1 files changed, 98 insertions, 0 deletions
diff --git a/osmpass/fetch.go b/osmpass/fetch.go
new file mode 100644
index 0000000..6abd136
--- /dev/null
+++ b/osmpass/fetch.go
@@ -0,0 +1,98 @@
+package osmpass
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"log/slog"
+	"net/http"
+	"os"
+
+	"github.com/schollz/progressbar/v3"
+)
+
+const francePBFRUL = "https://download.geofabrik.de/europe/france-latest.osm.pbf"
+
+// FetchFrancePBF downloads the full France OSM PBF to destPath, resuming a
+// partial download when the file already exists and skipping the download
+// entirely when it is already complete.
+func FetchFrancePBF(destPath string) error {
+	client := &http.Client{}
+
+	head, err := client.Head(francePBFRUL)
+	if err != nil {
+		return fmt.Errorf("failed to check remote size: %w", err)
+	}
+	head.Body.Close()
+	total := head.ContentLength
+	if total <= 0 {
+		return fmt.Errorf("unexpected remote content length: %d", total)
+	}
+
+	info, statErr := os.Stat(destPath)
+	var offset int64
+	if statErr == nil {
+		offset = info.Size()
+		if offset >= total {
+			slog.Info("Already downloaded", "file", destPath, "bytes", offset)
+			return nil
+		}
+		if offset > 0 {
+			slog.Info("Resuming download", "file", destPath, "bytes", offset, "remaining", total-offset)
+		}
+	} else if !errors.Is(statErr, os.ErrNotExist) {
+		return fmt.Errorf("failed to stat %s: %w", destPath, statErr)
+	}
+
+	req, err := http.NewRequest(http.MethodGet, francePBFRUL, nil)
+	if err != nil {
+		return err
+	}
+	req.Header.Set("User-Agent", "biking_home")
+	if offset > 0 {
+		req.Header.Set("Range", fmt.Sprintf("bytes=%d-", offset))
+	}
+
+	res, err := client.Do(req)
+	if err != nil {
+		return err
+	}
+	defer res.Body.Close()
+
+	resuming := offset > 0 && res.StatusCode == http.StatusPartialContent
+	if !resuming {
+		if res.StatusCode != http.StatusOK {
+			return fmt.Errorf("unexpected status code: %d", res.StatusCode)
+		}
+		if offset > 0 {
+			slog.Warn("Server ignored Range header, restarting download", "file", destPath)
+		}
+		offset = 0
+	}
+
+	mode := os.O_CREATE | os.O_WRONLY | os.O_TRUNC
+	if resuming {
+		mode = os.O_CREATE | os.O_WRONLY
+	}
+	file, err := os.OpenFile(destPath, mode, 0o644)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	bar := progressbar.DefaultBytes(res.ContentLength, "Downloading france-latest.osm.pbf")
+	if resuming {
+		// Re-append to the partial file when the server honored the range request.
+		if _, err := file.Seek(offset, io.SeekStart); err != nil {
+			return err
+		}
+	}
+	_, err = io.Copy(io.MultiWriter(file, bar), res.Body)
+	if err != nil {
+		return fmt.Errorf("failed to download %s: %w", francePBFRUL, err)
+	}
+	if size, err := file.Stat(); err == nil && size.Size() != total {
+		return fmt.Errorf("download incomplete: got %d bytes, want %d", size.Size(), total)
+	}
+	return nil
+}