pmomusic/pmoflac/src/decoder.rs

use tokio::{
    io::AsyncRead,
    sync::{mpsc, oneshot},
};

use crate::{
    common::ChannelReader,
    decoder_common::{
        spawn_ingest_task, spawn_writer_task, DecodedStream, CHANNEL_CAPACITY, DUPLEX_BUFFER_SIZE,
    },
    error::FlacError,
    pcm::StreamInfo,
    stream::ManagedAsyncReader,
    util::interleaved_i32_to_le_bytes,
};

/// Async stream alias for decoded FLAC audio.
pub type FlacDecodedStream = DecodedStream<FlacError>;

/// Decodes a FLAC stream into PCM audio data.
///
/// This function spawns background tasks to perform the decoding asynchronously.
/// The returned `FlacDecodedStream` implements `AsyncRead` for streaming the PCM output.
///
/// # Threading Model
///
/// - A Tokio task reads chunks from the input and forwards them via a channel
/// - A blocking task (via `spawn_blocking`) runs the FLAC decoder (claxon)
/// - Another Tokio task writes decoded PCM to an internal duplex stream
///
/// This architecture ensures true streaming: output is produced as input is consumed,
/// without buffering entire files.
///
/// # Arguments
///
/// * `reader` - Any async reader containing FLAC-encoded data
///
/// # Returns
///
/// A `FlacDecodedStream` that can be read to obtain PCM samples in little-endian
/// interleaved format. The stream's `info()` method provides metadata.
///
/// # Errors
///
/// Returns an error if:
/// - The input is not valid FLAC data
/// - An I/O error occurs while reading
/// - The decoder encounters corrupted data
///
/// # Example
///
/// ```no_run
/// use pmoflac::decode_flac_stream;
/// use tokio::io::AsyncReadExt;
///
/// # #[tokio::main]
/// # async fn main() -> Result<(), Box<dyn std::error::Error>> {
/// let flac_data: &[u8] = &[/* ... */];
/// let mut stream = decode_flac_stream(flac_data).await?;
///
/// let info = stream.info().clone();
/// println!("{} Hz, {} channels, {} bits/sample",
///     info.sample_rate, info.channels, info.bits_per_sample);
///
/// let mut pcm = Vec::new();
/// stream.read_to_end(&mut pcm).await?;
/// stream.wait().await?;
/// # Ok(())
/// # }
/// ```
pub async fn decode_flac_stream<R>(reader: R) -> Result<FlacDecodedStream, FlacError>
where
    R: AsyncRead + Unpin + Send + 'static,
{
    let (ingest_tx, ingest_rx) = mpsc::channel(CHANNEL_CAPACITY);
    spawn_ingest_task(reader, ingest_tx);

    let (pcm_tx, pcm_rx) = mpsc::channel(CHANNEL_CAPACITY);
    let (pcm_reader, pcm_writer) = tokio::io::duplex(DUPLEX_BUFFER_SIZE);
    let (info_tx, info_rx) = oneshot::channel::<Result<StreamInfo, FlacError>>();

    let blocking_handle = tokio::task::spawn_blocking(move || -> Result<(), FlacError> {
        let mut channel_reader = ChannelReader::<FlacError>::new(ingest_rx);
        let mut flac_reader = match claxon::FlacReader::new(&mut channel_reader) {
            Ok(reader) => reader,
            Err(err) => {
                let msg = err.to_string();
                let _ = info_tx.send(Err(FlacError::Decode(msg.clone())));
                return Err(FlacError::Decode(msg));
            }
        };

        let flac_info = flac_reader.streaminfo();
        let info = StreamInfo {
            sample_rate: flac_info.sample_rate,
            channels: flac_info.channels as u8,
            bits_per_sample: flac_info.bits_per_sample as u8,
            total_samples: flac_info.samples,
            max_block_size: flac_info.max_block_size,
            min_block_size: flac_info.min_block_size,
        };
        if info_tx.send(Ok(info.clone())).is_err() {
            return Ok(());
        }

        let mut blocks = flac_reader.blocks();
        let mut buffer = Vec::new();
        let mut interleaved = Vec::new();
        let mut pcm_bytes = Vec::new();
        loop {
            match blocks.read_next_or_eof(buffer) {
                Ok(Some(block)) => {
                    let frames = block.duration() as usize;
                    let channels = block.channels() as usize;

                    interleaved.clear();
                    interleaved.reserve(frames * channels);
                    for frame_idx in 0..frames {
                        for channel_idx in 0..channels {
                            interleaved.push(block.sample(channel_idx as u32, frame_idx as u32));
                        }
                    }

                    pcm_bytes.clear();
                    pcm_bytes.reserve(frames * channels * info.bytes_per_sample());
                    interleaved_i32_to_le_bytes(&interleaved, info.bits_per_sample, &mut pcm_bytes);
                    let chunk = std::mem::take(&mut pcm_bytes);
                    if pcm_tx.blocking_send(Ok(chunk)).is_err() {
                        break;
                    }

                    pcm_bytes = Vec::with_capacity(frames * channels * info.bytes_per_sample());
                    buffer = block.into_buffer();
                }
                Ok(None) => break,
                Err(err) => {
                    let msg = err.to_string();
                    let _ = pcm_tx.blocking_send(Err(FlacError::Decode(msg.clone())));
                    return Err(FlacError::Decode(msg));
                }
            }
        }

        Ok(())
    });

    let writer_handle = spawn_writer_task(pcm_rx, pcm_writer, blocking_handle, "flac-decode");

    let info = info_rx.await.map_err(|_| FlacError::ChannelClosed)??;
    let reader = ManagedAsyncReader::new("flac-decode-writer", pcm_reader, writer_handle);

    Ok(DecodedStream::new(info, reader))
}