See if a Flags enum is valid

// A faster solution
return (value & mask) == 0;

// Where mask == ~ (None | One | Eight)
// This constant can be precomputed at compile-time or run-time (like in Alternate 1)

I fell into applied algorithmics at the age of 16 or so. This eventually brought me to develop machine vision software as a professional. This is Dreamland for algorithm lovers.