make
To change the dimensions you can use the WIDTH
and HEIGHT
variables when
compiling (default 1024 x 800):
make clobber WIDTH=640 HEIGHT=480
to make the game 640 x 480. Obviously changing it to non numbers or negative numbers or too large numbers will cause problems.
There is an alternate version that is based on the author’s suggestion to change a value in order to see a bug that they avoided in their entry. See Alternate code below.
./prog
./prog
Press and hold the Left Arrow key until you turn 90-degrees to the left.
When you see a long tunnel, press the function key F2.
Use the Up, Down, Left and Right keys to change your path.
Press function higher numbered function keys (F3, F4, …) to go faster.
Press F1 to stop.
Turn 90-degrees to one side.
Press F4 to drill into some blocks for a bit.
Press F1 to stop.
Turn 180-degrees to face the way you came.
Press F2 and navigate back down the new hole until you reach the long tunnel again.
Press F1 to stop.
etc.
Press the ESC key.
The author suggested that one change q
to 0x82000820000000
instead of
0x820008202625a0
to see a bug that they avoided, writing:
To see this bug in action change the assignment above
to q=0x82000820000000`, and look towards the negative
direction of the main tunnel.
This alt version does this for you.
make alt
Use prog.alt
as you would prog
above.
Try locating the issue the author suggested exists.
You are in a twisty maze blocks, almost all alike.
Those blocks are generated by twisty maze of code blocks, almost all different.
Can you find your way around either and understand where you are?
How many function keys does your keyboard have? Can you generate very high number function keys such as F30 or F31? :-)
Textures? We don’t need no stinking textures.
Just run ./prog
with no arguments on an X11-compatible system. The scenery, though
pseudo-randomly generated should be strangely familiar, especially to those still
in their teenage years. You can navigate using the keyboard arrow keys and the
function keys. Your speed is a linear function of (n-1)
where Fn
is the last
function key you have pressed, and you always move in the direction you are
looking at. Be aware that you are probably some kind of a Creeper as is obvious
from looking at the program, hence your movement is destructive to the
environment. So after pressing F12 for warp speed, and moving around randomly,
you might find yourself in a 3D maze of twisty little passages, from which there
is only one Esc(ape) key. Note that in theory if you have a specialty keyboard
with 32 function keys you may even press F31 to reach infinite improbability
drive speed, though the author had never tested this feature. However a hidden
comment in keysymdef.h
suggests the existence of a mythical Sun Keyboard with 35
function keys :)
This entry, though pretty thoroughly obfuscated, is not entirely original, nor am I Notch. The inspiration for this entry is a demo code written by the Notch himself, which was released with the following license: “you may use the code in here for any purpose in any way you want, at your own risk”. That version will henceforth be referred to as the original. Though the original in itself was quite nice and rather dense piece of code, it was not written in C, nor entirely and meticulously obfuscated, a state of things I hopefully rectified.
Basing it on the original which already had a wonderful simple and concise ray casting through a generated voxel world rendering, I added a few tweaks of my own device:
This entry is rather portable I think on modern platforms. It assumes of course basic X-windows availability. There are several other salient assumptions though:
sizeof(int)==4
. This seems to cover pretty much everything
that supports X-windows so would be broken only on quite ancient machines,
embedded processors, or some big iron 64 bit system where sizeof(int)=8
.keysymdef.h
. I
have searched far and wide the lord Google’s domain, yet was unable to find a
version of the file where those assumptions were broken. This enables the
important support of up to 32 function keys keyboards without spilling over the
size limit, and allows us to use such fun expression for arrow view control as
the following:*(a&1?&C:&B)-=(.05 -a/2%2*.1)*!(a-1&4092^3920)
The entry, on purpose, (see the section about SIMD below) does not use enough bits of precision, so sometimes the ray will miss the intersection of two voxel faces and push on through. That’s why you may notice some noise along close up voxel edges. Fixing this issue would have taken the entry over the size limit regrettably.
Well, of course you have all the usual suspects, used for extreme compression, and such a nice layout: not very informative identifiers, which are being reused all over the place, some rather gruesome looking expressions, a whole load of magic constants with apparently no rhyme or reason (including a Lagrange polynomial), and a message from our friendly Cetaceans. However it also includes some clever and not so obvious though instructive techniques, to subtly make the code more magical and efficient, at the same time.
Those techniques are about the use of 64 bit integers in order to do SIMD work, in a portable way without ever using any SSE/AVX or other non-portable compiler inline functions! The irony though, is that when compiling the program on modern X86 architectures indeed SSE/AVX is used for the long longs. Following is some “Spoiler” information, So if you want to play tough, stop reading at this point.
So how is this done? Quite simply by using the whole long long
arithmetic’s
capability. I think the C99 standard assures us that long long
has at least 64
bits. Most of those bits are unused in normal programs, however here we use them
more wisely. For example, in the inner loop of the program you have the innocent
looking expression i+=d
. What it does in fact, is a SIMD addition of two
vectors, each with three signed fixed point coordinates, advancing our ray in 3D
space!
To spell it out, the macro producing such vectors is P
, and the assignment
q=0x820008202625a0
sets our creeper original position to about (32.5, 32.5019, 2441.406)
.
Evidently when doing such arithmetics, care must me taken to avoid overflow from
one component to the other, but since our voxel world is a torus of size 64 in
each dimension this hardly matters. A subtle issue, is how to deal with signed
quantities. In the i+=d
expression above i
has 3 positive components, but
d
has arbitrary signed components, as our ray may face in any direction. It
seems things should not work, specially as we defined both i
and d
as
unsigned
! However notice that the T
(and P
) macros convert float
to
signed long long
s.
Once sign is extended from a least component into higher bits, it seems at first
to be recipe for disaster, however everything works beautifully, and you get
precise component results when the final accumulation components in i
are
positive. This only gets broken a bit when the final accumulation has a least
significant negative component. To see this bug in action change the assignment
above to q=0x82000820000000
, and look towards the negative direction of the main
tunnel.
Similar technique is used on the RGB components so their brightness can be
adjusted simultaneously by the expression: o=o*b>>8
. Of course some spare bits
are needed between the components, so they are spread in the long long
and
reordered as RBG.