phase 0-2 complete

internal/radar/fetch.go

@@ -0,0 +1,51 @@
+package radar
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+	"time"
+)
+
+// nwsRadarBase is the NWS public product distribution server.
+// No credentials required; sn.last always holds the most recent scan.
+const nwsRadarBase = "https://tgftp.nws.noaa.gov/SL.us008001/DF.of/DC.radar"
+
+// Fetcher downloads NEXRAD Level 3 products from the NWS public server.
+type Fetcher struct {
+	client *http.Client
+}
+
+func NewFetcher(_ context.Context) (*Fetcher, error) {
+	return &Fetcher{client: &http.Client{Timeout: 30 * time.Second}}, nil
+}
+
+// FetchLatest downloads the latest Level 3 base reflectivity (0.5°) for site.
+func (f *Fetcher) FetchLatest(ctx context.Context, site string) ([]byte, error) {
+	return f.fetchDS(ctx, site, "p94r0")
+}
+
+// FetchVelocity downloads the latest Level 3 base velocity (0.5°) for site.
+func (f *Fetcher) FetchVelocity(ctx context.Context, site string) ([]byte, error) {
+	return f.fetchDS(ctx, site, "p99r0")
+}
+
+func (f *Fetcher) fetchDS(ctx context.Context, site, ds string) ([]byte, error) {
+	url := fmt.Sprintf("%s/DS.%s/SI.%s/sn.last", nwsRadarBase, ds, strings.ToLower(site))
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+	resp, err := f.client.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("NWS fetch: %w", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("NWS returned %s for %s", resp.Status, url)
+	}
+	return io.ReadAll(resp.Body)
+}