Programming

#include <time.h>
#include <iostream>
#include <string>
#include <deque>
#include <map>
#include <vector>
#include <cstdlib>

using namespace std;

const int  NPREF = 2;
const char NONWORD[] = "\n";    // cannot appear as real line: we remove newlines
const int  MAXGEN = 10000; // maximum words generated

typedef deque<string> Prefix;

map<Prefix, vector<string> > statetab; // prefix -> suffixes

void            build(Prefix&, istream&);
void            generate(int nwords);
void            add(Prefix&, const string&);

// markov main: markov-chain random text generation
int main(void)
{
        int     nwords = MAXGEN;
        Prefix prefix;  // current input prefix

        srand(time(NULL));
        for (int i = 0; i < NPREF; i++)
                add(prefix, NONWORD);
        build(prefix, cin);
        add(prefix, NONWORD);
        generate(nwords);
        return 0;
}

// build: read input words, build state table
void build(Prefix& prefix, istream& in)
{
        string buf;

        while (in >> buf)
                add(prefix, buf);
}

// add: add word to suffix deque, update prefix
void add(Prefix& prefix, const string& s)
{
        if (prefix.size() == NPREF) {
                statetab[prefix].push_back(s);
                prefix.pop_front();
        }
        prefix.push_back(s);
}

// generate: produce output, one word per line
void generate(int nwords)
{
        Prefix prefix;
        int i;

        for (i = 0; i < NPREF; i++)
                add(prefix, NONWORD);
        for (i = 0; i < nwords; i++) {
                vector<string>& suf = statetab[prefix];
                const string& w = suf[rand() % suf.size()];
                if (w == NONWORD)
                        break;
                cout << w << "\n";
                prefix.pop_front();     // advance
                prefix.push_back(w);
        }
}