Markov text generator (C++11)

markov.cpp

#include <string>
#include <vector>
#include <iostream>
#include <iterator>
#include <unordered_map>
#include <algorithm>
#include <stdexcept>
#include <cassert>
#include <cstdlib>
#include <ctime>
using namespace std;

class markov {
  public:
    markov(istream &input, size_t order) {
        vector<char> data { istreambuf_iterator<char>(input), istreambuf_iterator<char>() };
        // replace(data.begin(), data.end(), '\n', ' ');
        assert(data.size() > order);
        for(auto a = data.begin(), b = a + order; b != data.end(); ++a, ++b)
            prob[string(a, b)] += *b;
    }
    string generate(size_t nchars = 2000) {
        auto seed = max_element(prob.begin(), prob.end(), 
            [](const decltype(prob)::value_type &a, const decltype(prob)::value_type &b) {
                return a.second.size() < b.second.size();
            })->first;
        auto output = seed;
        try {
            while(output.size() < nchars) {
                auto & choices = prob.at(seed);
                auto choice = choices[rand() % choices.size()];
                output += choice;
                seed.erase(0, 1) += choice;
            }
        } catch(const out_of_range &e) { /* seed not found, just stop generating */ }
        return output;
    }
  private:
    unordered_map<string, string> prob;
};

int main(int argc, char **argv)
{
    if(argc != 2) {
        cout << "usage: " << argv[0] << " order <input.txt\n";
        return 1;
    }
    auto order = atoi(argv[1]);
    assert(order >= 0 && order <= 20);
    srand(time(0));
    markov m {cin, (unsigned)order};
    cout << m.generate() << "\n";
}

output.txt

$ g++ -Wall -Wextra -pedantic -std=c++11 -O2 markov.cpp -o markov

$ ./markov 12 <text_corpora/twain_sawyer.txt
                                                                             (Samuel Langhorne Clemens)




               T O M   S A W Y E R



CHAPTER XV

A FEW minutes later the widow's. I reckon there ain't no mistake 'bout
where I'LL go to. I been so wicked."

"Dad fetch it! This comes of playing with his grand secret
without Huck, you know!"

"Oh, that will be nice. They're so lovely, all spotted up."

"Yes, that's so. I didn't think of that. Oh, I know what is to become of a boy that will act like that. It makes
me feel so bad to think you could let me go to Sereny Harper and Ben Rogers in--because of course
there's got to be a Gang, or else there wouldn't be alive two days if that got found out.
YOU know that?"

"Yes, that's so," said Huck, nearly fainting.

They talked it all over, and as they sped by some outlying cottages that lay
near the village, and so he presently found
himself leaning to the impressive as it was fascinating.

Now a witness was called who testified that he began to doze, in spite of her, Tom knew
where the wind lay, now. So he forestalled what might be. Got bricks in it?--or old metal?"

"Old metal," said Tom. "You don't have any other name. Kings don't have any fun, anyway, all by himself that Huck had remained pirates. However, it seemed well
worth while to chance it,
so he fell to groaning with considerable, and
then added something to
himself, and then brought them back presently. I wish we could get hold of you I'll--"

She did not feel that it was time to wake up; this sort of life might be
romantic enough, in his blighted condition, but it was of no use. He
talked hopefully to Becky; but an age of anxious waiting passed and no
sounds came again.

The children groped their way to McDougal's cave, and the ferryboat was about at hand. The clanging bell had
been calling for half an hour. However, this time he thought he saw, that
Becky Thatcher, because pitfalls were somewhat common, and had about worn
himself out of temptation ro