FilenameSanitization.fs

    let [<Literal>] FilenameMaxLen: int = 64
    
    // Note: this is platform dependent, therefore Windows 
    // version of System.IO.Path.GetInvalidFileNameChars() is hardcoded here.
    let InvalidFilenameCharacters = [|
        '"'; '<'; '>'; '|'; '\000'; '\001'; '\002'; '\003'; '\004'; '\005'; '\006';
        '\007'; '\b'; '\009'; '\010'; '\011'; '\012'; '\013'; '\014'; '\015';
        '\016'; '\017'; '\018'; '\019'; '\020'; '\021'; '\022'; '\023'; '\024';
        '\025'; '\026'; '\027'; '\028'; '\029'; '\030'; '\031'; ':'; '*'; '?';
        '\\'; '/'
    |]
    let [<Literal>] FilenameReplacementCharacter = '_'

    let private InvalidRegexCharacters = Regex.Escape(String(InvalidFilenameCharacters));
    // note: this combines multiple occurrences to one match, and so might shorten the
    // filename.
    let private InvalidRegex = String.Format(@"([{0}]*\.+$)|([{0}]+)", InvalidRegexCharacters);

    // TODO: toolbox candidate.
    let private sanitizeFilename (fn: string) : string =
        // note: we want to truncate the string _before_ 
        // eliminating invalid characters, because the replacement 
        // algorithm might change.
        let fn = fn.[0..(min FilenameMaxLen fn.Length)-1]
        Regex.Replace(fn, InvalidRegex, "_");