Don't require the `--structure` argument.
{
private:
    using CodePoint = UInt32;
    using NGramHash = UInt32;
    struct Histogram
    {
        UInt64 total = 0;   
        UInt64 count_end = 0;
        using Buckets = boost::container::flat_map<CodePoint, UInt64>;
        Buckets buckets;
        void add(CodePoint code)
        {
            ++total;
            ++buckets[code];
        }
        void addEnd()
        {
            ++count_end;
        }
        CodePoint sample(UInt64 random, double end_multiplier) const