// SPDX-License-Identifier: GPL-3.0-or-later

package fim

import (
	"context"
	"encoding/json"
	"os"
	"path/filepath"
	"sync"
	"time"

	"codeberg.org/anassaeneroi/enodia-sentinal/go-agent/internal/config"
	"codeberg.org/anassaeneroi/enodia-sentinal/go-agent/internal/model"
)

const (
	baselineFilename = "fim-baseline.json"
	pacmanTimeout    = 10 * time.Minute
)

// Manager owns the deliberately separate, slow integrity work. Its scans run
// in at most one goroutine per engine and only hand completed alerts back to
// the polling loop. That keeps filesystem hashing and pacman from holding a
// detector, snapshot, or event-output lock.
type Manager struct {
	Config config.Config
	Now    func() time.Time
	Scan   func([]string) map[string]Entry
	Verify func(context.Context) (string, error)

	mu            sync.Mutex
	baseline      map[string]Entry
	initialized   bool
	lastScan      time.Time
	lastVerify    time.Time
	scanRunning   bool
	verifyRunning bool
	pendingAlerts []model.Alert
}

// New builds a manager with real host collection. Tests replace Scan or Verify
// so they never read the host or execute a package manager.
func New(cfg config.Config) *Manager {
	return &Manager{
		Config: cfg,
		Now:    time.Now,
		Scan:   ScanPaths,
		Verify: PacmanVerify,
	}
}

// Initialize loads a valid non-empty baseline or builds it once. Like Python,
// baseline creation is a local operator-state action: regular scans never
// silently accept a changed file as the new truth. The caller supplies LogDir
// only in explicit sidecar mode, so this baseline can never share or rewrite
// the Python service's production state.
func (m *Manager) Initialize() error {
	if !m.Config.FIMEnabled {
		return nil
	}
	m.mu.Lock()
	if m.initialized {
		m.mu.Unlock()
		return nil
	}
	m.mu.Unlock()

	if err := os.MkdirAll(m.Config.LogDir, 0o750); err != nil {
		return err
	}
	baseline := loadBaseline(filepath.Join(m.Config.LogDir, baselineFilename))
	if len(baseline) == 0 {
		// A missing, unreadable, malformed, or empty baseline has no trustworthy
		// historical truth. Rebuilding it is safer than treating every file as a
		// change, and it happens only during initialization—not a later scan.
		baseline = m.Scan(PathList(m.Config.FIMPaths))
		if err := writeBaseline(filepath.Join(m.Config.LogDir, baselineFilename), baseline); err != nil {
			return err
		}
	}
	m.mu.Lock()
	defer m.mu.Unlock()
	if !m.initialized {
		m.baseline = baseline
		m.initialized = true
	}
	return nil
}

// MaybeStart schedules due work and returns immediately. A short interval does
// not start overlapping scans, which bounds both disk pressure and pacman
// process count even if a scan takes longer than the configured cadence.
func (m *Manager) MaybeStart(now time.Time) {
	if !m.Config.FIMEnabled {
		return
	}
	m.mu.Lock()
	if !m.initialized {
		m.mu.Unlock()
		return
	}
	if !m.scanRunning && (m.lastScan.IsZero() || now.Sub(m.lastScan) >= m.Config.FIMScanInterval) {
		m.scanRunning = true
		m.lastScan = now
		// Copy state while protected, then release the mutex before filesystem
		// I/O. Holding the lock through hashing would block Drain and could make
		// regular detector sweeps wait behind a large tree.
		baseline := cloneEntries(m.baseline)
		paths := append([]string(nil), PathList(m.Config.FIMPaths)...)
		go m.runScan(baseline, paths)
	}
	if m.Config.FIMPackageVerify && !m.verifyRunning &&
		(m.lastVerify.IsZero() || now.Sub(m.lastVerify) >= m.Config.FIMPackageVerifyInterval) {
		m.verifyRunning = true
		m.lastVerify = now
		go m.runVerify()
	}
	m.mu.Unlock()
}

func (m *Manager) runScan(baseline map[string]Entry, paths []string) {
	// The baseline intentionally remains immutable after initialization. A
	// completed scan reports drift; it never turns observed drift into truth.
	alerts := DiffAlerts(Diff(baseline, m.Scan(paths)))
	m.mu.Lock()
	m.pendingAlerts = append(m.pendingAlerts, alerts...)
	m.scanRunning = false
	m.mu.Unlock()
}

func (m *Manager) runVerify() {
	// pacman can block on its database or a slow disk. Its own deadline keeps
	// the optional verification feature from accumulating background workers.
	ctx, cancel := context.WithTimeout(context.Background(), pacmanTimeout)
	defer cancel()
	alerts := VerifyAlerts(func() (string, error) { return m.Verify(ctx) })
	m.mu.Lock()
	m.pendingAlerts = append(m.pendingAlerts, alerts...)
	m.verifyRunning = false
	m.mu.Unlock()
}

// Drain returns completed background results once. The caller must route them
// through the shared Agent cooldown and snapshot paths before exposing them.
func (m *Manager) Drain() []model.Alert {
	m.mu.Lock()
	defer m.mu.Unlock()
	alerts := append([]model.Alert(nil), m.pendingAlerts...)
	m.pendingAlerts = nil
	return alerts
}

func loadBaseline(path string) map[string]Entry {
	entries := make(map[string]Entry)
	raw, err := os.ReadFile(path)
	if err != nil || json.Unmarshal(raw, &entries) != nil {
		return map[string]Entry{}
	}
	return entries
}

func writeBaseline(path string, entries map[string]Entry) error {
	raw, err := json.Marshal(entries)
	if err != nil {
		return err
	}
	raw = append(raw, '\n')
	// Write and rename in the same directory so a crash cannot leave a partly
	// written JSON file at the baseline path. The restrictive mode keeps hashes
	// and file metadata from becoming world-readable state.
	temporary := path + ".tmp"
	if err := os.WriteFile(temporary, raw, 0o640); err != nil {
		return err
	}
	if err := os.Rename(temporary, path); err != nil {
		return err
	}
	return os.Chmod(path, 0o640)
}

func cloneEntries(source map[string]Entry) map[string]Entry {
	result := make(map[string]Entry, len(source))
	for path, entry := range source {
		result[path] = entry
	}
	return result
}