public void CalcNewPathAndID(EpubItem item, int offset)
{
string oldAbsolutePath = item.AbsolutePath;
var oldFileName = oldAbsolutePath.getZipFileName();
var oldprefix = EpubItem.PrefixAsInt(oldFileName);
var fileName = oldFileName.StripPrefixFromFileName();
var path = oldAbsolutePath.GetZipPath();
if (!string.IsNullOrEmpty(path))
{
path += '/';
}
// note possible conflict as "cover" does not have a prefix
// which might conflict if there was also a page with 0000_cover.xhtml
// So, if page has a prefix, bump offest by one.
var bump = EpubItem.ExtractPrefixFromFileName(oldFileName) == null ? 0 : 1;
var newPrefix = (oldprefix + offset + bump).ToString("D4");
var newAbsolutePath = $"{path}{newPrefix}_{fileName}";
NewAbsolutePaths.Add(oldAbsolutePath, newAbsolutePath);
string newId = item.Id.StripDigits() + newPrefix;
NewItemIds.Add(item.Id, newId);
}
