ddex-builder 0.4.5

//! Buffer management for streaming DDEX XML output
//!
//! Handles chunked writing to disk/network with automatic flushing
//! and memory-bounded operations.

use std::collections::VecDeque;
use std::io::{Result as IoResult, Write as IoWrite};

/// Callback type for flush events
pub type FlushCallback = Box<dyn Fn(usize) + Send + Sync>;

/// Configuration for buffer management
#[derive(Debug, Clone)]
pub struct BufferConfig {
    /// Maximum buffer size before automatic flush
    pub max_buffer_size: usize,
    /// Number of buffers to keep in memory
    pub buffer_count: usize,
    /// Whether to enable compression
    pub enable_compression: bool,
}

impl Default for BufferConfig {
    fn default() -> Self {
        Self {
            max_buffer_size: 1024 * 1024, // 1MB per buffer
            buffer_count: 10,             // Up to 10MB total
            enable_compression: false,    // Disabled by default for simplicity
        }
    }
}

/// Manages buffered writing with automatic flushing and memory limits
pub struct BufferManager<W: IoWrite> {
    writer: W,
    config: BufferConfig,

    // Buffer management
    buffers: VecDeque<Vec<u8>>,
    current_buffer: Vec<u8>,

    // Statistics
    total_bytes_written: usize,
    total_flushes: usize,
    peak_buffer_size: usize,

    // Callbacks
    flush_callback: Option<FlushCallback>,
}

impl<W: IoWrite> BufferManager<W> {
    /// Create a new buffer manager with default configuration
    pub fn new(writer: W, max_buffer_size: usize) -> IoResult<Self> {
        let config = BufferConfig {
            max_buffer_size,
            ..BufferConfig::default()
        };
        Self::new_with_config(writer, config)
    }

    /// Create a new buffer manager with custom configuration
    pub fn new_with_config(writer: W, config: BufferConfig) -> IoResult<Self> {
        let buffer_capacity = config.max_buffer_size;
        Ok(BufferManager {
            writer,
            config,
            buffers: VecDeque::new(),
            current_buffer: Vec::with_capacity(buffer_capacity),
            total_bytes_written: 0,
            total_flushes: 0,
            peak_buffer_size: 0,
            flush_callback: None,
        })
    }

    /// Set a callback to be called when buffers are flushed
    pub fn set_flush_callback(&mut self, callback: FlushCallback) {
        self.flush_callback = Some(callback);
    }

    /// Write a chunk of data to the buffer
    pub fn write_chunk(&mut self, data: &[u8]) -> IoResult<()> {
        // If this chunk would overflow current buffer, flush it first
        if self.current_buffer.len() + data.len() > self.config.max_buffer_size {
            self.flush_current_buffer()?;
        }

        // If chunk is larger than max buffer size, write it directly
        if data.len() > self.config.max_buffer_size {
            self.write_directly(data)?;
            return Ok(());
        }

        // Add to current buffer
        self.current_buffer.extend_from_slice(data);

        // Update peak memory usage
        let current_memory = self.current_memory_usage();
        if current_memory > self.peak_buffer_size {
            self.peak_buffer_size = current_memory;
        }

        // Check if we need to flush due to buffer count limit
        if self.buffers.len() >= self.config.buffer_count {
            self.flush_oldest_buffer()?;
        }

        Ok(())
    }

    /// Flush the current buffer to the queue
    pub fn flush_current_buffer(&mut self) -> IoResult<()> {
        if !self.current_buffer.is_empty() {
            let buffer = std::mem::replace(
                &mut self.current_buffer,
                Vec::with_capacity(self.config.max_buffer_size),
            );
            self.buffers.push_back(buffer);

            // If we have too many buffers, flush the oldest one
            if self.buffers.len() > self.config.buffer_count {
                self.flush_oldest_buffer()?;
            }
        }
        Ok(())
    }

    /// Flush the oldest buffer to the writer
    pub fn flush_oldest_buffer(&mut self) -> IoResult<()> {
        if let Some(buffer) = self.buffers.pop_front() {
            self.write_buffer(&buffer)?;

            // Call flush callback if set
            if let Some(ref callback) = self.flush_callback {
                callback(buffer.len());
            }
        }
        Ok(())
    }

    /// Flush all buffers to the writer
    pub fn flush_all(&mut self) -> IoResult<()> {
        // Flush current buffer to queue first
        self.flush_current_buffer()?;

        // Flush all queued buffers
        while !self.buffers.is_empty() {
            self.flush_oldest_buffer()?;
        }

        // Ensure writer is flushed
        self.writer.flush()?;

        Ok(())
    }

    /// Write data directly to the writer without buffering
    fn write_directly(&mut self, data: &[u8]) -> IoResult<()> {
        // First flush any existing buffers to maintain order
        self.flush_all()?;

        // Write directly
        self.write_buffer(data)?;

        // Call flush callback for direct writes too
        if let Some(ref callback) = self.flush_callback {
            callback(data.len());
        }

        Ok(())
    }

    /// Write a buffer to the underlying writer
    fn write_buffer(&mut self, buffer: &[u8]) -> IoResult<()> {
        if self.config.enable_compression {
            // TODO: Implement compression if needed
            self.writer.write_all(buffer)?;
        } else {
            self.writer.write_all(buffer)?;
        }

        self.total_bytes_written += buffer.len();
        self.total_flushes += 1;

        Ok(())
    }

    /// Get current memory usage in bytes
    pub fn current_memory_usage(&self) -> usize {
        let buffered_size: usize = self.buffers.iter().map(|b| b.len()).sum();
        buffered_size + self.current_buffer.len()
    }

    /// Get current buffer size
    pub fn current_buffer_size(&self) -> usize {
        self.current_buffer.len()
    }

    /// Get total bytes written so far
    pub fn total_bytes_written(&self) -> usize {
        self.total_bytes_written
    }

    /// Get peak buffer size reached
    pub fn peak_buffer_size(&self) -> usize {
        self.peak_buffer_size
    }

    /// Get total number of flushes performed
    pub fn total_flushes(&self) -> usize {
        self.total_flushes
    }

    /// Get number of buffers currently queued
    pub fn queued_buffer_count(&self) -> usize {
        self.buffers.len()
    }

    /// Check if buffers are near capacity
    pub fn is_near_capacity(&self) -> bool {
        self.current_buffer.len() > (self.config.max_buffer_size * 3 / 4)
            || self.buffers.len() >= (self.config.buffer_count * 3 / 4)
    }

    /// Get buffer statistics
    pub fn get_stats(&self) -> BufferStats {
        BufferStats {
            current_memory_usage: self.current_memory_usage(),
            peak_memory_usage: self.peak_buffer_size,
            total_bytes_written: self.total_bytes_written,
            total_flushes: self.total_flushes,
            queued_buffers: self.buffers.len(),
            current_buffer_size: self.current_buffer.len(),
            is_near_capacity: self.is_near_capacity(),
        }
    }
}

/// Statistics for buffer management
#[derive(Debug, Default)]
pub struct BufferStats {
    /// Current memory usage in bytes
    pub current_memory_usage: usize,
    /// Peak memory usage reached
    pub peak_memory_usage: usize,
    /// Total bytes written through buffers
    pub total_bytes_written: usize,
    /// Number of times buffers were flushed
    pub total_flushes: usize,
    /// Number of buffers currently queued
    pub queued_buffers: usize,
    /// Size of the current active buffer
    pub current_buffer_size: usize,
    /// Whether we're near memory capacity
    pub is_near_capacity: bool,
}

impl<W: IoWrite> Drop for BufferManager<W> {
    /// Ensure all buffers are flushed when dropped
    fn drop(&mut self) {
        let _ = self.flush_all();
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::io::Cursor;

    #[test]
    fn test_basic_buffering() {
        let output = Vec::new();
        let cursor = Cursor::new(output);
        let mut buffer_manager = BufferManager::new(cursor, 100).unwrap();

        // Write some data
        buffer_manager.write_chunk(b"Hello, ").unwrap();
        buffer_manager.write_chunk(b"World!").unwrap();

        assert_eq!(buffer_manager.current_buffer_size(), 13);
        assert_eq!(buffer_manager.total_bytes_written(), 0); // Not flushed yet

        // Flush all
        buffer_manager.flush_all().unwrap();

        assert_eq!(buffer_manager.total_bytes_written(), 13);
        let output = buffer_manager.writer.clone().into_inner();
        assert_eq!(output, b"Hello, World!");
    }

    #[test]
    fn test_automatic_flushing() {
        let output = Vec::new();
        let cursor = Cursor::new(output);
        let mut buffer_manager = BufferManager::new(cursor, 10).unwrap(); // Small buffer

        // Write data that exceeds buffer size
        buffer_manager
            .write_chunk(b"This is a longer string")
            .unwrap();

        // Should have been written directly
        assert!(buffer_manager.total_bytes_written() > 0);
    }

    #[test]
    fn test_buffer_stats() {
        let output = Vec::new();
        let cursor = Cursor::new(output);
        let mut buffer_manager = BufferManager::new(cursor, 100).unwrap();

        buffer_manager.write_chunk(b"test data").unwrap();

        let stats = buffer_manager.get_stats();
        assert_eq!(stats.current_buffer_size, 9);
        assert_eq!(stats.total_bytes_written, 0);
        assert_eq!(stats.queued_buffers, 0);
    }

    #[test]
    fn test_flush_callback() {
        use std::sync::{Arc, Mutex};

        let output = Vec::new();
        let cursor = Cursor::new(output);
        let mut buffer_manager = BufferManager::new(cursor, 10).unwrap();

        let flush_count = Arc::new(Mutex::new(0));
        let flush_count_clone = flush_count.clone();

        buffer_manager.set_flush_callback(Box::new(move |_size| {
            let mut count = flush_count_clone.lock().unwrap();
            *count += 1;
        }));

        // Write enough to trigger flush
        buffer_manager
            .write_chunk(b"This will trigger a flush")
            .unwrap();
        buffer_manager.flush_all().unwrap();

        let count = *flush_count.lock().unwrap();
        assert!(count > 0);
    }
}