//
// This is example code from Chapter 23.4.1 "Implementation details" of
// "Programming -- Principles and Practice Using C++" by Bjarne Stroustrup
//

#include<string>
#include<vector>
#include<map>
#include<fstream>
#include<iostream>
#include <cstdlib>	// for exit()

using namespace std;

//------------------------------------------------------------------------------

// We define a Message as a pair of iterators into a vector<string> (our vector of lines):
typedef vector<string>::const_iterator Line_iter;

class Message {  // a Message points to the first and the last line of a message
    Line_iter first;
    Line_iter last;
public:
    Message(Line_iter p1, Line_iter p2) :first(p1), last(p2) { }
    Line_iter begin() const { return first; }
    Line_iter end() const { return last; }
    // ...
};

//------------------------------------------------------------------------------

typedef vector<Message>::const_iterator Mess_iter;

//------------------------------------------------------------------------------

// We define a Mail_file as a structure holding lines of text and messages:
struct Mail_file {        // a Mail_file holds all the lines from a file
                          // and simplifies access to messages
    string name;          // file name
    vector<string> lines; // the lines in order
    vector<Message> m;    // Messages in order

    Mail_file(const string& n);  // read file n into lines

    Mess_iter begin() const { return m.begin(); }
    Mess_iter end() const { return m.end(); }
};

//------------------------------------------------------------------------------

Mail_file::Mail_file(const string& n)
    // open file named "n"
    // read the lines from "n" into "lines"
    // find the messages in the lines and compose them in m
    // for simplicity assume every message is ended by a "---" line
{
    ifstream in(n.c_str());    // open the file
    if (!in) {
        cerr << "no " << n << '\n';
        exit(1);
    }
    
    string s;
    while (getline(in,s)) lines.push_back(s);    // build the vector of lines

    Line_iter first = lines.begin();    // build the vector of Messages
    for (Line_iter p = lines.begin(); p!=lines.end(); ++p) {
        if (*p == "----") {    // end of message
            m.push_back(Message(first,p));
            first = p+1;    // ---- not part of message
        }
    }
}

//------------------------------------------------------------------------------

int is_prefix(const string& s, const string& p)
    // is *p the first part of s?
{
    int n = p.size();
    if (string(s,0,n)==p) return n;
    return 0;
}

//------------------------------------------------------------------------------

bool find_from_addr(const Message* m, string& s)
{
    for(Line_iter p = m->begin(); p!=m->end(); ++p)
        if (int n = is_prefix(*p,"From: ")) {
            s = string(*p,n);
            return true;
        }
    return false;
}

//------------------------------------------------------------------------------

string find_subject(const Message* m)
{
    for(Line_iter p = m->begin(); p!=m->end(); ++p)
        if (int n = is_prefix(*p,"Subject: ")) return string(*p,n);
    return "";
}

//------------------------------------------------------------------------------

int main()
{
    Mail_file mfile("my-mail-file.txt");        // initialize mfile from a file
	
    // first gather messages from each sender together in a multimap:

    multimap<string, const Message*> sender;

    for (Mess_iter p = mfile.begin(); p!=mfile.end(); ++p) {
        const Message& m = *p;
        string s;
        if (find_from_addr(&m,s))
            sender.insert(make_pair(s,&m));
    }

    // now iterate through the multimap
    // and extract the subjects of John Doe's messages:
    typedef multimap<string, const Message*>::const_iterator MCI;

    pair<MCI,MCI> pp = sender.equal_range("John Doe <jdoe@machine.example>");
	for(MCI p = pp.first; p!=pp.second; ++p) {
        cout << find_subject(p->second) << '\n';
	}
}
//------------------------------------------------------------------------------