#ifndef TRANSPORT_H
#define TRANSPORT_H

/**
 * @file
 *
 * Defines a intermediate-level (as opposed to high-level or low-level) API for connected streams of data, presumeably
 * non-blocking ones.
 */
#include "error.h"

/**
 * Opaque transport state handle.
 *
 * Transports are reliable byte streams, connected with some endpoint over some medium. Common implementations are
 * e.g. TCP, SSL or fifo transports (using the OS file/socket API).
 *
 * Transports can be connected or unconnected. For synchronous opens (e.g. fifo_open_read()), the transport returned
 * will already be connected, meaning that the transport_callbacks::on_connect callback is unused. For async connects
 * such as sock_tcp_connect()/sock_ssl_connect(), the transport returned is *not* connected, and you must wait for
 * transport_callbacks::on_connect to be called before being able to send/recieve data on the transport.
 *
 * Once you have an opened transport, sending and receiving data is simple - just call transport_read()/transport_write().
 * These implement unbuffered I/O, so they may do partial reads/writes. In terms of the system read/write calls, the
 * main difference is in the error return codes. On EOF, instead of returning zero, they return ERR_EOF (or
 * ERR_WRITE_EOF for transport_write, for whoever knows what that means...). This means that when the underlying
 * transport is unable to fufill the request due to lack of data/buffer space, these can return zero to signifiy
 * something simliar to EAGAIN.
 *
 * The transport API also implements non-blocking/event-based operation (usually on top of libevent), although at a
 * slightly different level than the normal select/poll API. Instead of the user asking the transport to notify for
 * read/write after transport_read/transport_write return zero, the transport will take care of this itself. 
 * 
 * Specifically, the user can supply a mask of events they are currently interested in. By default, this should be the
 * full TRANSPORT_READ | TRANSPORT_WRITE, as the transport will take care of managing events by itself. If you wish to
 * e.g. throttle read/write, you may set a different event mask using transport_events(), which will prevent the
 * relevant callback from being triggered.
 *
 * For reads, the transport maintains a persistent read event, and will always call on_read when data is available on
 * the socket (i.e. normal select() semantics). If masked out using transport_events(), there should be no event
 * activity on the transport (i.e. the fd read event is removed).
 *
 * For writes, the transport maintains a write event that is disabled by default. If transport_write() returns zero, it will
 * become enabled *once*, and consequently trigger transport_callbacks::on_write *once*, after which you must call
 * transport_write() to possibly enable it again. If masked out using transport_events(), transport_write() will not
 * enable the write event, and any pending write event is cancelled. If masked back in using transport_events(), the
 * write event will *not* be registered, so if you have pending data, do a transport_write() after enabling
 * TRANSPORT_WRITE.
 *
 * Note that transport_write() returning fewer bytes than given will *not* enable the write event! You must call
 * transport_write() until you have either written all of your data, or it returns zero!
 */
struct transport;

/**
 * @see transport
 */
typedef struct transport transport_t;

/**
 * User callbacks for transports
 *
 * @see transport
 */
struct transport_callbacks {
    /**
     * The transport is now connected
     */
    void (*on_connect) (transport_t *transport, void *arg);

    /**
     * Data is now available for reading from the transport
     */
    void (*on_read) (transport_t *transport, void *arg);

    /**
     * The transport has become writeable
     */
    void (*on_write) (transport_t *transport, void *arg);

    /**
     * An asynchronous error has occured. This is only called for errors that occur while being called directly from
     * the underlying event loop, and never from inside an API function.
     *
     * You must call transport_destroy to release the transport.
     */
    void (*on_error) (transport_t *transport, const error_t *err, void *arg);
};

/**
 * Bitmask of available events
 *
 * @see transport
 */
enum transport_event {
    TRANSPORT_READ  = 0x01,
    TRANSPORT_WRITE = 0x02,
};

/**
 * User info required to build a transport
 *
 * @see transport
 */
struct transport_info {
    /** The callbacks table */
    const struct transport_callbacks *cb_tbl;

    /** The callback context argument */
    void *cb_arg;

    /** Initial event mask using transport_event flags */
    short ev_mask;
};

/**
 * Read a series of bytes from the transport into the given \a buf (up to \a len bytes). If succesfull, this returns
 * the number of bytes read (which will be less than or equal to \a len). If the transport is nonblocking, and there is
 * no data available, this returns zero, and need not be called again until transport_callbacks::on_read is invoked.
 *
 * On errors, this returns the negative error code, and more info via \a err. Note that as opposed to read(2), EOF is
 * handled as an error, returning ERR_EOF.
 *
 * @param transport the transport state
 * @param buf the buffer to read the bytes into
 * @param len the number of bytes to read into the buffer
 * @param err returned error info
 * @return bytes read, zero if none available, -err_t
 */
int transport_read (transport_t *transport, void *buf, size_t len, error_t *err);

/**
 * Write a series of bytes from the given \a buf (containing \a len bytes) to the transport. If succesfull, this
 * returns the number of bytes written (which may be less than \a len). If the transport is nonblocking, and the
 * operation would have blocked, no data will be written, and zero is returned; in this case, the transport's write
 * event is enabled (unless TRANSPORT_WRITE is masked out).
 *
 * On errors, this returns the negative error code, along with extended info via \a err.
 *
 * @param transport the transport state
 * @param buf the buffer to write the bytes from
 * @param len number of bytes to write
 * @param err returned error info
 * @return bytes written, zero if would have blocked, -err_t
 */
int transport_write (transport_t *transport, const void *buf, size_t len, error_t *err);

/**
 * Change the mask of enabled events.
 */
err_t transport_events (transport_t *transport, short mask);

/**
 * Install a new set of callback handlers, replacing the old ones.
 */
void transport_set_callbacks (transport_t *transport, const struct transport_callbacks *cb_tbl, void *cb_arg);

/**
 * Close and destroy the transport immediately, severing any established connection rudely.
 *
 * This will release all resources associated with the transport, including the transport itself, which must not be
 * used anymore.
 */
void transport_destroy (transport_t *transport);

#endif