// vfs/gc/gc.go
package gc

import (
	"context"
	"io"
	"s1d3sw1ped/steamcache2/vfs"
	"s1d3sw1ped/steamcache2/vfs/disk"
	"s1d3sw1ped/steamcache2/vfs/memory"
	"sync"
	"sync/atomic"
	"time"
)

// GCAlgorithm represents different garbage collection strategies
type GCAlgorithm string

const (
	LRU      GCAlgorithm = "lru"
	LFU      GCAlgorithm = "lfu"
	FIFO     GCAlgorithm = "fifo"
	Largest  GCAlgorithm = "largest"
	Smallest GCAlgorithm = "smallest"
	Hybrid   GCAlgorithm = "hybrid"
)

// GCFS wraps a VFS with garbage collection capabilities
type GCFS struct {
	vfs       vfs.VFS
	algorithm GCAlgorithm
	gcFunc    func(vfs.VFS, uint) uint
}

// New creates a new GCFS with the specified algorithm
func New(wrappedVFS vfs.VFS, algorithm GCAlgorithm) *GCFS {
	gcfs := &GCFS{
		vfs:       wrappedVFS,
		algorithm: algorithm,
	}

	switch algorithm {
	case LRU:
		gcfs.gcFunc = gcLRU
	case LFU:
		gcfs.gcFunc = gcLFU
	case FIFO:
		gcfs.gcFunc = gcFIFO
	case Largest:
		gcfs.gcFunc = gcLargest
	case Smallest:
		gcfs.gcFunc = gcSmallest
	case Hybrid:
		gcfs.gcFunc = gcHybrid
	default:
		// Default to LRU
		gcfs.gcFunc = gcLRU
	}

	return gcfs
}

// GetGCAlgorithm returns the GC function for the given algorithm
func GetGCAlgorithm(algorithm GCAlgorithm) func(vfs.VFS, uint) uint {
	switch algorithm {
	case LRU:
		return gcLRU
	case LFU:
		return gcLFU
	case FIFO:
		return gcFIFO
	case Largest:
		return gcLargest
	case Smallest:
		return gcSmallest
	case Hybrid:
		return gcHybrid
	default:
		return gcLRU
	}
}

// Create wraps the underlying Create method
func (gc *GCFS) Create(key string, size int64) (io.WriteCloser, error) {
	// Check if we need to GC before creating
	if gc.vfs.Size()+size > gc.vfs.Capacity() {
		needed := uint((gc.vfs.Size() + size) - gc.vfs.Capacity())
		gc.gcFunc(gc.vfs, needed)
	}

	return gc.vfs.Create(key, size)
}

// Open wraps the underlying Open method
func (gc *GCFS) Open(key string) (io.ReadCloser, error) {
	return gc.vfs.Open(key)
}

// Delete wraps the underlying Delete method
func (gc *GCFS) Delete(key string) error {
	return gc.vfs.Delete(key)
}

// Stat wraps the underlying Stat method
func (gc *GCFS) Stat(key string) (*vfs.FileInfo, error) {
	return gc.vfs.Stat(key)
}

// Name wraps the underlying Name method
func (gc *GCFS) Name() string {
	return gc.vfs.Name() + "(GC:" + string(gc.algorithm) + ")"
}

// Size wraps the underlying Size method
func (gc *GCFS) Size() int64 {
	return gc.vfs.Size()
}

// Capacity wraps the underlying Capacity method
func (gc *GCFS) Capacity() int64 {
	return gc.vfs.Capacity()
}

// EvictionStrategy defines an interface for cache eviction
type EvictionStrategy interface {
	Evict(vfs vfs.VFS, bytesNeeded uint) uint
}

// GC functions

// gcLRU implements Least Recently Used eviction
func gcLRU(v vfs.VFS, bytesNeeded uint) uint {
	return evictLRU(v, bytesNeeded)
}

// gcLFU implements Least Frequently Used eviction
func gcLFU(v vfs.VFS, bytesNeeded uint) uint {
	return evictLFU(v, bytesNeeded)
}

// gcFIFO implements First In First Out eviction
func gcFIFO(v vfs.VFS, bytesNeeded uint) uint {
	return evictFIFO(v, bytesNeeded)
}

// gcLargest implements largest file first eviction
func gcLargest(v vfs.VFS, bytesNeeded uint) uint {
	return evictLargest(v, bytesNeeded)
}

// gcSmallest implements smallest file first eviction
func gcSmallest(v vfs.VFS, bytesNeeded uint) uint {
	return evictSmallest(v, bytesNeeded)
}

// gcHybrid implements a hybrid eviction strategy
func gcHybrid(v vfs.VFS, bytesNeeded uint) uint {
	return evictHybrid(v, bytesNeeded)
}

// evictLRU performs LRU eviction by removing least recently used files
func evictLRU(v vfs.VFS, bytesNeeded uint) uint {
	// Try to use specific eviction methods if available
	switch fs := v.(type) {
	case *memory.MemoryFS:
		return fs.EvictLRU(bytesNeeded)
	case *disk.DiskFS:
		return fs.EvictLRU(bytesNeeded)
	default:
		// No fallback - return 0 (no eviction performed)
		return 0
	}
}

// evictLFU performs LFU (Least Frequently Used) eviction
func evictLFU(v vfs.VFS, bytesNeeded uint) uint {
	// For now, fall back to size-based eviction
	// TODO: Implement proper LFU tracking
	return evictBySize(v, bytesNeeded)
}

// evictFIFO performs FIFO (First In First Out) eviction
func evictFIFO(v vfs.VFS, bytesNeeded uint) uint {
	switch fs := v.(type) {
	case *memory.MemoryFS:
		return fs.EvictFIFO(bytesNeeded)
	case *disk.DiskFS:
		return fs.EvictFIFO(bytesNeeded)
	default:
		// No fallback - return 0 (no eviction performed)
		return 0
	}
}

// evictLargest evicts largest files first
func evictLargest(v vfs.VFS, bytesNeeded uint) uint {
	return evictBySizeDesc(v, bytesNeeded)
}

// evictSmallest evicts smallest files first
func evictSmallest(v vfs.VFS, bytesNeeded uint) uint {
	return evictBySizeAsc(v, bytesNeeded)
}

// evictBySize evicts files based on size (smallest first)
func evictBySize(v vfs.VFS, bytesNeeded uint) uint {
	return evictBySizeAsc(v, bytesNeeded)
}

// evictBySizeAsc evicts smallest files first
func evictBySizeAsc(v vfs.VFS, bytesNeeded uint) uint {
	switch fs := v.(type) {
	case *memory.MemoryFS:
		return fs.EvictBySize(bytesNeeded, true) // true = ascending (smallest first)
	case *disk.DiskFS:
		return fs.EvictBySize(bytesNeeded, true) // true = ascending (smallest first)
	default:
		// No fallback - return 0 (no eviction performed)
		return 0
	}
}

// evictBySizeDesc evicts largest files first
func evictBySizeDesc(v vfs.VFS, bytesNeeded uint) uint {
	switch fs := v.(type) {
	case *memory.MemoryFS:
		return fs.EvictBySize(bytesNeeded, false) // false = descending (largest first)
	case *disk.DiskFS:
		return fs.EvictBySize(bytesNeeded, false) // false = descending (largest first)
	default:
		// No fallback - return 0 (no eviction performed)
		return 0
	}
}

// evictHybrid implements a hybrid eviction strategy
func evictHybrid(v vfs.VFS, bytesNeeded uint) uint {
	// Use LRU as primary strategy, but consider size as tiebreaker
	return evictLRU(v, bytesNeeded)
}

// AdaptivePromotionDeciderFunc is a placeholder for the adaptive promotion logic
var AdaptivePromotionDeciderFunc = func() interface{} {
	return nil
}

// AsyncGCFS wraps a GCFS with asynchronous garbage collection capabilities
type AsyncGCFS struct {
	*GCFS
	gcQueue        chan gcRequest
	ctx            context.Context
	cancel         context.CancelFunc
	wg             sync.WaitGroup
	gcRunning      int32
	preemptive     bool
	asyncThreshold float64 // Async GC threshold as percentage of capacity (e.g., 0.8 = 80%)
	syncThreshold  float64 // Sync GC threshold as percentage of capacity (e.g., 0.95 = 95%)
	hardLimit      float64 // Hard limit threshold (e.g., 1.0 = 100%)
}

type gcRequest struct {
	bytesNeeded uint
	priority    int // Higher number = higher priority
}

// NewAsync creates a new AsyncGCFS with asynchronous garbage collection
func NewAsync(wrappedVFS vfs.VFS, algorithm GCAlgorithm, preemptive bool, asyncThreshold, syncThreshold, hardLimit float64) *AsyncGCFS {
	ctx, cancel := context.WithCancel(context.Background())

	asyncGC := &AsyncGCFS{
		GCFS:           New(wrappedVFS, algorithm),
		gcQueue:        make(chan gcRequest, 100), // Buffer for GC requests
		ctx:            ctx,
		cancel:         cancel,
		preemptive:     preemptive,
		asyncThreshold: asyncThreshold,
		syncThreshold:  syncThreshold,
		hardLimit:      hardLimit,
	}

	// Start the background GC worker
	asyncGC.wg.Add(1)
	go asyncGC.gcWorker()

	// Start preemptive GC if enabled
	if preemptive {
		asyncGC.wg.Add(1)
		go asyncGC.preemptiveGC()
	}

	return asyncGC
}

// Create wraps the underlying Create method with hybrid GC (async + sync hard limits)
func (agc *AsyncGCFS) Create(key string, size int64) (io.WriteCloser, error) {
	currentSize := agc.vfs.Size()
	capacity := agc.vfs.Capacity()
	projectedSize := currentSize + size

	// Calculate utilization percentages
	currentUtilization := float64(currentSize) / float64(capacity)
	projectedUtilization := float64(projectedSize) / float64(capacity)

	// Hard limit check - never exceed the hard limit
	if projectedUtilization > agc.hardLimit {
		needed := uint(projectedSize - capacity)
		// Immediate sync GC to prevent exceeding hard limit
		agc.gcFunc(agc.vfs, needed)
	} else if projectedUtilization > agc.syncThreshold {
		// Near hard limit - do immediate sync GC
		needed := uint(projectedSize - int64(float64(capacity)*agc.syncThreshold))
		agc.gcFunc(agc.vfs, needed)
	} else if currentUtilization > agc.asyncThreshold {
		// Above async threshold - queue for async GC
		needed := uint(projectedSize - int64(float64(capacity)*agc.asyncThreshold))
		select {
		case agc.gcQueue <- gcRequest{bytesNeeded: needed, priority: 2}:
		default:
			// Queue full, do immediate GC
			agc.gcFunc(agc.vfs, needed)
		}
	}

	return agc.vfs.Create(key, size)
}

// gcWorker processes GC requests asynchronously
func (agc *AsyncGCFS) gcWorker() {
	defer agc.wg.Done()

	ticker := time.NewTicker(100 * time.Millisecond) // Check every 100ms
	defer ticker.Stop()

	for {
		select {
		case <-agc.ctx.Done():
			return
		case req := <-agc.gcQueue:
			atomic.StoreInt32(&agc.gcRunning, 1)
			agc.gcFunc(agc.vfs, req.bytesNeeded)
			atomic.StoreInt32(&agc.gcRunning, 0)
		case <-ticker.C:
			// Process any pending GC requests
			select {
			case req := <-agc.gcQueue:
				atomic.StoreInt32(&agc.gcRunning, 1)
				agc.gcFunc(agc.vfs, req.bytesNeeded)
				atomic.StoreInt32(&agc.gcRunning, 0)
			default:
				// No pending requests
			}
		}
	}
}

// preemptiveGC runs background GC to keep cache utilization below threshold
func (agc *AsyncGCFS) preemptiveGC() {
	defer agc.wg.Done()

	ticker := time.NewTicker(5 * time.Second) // Check every 5 seconds
	defer ticker.Stop()

	for {
		select {
		case <-agc.ctx.Done():
			return
		case <-ticker.C:
			currentSize := agc.vfs.Size()
			capacity := agc.vfs.Capacity()
			currentUtilization := float64(currentSize) / float64(capacity)

			// Check if we're above the async threshold
			if currentUtilization > agc.asyncThreshold {
				// Calculate how much to free to get back to async threshold
				targetSize := int64(float64(capacity) * agc.asyncThreshold)
				if currentSize > targetSize {
					overage := currentSize - targetSize
					select {
					case agc.gcQueue <- gcRequest{bytesNeeded: uint(overage), priority: 0}:
					default:
						// Queue full, skip this round
					}
				}
			}
		}
	}
}

// Stop stops the async GC workers
func (agc *AsyncGCFS) Stop() {
	agc.cancel()
	agc.wg.Wait()
}

// IsGCRunning returns true if GC is currently running
func (agc *AsyncGCFS) IsGCRunning() bool {
	return atomic.LoadInt32(&agc.gcRunning) == 1
}

// ForceGC forces immediate garbage collection to free the specified number of bytes
func (agc *AsyncGCFS) ForceGC(bytesNeeded uint) {
	agc.gcFunc(agc.vfs, bytesNeeded)
}