velocidb 0.1.0

// Cloud Virtual File System for remote object storage
// Supports S3, Azure Blob, Google Cloud Storage via object_store crate

#[cfg(feature = "cloud-vfs")]
use object_store::{ObjectStore, path::Path as ObjectPath};

use crate::async_io::AsyncVfs;
use crate::storage::{Page, PAGE_SIZE};
use crate::types::{PageId, Result, VelociError};
use async_trait::async_trait;
use parking_lot::RwLock;
use std::collections::HashMap;
use std::sync::Arc;

#[cfg(feature = "cloud-vfs")]
use bytes::Bytes;

/// Cloud VFS implementation for remote object storage
/// Provides transparent access to database files stored in cloud storage
#[cfg(feature = "cloud-vfs")]
pub struct CloudVfs {
    /// Object store client (S3, Azure, GCS)
    store: Arc<dyn ObjectStore>,
    /// Path to the database file in object storage
    db_path: ObjectPath,
    /// Local page cache
    cache: Arc<RwLock<HashMap<PageId, Page>>>,
    /// Cache capacity
    cache_capacity: usize,
    /// Number of pages (cached metadata)
    num_pages: Arc<RwLock<u64>>,
    /// Pending writes (write-back cache)
    pending_writes: Arc<RwLock<HashMap<PageId, Page>>>,
}

#[cfg(feature = "cloud-vfs")]
impl CloudVfs {
    pub async fn new(
        store: Arc<dyn ObjectStore>,
        db_path: impl Into<ObjectPath>,
        cache_capacity: usize,
    ) -> Result<Self> {
        let db_path = db_path.into();

        // Try to get metadata to determine file size
        let num_pages = match store.head(&db_path).await {
            Ok(meta) => {
                let file_size = meta.size;
                (file_size + PAGE_SIZE - 1) / PAGE_SIZE
            }
            Err(_) => {
                // File doesn't exist yet
                0
            }
        };

        Ok(Self {
            store,
            db_path,
            cache: Arc::new(RwLock::new(HashMap::new())),
            cache_capacity,
            num_pages: Arc::new(RwLock::new(num_pages as u64)),
            pending_writes: Arc::new(RwLock::new(HashMap::new())),
        })
    }

    /// Fetch a page from remote storage
    async fn fetch_page_from_remote(&self, page_id: PageId) -> Result<Page> {
        let offset = page_id * PAGE_SIZE as u64;
        let range = offset..(offset + PAGE_SIZE as u64);

        // Perform range read
        let result = self.store.get_range(&self.db_path, range).await
            .map_err(|e| VelociError::IoError(format!("Cloud fetch error: {}", e)))?;

        let mut page = Page::new();
        let bytes_read = result.len().min(PAGE_SIZE);
        page.data_mut()[..bytes_read].copy_from_slice(&result[..bytes_read]);

        Ok(page)
    }

    /// Flush a page to remote storage
    async fn flush_page_to_remote(&self, page_id: PageId, page: &Page) -> Result<()> {
        let offset = page_id * PAGE_SIZE as u64;

        // For cloud storage, we need to read-modify-write the entire file
        // This is simplified; production would use multipart uploads

        // Read entire file
        let file_data = self.store.get(&self.db_path).await
            .map(|r| r.bytes().to_vec())
            .unwrap_or_else(|_| Vec::new());

        // Ensure file is large enough
        let required_size = (offset + PAGE_SIZE as u64) as usize;
        let mut new_data = if file_data.len() < required_size {
            let mut v = file_data;
            v.resize(required_size, 0);
            v
        } else {
            file_data
        };

        // Update the page
        new_data[offset as usize..(offset as usize + PAGE_SIZE)]
            .copy_from_slice(page.data());

        // Write back
        let bytes = Bytes::from(new_data);
        self.store.put(&self.db_path, bytes).await
            .map_err(|e| VelociError::IoError(format!("Cloud write error: {}", e)))?;

        Ok(())
    }

    /// Evict LRU page from cache
    fn evict_lru(&self) -> Result<()> {
        let mut cache = self.cache.write();
        
        if cache.len() >= self.cache_capacity {
            // Simple eviction: remove first entry
            if let Some(&page_id) = cache.keys().next() {
                cache.remove(&page_id);
            }
        }

        Ok(())
    }
}

#[cfg(feature = "cloud-vfs")]
#[async_trait]
impl AsyncVfs for CloudVfs {
    async fn read_page(&self, page_id: PageId) -> Result<Page> {
        // Check pending writes first
        {
            let pending = self.pending_writes.read();
            if let Some(page) = pending.get(&page_id) {
                return Ok(page.clone());
            }
        }

        // Check cache
        {
            let cache = self.cache.read();
            if let Some(page) = cache.get(&page_id) {
                return Ok(page.clone());
            }
        }

        // Cache miss - fetch from remote
        let page = self.fetch_page_from_remote(page_id).await?;

        // Add to cache
        self.evict_lru()?;
        self.cache.write().insert(page_id, page.clone());

        Ok(page)
    }

    async fn write_page(&self, page_id: PageId, page: &Page) -> Result<()> {
        // Write to pending writes (write-back cache)
        self.pending_writes.write().insert(page_id, page.clone());

        // Update cache
        self.cache.write().insert(page_id, page.clone());

        // Update page count
        let mut num_pages = self.num_pages.write();
        if page_id >= *num_pages {
            *num_pages = page_id + 1;
        }

        Ok(())
    }

    async fn allocate_page(&self) -> Result<PageId> {
        let mut num_pages = self.num_pages.write();
        let page_id = *num_pages;
        *num_pages += 1;

        // Initialize the page in pending writes
        let page = Page::new();
        drop(num_pages);
        
        self.write_page(page_id, &page).await?;

        Ok(page_id)
    }

    async fn flush(&self) -> Result<()> {
        // Flush all pending writes to remote storage
        let pending = self.pending_writes.read().clone();
        drop(pending);

        let mut pending_write = self.pending_writes.write();
        
        for (page_id, page) in pending_write.drain() {
            self.flush_page_to_remote(page_id, &page).await?;
        }

        Ok(())
    }

    async fn num_pages(&self) -> u64 {
        *self.num_pages.read()
    }

    async fn sync(&self) -> Result<()> {
        self.flush().await
    }
}

/// Prefetching strategy for cloud storage
#[cfg(feature = "cloud-vfs")]
pub struct CloudPrefetcher {
    vfs: Arc<CloudVfs>,
}

#[cfg(feature = "cloud-vfs")]
impl CloudPrefetcher {
    pub fn new(vfs: Arc<CloudVfs>) -> Self {
        Self { vfs }
    }

    /// Prefetch a range of pages
    pub async fn prefetch_range(&self, start_page: PageId, count: usize) -> Result<()> {
        let mut handles = Vec::new();

        for i in 0..count {
            let page_id = start_page + i as u64;
            let vfs = Arc::clone(&self.vfs);

            let handle = tokio::spawn(async move {
                vfs.read_page(page_id).await
            });

            handles.push(handle);
        }

        // Wait for all prefetches
        for handle in handles {
            let _ = handle.await;
        }

        Ok(())
    }

    /// Prefetch pages based on access pattern
    pub async fn prefetch_sequential(&self, current_page: PageId, window: usize) -> Result<()> {
        // Prefetch next `window` pages
        self.prefetch_range(current_page + 1, window).await
    }
}

/// Cloud VFS configuration
#[cfg(feature = "cloud-vfs")]
#[derive(Debug, Clone)]
pub struct CloudVfsConfig {
    /// Cache capacity in pages
    pub cache_capacity: usize,
    /// Prefetch window size
    pub prefetch_window: usize,
    /// Write-back cache enabled
    pub write_back_cache: bool,
    /// Batch size for multipart uploads
    pub multipart_batch_size: usize,
}

#[cfg(feature = "cloud-vfs")]
impl Default for CloudVfsConfig {
    fn default() -> Self {
        Self {
            cache_capacity: 1024,
            prefetch_window: 16,
            write_back_cache: true,
            multipart_batch_size: 64,
        }
    }
}

// Stub implementations when cloud-vfs feature is disabled
#[cfg(not(feature = "cloud-vfs"))]
pub struct CloudVfs;

#[cfg(not(feature = "cloud-vfs"))]
impl CloudVfs {
    pub fn new() -> Self {
        Self
    }
}

#[cfg(not(feature = "cloud-vfs"))]
pub struct CloudPrefetcher;

#[cfg(not(feature = "cloud-vfs"))]
impl CloudPrefetcher {
    pub fn new(_vfs: &CloudVfs) -> Self {
        Self
    }
}

#[cfg(not(feature = "cloud-vfs"))]
#[derive(Debug, Clone)]
pub struct CloudVfsConfig;

#[cfg(not(feature = "cloud-vfs"))]
impl Default for CloudVfsConfig {
    fn default() -> Self {
        Self
    }
}

#[cfg(test)]
#[cfg(feature = "cloud-vfs")]
mod tests {
    use super::*;

    // Note: These tests require object_store and actual cloud credentials
    // In practice, use LocalFileSystem from object_store for testing

    #[tokio::test]
    async fn test_cloud_vfs_placeholder() {
        // Placeholder test - actual tests would require cloud storage setup
        assert!(true);
    }
}