use std::fs::{self, File, OpenOptions};
use std::io::{Read, Seek, SeekFrom, Write};


use crc32fast::Hasher;

use crate::error::Error;
use crate::location::Location;
use crate::OurDB;

// Header size: 2 bytes (size) + 4 bytes (CRC32) + 6 bytes (previous location)
pub const HEADER_SIZE: usize = 12;

impl OurDB {
    /// Selects and opens a database file for read/write operations
    pub(crate) fn db_file_select(&mut self, file_nr: u16) -> Result<(), Error> {
        // No need to check if file_nr > 65535 as u16 can't exceed that value

        let path = self.path.join(format!("{}.db", file_nr));

        // Always close the current file if it's open
        self.file = None;

        // Create file if it doesn't exist
        if !path.exists() {
            self.create_new_db_file(file_nr)?;
        }

        // Open the file fresh
        let file = OpenOptions::new()
            .read(true)
            .write(true)
            .open(&path)?;
            
        self.file = Some(file);
        self.file_nr = file_nr;

        Ok(())
    }

    /// Creates a new database file
    pub(crate) fn create_new_db_file(&mut self, file_nr: u16) -> Result<(), Error> {
        let new_file_path = self.path.join(format!("{}.db", file_nr));
        let mut file = File::create(&new_file_path)?;
        
        // Write a single byte to make all positions start from 1
        file.write_all(&[0u8])?;
        
        Ok(())
    }

    /// Gets the file number to use for the next write operation
    pub(crate) fn get_file_nr(&mut self) -> Result<u16, Error> {
        // For keysize 2, 3, or 4, we can only use file_nr 0
        if self.lookup.keysize() <= 4 {
            let path = self.path.join("0.db");
            
            if !path.exists() {
                self.create_new_db_file(0)?;
            }
            
            return Ok(0);
        }
        
        // For keysize 6, we can use multiple files
        let path = self.path.join(format!("{}.db", self.last_used_file_nr));
        
        if !path.exists() {
            self.create_new_db_file(self.last_used_file_nr)?;
            return Ok(self.last_used_file_nr);
        }

        let metadata = fs::metadata(&path)?;
        if metadata.len() >= self.file_size as u64 {
            self.last_used_file_nr += 1;
            self.create_new_db_file(self.last_used_file_nr)?;
        }

        Ok(self.last_used_file_nr)
    }

    /// Stores data at the specified ID with history tracking
    pub(crate) fn set_(&mut self, id: u32, old_location: Location, data: &[u8]) -> Result<(), Error> {
        // Validate data size - maximum is u16::MAX (65535 bytes or ~64KB)
        if data.len() > u16::MAX as usize {
            return Err(Error::InvalidOperation(
                format!("Data size exceeds maximum allowed size of {} bytes", u16::MAX)
            ));
        }

        // Get file number to use
        let file_nr = self.get_file_nr()?;
        
        // Select the file
        self.db_file_select(file_nr)?;

        // Get current file position for lookup
        let file = self.file.as_mut().ok_or_else(|| Error::Other("No file open".to_string()))?;
        file.seek(SeekFrom::End(0))?;
        let position = file.stream_position()? as u32;

        // Create new location
        let new_location = Location {
            file_nr,
            position,
        };

        // Calculate CRC of data
        let crc = calculate_crc(data);

        // Create header
        let mut header = vec![0u8; HEADER_SIZE];

        // Write size (2 bytes)
        let size = data.len() as u16; // Safe now because we've validated the size
        header[0] = (size & 0xFF) as u8;
        header[1] = ((size >> 8) & 0xFF) as u8;

        // Write CRC (4 bytes)
        header[2] = (crc & 0xFF) as u8;
        header[3] = ((crc >> 8) & 0xFF) as u8;
        header[4] = ((crc >> 16) & 0xFF) as u8;
        header[5] = ((crc >> 24) & 0xFF) as u8;

        // Write previous location (6 bytes)
        let prev_bytes = old_location.to_bytes();
        for (i, &byte) in prev_bytes.iter().enumerate().take(6) {
            header[6 + i] = byte;
        }

        // Write header
        file.write_all(&header)?;

        // Write actual data
        file.write_all(data)?;
        file.flush()?;

        // Update lookup table with new position
        self.lookup.set(id, new_location)?;

        Ok(())
    }

    /// Retrieves data at the specified location
    pub(crate) fn get_(&mut self, location: Location) -> Result<Vec<u8>, Error> {
        if location.position == 0 {
            return Err(Error::NotFound(format!("Record not found, location: {:?}", location)));
        }

        // Select the file
        self.db_file_select(location.file_nr)?;

        let file = self.file.as_mut().ok_or_else(|| Error::Other("No file open".to_string()))?;

        // Read header
        file.seek(SeekFrom::Start(location.position as u64))?;
        let mut header = vec![0u8; HEADER_SIZE];
        file.read_exact(&mut header)?;

        // Parse size (2 bytes)
        let size = u16::from(header[0]) | (u16::from(header[1]) << 8);

        // Parse CRC (4 bytes)
        let stored_crc = u32::from(header[2]) 
                       | (u32::from(header[3]) << 8) 
                       | (u32::from(header[4]) << 16) 
                       | (u32::from(header[5]) << 24);

        // Read data
        let mut data = vec![0u8; size as usize];
        file.read_exact(&mut data)?;

        // Verify CRC
        let calculated_crc = calculate_crc(&data);
        if calculated_crc != stored_crc {
            return Err(Error::DataCorruption("CRC mismatch: data corruption detected".to_string()));
        }

        Ok(data)
    }

    /// Retrieves the previous position for a record (for history tracking)
    pub(crate) fn get_prev_pos_(&mut self, location: Location) -> Result<Location, Error> {
        if location.position == 0 {
            return Err(Error::NotFound("Record not found".to_string()));
        }

        // Select the file
        self.db_file_select(location.file_nr)?;

        let file = self.file.as_mut().ok_or_else(|| Error::Other("No file open".to_string()))?;

        // Skip size and CRC (6 bytes)
        file.seek(SeekFrom::Start(location.position as u64 + 6))?;

        // Read previous location (6 bytes)
        let mut prev_bytes = vec![0u8; 6];
        file.read_exact(&mut prev_bytes)?;

        // Create location from bytes
        Location::from_bytes(&prev_bytes, 6)
    }

    /// Deletes the record at the specified location
    pub(crate) fn delete_(&mut self, id: u32, location: Location) -> Result<(), Error> {
        if location.position == 0 {
            return Err(Error::NotFound("Record not found".to_string()));
        }

        // Select the file
        self.db_file_select(location.file_nr)?;

        let file = self.file.as_mut().ok_or_else(|| Error::Other("No file open".to_string()))?;

        // Read size first
        file.seek(SeekFrom::Start(location.position as u64))?;
        let mut size_bytes = vec![0u8; 2];
        file.read_exact(&mut size_bytes)?;
        let size = u16::from(size_bytes[0]) | (u16::from(size_bytes[1]) << 8);

        // Write zeros for the entire record (header + data)
        let zeros = vec![0u8; HEADER_SIZE + size as usize];
        file.seek(SeekFrom::Start(location.position as u64))?;
        file.write_all(&zeros)?;

        // Clear lookup entry
        self.lookup.delete(id)?;

        Ok(())
    }

    /// Condenses the database by removing empty records and updating positions
    pub fn condense(&mut self) -> Result<(), Error> {
        // Create a temporary directory
        let temp_path = self.path.join("temp");
        fs::create_dir_all(&temp_path)?;

        // Get all file numbers
        let mut file_numbers = Vec::new();
        for entry in fs::read_dir(&self.path)? {
            let entry = entry?;
            let path = entry.path();
            
            if path.is_file() && path.extension().map_or(false, |ext| ext == "db") {
                if let Some(stem) = path.file_stem() {
                    if let Ok(file_nr) = stem.to_string_lossy().parse::<u16>() {
                        file_numbers.push(file_nr);
                    }
                }
            }
        }

        // Process each file
        for file_nr in file_numbers {
            let src_path = self.path.join(format!("{}.db", file_nr));
            let temp_file_path = temp_path.join(format!("{}.db", file_nr));
            
            // Create new file
            let mut temp_file = File::create(&temp_file_path)?;
            temp_file.write_all(&[0u8])?; // Initialize with a byte
            
            // Open source file
            let mut src_file = File::open(&src_path)?;
            
            // Read and process records
            let mut buffer = vec![0u8; 1024]; // Read in chunks
            let mut _position = 0;
            
            while let Ok(bytes_read) = src_file.read(&mut buffer) {
                if bytes_read == 0 {
                    break;
                }
                
                // Process the chunk
                // This is a simplified version - in a real implementation,
                // you would need to handle records that span chunk boundaries
                
                _position += bytes_read;
            }
            
            // TODO: Implement proper record copying and position updating
            // This would involve:
            // 1. Reading each record from the source file
            // 2. If not deleted (all zeros), copy to temp file
            // 3. Update lookup table with new positions
        }
        
        // TODO: Replace original files with temp files
        
        // Clean up
        fs::remove_dir_all(&temp_path)?;
        
        Ok(())
    }
}

/// Calculates CRC32 for the data
fn calculate_crc(data: &[u8]) -> u32 {
    let mut hasher = Hasher::new();
    hasher.update(data);
    hasher.finalize()
}

#[cfg(test)]
mod tests {
    use std::path::PathBuf;
    
    use crate::{OurDB, OurDBConfig, OurDBSetArgs};
    use std::env::temp_dir;
    use std::time::{SystemTime, UNIX_EPOCH};

    fn get_temp_dir() -> PathBuf {
        let timestamp = SystemTime::now()
            .duration_since(UNIX_EPOCH)
            .unwrap()
            .as_secs();
        temp_dir().join(format!("ourdb_backend_test_{}", timestamp))
    }

    #[test]
    fn test_backend_operations() {
        let temp_dir = get_temp_dir();
        
        let config = OurDBConfig {
            path: temp_dir.clone(),
            incremental_mode: false,
            file_size: None,
            keysize: None,
            reset: None,            // Don't reset existing database
        };
        
        let mut db = OurDB::new(config).unwrap();
        
        // Test set and get
        let test_data = b"Test data for backend operations";
        let id = 1;
        
        db.set(OurDBSetArgs { id: Some(id), data: test_data }).unwrap();
        
        let retrieved = db.get(id).unwrap();
        assert_eq!(retrieved, test_data);
        
        // Clean up
        db.destroy().unwrap();
    }
}