package gc

import (
	"context"
	"fmt"
	"sync"
	"time"
)

// DB provides the database operations needed by GC.
type DB interface {
	// FindUnreferencedBlobs returns digests and sizes of blobs with no
	// manifest_blobs entries, deletes those blob rows in a transaction,
	// and returns the results.
	FindAndDeleteUnreferencedBlobs() ([]UnreferencedBlob, error)
	// BlobExistsByDigest checks whether a blob row exists for the given digest.
	BlobExistsByDigest(digest string) (bool, error)
}

// Storage provides filesystem operations for blob cleanup.
type Storage interface {
	Delete(digest string) error
	ListBlobDigests() ([]string, error)
}

// UnreferencedBlob is a blob that has no manifest references.
type UnreferencedBlob struct {
	Digest string
	Size   int64
}

// Result records the outcome of a GC run.
type Result struct {
	BlobsRemoved int
	BytesFreed   int64
	Duration     time.Duration
}

// Collector performs garbage collection of unreferenced blobs.
type Collector struct {
	db      DB
	storage Storage
	mu      sync.Mutex // registry-wide GC lock
}

// New creates a new garbage collector.
func New(db DB, storage Storage) *Collector {
	return &Collector{db: db, storage: storage}
}

// Run executes the two-phase GC algorithm per ARCHITECTURE.md §9.
// Phase 1 (DB): find unreferenced blobs, delete rows in a transaction.
// Phase 2 (filesystem): delete blob files, clean up empty dirs.
// Returns ErrGCRunning if another GC run is already in progress.
func (c *Collector) Run(ctx context.Context) (*Result, error) {
	if !c.mu.TryLock() {
		return nil, ErrGCRunning
	}
	defer c.mu.Unlock()

	start := time.Now()

	// Check for cancellation.
	if err := ctx.Err(); err != nil {
		return nil, fmt.Errorf("gc: %w", err)
	}

	// Phase 1: Mark and sweep in DB.
	unreferenced, err := c.db.FindAndDeleteUnreferencedBlobs()
	if err != nil {
		return nil, fmt.Errorf("gc: phase 1: %w", err)
	}

	// Phase 2: Delete files from storage.
	var bytesFreed int64
	for _, blob := range unreferenced {
		if err := ctx.Err(); err != nil {
			// Return partial result on cancellation.
			return &Result{
				BlobsRemoved: len(unreferenced),
				BytesFreed:   bytesFreed,
				Duration:     time.Since(start),
			}, fmt.Errorf("gc: phase 2 interrupted: %w", err)
		}
		// Best-effort file deletion. If the file is already gone (e.g.,
		// crash recovery), that's fine.
		if err := c.storage.Delete(blob.Digest); err != nil {
			// Log but continue — orphaned files are harmless and will
			// be caught by reconcile.
			continue
		}
		bytesFreed += blob.Size
	}

	return &Result{
		BlobsRemoved: len(unreferenced),
		BytesFreed:   bytesFreed,
		Duration:     time.Since(start),
	}, nil
}

// Reconcile scans the filesystem for blob files with no matching DB row
// and deletes them. This handles crash recovery — files left behind when
// the process crashed after Phase 1 (DB cleanup) but before Phase 2
// (file cleanup) completed.
func (c *Collector) Reconcile(ctx context.Context) (*Result, error) {
	if !c.mu.TryLock() {
		return nil, ErrGCRunning
	}
	defer c.mu.Unlock()

	start := time.Now()

	digests, err := c.storage.ListBlobDigests()
	if err != nil {
		return nil, fmt.Errorf("gc: list blob files: %w", err)
	}

	var removed int
	var bytesFreed int64
	for _, digest := range digests {
		if err := ctx.Err(); err != nil {
			return &Result{
				BlobsRemoved: removed,
				BytesFreed:   bytesFreed,
				Duration:     time.Since(start),
			}, fmt.Errorf("gc: reconcile interrupted: %w", err)
		}

		exists, err := c.db.BlobExistsByDigest(digest)
		if err != nil {
			continue
		}
		if !exists {
			if err := c.storage.Delete(digest); err != nil {
				continue
			}
			removed++
		}
	}

	return &Result{
		BlobsRemoved: removed,
		BytesFreed:   bytesFreed,
		Duration:     time.Since(start),
	}, nil
}

// Lock acquires the GC lock, blocking new blob uploads.
// Returns a function to release the lock.
func (c *Collector) Lock() func() {
	c.mu.Lock()
	return c.mu.Unlock
}

// TryLock attempts to acquire the GC lock without blocking.
// Returns true if the lock was acquired.
func (c *Collector) TryLock() bool {
	return c.mu.TryLock()
}