package grib

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

	"git.intra.yksa.space/gsn/predictor/internal/pkg/errcodes"
	"golang.org/x/sync/errgroup"
)

type Downloader struct {
	Dir        string
	Parallel   int
	Client     *http.Client
	DatasetURL string
}

func (d *Downloader) fileURL(run string, hour int, step int) string {
	return fmt.Sprintf("%s/gfs.%s/%02d/atmos/gfs.t%02dz.pgrb2.0p50.f%03d", d.DatasetURL, run, hour, hour, step)
}

func (d *Downloader) fetch(ctx context.Context, url, dst string) (err error) {
	// Check if final file already exists
	if _, err := os.Stat(dst); err == nil {
		return nil
	}

	tmp := dst + ".part"

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

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

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

	req, _ := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	resp, err := d.Client.Do(req)
	if err != nil {
		return err
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return errcodes.Wrap(errcodes.ErrDownload, "bad status: "+resp.Status)
	}

	if _, err := io.Copy(f, resp.Body); err != nil {
		return err
	}

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

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

func (d *Downloader) Run(ctx context.Context, run time.Time) error {
	runStr := run.Format("20060102")
	hour := run.Hour()
	g, ctx := errgroup.WithContext(ctx)
	sem := make(chan struct{}, d.Parallel)
	for _, step := range steps {
		step := step
		sem <- struct{}{}
		g.Go(func() error {
			defer func() { <-sem }()
			url := d.fileURL(runStr, hour, step)
			dst := filepath.Join(d.Dir, fileName(run, step))
			return d.fetch(ctx, url, dst)
		})
	}
	return g.Wait()
}