package grib

import (
	"context"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"time"

	"git.intra.yksa.space/gsn/predictor/internal/pkg/errcodes"
	"github.com/aws/aws-sdk-go-v2/aws"
	"github.com/aws/aws-sdk-go-v2/config"
	"github.com/aws/aws-sdk-go-v2/service/s3"
	"golang.org/x/sync/errgroup"
)

// S3Downloader downloads GRIB files from AWS S3
type S3Downloader struct {
	Dir      string
	Parallel int
	Bucket   string
	Region   string
	Client   *s3.Client
}

// NewS3Downloader creates a new S3 downloader with anonymous access
func NewS3Downloader(dir string, parallel int, bucket, region string) (*S3Downloader, error) {
	// Create AWS config with anonymous credentials for public bucket
	cfg, err := config.LoadDefaultConfig(context.Background(),
		config.WithRegion(region),
		config.WithCredentialsProvider(aws.AnonymousCredentials{}),
	)
	if err != nil {
		return nil, errcodes.Wrap(err, "failed to load AWS config")
	}

	client := s3.NewFromConfig(cfg)

	return &S3Downloader{
		Dir:      dir,
		Parallel: parallel,
		Bucket:   bucket,
		Region:   region,
		Client:   client,
	}, nil
}

// s3Key generates the S3 key for a GRIB file
// Path format: gfs.YYYYMMDD/HH/atmos/gfs.tHHz.pgrb2.0p50.fFFF
func (d *S3Downloader) s3Key(run string, hour int, step int) string {
	return fmt.Sprintf("gfs.%s/%02d/atmos/gfs.t%02dz.pgrb2.0p50.f%03d", run, hour, hour, step)
}

// CheckFileExists checks if a file exists in S3 using HeadObject
func (d *S3Downloader) CheckFileExists(ctx context.Context, key string) (bool, int64, error) {
	input := &s3.HeadObjectInput{
		Bucket: aws.String(d.Bucket),
		Key:    aws.String(key),
	}

	result, err := d.Client.HeadObject(ctx, input)
	if err != nil {
		// Check if error is NotFound
		// AWS SDK v2 doesn't export specific error types, check error string
		if isNotFoundError(err) {
			return false, 0, nil
		}
		return false, 0, errcodes.Wrap(err, "failed to check file existence")
	}

	size := int64(0)
	if result.ContentLength != nil {
		size = *result.ContentLength
	}

	return true, size, nil
}

// isNotFoundError checks if error is a NotFound error
func isNotFoundError(err error) bool {
	if err == nil {
		return false
	}
	// AWS SDK v2 error handling
	errStr := err.Error()
	return contains(errStr, "NotFound") || contains(errStr, "404") || contains(errStr, "NoSuchKey")
}

func contains(s, substr string) bool {
	return len(s) >= len(substr) && (s == substr || len(s) > len(substr) && findSubstring(s, substr))
}

func findSubstring(s, substr string) bool {
	for i := 0; i <= len(s)-len(substr); i++ {
		if s[i:i+len(substr)] == substr {
			return true
		}
	}
	return false
}

// ListAvailableFiles lists all available files for a given run
func (d *S3Downloader) ListAvailableFiles(ctx context.Context, run string, hour int) ([]string, error) {
	prefix := fmt.Sprintf("gfs.%s/%02d/atmos/", run, hour)

	input := &s3.ListObjectsV2Input{
		Bucket: aws.String(d.Bucket),
		Prefix: aws.String(prefix),
	}

	var files []string
	paginator := s3.NewListObjectsV2Paginator(d.Client, input)

	for paginator.HasMorePages() {
		page, err := paginator.NextPage(ctx)
		if err != nil {
			return nil, errcodes.Wrap(err, "failed to list S3 objects")
		}

		for _, obj := range page.Contents {
			if obj.Key != nil {
				files = append(files, *obj.Key)
			}
		}
	}

	return files, nil
}

// fetchFromS3 downloads a file from S3 to local disk with retry logic
func (d *S3Downloader) fetchFromS3(ctx context.Context, key, dst string) (err error) {
	// Check if final file already exists
	if _, err := os.Stat(dst); err == nil {
		return nil
	}

	const maxRetries = 3
	var lastErr error

	for attempt := 0; attempt < maxRetries; attempt++ {
		if attempt > 0 {
			// Exponential backoff: 2s, 4s, 8s
			waitTime := time.Duration(1<<uint(attempt)) * time.Second
			time.Sleep(waitTime)
		}

		lastErr = d.fetchFromS3Once(ctx, key, dst)
		if lastErr == nil {
			return nil
		}
	}

	return errcodes.Wrap(lastErr, fmt.Sprintf("failed after %d retries", maxRetries))
}

// fetchFromS3Once performs a single download attempt
func (d *S3Downloader) fetchFromS3Once(ctx context.Context, key, dst string) (err error) {
	tmp := dst + ".part"

	// Remove old .part file if it exists
	os.Remove(tmp)

	f, err := os.Create(tmp)
	if err != nil {
		return err
	}

	fileClosed := false
	// Cleanup .part file on any error (using named return value)
	defer func() {
		if !fileClosed {
			f.Close()
		}
		if err != nil {
			os.Remove(tmp)
		}
	}()

	// Check if file exists in S3
	exists, size, checkErr := d.CheckFileExists(ctx, key)
	if checkErr != nil {
		return errcodes.Wrap(checkErr, "failed to check S3 file existence")
	}
	if !exists {
		return errcodes.Wrap(errcodes.ErrDownload, fmt.Sprintf("file not found in S3: %s", key))
	}

	// Download from S3
	input := &s3.GetObjectInput{
		Bucket: aws.String(d.Bucket),
		Key:    aws.String(key),
	}

	result, err := d.Client.GetObject(ctx, input)
	if err != nil {
		return errcodes.Wrap(err, "failed to get S3 object")
	}
	defer result.Body.Close()

	// Copy to local file
	written, err := io.Copy(f, result.Body)
	if err != nil {
		return errcodes.Wrap(err, fmt.Sprintf("failed to write S3 object to file %s", dst))
	}

	// Verify size if available
	if size > 0 && written != size {
		return errcodes.Wrap(errcodes.ErrDownload, fmt.Sprintf("size mismatch: got %d bytes, expected %d", written, size))
	}

	// Close file before rename
	if err := f.Close(); err != nil {
		return err
	}
	fileClosed = true

	// If rename fails, err will be set and defer will cleanup .part file
	return os.Rename(tmp, dst)
}

// Run downloads all required GRIB files for a forecast run
func (d *S3Downloader) Run(ctx context.Context, run time.Time) error {
	runStr := run.Format("20060102")
	hour := run.Hour()

	// First, list available files to verify they exist
	availableFiles, err := d.ListAvailableFiles(ctx, runStr, hour)
	if err != nil {
		return errcodes.Wrap(err, "failed to list available files")
	}

	if len(availableFiles) == 0 {
		return errcodes.Wrap(errcodes.ErrDownload, fmt.Sprintf("no files found for run %s/%02d", runStr, hour))
	}

	// Build a map of available files for quick lookup
	availableMap := make(map[string]bool)
	for _, file := range availableFiles {
		availableMap[file] = true
	}

	g, ctx := errgroup.WithContext(ctx)
	sem := make(chan struct{}, d.Parallel)

	for _, step := range steps {
		step := step
		key := d.s3Key(runStr, hour, step)

		// Check if file is available in S3
		if !availableMap[key] {
			// Log warning but don't fail - some forecast hours might not be available yet
			continue
		}

		sem <- struct{}{}
		g.Go(func() error {
			defer func() { <-sem }()
			dst := filepath.Join(d.Dir, fileName(run, step))
			return d.fetchFromS3(ctx, key, dst)
		})
	}

	return g.Wait()
}