@menu
* Kernel::                      The fundamental components.
* Input::                       Keyboard, mouse, and controller input.
* Math::                        Linear algebra and more.
* Graphics::                    Eye candy.
@end menu

@node Kernel
@section Kernel

At the very core of Chickadee, in the @code{(chickadee)} module, lies
an event loop.  This loop, or ``kernel'', is responsible for creating
and managing the game window, dispatching input events, ensuring that
the game is updated at the desired interval, and rendering graphics.
The kernel implements what is known as a ``fixed timestep'' game loop,
meaning that the game simulation will be advanced by a fixed interval
of time and will never vary from frame to frame, unlike some other
styles of game loops.  The appropriately named @code{run-game} and
@code{abort-game} procedures are the entry and exit points to the
Chickadee kernel.

On its own, the kernel does not do very much at all.  In order to
actually respond to input events, update game state, or draw something
to the game window, a hacker with a penchant for game development must
latch onto extension points built into the kernel, called ``hooks'',
and specify what action ought to be taken for any given event.  For
example, the @code{key-press-hook} can be used to respond to the
@code{a} key being pressed by swinging the player's mighty sword.
There are many hooks available, so read on to learn about all of them.
For information about using Guile's hook API, see @xref{Hooks,,,
guile, GNU Guile Reference Manual}.

@deffn {Scheme Procedure} run-game [#:window-title "Chickadee!"]
       [#:window-width 640] [#:window-height 480] [#:window-fullscreen? #f]
       [#:update-hz 60]
Start the event loop.  This procedure will not return until
@code{abort-game} is called.

The @code{update-hook} will be run @var{update-hz} times per second.

A new graphical window will be opened with @var{window-width} x
@var{window-height} as its dimensions, @var{window-title} as its
title, and in fullscreen mode if @var{window-fullscreen?} is
@code{#t}.
@end deffn

@deffn {Scheme Procedure} abort-game
Stop the currently running Chickadee event loop.
@end deffn

@deffn {Scheme Procedure} time
Return the current game time in milliseconds.
@end deffn

@defvr {Scheme Variable} load-hook
A hook that is run once when the event loop boots, before any other
hook is run.  This hook is run with zero arguments.

@example
(add-hook! load-hook (lambda () (display "hello!\n")))
@end example

@end defvr

@defvr {Scheme Variable} update-hook
A hook that is run every time the game simulation should be advanced.
This hook is run with a single argument @var{dt}, the fixed timestep
that was configured when the event loop was started, in milliseconds.

@example
(add-hook! update-hook (lambda (dt) (display "tick!\n")))
@end example

@end defvr

@defvr {Scheme Variable} before-draw-hook
A hook that is run before a frame is rendered.  This hook is run with
zero arguments.

@example
(add-hook! before-draw-hook (lambda () (display "about to draw!\n")))
@end example

@end defvr

@defvr {Scheme Variable} after-draw-hook
A hook that is run after a frame is rendered.  This hook is run with
zero arguments.

@example
(add-hook! after-draw-hook (lambda () (display "done drawing!\n")))
@end example

Combined with @code{before-draw-hook}, one can perform a frames per
second calculation to monitor game performance and stability.

@end defvr

@defvr {Scheme Variable} draw-hook
A hook that is run each time a frame should be rendered.  This hook is
run with a single argument @var{alpha}, a value in the range [0, 1]
which represents how much time has past since the last game state
update relative to the upcoming game state update, as a percentage.
Because the game state is updated independent of rendering, it is
often the case that rendering is occuring between two updates.  If the
game is rendered as it was during the last update, a strange
side-effect will occur that makes animation appear rough or
``choppy''.  To counter this, the @var{alpha} value can be used to
perfrom a linear interpolation of a moving object between its current
position and its previous position.  This odd trick has the pleasing
result of making the animation look smooth again, but requires keeping
track of previous state.

(TODO: Add example of linear interpolation)

@example
(add-hook! draw-hook (lambda (alpha) (display "<(._.<) \n")))
@end example

@end defvr

@node Input
@section Input

@node Math
@section Math

@node Graphics
@section Graphics