an unencodable-value error for rejection, or a codec-domain
    /// error when replacement encoding fails.
    ///
    /// # Panics
    ///
    /// Panics if replacement encoding violates the codec length contract or
    /// the replacement cannot be encoded.
    #[inline]
    fn apply_unencodable_action(
        &mut self,
        action: EncodeUnencodableAction<C::Value>,
        attempt: EncodeAttempt<'_, C::Value, C::Unit>,
    ) -> Result<EncodeOutcome, TranscodeEncodeErrorOf<C>> {
        match action {
            EncodeUnencodableAction::Reject => {
                // Keep the streaming encoder usable for non-Clone values.
                // The failure still carries the absolute input index; callers
                // that need value context can add it in a higher-level facade.
                Err(TranscodeEncodeError::unencodable_without_context(attempt.input_index()))
            }
            EncodeUnencodableAction::Skip => Ok(EncodeOutcome::consumed(0)),
            EncodeUnencodableAction::Replace { value } => self.encode_replacement_value(value, attempt),
        }
    }

    /// Encodes a hook-provided replacement value.
    ///
    /// # Parameters
    ///
    /// - `value`: Replacement value selected by hooks.
    /// - `attempt`: Original value, absolute indices, and writable output
    ///   buffer.
    ///
    /// # Returns
    ///
    /// Returns the encode outcome for the replacement.
    ///
    /// # Errors
    ///
    /// Returns a codec error when replacement encoding fails.
    ///
    /// # Panics
    ///
    /// Panics when hooks return a replacement value that the codec cannot
    /// encode, [`Codec::encode_len`] exceeds
    /// [`Codec::MAX_ENCODE_UNITS_PER_VALUE`], or [`Codec::encode`] writes a
    /// different number of units than planned.
    fn encode_replacement_value(
        &mut self,
        value: C::Value,
        attempt: EncodeAttempt<'_, C::Value, C::Unit>,
    ) -> Result<EncodeOutcome, TranscodeEncodeErrorOf<C>> {
        assert!(
            self.codec.can_encode_value(&value),
            "EncodeUnencodableAction::Replace returned an unencodable replacement value",
        );
        let required = self.codec.encode_len(&value);
        assert!(
            required <= C::MAX_ENCODE_UNITS_PER_VALUE,
            "Codec::encode_len exceeded Codec::MAX_ENCODE_UNITS_PER_VALUE",
        );
        if attempt.available_output() < required {
            return Ok(EncodeOutcome::need_output(
                NonZeroUsize::new(required).expect("required output is non-zero when capacity is insufficient"),
            ));
        }
        let (_, input_index, output, output_index) = attempt.into_parts();
        let written = unsafe {
            // SAFETY: The capacity check above reserves the exact replacement
            // value width, and the hook contract requires encodability.
            self.codec.encode(&value, output, output_index)
        }
        .map_err(|error| TranscodeEncodeError::domain_main(error, input_index))?;
        assert!(
            written == required,
            "Codec::encode wrote a different length than Codec::encode_len",
        );
        Ok(EncodeOutcome::consumed(written))
    }
}

impl<C, H> Default for TranscodeEncodeEngine<C, H>
where
    C: Codec + Default,
    H: TranscodeEncodeHooks<C> + Default,
{
    /// Creates a default buffered encoder engine.
    ///
    /// # Returns
    ///
    /// Returns an engine with default codec and hooks.
    #[inline]
    fn default() -> Self {
        Self::new(C::default(), H::default())
    }
}

impl<C, H> Transcoder for TranscodeEncodeEngine<C, H>
where
    C: Codec,
    H: TranscodeEncodeHooks<C>,
{
    /// Logical values borrowed from the caller during encoding.
    type Input = C::Value;
    /// Encoded storage units written into caller-owned buffers.
    type Output = C::Unit;
    /// Framework failures or codec-domain errors from encoding.
    type Error = TranscodeEncodeErrorOf<C>;

    /// Gets a conservative upper bound for output units needed for
    /// `input_len` values.
    ///
    /// This bound covers only the streaming encode phase. It is delegated to
    /// [`TranscodeEncodeHooks::max_transcode_output_len`], so it includes hook
    /// policy and is valid for every reachable transient codec and hook state.
    /// Downstream encoders must use this engine-level API for capacity planning
    /// instead of recomputing the bound from [`Codec`] constants.
    ///
    /// # Parameters
    ///
    /// - `input_len`: Number of input values the caller plans to encode.
    ///
    /// # Returns
    ///
    /// Returns a conservative upper bound for output units.
    ///
    /// # Errors
    ///
    /// Returns a capacity error when hook planning overflows.
    #[inline]
    fn max_transcode_output_len(&self, input_len: usize) -> Result<usize, CapacityError> {
        TranscodeEncodeEngine::max_transcode_output_len(self, input_len)
    }

    /// Gets the global maximum output units emitted by stream reset.
    ///
    /// # Returns
    ///
    /// Returns the codec's reset-output upper bound. This query never fails.
    #[inline]
    fn max_reset_output_len(&self) -> Result<usize, CapacityError> {
        TranscodeEncodeEngine::max_reset_output_len(self)
    }

    /// Gets the global maximum output units emitted by finishing codec and hook
    /// state.
    ///
    /// Returns the sum of [`Codec::MAX_ENCODE_FINISH_UNITS`] and the
    /// hook-provided final-output bound. The codec finish portion covers units
    /// written by [`Codec::encode_finish`]; hook implementations must not
    /// include that portion in
    /// [`TranscodeEncodeHooks::max_finish_output_len`]. Both component bounds
    /// cover every reachable transient state.
    ///
    /// # Returns
    ///
    /// Returns the combined codec-finish and hook-finish output bound.
    ///
    /// # Errors
    ///
    /// Returns [`CapacityError::OutputLengthOverflow`] when the sum overflows.
    #[inline]
    fn max_finish_output_len(&self) -> Result<usize, CapacityError> {
        TranscodeEncodeEngine::max_finish_output_len(self)
    }

    /// Resets codec encode state, hook-owned state, and stream-start output.
    ///
    /// # Parameters
    ///
    /// - `output`: Complete output unit slice visible to the encoder.
    /// - `output_index`: Absolute output unit index where writing starts.
    ///
    /// # Returns
    ///
    /// Returns the number of reset units written.
    ///
    /// # Errors
    ///
    /// Returns framework errors when the caller provides invalid or
    /// insufficient output capacity. Returns domain errors when codec reset or
    /// hook reset handling fails. Capacity and index failures occur before any
    /// reset state is changed. Once reset execution starts, an error poisons
    /// the engine until a later reset succeeds.
    ///
    /// # Panics
    ///
    /// Panics if the codec reports reset output beyond its declared bound.
    #[inline]
    fn reset(&mut self, output: &mut [C::Unit], output_index: usize) -> Result<usize, TranscodeEncodeErrorOf<C>> {
        TranscodeEncodeEngine::reset(self, output, output_index)
    }

    /// Encodes values into a caller-provided output buffer.
    ///
    /// The engine stops before consuming the next input value when the current
    /// output buffer does not satisfy that value's planned capacity bound.
    ///
    /// # Parameters
    ///
    /// - `input`: Complete input value slice visible to the encoder.
    /// - `input_index`: Absolute input value index where encoding starts.
    /// - `output`: Complete output unit slice visible to the encoder.
    /// - `output_index`: Absolute output unit index where writing starts.
    ///
    /// # Returns
    ///
    /// Returns progress describing input values consumed, output units written,
    /// and why encoding stopped.
    ///
    /// # Errors
    ///
    /// Returns framework errors when `input_index` is outside `input` or
    /// `output_index` is outside `output`, and domain errors when codec or
    /// hook processing fails or rejects a value. Returns
    /// [`TranscodeFailure::TranscodeBeforeReset`] when the engine has not
    /// completed its first reset. Returns
    /// [`TranscodeFailure::TranscodeAfterFinish`] when the logical stream was
    /// already finished and has not been reset, or
    /// [`TranscodeFailure::LifecyclePoisoned`] when an earlier reset or finish
    /// failed after execution started.
    ///
    /// # Panics
    ///
    /// Panics if the codec violates its declared encode length or hooks
    /// return an unencodable replacement.
    #[inline]
    fn transcode(
        &mut self,
        input: &[C::Value],
        input_index: usize,
        output: &mut [C::Unit],
        output_index: usize,
    ) -> Result<TranscodeProgress, TranscodeEncodeErrorOf<C>> {
        TranscodeEncodeEngine::transcode(self, input, input_index, output, output_index)
    }

    /// Finishes codec and hook-owned output after EOF.
    ///
    /// Finalization first finishes encode-side codec state through
    /// [`Codec::encode_finish`], then lets hook implementations finish their
    /// own retained state. The caller must provide enough output capacity for
    /// [`TranscodeEncodeEngine::max_finish_output_len`], which includes both
    /// the codec finish bound and the hook-owned finish bound.
    ///
    /// # Parameters
    ///
    /// - `output`: Complete output unit slice visible to the encoder.
    /// - `output_index`: Absolute output unit index where writing starts.
    ///
    /// # Returns
    ///
    /// Returns the number of units written by finalization.
    ///
    /// # Errors
    ///
    /// Returns framework errors when the caller provides invalid or
    /// insufficient output capacity. Returns domain errors when codec finish or
    /// hook finalization fails. Returns
    /// [`TranscodeFailure::FinishBeforeReset`] when the engine has not
    /// completed its first reset. Returns
    /// [`TranscodeFailure::FinishAfterFinish`] when the logical stream was
    /// already finished and has not been reset, or
    /// [`TranscodeFailure::LifecyclePoisoned`] when an earlier reset or finish
    /// failed after execution started. Capacity and index failures occur before
    /// finish execution and remain retryable; later failures poison the engine
    /// until reset succeeds.
    ///
    /// # Panics
    ///
    /// Panics when the codec finish writes beyond
    /// [`Codec::MAX_ENCODE_FINISH_UNITS`] or when the combined codec and hook
    /// finalization writes beyond
    /// [`TranscodeEncodeEngine::max_finish_output_len`].
    #[inline]
    fn finish(&mut self, output: &mut [C::Unit], output_index: usize) -> Result<usize, TranscodeEncodeErrorOf<C>> {
        TranscodeEncodeEngine::finish(self, output, output_index)
    }
}

impl<C, H> TranscodeEncoder for TranscodeEncodeEngine<C, H>
where
    C: Codec,
    H: TranscodeEncodeHooks<C>,
{
    /// Codec-specific source error retained by domain failures.
    type EncodeError = C::EncodeError;
}