//////////////////////////////////////////////////////////////
//
// DrivenEngine
//
// This module achieves two goals:
//
//    * Makes it possible to do replay logging.
//    * Makes the engine act like a deterministic state machine.
//
// We want to implement replay logging.  That means we want to be able to run
// the engine, keeping a log, then "replay" that execution later.
//
// Unfortunately, I/O makes this difficult.  Suppose that during the execution
// of the server, the server reads a lua source file.  Later, we want to replay
// that execution, but the lua source file has changed on disk.  When the engine
// reads the lua source file, we need it to "read" the data that *used* to be
// there, not the data that's currently there.
//
// To make it work, all I/O has to operate in one of two modes: either it can be
// real I/O, or it can be simulated I/O that retrieves data from the logfile.
//
// This module, DrivenEngine, provides an API for the engine to do all its I/O.
// It provides a variety of methods to open sockets, read and write sockets,
// read lua source, get the clock, and so forth.
//
// But in reality, these I/O functions don't ever call operating system
// functions like "read" or "write" or "connect."  They don't call the operating
// system at all - not even indirectly, through a wrapper.  Therefore, they
// can't really do any I/O.  When you use one of these I/O functions to (say)
// write some data to a communication channel, the only thing that happens is
// that the data is put into a buffer.  The actual transmission of the data
// happens elsewhere, in what is called the "Driver."  Likewise, when you use
// one of these I/O functions to read data, it only returns data that was
// previously stored by the "Driver."
//
// The "Driver" is a module that implements the actual I/O.  It is highly
// OS-dependent code, because it contains code to manipulate sockets, time
// clocks, and the like.
//
// From the perspective of the driver, the DrivenEngine is a C++ object that
// acts like a state machine.  This state machine is driven forward by I/O
// events. The DrivenEngine provides an API where the driver can feed in these
// I/O events.
//
// Notice that the usual call graph is inverted: in most application programs,
// the application calls the operating system to do I/O.  But when using class
// DrivenEngine, it's the other way around: the driver calls into class
// DrivenEngine to drive it forward.  I/O routines drive computation. 
//
// So the upshot of all this is that the DrivenEngine is a deterministic state
// machine, free of all OS-specific code.
//
//////////////////////////////////////////////////////////////
//
// Here are the rules for what the driver must do:
//
//  * Before doing anything else, the driver must select one of the three
//    logmodes.
//
//  * If 'logmode_replay' is selected, then the driver must proceed to invoke
//    'drv_step_logfile' over and over until it returns false.  In replay mode,
//    the driver should not do anything else.
//
//  * If 'logmode_write' or 'logmode_none' is selected, the driver must proceed
//    to drive the application.  Follow the remainder of these steps.
//
//  * Read the lua source from disk, and call 'drv_set_lua_source'.
//
//  * Invoke the DrivenEngine's init callback by calling
//    'drv_invoke_engine_init'.
//
//  * Open a hardwired list of ports for listening.
//
//  * Repeat the following steps over and over:
//
//      - List all existing channels using drv_list_channels.
//
//      - If there are any new channels in the channel list, use
//        drv_get_channel_target to fetch the target host, then create a socket
//        and open the connection.  Associate the socket with the channel.
//
//      - If the channel list is missing a previously-known channel, then that
//        channel was deleted by the engine.  Clean up the channel's data and
//        close the socket, if any.
//
//      - Do an OS 'poll'.  The poll should include the sockets for all channels
//        in the channel list, all listening ports, and stdio.
//
//      - If the poll indicates that a listening port has acceptable
//        connections, accept and call drv_notify_accept.  Associate the
//        accepted socket with the channel.
//
//      - If the poll indicates that a connection can accept outgoing data, use
//        drv_peek_outgoing to fetch some data to write, and write it.  Use
//        drv_sent_outgoing_bytes to indicate that the data was sent.
//
//      - If the poll indicates that a connection has incoming data, read the
//        data then push it into the channel using drv_recv_incoming.
//
//      - If the poll indicates that STDIO can be read/written, use
//        drv_peek_outgoing, drv_sent_outgoing, and drv_recv_incoming in the
//        same manner as you would for a socket.
//
//      - Use 'drv_invoke_engine_update' to invoke the engine's update
//        callback.
//
//////////////////////////////////////////////////////////////

#ifndef DRIVENENGINE_HPP
#define DRIVENENGINE_HPP

#include <memory>
#include <string>
#include "streambuffer.hpp"
#include "util.hpp"

class Channel {
private:
    int chid_;
    std::unique_ptr<StreamBuffer> sb_in_;
    std::unique_ptr<StreamBuffer> sb_out_;
    int port_;
    bool remote_closed_;
    std::string target_;
    
public:
    // Get the buffers associated with this channel.
    //
    StreamBuffer *out();
    StreamBuffer *in();
    const StreamBuffer *out() const;
    const StreamBuffer *in() const;

    // If this is a socket connection, the receiver's port number.
    //
    int port() { return port_; }

    // If this is an outgoing socket connection, get the target host.
    const std::string &target() { return target_; }

    // True if the remote closed the connection, or a failure occurred.
    //
    bool remote_closed() const { return remote_closed_; }

    // If communications were closed by the remote, there may
    // be an error message.
    //
    std::string errmsg() const { return errmsg_; }
};

class DrivenEngine {
public:
    // Constructor.
    //
    // Most initialization is achieved by 'drv_xxx' functions, so
    // this constructor takes no arguments.
    //
    DrivenEngine();

    // Destructor.
    //
    // It is necessary to delete all channels before deleting the
    // DrivenEngine.  The destructor will verify that this has been done.
    //
    ~DrivenEngine();

    //////////////////////////////////////////////////////////////
    //
    // The following methods are the 'engine' side of the pipe.
    //
    //////////////////////////////////////////////////////////////

    // The initialization function. You should override this in a subclass.
    // This will be called to initialize the logic engine, shortly after the lua
    // source is loaded.
    //
    virtual void init() { }

    // The update function. You should override this in a subclass.  This will
    // be called to give the engine a chance to respond to new data.
    //
    virtual void update() {}

    // Get the current time.
    //
    // DRIVER: This returns the time most recently stored by the driver
    // using drv_set_clock.
    //
    double get_clock();

    // Create a channel and open an outgoing connection.
    //
    // DRIVER: The channel object is created instantly, but it does nothing
    // until the driver notices the new channel.  The driver is responsible for
    // actually opening the connection and relaying data into the channel using
    // drv_get_target, drv_peek_outgoing, drv_sent_outgoing, drv_recv_incoming.
    //
    std::unique_ptr<Channel> new_outgoing_channel(const std::string &target)

    // Create a new channel from any pending incoming connection. If there is no
    // incoming connection, returns nullptr.
    //
    // DRIVER: The driver must be hardwired to know what ports to listen on.
    // When the driver notices a new incoming connection, it calls
    // drv_notify_accept, which triggers the creation of the channel.  The
    // channel is put into the incoming channel queue, which is fetched by this
    // method. The driver is responsible for relaying data into the channel
    // using drv_get_target, drv_peek_outgoing, drv_sent_outgoing,
    // drv_recv_incoming.
    //
    std::unique_ptr<Channel> new_incoming_channel();

    // Obtain the stdio channel. There is only one stdio channel.  It is owned
    // by the DrivenEngine. It is an error to delete the stdio channel.
    //
    // DRIVER: the stdio channel is created automatically when the DrivenEngine
    // is created.  The driver is responsible for relaying data into the channel
    // using drv_get_target, drv_peek_outgoing, drv_sent_outgoing,
    // drv_recv_incoming.
    //
    Channel *get_stdio_channel();

    // Fetches the entire contents of the lua source directory. The keys in the
    // map are filenames, and the values in the map are file contents.  By
    // default, the lua source is actually read from disk just once. If you call
    // get_lua_source a second time, it will return a nullptr.  The nullptr
    // indicates that the source has not been refreshed recently.  If you want
    // to reread the source code, you must trigger the process by calling
    // 'refresh_lua_source'.  After some delay, it will again be possible to
    // get_lua_source.
    //
    // DRIVER: the driver is responsible for storing the lua source into the
    // DrivenEngine, once, at startup, using drv_set_source. The driver will
    // periodically poll to see if the engine has called rescan_lua_source,
    // using drv_get_rescan
    //
    std::unique_ptr<util::StringMap> get_lua_source();

    // Rescan the lua source directory. The lua source directory is read once,
    // automatically, at engine creation time.  If you want to read it again,
    // you must trigger a rescan.  The rescan is not instantaneous.
    //
    // DRIVER: this merely sets a flag, which the driver will notice later,
    // causing the driver to update the lua source.
    //
    void rescan_lua_source();

    //////////////////////////////////////////////////////////////
    //
    // The following methods are the 'driver' side of the pipe.
    //
    //////////////////////////////////////////////////////////////

    // Run in logging mode.  In this mode, the 'drv' methods will write records
    // into the logfile, making it possible to replay the exact sequence of drv
    // methods that were invoked.  The replay log will be flushed and closed
    // automatically in the event of a crash, or if it exceeds the maximum size.
    //
    void drv_logmode_write(const std::string &filename, int64_t maxsize);

    // Run in replay mode.  In this mode, the only function the driver may call
    // is 'drv_step_logfile.'  Each time this method is called, a single drv_xxx
    // method will be called, as requested by the logfile.
    //
    void drv_logmode_replay(const std::string &filename);
    
    // Run in non-logging mode. In this mode, no logfile will be read or written
    // in this mode.  
    //
    void drv_logmode_none();
    
    // Invoke the engine's init function.  The driver must call drv_set_lua_source
    // before calling this.
    //
    void drv_invoke_engine_init();

    // Invoke the engine's update function.  This typically causes the engine
    // to check the I/O buffers, and respond to I/O, if any.  It also typically
    // causes the engine to check the clock, and do any scheduled calculation.
    //
    void drv_invoke_engine_update();

    // Get a list of all non-closed existing channels. This may include new
    // channels that were created using 'new_outgoing_channel'. It may also be
    // missing channels that were deleted.  It is up to the driver to update its
    // internal data structures to 'match' this list.  Channel number zero is
    // always present, it is the stdio channel.
    //
    void drv_list_channels(std::vector<int> &channels);

    // Get the target of a channel. A target is a string like
    // "www.whatever.com:80".  It indicates the host and port that the channel
    // is supposed to be talking to.  Non-socket channels and incoming channels
    // have empty targets.
    //
    std::string drv_get_target(int chid);

    // Get a pointer to the bytes in the outgoing buffer.  The pointer returned
    // here is naturally only valid until the buffer is changed.  This function
    // is used for all channels, including sockets and stdio.
    //
    void drv_peek_outgoing(int chid, int *nbytes, const char **bytes);

    // Notifies the channel that some bytes were transmitted.  This causes those
    // bytes to be removed from the outgoing buffer.  This function is used for
    // all channels, including sockets and stdio.
    //
    void drv_sent_outgoing(int chid, int nbytes);
    
    // Notifies the channel that some bytes were received.  This causes those
    // bytes to be appended to the incoming buffer.  This function is used for
    // all channels, including sockets and stdio.
    //
    void drv_recv_incoming(int chid, int nbytes, char *bytes);

    // Notify the channel that the connection was closed.  This includes all
    // sorts of closes, including friendly termination, all the way to network
    // failure.  Closing the channel doesn't delete it.  The engine is
    // responsible for noticing that the channel closed and the engine must
    // delete it.  Closing a channel prevents it from showing up in
    // 'drv_list_channels'.  
    //
    void drv_notify_close(int chid);

    // Notify the DrivenEngine that somebody connected to an incoming port.
    // This will cause the DrivenEngine to allocate a new channel and put the
    // new channel into the incoming channels queue.  Returns the new channel
    // ID.  The new incoming channel appears in the 'drv_list_channels' list,
    // even before the engine pops the channel from the incoming channels queue.
    //
    int drv_notify_accept(int port);

    // Set the clock.  The driver is expected to periodically check the system
    // clock and feed the value into the engine.
    //
    void drv_set_clock(double t);

    // Set the lua source code.  The driver is expected to read the lua source
    // code and store it (using this function) once before invoking 
    //
    void drv_set_lua_source(const util::StringMap &source);

    // Check the 'rescan_lua_source' flag.  If this flag is set, it means
    // that the engine wants the driver to rescan the lua source code.
    // When the driver sees this flag, it should rescan the source and call
    // drv_set_source.
    //
    bool drv_get_rescan_lua_source();

    // In replay mode, perform a single step of the logfile.  Returns true
    // if the logfile was not empty.
    //
    bool drv_step_logfile();

private:
    Channel *stdio_channel_;
    int next_channel_id_;
    std::map<int, Channel *> channels_;
    Channel *recent_channel_;
    std::vector<Channel*> accepted_channels_;
    bool rescan_lua_source_;
    std::unique_ptr<util::StringMap> lua_source_;
};

#endif // DRIVENENGINE_HPP