refactor(countrw): move to iout

iout/copy_range.go

@@ -0,0 +1,23 @@
+package iout
+
+import (
+	"errors"
+	"fmt"
+	"io"
+)
+
+func CopyRange(w io.Writer, r io.Reader, start, length int64) error {
+	if _, err := io.CopyN(io.Discard, r, start); err != nil && !errors.Is(err, io.EOF) {
+		return fmt.Errorf("discard %d: %w", start, err)
+	}
+	if length == 0 {
+		if _, err := io.Copy(w, r); err != nil && !errors.Is(err, io.EOF) {
+			return fmt.Errorf("direct copy: %w", err)
+		}
+		return nil
+	}
+	if _, err := io.CopyN(w, io.MultiReader(r, NewNullReader()), length); err != nil && !errors.Is(err, io.EOF) {
+		return fmt.Errorf("copy %d: %w", length, err)
+	}
+	return nil
+}