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#ifndef TRANSPORT_H

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#define TRANSPORT_H

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/**

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 * @file

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 *

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 * Defines a intermediate-level (as opposed to high-level or low-level) API for connected streams of data, presumeably

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 * non-blocking ones.

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 */

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#include "error.h"

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/**

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 * Opaque transport state handle.

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 *

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 * Transports are reliable byte streams, connected with some endpoint over some medium. Common implementations are

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 * e.g. TCP, SSL or fifo transports (using the OS file/socket API).

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 *

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 * Transports can be connected or unconnected. For synchronous opens (e.g. fifo_open_read()), the transport returned

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 * will already be connected, meaning that the transport_callbacks::on_connect callback is unused. For async connects

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 * such as sock_tcp_connect()/sock_ssl_connect(), the transport returned is *not* connected, and you must wait for

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 * transport_callbacks::on_connect to be called before being able to send/recieve data on the transport.

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 *

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 * Once you have an opened transport, sending and receiving data is simple - just call transport_read()/transport_write().

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 * These implement unbuffered I/O, so they may do partial reads/writes. In terms of the system read/write calls, the

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 * main difference is in the error return codes. On EOF, instead of returning zero, they return ERR_EOF (or

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 * ERR_WRITE_EOF for transport_write, for whoever knows what that means...). This means that when the underlying

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 * transport is unable to fufill the request due to lack of data/buffer space, these can return zero to signifiy

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 * something simliar to EAGAIN.

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 *

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 * The transport API also implements non-blocking/event-based operation (usually on top of libevent), although at a

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 * slightly different level than the normal select/poll API. Instead of the user asking the transport to notify for

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 * read/write after transport_read/transport_write return zero, the transport will take care of this itself. 

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 * 

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 * Specifically, the user can supply a mask of events they are currently interested in. By default, this should be the

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 * full TRANSPORT_READ | TRANSPORT_WRITE, as the transport will take care of managing events by itself. If you wish to

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 * e.g. throttle read/write, you may set a different event mask using transport_events(), which will prevent the

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 * relevant callback from being triggered.

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 *

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 * For reads, the transport maintains a persistent read event, and will always call on_read when data is available on

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 * the socket (i.e. normal select() semantics). If masked out using transport_events(), there should be no event

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 * activity on the transport (i.e. the fd read event is removed).

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 *

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 * For writes, the transport maintains a write event that is disabled by default. If transport_write() returns zero, it will

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 * become enabled *once*, and consequently trigger transport_callbacks::on_write *once*, after which you must call

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 * transport_write() to possibly enable it again. If masked out using transport_events(), transport_write() will not

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 * enable the write event, and any pending write event is cancelled. If masked back in using transport_events(), the

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 * write event will *not* be registered, so if you have pending data, do a transport_write() after enabling

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 * TRANSPORT_WRITE.

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 *

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 * Note that transport_write() returning fewer bytes than given will *not* enable the write event! You must call

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 * transport_write() until you have either written all of your data, or it returns zero!

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 */

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struct transport;

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/**

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 * @see transport

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 */

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typedef struct transport transport_t;

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/**

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 * User callbacks for transports

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 *

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 * @see transport

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 */

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struct transport_callbacks {

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    /**

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     * The transport is now connected

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     */

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    void (*on_connect) (transport_t *transport, void *arg);

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    /**

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     * Data is now available for reading from the transport

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     */

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    void (*on_read) (transport_t *transport, void *arg);

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    /**

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     * The transport has become writeable

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     */

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    void (*on_write) (transport_t *transport, void *arg);

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    /**

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     * An asynchronous error has occured. This is only called for errors that occur while being called directly from

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     * the underlying event loop, and never from inside an API function.

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     *

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     * You must call transport_destroy to release the transport.

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     */

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    void (*on_error) (transport_t *transport, const error_t *err, void *arg);

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};

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/**

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 * Bitmask of available events

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 *

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 * @see transport

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 */

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enum transport_event {

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    TRANSPORT_READ  = 0x01,

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    TRANSPORT_WRITE = 0x02,

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};

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/**

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 * User info required to build a transport

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 *

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 * @see transport

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 */

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struct transport_info {

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    /** The callbacks table */

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    const struct transport_callbacks *cb_tbl;

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    /** The callback context argument */

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    void *cb_arg;

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    /** Initial event mask using transport_event flags */

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    short ev_mask;

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};

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/**

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 * Read a series of bytes from the transport into the given \a buf (up to \a len bytes). If succesfull, this returns

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 * the number of bytes read (which will be less than or equal to \a len). If the transport is nonblocking, and there is

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 * no data available, this returns zero, and need not be called again until transport_callbacks::on_read is invoked.

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 *

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 * On errors, this returns the negative error code, and more info via \a err. Note that as opposed to read(2), EOF is

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 * handled as an error, returning ERR_EOF.

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 *

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 * @param transport the transport state

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 * @param buf the buffer to read the bytes into

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 * @param len the number of bytes to read into the buffer

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 * @param err returned error info

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 * @return bytes read, zero if none available, -err_t

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 */

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int transport_read (transport_t *transport, void *buf, size_t len, error_t *err);

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/**

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 * Write a series of bytes from the given \a buf (containing \a len bytes) to the transport. If succesfull, this

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 * returns the number of bytes written (which may be less than \a len). If the transport is nonblocking, and the

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 * operation would have blocked, no data will be written, and zero is returned; in this case, the transport's write

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 * event is enabled (unless TRANSPORT_WRITE is masked out).

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 *

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 * On errors, this returns the negative error code, along with extended info via \a err.

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 *

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 * @param transport the transport state

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 * @param buf the buffer to write the bytes from

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 * @param len number of bytes to write

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 * @param err returned error info

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 * @return bytes written, zero if would have blocked, -err_t

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 */

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int transport_write (transport_t *transport, const void *buf, size_t len, error_t *err);

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/**

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 * Change the mask of enabled events.

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 */

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err_t transport_events (transport_t *transport, short mask);

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/**

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 * Install a new set of callback handlers, replacing the old ones.

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 */

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void transport_set_callbacks (transport_t *transport, const struct transport_callbacks *cb_tbl, void *cb_arg);

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/**

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 * Close and destroy the transport immediately, severing any established connection rudely.

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 *

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 * This will release all resources associated with the transport, including the transport itself, which must not be

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 * used anymore.

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 */

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void transport_destroy (transport_t *transport);

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#endif