package wal

import (
	"errors"
	"fmt"
	"os"

	"github.com/dailz/go-kv/manifest"
)

// RecoveryResult holds the outcome of a WAL recovery pass.
type RecoveryResult struct {
	NextSequence    uint64
	NextSegmentID   uint64
	ReplayedEntries int
	Truncated       bool
	// TruncateError is informational only: non-nil means "tail corruption
	// was found and repair was attempted". It does NOT report persistence
	// failures — those cause Recover to return an error instead.
	TruncateError error
}

// Recover performs a full WAL recovery: reads the recovery checkpoint from
// MANIFEST, scans segments, replays entries, and persists tail truncation
// per design §3.2 line 787-800.
func Recover(dir string, replayer BatchReplayer) (*RecoveryResult, error) {
	if replayer == nil {
		return nil, fmt.Errorf("wal: recover: replayer is nil")
	}

	// Step 1: Determine recovery segment ID from MANIFEST.
	recoverySegmentID, err := resolveRecoverySegmentID(dir)
	if err != nil {
		return nil, fmt.Errorf("wal: recover: resolve segment id: %w", err)
	}

	// Step 2: Scan and replay segments.
	nextSequence, err := RecoverFromSegments(dir, recoverySegmentID, replayer)
	if err != nil {
		if !IsTailCorruption(err) {
			return nil, fmt.Errorf("wal: recover: %w", err)
		}

		// Step 3: Tail corruption — truncate the last segment per design
		// §3.2 line 787-800.
		result := &RecoveryResult{
			NextSequence:  nextSequence,
			Truncated:     true,
			TruncateError: err, // informational: tail corruption was detected
		}

		segments, scanErr := ScanSegments(dir, recoverySegmentID)
		if scanErr != nil {
			return nil, fmt.Errorf("wal: recover: scan after tail corruption: %w", scanErr)
		}
		if len(segments) > 0 {
			result.NextSegmentID = segments[len(segments)-1].SegmentID + 1
		} else {
			result.NextSegmentID = recoverySegmentID
		}

		if len(segments) > 0 {
			// After C4 fix, TailCorruptionError is only returned for the last
			// segment. Use tce.SegmentPath as authoritative truncation target
			// (defensive: fall back to segments[last] if missing).
			corruptedPath := segments[len(segments)-1].FilePath
			var tce *TailCorruptionError
			if errors.As(err, &tce) && tce.SegmentPath != "" {
				corruptedPath = tce.SegmentPath
			}

			lastCompleteBatchEnd, findErr := findLastCompleteBatchEnd(corruptedPath)
			if findErr != nil {
				return nil, fmt.Errorf("wal: recover: find truncation offset: %w", findErr)
			}

			// Phase 1: truncated segment is always segments[last], no
			// trailing empty segments to clean up. C4 fix means non-last
			// segment corruption hard-fails above, so we never reach here
			// with a non-last corrupted segment.
			var emptyTrailing []string

			if err := truncateAndPersist(corruptedPath, lastCompleteBatchEnd, dir, emptyTrailing); err != nil {
				// Per design §3.2 line 799: DB must NOT enter writable state.
				return nil, fmt.Errorf("wal: recover: persist tail truncation: %w", err)
			}
		}

		result.ReplayedEntries = 0 // Phase 1: replayer interface doesn't expose count

		// Per design §3.2 line 280, recovery repair must NOT update MANIFEST.
		// The truncated WAL state is persisted via ftruncate + fsync segment
		// + fsync dir (see truncateAndPersist). MANIFEST can only advance via
		// checkpoint (MemTable flush) in future phases.

		return result, nil
	}

	// Step 4: Successful recovery — compute result.
	segments, scanErr := ScanSegments(dir, recoverySegmentID)
	if scanErr != nil {
		return nil, fmt.Errorf("wal: recover: scan after replay: %w", scanErr)
	}

	result := &RecoveryResult{
		NextSequence:  nextSequence,
		NextSegmentID: recoverySegmentID,
		Truncated:     false,
	}
	if len(segments) > 0 {
		result.NextSegmentID = segments[len(segments)-1].SegmentID + 1
	}

	// Per design §3.2 line 280, recovery must NOT update MANIFEST.
	// RecoveryResult.NextSegmentID is in-memory only, consumed by DB.Open to
	// seed the new WalWriter. MANIFEST stays at its pre-recovery value.

	return result, nil
}

// resolveRecoverySegmentID returns the recovery start segment ID from MANIFEST.
// MANIFEST is the only authoritative source of recovery start per design §3.2
// line 600-06. CURRENT is a write-side hint and must NOT be used here.
func resolveRecoverySegmentID(dir string) (uint64, error) {
	mf, err := manifest.Load(dir)
	if err != nil {
		return 0, fmt.Errorf("load manifest: %w", err)
	}
	return mf.RecoverySegmentID, nil
}

// segmentFsyncFn is the package-level indirection for fsyncing a truncated
// segment file. Tests that override this must not use t.Parallel().
// Same pattern as dirFsyncFn (see wal/dir_fsync.go).
var segmentFsyncFn = segmentFsync

func segmentFsync(filePath string) error {
	f, err := os.OpenFile(filePath, os.O_WRONLY, 0o644)
	if err != nil {
		return fmt.Errorf("open for fsync: %w", err)
	}
	defer f.Close()
	if err := f.Sync(); err != nil {
		return fmt.Errorf("fsync: %w", err)
	}
	return nil
}

// truncateAndPersist executes the 4-step tail-truncation protocol per
// design §3.2 line 787-794. Any step failure is fatal: per line 799,
// DB must NOT enter writable state if truncation cannot be persisted.
func truncateAndPersist(
	filePath string,
	lastCompleteBatchEnd int64,
	dir string,
	emptyTrailingSegments []string,
) error {
	// Step 1: ftruncate
	if lastCompleteBatchEnd < 0 {
		return fmt.Errorf("wal: invalid truncate offset %d", lastCompleteBatchEnd)
	}
	if err := os.Truncate(filePath, lastCompleteBatchEnd); err != nil {
		return fmt.Errorf("ftruncate %s to %d: %w", filePath, lastCompleteBatchEnd, err)
	}

	// Step 2: fsync the truncated segment
	if err := segmentFsyncFn(filePath); err != nil {
		return fmt.Errorf("fsync truncated segment: %w", err)
	}

	// Step 3: delete empty trailing segments
	for _, segPath := range emptyTrailingSegments {
		if err := os.Remove(segPath); err != nil {
			return fmt.Errorf("remove empty segment %s: %w", segPath, err)
		}
	}

	// Step 4: fsync WAL directory (reuses C6's dirFsyncFn)
	if err := dirFsyncFn(dir); err != nil {
		return fmt.Errorf("fsync WAL dir after truncation: %w", err)
	}
	return nil
}

// findLastCompleteBatchEnd walks the segment file, runs physical records
// through the FragmentCollector state machine, and returns the byte offset
// of the END of the last complete WAL Batch.
//
// This is the correct truncation target per design §3.2 line 786. A previous
// version (findValidOffset) only checked physical record CRCs, missing the
// case where a First + Middle* fragment chain has no Last (H8 bug): physical
// CRCs pass but no complete batch exists at that offset.
//
// Block-boundary handling: WAL format allows a full block to end with zero
// padding when the next record doesn't fit (see BlockWriter.paddingNeeded).
// This function CONTINUES to the next block on padding in a full block, and
// only RETURNS on padding in a short (final) block or actual corruption.
func findLastCompleteBatchEnd(filePath string) (int64, error) {
	f, err := os.Open(filePath)
	if err != nil {
		return 0, fmt.Errorf("open %s: %w", filePath, err)
	}
	defer f.Close()

	if _, err := f.Seek(WalFileHeaderSize, 0); err != nil {
		return 0, fmt.Errorf("seek past header: %w", err)
	}

	collector := NewFragmentCollector()
	lastCompleteEnd := int64(WalFileHeaderSize)
	blockStartOffset := int64(WalFileHeaderSize)
	buf := make([]byte, WalBlockSize)

	for {
		n, readErr := f.Read(buf)
		if n > 0 {
			blockData := buf[:n]
			isFullBlock := n == WalBlockSize && readErr == nil
			pos := 0
			for pos < len(blockData) {
				remaining := len(blockData) - pos

				if remaining < PhysicalRecordHeaderSize {
					if isFullBlock {
						break // padding in full block, continue to next block
					}
					return lastCompleteEnd, nil // tail padding in short block
				}

				if isAllZeros(blockData[pos : pos+PhysicalRecordHeaderSize]) {
					if isFullBlock {
						break // zero-led padding in full block, continue
					}
					return lastCompleteEnd, nil // tail padding
				}

				rec, consumed, err := DecodePhysicalRecord(blockData[pos:])
				if err != nil {
					return lastCompleteEnd, nil // physical corruption
				}
				if err := collector.Append(rec.Type, rec.Payload); err != nil {
					return lastCompleteEnd, nil // fragment state machine rejected
				}

				pos += consumed
				recordEndAbsolute := blockStartOffset + int64(pos)

				if collector.IsComplete() {
					lastCompleteEnd = recordEndAbsolute
					collector.Reset()
				}
			}
			blockStartOffset += int64(n)
		}
		if readErr != nil || n < WalBlockSize {
			break
		}
	}

	return lastCompleteEnd, nil
}