qubit-codec 0.9.0

// =============================================================================
//    Copyright (c) 2026 Haixing Hu.
//
//    SPDX-License-Identifier: Apache-2.0
//
//    Licensed under the Apache License, Version 2.0.
// =============================================================================
//! Buffered converter adapter backed by two low-level codecs.

use core::fmt;

use super::{
    CodecTranscodeDecodeHooks,
    CodecTranscodeEncodeHooks,
};
use crate::{
    CapacityError,
    Codec,
    CodecConvertError,
    CodecDecodeError,
    CodecEncodeError,
    TranscodeConvertEngine,
    TranscodeConvertEngineError,
    TranscodeConverter,
    TranscodeDecodeEngineError,
    TranscodeEncodeEngineError,
    TranscodeError,
    TranscodeProgress,
    Transcoder,
};

/// Strict codec-backed converter error type.
type CodecTranscodeConvertError<D, E> =
    CodecConvertError<<D as Codec>::DecodeError, <E as Codec>::EncodeError>;

type CodecTranscodeConvertEngineError<D, E> = TranscodeConvertEngineError<
    TranscodeDecodeEngineError<
        <D as Codec>::DecodeError,
        CodecDecodeError<<D as Codec>::DecodeError>,
    >,
    TranscodeEncodeEngineError<
        <E as Codec>::EncodeError,
        CodecEncodeError<<E as Codec>::EncodeError>,
    >,
>;

/// Converts source units to target units through a decoded value by using
/// codecs.
///
/// The converter decodes one source value with the decoder codec, then encodes
/// that value with the encoder codec. If the current output buffer cannot hold
/// the encoded value, the already decoded value is retained by the common
/// converter engine and must be drained before more source input is consumed.
/// Incomplete source tails are left in the caller-provided input slice; callers
/// own input-buffer refill and EOF incomplete-tail policy.
///
/// Because finalization receives no source input, the source codec should have
/// locally decidable decode boundaries for the default converter bridge. Source
/// formats that require EOF-aware maximal-munch parsing or delayed boundary
/// decisions should implement that source-side policy in a custom transcoder or
/// facade before conversion.
///
/// # Type Parameters
///
/// - `D`: Low-level codec used to decode source units.
/// - `E`: Low-level codec used to encode target units.
pub struct CodecTranscodeConverter<D, E>
where
    D: Codec,
    E: Codec<Value = D::Value>,
{
    /// Common buffered converter engine.
    engine: TranscodeConvertEngine<
        D,
        E,
        CodecTranscodeDecodeHooks,
        CodecTranscodeEncodeHooks,
    >,
}

impl<D, E> fmt::Debug for CodecTranscodeConverter<D, E>
where
    D: Codec,
    E: Codec<Value = D::Value>,
    TranscodeConvertEngine<
        D,
        E,
        CodecTranscodeDecodeHooks,
        CodecTranscodeEncodeHooks,
    >: fmt::Debug,
{
    /// Formats the wrapped converter engine for debugging.
    ///
    /// # Parameters
    ///
    /// - `f`: Destination formatter.
    ///
    /// # Returns
    ///
    /// Returns `fmt::Result` from the formatter.
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("CodecTranscodeConverter")
            .field("engine", &self.engine)
            .finish()
    }
}

impl<D, E> CodecTranscodeConverter<D, E>
where
    D: Codec,
    E: Codec<Value = D::Value>,
{
    /// Creates a buffered converter backed by decoder and encoder codecs.
    ///
    /// # Parameters
    ///
    /// - `decoder`: Low-level codec used to decode source units.
    /// - `encoder`: Low-level codec used to encode target units.
    ///
    /// # Returns
    ///
    /// Returns a buffered converter adapter for the supplied codecs.
    #[inline(always)]
    #[must_use]
    pub fn new(decoder: D, encoder: E) -> Self {
        Self {
            engine: TranscodeConvertEngine::new(
                decoder,
                encoder,
                CodecTranscodeDecodeHooks,
                CodecTranscodeEncodeHooks,
            ),
        }
    }

    /// Returns an upper bound for target units produced from `input_len` units.
    ///
    /// This concrete adapter method is available even when `D::Value` does not
    /// implement [`Default`].
    ///
    /// # Parameters
    ///
    /// - `input_len`: Source units the caller plans to convert.
    ///
    /// # Returns
    ///
    /// Returns a conservative upper bound for produced target units.
    #[must_use = "capacity planning can fail on overflow"]
    #[inline(always)]
    pub fn max_output_len(
        &self,
        input_len: usize,
    ) -> Result<usize, CapacityError> {
        self.engine.max_output_len(input_len)
    }

    /// Returns the maximum target units emitted by finishing internal state.
    ///
    /// # Returns
    ///
    /// Returns a conservative upper bound for remaining converter-final output.
    #[must_use = "capacity planning can fail on overflow"]
    #[inline(always)]
    pub fn max_finish_output_len(&self) -> Result<usize, CapacityError> {
        self.engine.max_finish_output_len()
    }

    /// Returns the maximum target units emitted when resetting stream state.
    #[must_use = "capacity planning can fail on overflow"]
    #[inline(always)]
    pub fn max_reset_output_len(&self) -> Result<usize, CapacityError> {
        self.engine.max_reset_output_len()
    }

    /// Clears retained pending output and hook state and emits stream-start
    /// encode output.
    ///
    /// `D::Value: Default` is required so the engine can allocate scratch
    /// storage for any stream-start values the source decoder emits through
    /// [`Codec::decode_reset`](crate::Codec::decode_reset) before they are
    /// piped through the target encoder. Stateless decoders never reach the
    /// allocating path; the bound is consulted only when
    /// [`Codec::MAX_DECODE_RESET_VALUES`](crate::Codec::MAX_DECODE_RESET_VALUES)
    /// is non-zero.
    #[inline(always)]
    pub fn reset(
        &mut self,
        output: &mut [E::Unit],
        output_index: usize,
    ) -> Result<usize, TranscodeError<CodecTranscodeConvertError<D, E>>>
    where
        D::Value: Default,
    {
        self.engine.reset(output, output_index).map_err(|error| {
            error.map_domain(flatten_convert_engine_error::<D, E>)
        })
    }

    /// Converts source units into target units.
    ///
    /// This is the main streaming operation and does not require `D::Value` to
    /// implement [`Default`].
    ///
    /// # Parameters
    ///
    /// - `input`: Source unit slice.
    /// - `input_index`: Absolute source index where conversion starts.
    /// - `output`: Target unit slice.
    /// - `output_index`: Absolute target index where writing starts.
    ///
    /// # Returns
    ///
    /// Returns conversion progress for consumed/produced counters and stop
    /// reason.
    ///
    /// # Errors
    ///
    /// Returns converter error when source or target indices are invalid, or
    /// when decoding/encoding fails under current policy.
    #[inline(always)]
    pub fn transcode(
        &mut self,
        input: &[D::Unit],
        input_index: usize,
        output: &mut [E::Unit],
        output_index: usize,
    ) -> Result<
        TranscodeProgress,
        TranscodeError<CodecTranscodeConvertError<D, E>>,
    > {
        self.engine
            .transcode(input, input_index, output, output_index)
            .map_err(|error| {
                error.map_domain(flatten_convert_engine_error::<D, E>)
            })
    }

    /// Finishes internally retained output after EOF.
    ///
    /// Finalization delegates to the reusable converter engine. It drains
    /// retained pending output, encodes source-side decode flush values, and
    /// then finishes target-side encode hook state.
    ///
    /// # Parameters
    ///
    /// - `output`: Target unit slice for finalization output.
    /// - `output_index`: Absolute target output index where writing starts.
    ///
    /// # Returns
    ///
    /// Returns the number of target units written by finalization.
    ///
    /// # Errors
    ///
    /// Returns a finish error for pending output that cannot be finalized.
    #[inline(always)]
    pub fn finish(
        &mut self,
        output: &mut [E::Unit],
        output_index: usize,
    ) -> Result<usize, TranscodeError<CodecTranscodeConvertError<D, E>>>
    where
        D::Value: Default,
    {
        self.engine.finish(output, output_index).map_err(|error| {
            error.map_domain(flatten_convert_engine_error::<D, E>)
        })
    }
}

impl<D, E> Transcoder<D::Unit, E::Unit> for CodecTranscodeConverter<D, E>
where
    D: Codec,
    E: Codec<Value = D::Value>,
    D::Value: Default,
{
    type Error = CodecConvertError<D::DecodeError, E::EncodeError>;

    /// Returns an upper bound for target units produced from `input_len` units.
    ///
    /// # Parameters
    ///
    /// - `input_len`: Source units the caller plans to convert.
    ///
    /// # Returns
    ///
    /// Returns a conservative upper bound for produced target units.
    #[inline(always)]
    fn max_output_len(&self, input_len: usize) -> Result<usize, CapacityError> {
        CodecTranscodeConverter::max_output_len(self, input_len)
    }

    /// Returns the maximum target units emitted by finishing internal state.
    ///
    /// # Returns
    ///
    /// Returns a conservative upper bound for remaining converter-final output.
    #[inline(always)]
    fn max_finish_output_len(&self) -> Result<usize, CapacityError> {
        CodecTranscodeConverter::max_finish_output_len(self)
    }

    /// Returns the maximum target units emitted when resetting stream state.
    #[inline(always)]
    fn max_reset_output_len(&self) -> Result<usize, CapacityError> {
        CodecTranscodeConverter::max_reset_output_len(self)
    }

    /// Clears retained pending output, resets component state, and emits
    /// stream-start encode output.
    #[inline(always)]
    fn reset(
        &mut self,
        output: &mut [E::Unit],
        output_index: usize,
    ) -> Result<usize, TranscodeError<Self::Error>> {
        CodecTranscodeConverter::reset(self, output, output_index)
    }

    /// Converts source units into target units.
    ///
    /// # Parameters
    ///
    /// - `input`: Source unit slice.
    /// - `input_index`: Absolute source index where conversion starts.
    /// - `output`: Target unit slice.
    /// - `output_index`: Absolute target index where writing starts.
    ///
    /// # Returns
    ///
    /// Returns conversion progress for consumed/produced counters and stop
    /// reason.
    ///
    /// # Errors
    ///
    /// Returns converter error when source or target indices are invalid, or
    /// when decoding/encoding fails under current policy.
    #[inline(always)]
    fn transcode(
        &mut self,
        input: &[D::Unit],
        input_index: usize,
        output: &mut [E::Unit],
        output_index: usize,
    ) -> Result<TranscodeProgress, TranscodeError<Self::Error>> {
        CodecTranscodeConverter::transcode(
            self,
            input,
            input_index,
            output,
            output_index,
        )
    }

    /// Finishes internally retained output after EOF.
    ///
    /// # Parameters
    ///
    /// - `output`: Target unit slice for finalization output.
    /// - `output_index`: Absolute target output index where writing starts.
    ///
    /// # Returns
    ///
    /// Returns the number of target units written by finalization.
    ///
    /// # Errors
    ///
    /// Returns a finish error for pending output that cannot be finalized.
    #[inline(always)]
    fn finish(
        &mut self,
        output: &mut [E::Unit],
        output_index: usize,
    ) -> Result<usize, TranscodeError<Self::Error>> {
        CodecTranscodeConverter::finish(self, output, output_index)
    }
}

impl<D, E> TranscodeConverter<D::Unit, E::Unit>
    for CodecTranscodeConverter<D, E>
where
    D: Codec,
    E: Codec<Value = D::Value>,
    D::Value: Default,
{
    // empty
}

impl<D, E> Default for CodecTranscodeConverter<D, E>
where
    D: Codec,
    E: Codec<Value = D::Value>,
    TranscodeConvertEngine<
        D,
        E,
        CodecTranscodeDecodeHooks,
        CodecTranscodeEncodeHooks,
    >: Default,
{
    /// Creates a default codec-backed buffered converter.
    ///
    /// # Returns
    ///
    /// Returns a converter with default codecs and hooks.
    #[inline(always)]
    fn default() -> Self {
        Self {
            engine: TranscodeConvertEngine::default(),
        }
    }
}

#[inline(always)]
fn flatten_convert_engine_error<D, E>(
    error: CodecTranscodeConvertEngineError<D, E>,
) -> CodecTranscodeConvertError<D, E>
where
    D: Codec,
    E: Codec<Value = D::Value>,
{
    match error {
        TranscodeConvertEngineError::Decode(error) => match error {
            TranscodeDecodeEngineError::Codec(error)
            | TranscodeDecodeEngineError::Hook(error) => {
                CodecConvertError::decode(error)
            }
        },
        TranscodeConvertEngineError::Encode(error) => match error {
            TranscodeEncodeEngineError::Codec(error)
            | TranscodeEncodeEngineError::Hook(error) => {
                CodecConvertError::encode(error)
            }
        },
    }
}