package grib

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

	"golang.org/x/sync/errgroup"
)

// NOMADS only.
const nomadsRoot = "https://nomads.ncep.noaa.gov/pub/data/nccf/com/gfs/prod"

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

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", nomadsRoot, run, hour, hour, step)
}

func (d *Downloader) fetch(ctx context.Context, url, dst string) error {
	if _, err := os.Stat(dst); err == nil {
		return nil
	}
	tmp := dst + ".part"
	f, err := os.Create(tmp)
	if err != nil {
		return err
	}
	defer f.Close()
	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 fmt.Errorf("bad status %s", resp.Status)
	}
	if _, err := io.Copy(f, resp.Body); err != nil {
		return err
	}
	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()
}