package detect

import (
	"log"
	"sync"
	"time"

	"codeberg.org/pata1704/guenther/pkg/types"
)

// ScalingLevel represents the current detector complexity level.
type ScalingLevel int

const (
	LevelNormal   ScalingLevel = iota // SEAD Ensemble (full accuracy)
	LevelHigh                         // COPOD (reduced complexity)
	LevelCritical                     // MAD (minimal overhead)
)

// levelName maps ScalingLevel to a human-readable string for logging.
var levelName = map[ScalingLevel]string{
	LevelNormal:   "SEAD Ensemble (Normal)",
	LevelHigh:     "COPOD (High Load)",
	LevelCritical: "MAD (Critical Load)",
}

// ── SwitchableDetector ───────────────────────────────────────────────────────

// SwitchableDetector wraps a SEADDetector and allows runtime switching to
// lighter-weight sub-detectors (COPOD, MAD) under high CPU load.
//
// State consistency guarantee: all base detectors are kept up-to-date
// regardless of which one is currently active. This ensures a clean
// transition back to SEAD without stale internal state.
//
// Update-deduplication contract:
//
//	SEAD.Score()  calls d.Score() on every base detector, which self-updates.
//	              → no separate Update() call needed; doing so would double-count.
//	SEAD.Update() calls d.Update() on every base detector directly.
//	              → used here when we need to advance inactive detectors
//	                without scoring through SEAD.
//
// For LevelHigh / LevelCritical we call:
//
//	s.ensemble.Update(vector)  → advances MAD, RRCF variants via d.Update()
//	                             COPOD.Update() = COPOD.update() (buffer append only)
//	active.Score(vector)       → scores + self-updates the active detector
//	                             (COPOD.Score calls update internally again)
//
// This means COPOD receives one Update() + one self-update from Score() per tick.
// That is intentional: Update() appends to the sliding window buffer; Score()
// computes the copula and then appends the scored point (score-then-insert).
// The two operations are not idempotent and must both run for correct behaviour.
// RRCF and MAD are updated via SEAD.Update() only; their Score() methods are
// not called when inactive so they do not double-count.
type SwitchableDetector struct {
	mu sync.RWMutex

	ensemble *SEADDetector
	copod    AnomalyDetector // may be nil if COPOD is not configured
	mad      AnomalyDetector // may be nil if MAD is not configured

	activeLevel ScalingLevel
}

// NewSwitchableDetector creates a SwitchableDetector backed by the given
// SEADDetector. COPOD and MAD sub-detectors are extracted from the ensemble
// for direct access during high-load switching.
//
// If a sub-detector is not present in the ensemble, the corresponding field
// is nil and Score() falls back to the ensemble for that level.
func NewSwitchableDetector(ensemble *SEADDetector) *SwitchableDetector {
	return &SwitchableDetector{
		ensemble:    ensemble,
		copod:       ensemble.GetDetector("COPOD"),
		mad:         ensemble.GetDetector("MAD"),
		activeLevel: LevelNormal,
	}
}

// Fit trains all underlying detectors on the given baseline vectors.
func (s *SwitchableDetector) Fit(vectors []types.FeatureVector) error {
	return s.ensemble.Fit(vectors)
}

// Update advances the internal state of all base detectors without scoring.
// Safe for concurrent use.
func (s *SwitchableDetector) Update(vector types.FeatureVector) error {
	return s.ensemble.Update(vector)
}

// Score returns an AnomalyResult from the currently active detector.
//
// All inactive detectors are kept current via SEAD.Update() so that
// switching back to a heavier detector does not produce stale scores.
// Safe for concurrent use.
func (s *SwitchableDetector) Score(vector types.FeatureVector) (types.AnomalyResult, error) {
	s.mu.RLock()
	level := s.activeLevel
	s.mu.RUnlock()

	// LevelNormal: SEAD.Score() handles everything internally.
	// It scores all base detectors (which self-update) and applies
	// MWU weight adaptation. No separate Update() needed.
	if level == LevelNormal {
		return s.ensemble.Score(vector)
	}

	// LevelHigh / LevelCritical:
	// 1. Advance all base detectors via SEAD.Update() so inactive detectors
	//    (MAD, RRCF variants for LevelHigh; RRCF, COPOD for LevelCritical)
	//    maintain current state. SEAD weight adaptation is NOT performed here
	//    because we are bypassing SEAD.Score().
	if err := s.ensemble.Update(vector); err != nil {
		// Non-fatal: log and continue. A single missed update is acceptable;
		// the detector will resync on the next tick.
		log.Printf("scaling: ensemble update error at level %s: %v", levelName[level], err)
	}

	// 2. Score via the active sub-detector.
	//    COPOD.Score() additionally self-updates (score-then-insert), which is
	//    correct and complementary to the Update() call above (see type doc).
	//    MAD.Update() internally calls Score(), so it is already current after
	//    the SEAD.Update() call; MAD.Score() here is pure scoring only.
	switch level {
	case LevelHigh:
		if s.copod == nil {
			log.Printf("scaling: COPOD unavailable at LevelHigh, falling back to ensemble")
			return s.ensemble.Score(vector)
		}
		res, err := s.copod.Score(vector)
		if err != nil {
			return res, err
		}
		res.Method = "COPOD (High Load)"
		return res, nil

	case LevelCritical:
		if s.mad == nil {
			log.Printf("scaling: MAD unavailable at LevelCritical, falling back to ensemble")
			return s.ensemble.Score(vector)
		}
		res, err := s.mad.Score(vector)
		if err != nil {
			return res, err
		}
		res.Method = "MAD (Critical Load)"
		return res, nil

	default:
		return s.ensemble.Score(vector)
	}
}

// Switch atomically changes the active detection level.
// It is a no-op if the requested level equals the current level.
// Safe for concurrent use.
func (s *SwitchableDetector) Switch(level ScalingLevel) {
	s.mu.Lock()
	defer s.mu.Unlock()

	if s.activeLevel == level {
		return
	}
	log.Printf("[SCALING] %s → %s", levelName[s.activeLevel], levelName[level])
	s.activeLevel = level
}

// ── ScalingController ────────────────────────────────────────────────────────

// ScalingController monitors CPU load and drives a SwitchableDetector through
// its scaling levels (Normal → High → Critical and back).
//
// Level transitions follow a two-phase commit pattern:
//
//  1. A CPU measurement moves the desired level to a "pending" state.
//  2. Only after the pending level has been stable for the configured
//     duration is Switch() called on the detector.
//
// This prevents rapid oscillation under bursty workloads.
//
// Hysteresis rules (in the dead-band between downThres and highThres):
//
//	Critical → High  (one step down, not straight to Normal)
//	High     → High  (stays until CPU drops below downThres)
//	Normal   → Normal
//
// ScalingController is not safe for concurrent use. ObserveCPU must be
// called from a single goroutine (the DetectionLayer's processing loop).
type ScalingController struct {
	detector *SwitchableDetector

	// Thresholds (CPU percent, 0–100)
	highThres float64
	critThres float64
	downThres float64

	// Required stable duration before a level transition is committed.
	highDur time.Duration
	critDur time.Duration
	downDur time.Duration

	// currentLevel is the level that has been committed to the detector.
	currentLevel ScalingLevel

	// pendingLevel is the desired level based on recent CPU measurements.
	// It must remain stable for the corresponding duration before becoming current.
	pendingLevel ScalingLevel

	// pendingStart is the time at which pendingLevel last changed.
	// The pending level is committed when time.Since(pendingStart) >= required duration.
	pendingStart time.Time
}

// NewScalingController constructs a ScalingController.
// Duration arguments are in seconds (float64 to match YAML config values).
func NewScalingController(
	detector *SwitchableDetector,
	highThres, critThres, downThres float64,
	highDurSec, critDurSec, downDurSec float64,
) *ScalingController {
	return &ScalingController{
		detector:     detector,
		highThres:    highThres,
		critThres:    critThres,
		downThres:    downThres,
		highDur:      time.Duration(highDurSec * float64(time.Second)),
		critDur:      time.Duration(critDurSec * float64(time.Second)),
		downDur:      time.Duration(downDurSec * float64(time.Second)),
		currentLevel: LevelNormal,
		pendingLevel: LevelNormal,
		pendingStart: time.Now(), // explicit init avoids zero-time edge case
	}
}

// ObserveCPU processes a single CPU measurement and, if warranted, triggers
// a level switch on the underlying SwitchableDetector.
//
// Must be called from a single goroutine only (not safe for concurrent use).
func (c *ScalingController) ObserveCPU(cpuPercent float64) {
	now := time.Now()

	desired := c.desiredLevel(cpuPercent)

	// Phase 1: desired level changed → restart the stability timer.
	if desired != c.pendingLevel {
		c.pendingLevel = desired
		c.pendingStart = now
		return
	}

	// Phase 2: desired level has been stable – check if duration is met.
	if now.Sub(c.pendingStart) < c.durationFor(desired) {
		return
	}

	if desired != c.currentLevel {
		c.currentLevel = desired
		c.detector.Switch(desired)
	}
	c.pendingStart = now
}

// desiredLevel computes the target ScalingLevel for a given CPU measurement,
// applying hysteresis in the dead-band between downThres and highThres.
func (c *ScalingController) desiredLevel(cpuPercent float64) ScalingLevel {
	switch {
	case cpuPercent > c.critThres:
		return LevelCritical
	case cpuPercent > c.highThres:
		return LevelHigh
	case cpuPercent < c.downThres:
		return LevelNormal
	default:
		// Dead-band: degrade at most one step to avoid jumping straight
		// from Critical to Normal on a brief CPU dip.
		switch c.currentLevel {
		case LevelCritical:
			return LevelHigh
		case LevelHigh:
			return LevelHigh
		default:
			return LevelNormal
		}
	}
}

// durationFor returns the required stable duration for a given target level.
func (c *ScalingController) durationFor(level ScalingLevel) time.Duration {
	switch level {
	case LevelCritical:
		return c.critDur
	case LevelHigh:
		return c.highDur
	default:
		return c.downDur
	}
}