//! Log rotation via segment files with global offset addressing.
//!
//! Each segment file is named `{base_offset:016x}.wal` where `base_offset` is
//! the global WAL offset where that segment begins. Reads resolve the correct
//! segment via binary search, and writes rotate to a new segment when the
//! current one exceeds `MAX_SEGMENT_SIZE`.
//!
//! # Cleanup
//!
//! `SegmentManager::cleanup(min_cursor)` deletes segments whose entire range
//! is below `min_cursor`, freeing disk space after consumers have advanced.

use crate::error::{QuarantineError, Result};
use crate::format::{FileHeader, HEADER_SIZE};
use std::fs;
use std::path::{Path, PathBuf};
use tracing::{debug, info, instrument, warn};

/// Default maximum segment size (1 GB).
pub const DEFAULT_MAX_SEGMENT_SIZE: u64 = 1024 * 1024 * 1024;

/// A single WAL segment file.
#[derive(Debug, Clone)]
pub struct Segment {
    /// Global WAL offset where this segment starts.
    pub base_offset: u64,
    /// Path to the segment file.
    pub path: PathBuf,
    /// Current file size in bytes.
    pub size: u64,
}

impl Segment {
    /// Format a segment filename from its base offset.
    pub fn filename(base_offset: u64) -> String {
        format!("{:016x}.wal", base_offset)
    }

    /// Parse a base offset from a segment filename.
    pub fn parse_filename(name: &str) -> Option<u64> {
        let stem = name.strip_suffix(".wal")?;
        if stem.len() != 16 {
            return None;
        }
        u64::from_str_radix(stem, 16).ok()
    }
}

/// Manages multiple WAL segment files.
pub struct SegmentManager {
    /// Directory containing segment files.
    data_dir: PathBuf,
    /// Segments sorted by base_offset.
    segments: Vec<Segment>,
    /// Maximum size per segment before rotation.
    max_segment_size: u64,
}

impl SegmentManager {
    /// Open an existing segment directory, scanning for segment files.
    #[instrument(skip_all, fields(data_dir = %data_dir.as_ref().display()))]
    pub fn open(data_dir: impl AsRef<Path>, max_segment_size: u64) -> Result<Self> {
        let data_dir = data_dir.as_ref().to_path_buf();
        fs::create_dir_all(&data_dir).map_err(|e| QuarantineError::io(&data_dir, e))?;

        let mut segments = Vec::new();

        let entries = fs::read_dir(&data_dir).map_err(|e| QuarantineError::io(&data_dir, e))?;
        for entry in entries {
            let entry = entry.map_err(|e| QuarantineError::io(&data_dir, e))?;
            let name = entry.file_name();
            let name_str = name.to_string_lossy();

            if let Some(base_offset) = Segment::parse_filename(&name_str) {
                let meta = entry.metadata().map_err(|e| QuarantineError::io(entry.path(), e))?;
                segments.push(Segment { base_offset, path: entry.path(), size: meta.len() });
            }
        }

        segments.sort_by_key(|s| s.base_offset);

        debug!(segment_count = segments.len(), "SegmentManager opened");
        let mgr = Self { data_dir, segments, max_segment_size };

        // Initialize metrics
        mgr.update_metrics();

        Ok(mgr)
    }

    /// Rescan the data directory for new segment files.
    ///
    /// This is used by read-only journal instances that need to discover
    /// segments created by a separate writer instance.
    #[instrument(skip(self), fields(data_dir = %self.data_dir.display()))]
    pub fn refresh(&mut self) -> Result<()> {
        let mut segments = Vec::new();

        let entries =
            fs::read_dir(&self.data_dir).map_err(|e| QuarantineError::io(&self.data_dir, e))?;
        for entry in entries {
            let entry = entry.map_err(|e| QuarantineError::io(&self.data_dir, e))?;
            let name = entry.file_name();
            let name_str = name.to_string_lossy();

            if let Some(base_offset) = Segment::parse_filename(&name_str) {
                let meta = entry.metadata().map_err(|e| QuarantineError::io(entry.path(), e))?;
                segments.push(Segment { base_offset, path: entry.path(), size: meta.len() });
            }
        }

        segments.sort_by_key(|s| s.base_offset);
        debug!(segment_count = segments.len(), "SegmentManager refreshed");
        self.segments = segments;

        // Update metrics after refresh
        self.update_metrics();

        Ok(())
    }

    /// Get all segments, sorted by base_offset.
    pub fn segments(&self) -> &[Segment] {
        &self.segments
    }

    /// Find the segment containing the given global offset.
    ///
    /// Uses binary search: finds the last segment whose `base_offset <= offset`.
    pub fn resolve_segment(&self, offset: u64) -> Option<&Segment> {
        if self.segments.is_empty() {
            return None;
        }

        // Binary search for the largest base_offset <= offset
        let idx = match self.segments.binary_search_by_key(&offset, |s| s.base_offset) {
            Ok(exact) => exact,
            Err(insert) => {
                if insert == 0 {
                    return None; // offset is before all segments
                }
                insert - 1
            }
        };

        Some(&self.segments[idx])
    }

    /// Get the current (latest) segment, if any.
    pub fn current_segment(&self) -> Option<&Segment> {
        self.segments.last()
    }

    /// Check if the current segment needs rotation.
    pub fn needs_rotation(&self, current_segment_size: u64) -> bool {
        current_segment_size >= self.max_segment_size
    }

    /// Update the cached size of the current (latest) segment.
    ///
    /// Call this after appending data to keep the cached size in sync with
    /// the actual file size. This ensures that `read()` operations can use
    /// the cached size for bounds checking without a disk stat call.
    pub fn update_current_segment_size(&mut self, new_size: u64) {
        if let Some(segment) = self.segments.last_mut() {
            segment.size = new_size;
        }
    }

    /// Create a new segment with the given base offset.
    ///
    /// Writes a v2 FileHeader to the new file and adds it to the segment list.
    #[instrument(skip(self), fields(base_offset))]
    pub fn create_segment(&mut self, base_offset: u64) -> Result<&Segment> {
        let filename = Segment::filename(base_offset);
        let path = self.data_dir.join(&filename);

        // Write header
        let header = FileHeader::new();
        let mut buf = Vec::with_capacity(HEADER_SIZE);
        header.write_to(&mut buf)?;
        fs::write(&path, &buf).map_err(|e| QuarantineError::io(&path, e))?;

        let segment = Segment { base_offset, path, size: HEADER_SIZE as u64 };

        self.segments.push(segment);

        // Update metrics
        self.update_metrics();

        info!(base_offset, filename, "Created new segment");

        self.segments.last().ok_or_else(|| {
            QuarantineError::IoGeneric(std::io::Error::other("segment list unexpectedly empty"))
        })
    }

    /// Delete segments whose entire range is below `min_cursor`.
    ///
    /// A segment can be deleted if the *next* segment's base_offset <= min_cursor,
    /// meaning no reads will ever need the deleted segment.
    ///
    /// Returns the number of bytes freed.
    #[instrument(skip(self))]
    pub fn cleanup(&mut self, min_cursor: u64) -> Result<u64> {
        let mut freed = 0u64;
        let mut to_remove = Vec::new();

        for (i, _segment) in self.segments.iter().enumerate() {
            // Can only delete if there's a next segment and it starts at or below min_cursor
            if i + 1 < self.segments.len() && self.segments[i + 1].base_offset <= min_cursor {
                to_remove.push(i);
            }
        }

        // Remove in reverse order to preserve indices
        for &idx in to_remove.iter().rev() {
            let segment = &self.segments[idx];
            info!(
                base_offset = segment.base_offset,
                size = segment.size,
                path = %segment.path.display(),
                "Deleting old segment"
            );
            match fs::remove_file(&segment.path) {
                Ok(()) => {
                    freed += segment.size;
                    self.segments.remove(idx);
                }
                Err(e) => {
                    warn!(
                        error = %e,
                        path = %segment.path.display(),
                        "Failed to delete segment file, keeping in list"
                    );
                }
            }
        }

        if freed > 0 {
            info!(
                freed_bytes = freed,
                remaining_segments = self.segments.len(),
                "Cleanup complete"
            );

            // Update metrics after cleanup
            self.update_metrics();
        }

        Ok(freed)
    }

    /// Get the data directory path.
    pub fn data_dir(&self) -> &Path {
        &self.data_dir
    }

    /// Update metrics for disk usage and segment count.
    fn update_metrics(&self) {
        let total_disk_usage: u64 = self.segments.iter().map(|s| s.size).sum();
        metrics::gauge!("stemedb_wal_disk_usage_bytes").set(total_disk_usage as f64);
        metrics::gauge!("stemedb_wal_segments_count").set(self.segments.len() as f64);
    }
}

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

    #[test]
    fn test_segment_name_roundtrip() {
        let offsets = [0u64, 1, 255, 65536, 0xDEAD_BEEF, u64::MAX];
        for offset in offsets {
            let name = Segment::filename(offset);
            let parsed = Segment::parse_filename(&name);
            assert_eq!(parsed, Some(offset), "Roundtrip failed for offset {}", offset);
        }
    }

    #[test]
    fn test_parse_filename_rejects_invalid() {
        assert_eq!(Segment::parse_filename("not_a_wal.txt"), None);
        assert_eq!(Segment::parse_filename("short.wal"), None);
        assert_eq!(Segment::parse_filename("0000000000000000.log"), None);
        assert_eq!(Segment::parse_filename(""), None);
        // Too many hex digits
        assert_eq!(Segment::parse_filename("00000000000000000.wal"), None);
    }

    #[test]
    fn test_resolve_segment_binary_search() {
        let dir = tempdir().expect("tempdir");
        let mut mgr = SegmentManager::open(dir.path(), DEFAULT_MAX_SEGMENT_SIZE).expect("open");

        // Create segments at offsets 0, 1000, 2000
        mgr.create_segment(0).expect("seg 0");
        mgr.create_segment(1000).expect("seg 1000");
        mgr.create_segment(2000).expect("seg 2000");

        // Offset 0 -> segment 0
        assert_eq!(mgr.resolve_segment(0).map(|s| s.base_offset), Some(0));
        // Offset 500 -> segment 0
        assert_eq!(mgr.resolve_segment(500).map(|s| s.base_offset), Some(0));
        // Offset 999 -> segment 0
        assert_eq!(mgr.resolve_segment(999).map(|s| s.base_offset), Some(0));
        // Offset 1000 -> segment 1000
        assert_eq!(mgr.resolve_segment(1000).map(|s| s.base_offset), Some(1000));
        // Offset 1500 -> segment 1000
        assert_eq!(mgr.resolve_segment(1500).map(|s| s.base_offset), Some(1000));
        // Offset 2000 -> segment 2000
        assert_eq!(mgr.resolve_segment(2000).map(|s| s.base_offset), Some(2000));
        // Offset 99999 -> segment 2000
        assert_eq!(mgr.resolve_segment(99999).map(|s| s.base_offset), Some(2000));
    }

    #[test]
    fn test_resolve_segment_empty() {
        let dir = tempdir().expect("tempdir");
        let mgr = SegmentManager::open(dir.path(), DEFAULT_MAX_SEGMENT_SIZE).expect("open");
        assert!(mgr.resolve_segment(0).is_none());
    }

    #[test]
    fn test_rotation_creates_new_segment() {
        let dir = tempdir().expect("tempdir");
        // Small threshold for testing: 1KB
        let mut mgr = SegmentManager::open(dir.path(), 1024).expect("open");

        mgr.create_segment(0).expect("create seg 0");
        assert_eq!(mgr.segments().len(), 1);

        // Simulate that segment 0 grew beyond threshold
        assert!(mgr.needs_rotation(2048));
        assert!(!mgr.needs_rotation(512));

        mgr.create_segment(2048).expect("create seg 2048");
        assert_eq!(mgr.segments().len(), 2);
    }

    #[test]
    fn test_cleanup_deletes_old_segments() {
        let dir = tempdir().expect("tempdir");
        let mut mgr = SegmentManager::open(dir.path(), DEFAULT_MAX_SEGMENT_SIZE).expect("open");

        mgr.create_segment(0).expect("seg 0");
        mgr.create_segment(1000).expect("seg 1000");
        mgr.create_segment(2000).expect("seg 2000");
        assert_eq!(mgr.segments().len(), 3);

        // Cleanup with min_cursor=1500: can delete seg 0 (next seg starts at 1000 <= 1500)
        let freed = mgr.cleanup(1500).expect("cleanup");
        assert!(freed > 0);
        assert_eq!(mgr.segments().len(), 2);
        assert_eq!(mgr.segments()[0].base_offset, 1000);

        // Cleanup with min_cursor=2500: can delete seg 1000 (next starts at 2000 <= 2500)
        let freed = mgr.cleanup(2500).expect("cleanup");
        assert!(freed > 0);
        assert_eq!(mgr.segments().len(), 1);
        assert_eq!(mgr.segments()[0].base_offset, 2000);

        // Last segment is never deleted
        let freed = mgr.cleanup(u64::MAX).expect("cleanup");
        assert_eq!(freed, 0);
        assert_eq!(mgr.segments().len(), 1);
    }

    #[test]
    fn test_segment_manager_scans_existing_files() {
        let dir = tempdir().expect("tempdir");

        // Create segments manually, then reopen
        {
            let mut mgr = SegmentManager::open(dir.path(), DEFAULT_MAX_SEGMENT_SIZE).expect("open");
            mgr.create_segment(0).expect("seg 0");
            mgr.create_segment(5000).expect("seg 5000");
            mgr.create_segment(10000).expect("seg 10000");
        }

        // Reopen and verify scan
        let mgr = SegmentManager::open(dir.path(), DEFAULT_MAX_SEGMENT_SIZE).expect("reopen");
        assert_eq!(mgr.segments().len(), 3);
        assert_eq!(mgr.segments()[0].base_offset, 0);
        assert_eq!(mgr.segments()[1].base_offset, 5000);
        assert_eq!(mgr.segments()[2].base_offset, 10000);
    }

    #[test]
    fn test_segment_file_has_valid_header() {
        let dir = tempdir().expect("tempdir");
        let mut mgr = SegmentManager::open(dir.path(), DEFAULT_MAX_SEGMENT_SIZE).expect("open");
        mgr.create_segment(0).expect("seg 0");

        // Read the file and verify header
        let data = std::fs::read(&mgr.segments()[0].path).expect("read");
        assert_eq!(data.len(), HEADER_SIZE);
        assert_eq!(&data[0..4], b"STEM");
        assert_eq!(data[4], 2); // version
    }
}