use std::fs;
use std::path::Path;

use super::error::WalError;

/// File name for the checkpoint metadata.
const CHECKPOINT_FILE: &str = "checkpoint.meta";

/// Temporary file used for atomic write.
const CHECKPOINT_TMP: &str = "checkpoint.meta.tmp";

/// Manages checkpoint metadata for the WAL.
///
/// A checkpoint marks the sequence number through which all signal events
/// have been materialized to durable storage. On recovery, the WAL only
/// needs to replay events after the checkpoint.
///
/// Checkpoint writes are atomic: write to a temp file, fsync, then rename.
pub struct CheckpointManager;

impl CheckpointManager {
    /// Write a checkpoint with the given sequence number and timestamp.
    ///
    /// Uses write-to-temp-then-rename for atomicity on POSIX systems.
    ///
    /// # Errors
    ///
    /// Returns `WalError::Io` on filesystem failure.
    pub fn write(dir: &Path, seq: u64, ts: u64) -> Result<(), WalError> {
        let content = format!("seq={seq}\nts={ts}\n");
        let tmp_path = dir.join(CHECKPOINT_TMP);
        let final_path = dir.join(CHECKPOINT_FILE);

        fs::write(&tmp_path, content.as_bytes())?;

        // fsync the temp file to ensure contents are durable before rename
        let file = fs::File::open(&tmp_path)?;
        file.sync_all()?;
        drop(file);

        // Atomic rename (POSIX guarantees)
        fs::rename(&tmp_path, &final_path)?;

        // Fsync the directory to ensure the rename (directory entry update)
        // is durable. Without this, a crash after rename but before the
        // directory metadata is flushed could lose the checkpoint file.
        let dir_fd = fs::File::open(dir)?;
        dir_fd.sync_all()?;

        Ok(())
    }

    /// Read the checkpoint metadata.
    ///
    /// Returns `None` if the checkpoint file does not exist (fresh WAL).
    /// Returns `Some((seq, ts))` on success.
    ///
    /// # Errors
    ///
    /// Returns `WalError::Io` on read failure, or `WalError::Corruption`
    /// if the file exists but cannot be parsed.
    pub fn read(dir: &Path) -> Result<Option<(u64, u64)>, WalError> {
        let path = dir.join(CHECKPOINT_FILE);
        let content = match fs::read_to_string(&path) {
            Ok(c) => c,
            Err(e) if e.kind() == std::io::ErrorKind::NotFound => return Ok(None),
            Err(e) => return Err(WalError::Io(e)),
        };

        let mut seq: Option<u64> = None;
        let mut ts: Option<u64> = None;

        for line in content.lines() {
            let line = line.trim();
            if line.is_empty() {
                continue;
            }
            if let Some(val) = line.strip_prefix("seq=") {
                seq = Some(val.parse::<u64>().map_err(|_| WalError::Corruption {
                    message: format!("invalid seq value in checkpoint: '{val}'"),
                })?);
            } else if let Some(val) = line.strip_prefix("ts=") {
                ts = Some(val.parse::<u64>().map_err(|_| WalError::Corruption {
                    message: format!("invalid ts value in checkpoint: '{val}'"),
                })?);
            }
        }

        match (seq, ts) {
            (Some(s), Some(t)) => Ok(Some((s, t))),
            _ => Err(WalError::Corruption {
                message: "checkpoint file missing seq or ts field".into(),
            }),
        }
    }
}

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

    #[test]
    fn write_and_read_roundtrip() {
        let dir = tempfile::tempdir().expect("tempdir creation should succeed");
        CheckpointManager::write(dir.path(), 1000, 2_000_000_000).expect("write should succeed");
        let result = CheckpointManager::read(dir.path()).expect("read should succeed");
        assert_eq!(result, Some((1000, 2_000_000_000)));
    }

    #[test]
    fn read_missing_returns_none() {
        let dir = tempfile::tempdir().expect("tempdir creation should succeed");
        let result = CheckpointManager::read(dir.path()).expect("read should succeed");
        assert_eq!(result, None);
    }

    #[test]
    fn overwrite_updates_values() {
        let dir = tempfile::tempdir().expect("tempdir creation should succeed");
        CheckpointManager::write(dir.path(), 100, 200).expect("write should succeed");
        CheckpointManager::write(dir.path(), 500, 600).expect("write should succeed");
        let result = CheckpointManager::read(dir.path()).expect("read should succeed");
        assert_eq!(result, Some((500, 600)));
    }

    #[test]
    fn corrupt_file_returns_error() {
        let dir = tempfile::tempdir().expect("tempdir creation should succeed");
        let path = dir.path().join("checkpoint.meta");
        fs::write(&path, "garbage data").expect("write should succeed");
        let result = CheckpointManager::read(dir.path());
        assert!(result.is_err());
    }

    #[test]
    fn partial_file_returns_error() {
        let dir = tempfile::tempdir().expect("tempdir creation should succeed");
        let path = dir.path().join("checkpoint.meta");
        fs::write(&path, "seq=100\n").expect("write should succeed"); // missing ts
        let result = CheckpointManager::read(dir.path());
        assert!(result.is_err());
    }

    #[test]
    fn max_u64_values() {
        let dir = tempfile::tempdir().expect("tempdir creation should succeed");
        CheckpointManager::write(dir.path(), u64::MAX, u64::MAX).expect("write should succeed");
        let result = CheckpointManager::read(dir.path()).expect("read should succeed");
        assert_eq!(result, Some((u64::MAX, u64::MAX)));
    }

    #[test]
    fn zero_values() {
        let dir = tempfile::tempdir().expect("tempdir creation should succeed");
        CheckpointManager::write(dir.path(), 0, 0).expect("write should succeed");
        let result = CheckpointManager::read(dir.path()).expect("read should succeed");
        assert_eq!(result, Some((0, 0)));
    }

    mod proptests {
        use super::*;
        use proptest::prelude::*;

        proptest! {
            #[test]
            fn roundtrip(seq: u64, ts: u64) {
                let dir = tempfile::tempdir().expect("tempdir creation should succeed");
                CheckpointManager::write(dir.path(), seq, ts)?;
                let result = CheckpointManager::read(dir.path())?;
                prop_assert_eq!(result, Some((seq, ts)));
            }
        }
    }
}