//! Block-level Checksum for Data Integrity
//!
//! Reference: ZFS/Btrfs checksum verification
//! - ZFS: Fletcher4/SHA256 per-block checksum
//! - Btrfs: CRC32C per-block checksum
//!
//! MarkBase uses SHA-256 (32 bytes) per 4KB block for integrity verification.

use std::path::PathBuf;
use std::io::{Read, Write};

use sha2::{Sha256, Digest};
use serde::{Serialize, Deserialize};

use super::{VfsBackend, VfsFile, VfsError};

pub const BLOCK_SIZE: usize = 4096;
pub const HASH_SIZE: usize = 32;  // SHA-256
pub const CHECKSUM_DIR: &str = ".checksums";
pub const CHECKSUM_EXT: &str = ".checksums";

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct VfsBlockChecksum {
    pub offset: u64,      // Block offset (multiple of BLOCK_SIZE)
    pub hash: Vec<u8>,    // SHA-256 hash (32 bytes)
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct VfsChecksumFile {
    pub block_size: usize,
    pub algorithm: String,  // "sha256"
    pub blocks: Vec<VfsBlockChecksum>,
    pub file_size: u64,     // Original file size
}

impl VfsChecksumFile {
    pub fn new(file_size: u64) -> Self {
        Self {
            block_size: BLOCK_SIZE,
            algorithm: "sha256".to_string(),
            blocks: Vec::new(),
            file_size,
        }
    }

    pub fn from_bytes(data: &[u8]) -> Result<Self, VfsError> {
        serde_json::from_slice(data)
            .map_err(|e| VfsError::Io(format!("checksum parse failed: {}", e)))
    }

    pub fn to_bytes(&self) -> Result<Vec<u8>, VfsError> {
        serde_json::to_vec(self)
            .map_err(|e| VfsError::Io(format!("checksum serialize failed: {}", e)))
    }

    pub fn get_checksum(&self, offset: u64) -> Option<&[u8]> {
        self.blocks.iter()
            .find(|b| b.offset == offset)
            .map(|b| b.hash.as_slice())
    }

    pub fn set_checksum(&mut self, offset: u64, hash: Vec<u8>) {
        if let Some(block) = self.blocks.iter_mut().find(|b| b.offset == offset) {
            block.hash = hash;
        } else {
            self.blocks.push(VfsBlockChecksum { offset, hash });
            self.blocks.sort_by_key(|b| b.offset);
        }
    }

    pub fn block_count(&self) -> usize {
        (self.file_size as usize / BLOCK_SIZE) + 
            if !(self.file_size as usize).is_multiple_of(BLOCK_SIZE) { 1 } else { 0 }
    }
}

pub fn compute_block_hash(data: &[u8]) -> Vec<u8> {
    let mut hasher = Sha256::new();
    hasher.update(data);
    hasher.finalize().to_vec()
}

pub fn verify_block_hash(data: &[u8], expected: &[u8]) -> bool {
    let actual = compute_block_hash(data);
    actual == expected
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ChecksumMode {
    Lazy,      // Only verify on scrub (default)
    OnRead,    // Verify every read
}

#[derive(Debug, Clone)]
pub struct ChecksumConfig {
    pub mode: ChecksumMode,
    pub cache_verified: bool,
}

impl Default for ChecksumConfig {
    fn default() -> Self {
        Self {
            mode: ChecksumMode::Lazy,
            cache_verified: true,
        }
    }
}

#[derive(Debug)]
pub struct ScrubResult {
    pub path: PathBuf,
    pub total_blocks: usize,
    pub verified_blocks: usize,
    pub corrupted_blocks: Vec<u64>,
    pub repaired_blocks: Vec<u64>,
    pub repair_failed: bool,
}

impl ScrubResult {
    pub fn is_clean(&self) -> bool {
        self.corrupted_blocks.is_empty()
    }

    pub fn repair_success_rate(&self) -> f64 {
        if self.corrupted_blocks.is_empty() {
            1.0
        } else {
            self.repaired_blocks.len() as f64 / self.corrupted_blocks.len() as f64
        }
    }
}

pub fn checksum_path_for_file(file_path: &PathBuf, root: &PathBuf) -> PathBuf {
    let relative = file_path.strip_prefix(root)
        .unwrap_or(file_path);
    root.join(CHECKSUM_DIR)
        .join(relative)
        .with_extension(CHECKSUM_EXT)
}

pub fn ensure_checksum_dir(root: &PathBuf, backend: &dyn VfsBackend) -> Result<(), VfsError> {
    let checksum_dir = root.join(CHECKSUM_DIR);
    if !backend.exists(&checksum_dir) {
        backend.create_dir(&checksum_dir, 0o755)?;
    }
    Ok(())
}

/// Scrub a single file to verify integrity
///
/// This reads the file and verifies each block checksum.
/// If repair=true and corrupted blocks are found, attempts to repair from RAID/Dedup.
pub fn scrub_file(
    backend: &dyn VfsBackend,
    file_path: &PathBuf,
    root_path: &PathBuf,
    repair: bool,
) -> Result<ScrubResult, VfsError> {
    let checksum_path = checksum_path_for_file(file_path, root_path);
    
    if !backend.exists(&checksum_path) {
        return Ok(ScrubResult {
            path: file_path.clone(),
            total_blocks: 0,
            verified_blocks: 0,
            corrupted_blocks: vec![],
            repaired_blocks: vec![],
            repair_failed: false,
        });
    }
    
    let checksum_file_data = {
        let mut checksum_file = backend.open_file(&checksum_path, &super::open_flags::OpenFlags::new().read())?;
        checksum_file.read_all()?
    };
    let checksum_data = VfsChecksumFile::from_bytes(&checksum_file_data)?;
    
    let mut file_handle = backend.open_file(file_path, &super::open_flags::OpenFlags::new().read())?;
    let stat = file_handle.stat()?;
    let file_size = stat.size;
    
    let block_count = checksum_data.block_count();
    let mut verified_blocks = 0;
    let mut corrupted_blocks: Vec<u64> = vec![];
    let mut repaired_blocks: Vec<u64> = vec![];
    
    for block_idx in 0..block_count {
        let offset = (block_idx as u64) * BLOCK_SIZE as u64;
        let block_size = if offset + BLOCK_SIZE as u64 <= file_size {
            BLOCK_SIZE
        } else {
            (file_size - offset) as usize
        };
        
        let mut buffer = vec![0u8; block_size];
        let bytes_read = file_handle.read_at(&mut buffer, offset)?;
        
        if bytes_read != block_size {
            corrupted_blocks.push(offset);
            continue;
        }
        
        let expected_hash = checksum_data.get_checksum(offset);
        if expected_hash.is_none() {
            verified_blocks += 1;
            continue;
        }
        
        let is_valid = verify_block_hash(&buffer, expected_hash.unwrap());
        if is_valid {
            verified_blocks += 1;
        } else {
            corrupted_blocks.push(offset);
            
            if repair {
                if repair_block(backend, file_path, offset, &buffer).is_ok() {
                    repaired_blocks.push(offset);
                }
            }
        }
    }
    
    let corrupted_count = corrupted_blocks.len();
    let repaired_count = repaired_blocks.len();
    
    Ok(ScrubResult {
        path: file_path.clone(),
        total_blocks: block_count,
        verified_blocks,
        corrupted_blocks,
        repaired_blocks,
        repair_failed: repair && repaired_count < corrupted_count,
    })
}

/// Scrub all files in a directory
///
/// Recursively walks the directory and scrubs all files with checksums.
pub fn scrub_all(
    backend: &dyn VfsBackend,
    root_path: &PathBuf,
    repair: bool,
) -> Result<Vec<ScrubResult>, VfsError> {
    let mut results = vec![];
    
    let checksum_dir = root_path.join(CHECKSUM_DIR);
    if !backend.exists(&checksum_dir) {
        return Ok(results);
    }
    
    scrub_recursive(backend, root_path, root_path, repair, &mut results)?;
    
    Ok(results)
}

fn scrub_recursive(
    backend: &dyn VfsBackend,
    current_path: &PathBuf,
    root_path: &PathBuf,
    repair: bool,
    results: &mut Vec<ScrubResult>,
) -> Result<(), VfsError> {
    let entries = backend.read_dir(current_path)?;
    
    for entry in entries {
        let entry_path = current_path.join(&entry.name);
        
        if entry.stat.is_dir {
            if entry.name != CHECKSUM_DIR {
                scrub_recursive(backend, &entry_path, root_path, repair, results)?;
            }
        } else if !entry.name.ends_with(CHECKSUM_EXT) {
            let result = scrub_file(backend, &entry_path, root_path, repair)?;
            results.push(result);
        }
    }
    
    Ok(())
}

/// Attempt to repair a corrupted block
///
/// Tries RAID repair first (if backend is RAID), then Dedup repair.
pub fn repair_block(
    _backend: &dyn VfsBackend,
    _file_path: &PathBuf,
    _offset: u64,
    _expected_checksum: &[u8],
) -> Result<Vec<u8>, VfsError> {
    // Try Dedup repair first (check if block exists in dedup store)
    // This requires the backend to have dedup integration
    
    // For now, return error - RAID/Dedup repair requires specific backend types
    Err(VfsError::Io("block repair requires RAID or Dedup backend (Phase 4/6)".to_string()))
}

/// Repair block from DedupStore
///
/// This is called when checksum detects corruption and dedup store is available.
pub fn repair_block_from_dedup(
    dedup_store: &super::dedup::DedupStore,
    checksum_hash: &[u8],
) -> Result<Vec<u8>, VfsError> {
    dedup_store.repair_from_checksum(checksum_hash)
}

/// Create checksums for a file
///
/// This reads the file and computes checksums for all blocks.
pub fn create_checksums_for_file(
    backend: &dyn VfsBackend,
    file_path: &PathBuf,
    root_path: &PathBuf,
) -> Result<(), VfsError> {
    ensure_checksum_dir(root_path, backend)?;
    
    let mut file_handle = backend.open_file(file_path, &super::open_flags::OpenFlags::new().read())?;
    let stat = file_handle.stat()?;
    let file_size = stat.size;
    
    let mut checksum_data = VfsChecksumFile::new(file_size);
    
    let block_count = checksum_data.block_count();
    
    for block_idx in 0..block_count {
        let offset = (block_idx as u64) * BLOCK_SIZE as u64;
        let block_size = if offset + BLOCK_SIZE as u64 <= file_size {
            BLOCK_SIZE
        } else {
            (file_size - offset) as usize
        };
        
        let mut buffer = vec![0u8; block_size];
        let bytes_read = file_handle.read_at(&mut buffer, offset)?;
        
        if bytes_read > 0 {
            let hash = compute_block_hash(&buffer[..bytes_read]);
            checksum_data.set_checksum(offset, hash);
        }
    }
    
    let checksum_path = checksum_path_for_file(file_path, root_path);
    let checksum_bytes = checksum_data.to_bytes()?;
    
    let mut checksum_file = backend.open_file(
        &checksum_path,
        &super::open_flags::OpenFlags::new().write().create().truncate(),
    )?;
    checksum_file.write_all(&checksum_bytes)?;
    checksum_file.flush()?;
    
    Ok(())
}

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

    #[test]
    fn test_compute_block_hash() {
        let data = b"test block data for hashing";
        let hash = compute_block_hash(data);
        assert_eq!(hash.len(), HASH_SIZE);
        
        let hash2 = compute_block_hash(data);
        assert_eq!(hash, hash2);
    }

    #[test]
    fn test_verify_block_hash() {
        let data = b"test block data";
        let hash = compute_block_hash(data);
        assert!(verify_block_hash(data, &hash));
        
        let wrong_data = b"wrong block data";
        assert!(!verify_block_hash(wrong_data, &hash));
    }

    #[test]
    fn test_checksum_file_roundtrip() {
        let mut checksum_file = VfsChecksumFile::new(8192);
        checksum_file.set_checksum(0, compute_block_hash(b"block0"));
        checksum_file.set_checksum(4096, compute_block_hash(b"block1"));
        
        let bytes = checksum_file.to_bytes().unwrap();
        let decoded = VfsChecksumFile::from_bytes(&bytes).unwrap();
        
        assert_eq!(decoded.block_size, BLOCK_SIZE);
        assert_eq!(decoded.blocks.len(), 2);
        assert_eq!(decoded.file_size, 8192);
    }

    #[test]
    fn test_checksum_file_get_set() {
        let mut checksum_file = VfsChecksumFile::new(4096);
        
        let hash = compute_block_hash(b"test");
        checksum_file.set_checksum(0, hash.clone());
        
        let retrieved = checksum_file.get_checksum(0);
        assert!(retrieved.is_some());
        assert_eq!(retrieved.unwrap(), hash.as_slice());
        
        checksum_file.set_checksum(0, compute_block_hash(b"new"));
        let updated = checksum_file.get_checksum(0).unwrap();
        assert_ne!(updated, hash.as_slice());
    }

    #[test]
    fn test_block_count_calculation() {
        let checksum_file = VfsChecksumFile::new(4096);
        assert_eq!(checksum_file.block_count(), 1);
        
        let checksum_file = VfsChecksumFile::new(8192);
        assert_eq!(checksum_file.block_count(), 2);
        
        let checksum_file = VfsChecksumFile::new(4097);
        assert_eq!(checksum_file.block_count(), 2);
        
        let checksum_file = VfsChecksumFile::new(0);
        assert_eq!(checksum_file.block_count(), 0);
    }

    #[test]
    fn test_scrub_result_metrics() {
        let result = ScrubResult {
            path: PathBuf::from("/test"),
            total_blocks: 10,
            verified_blocks: 10,
            corrupted_blocks: vec![],
            repaired_blocks: vec![],
            repair_failed: false,
        };
        assert!(result.is_clean());
        assert_eq!(result.repair_success_rate(), 1.0);
        
        let result2 = ScrubResult {
            path: PathBuf::from("/test"),
            total_blocks: 10,
            verified_blocks: 8,
            corrupted_blocks: vec![4096, 8192],
            repaired_blocks: vec![4096],
            repair_failed: false,
        };
        assert!(!result2.is_clean());
        assert_eq!(result2.repair_success_rate(), 0.5);
    }
}