A random dungeon generator that fits on a business card

generate.c

#include <time.h> //  Robert Nystrom
#include <stdio.h> // @munificentbob
#include <stdlib.h> //     for Ginny
#define  r return    //    2008-2019
#define  l(a, b, c, d) for (i y=a;y\
<b; y++) for (int x = c; x < d; x++)
typedef int i;const i H=40;const i W
=80;i m[40][80];i g(i x){r rand()%x;
}void cave(i s){i w=g(10)+5;i h=g(6)
+3;i t=g(W-w-2)+1;i u=g(H-h-2)+1;l(u
-1,u+h+2,t-1            ,t+w+2)if(m[
y][x]=='.'                  )r;i d=0
;i e,f        ;if(!s){l(      u-1,u+
h+2,t-    1,t+w+2){i s=x<t     ||x>t
+w;i    t=y<u||           y>    u+h;
if(s    ^t&&              m[      y]
[x    ]=='#'    ){d++;    if(g    (d
)     ==0)    e=x,f=y;    }}if    (d
==    0)r;    }l(u-1,u    +h+2    ,t
-1    ,t+w    +2){i s=    x< t    ||
x>    t+w;    i t= y<u    ||y>    u+
h;    m[y]      [x]= s    &&t?   '!'
:s^t    ?'#'                    :'.'
;}if    (d>0)m                  [f][
e]=g(2    )?'\'':'+';for(i j=0;j<(s?
1:g(6)        +1);j++)m[g(h)+u][g(w)
+t]=s?'@'                 :g(4) ==0?
'$':65+g(62)              ;}i main(i
argc, const char* argv[]) {srand((i)
time(NULL));l(0, H, 0,W)m[y][x]=' ';
for(i j=0;j<1000;j++)cave(j==0);l(0,
H,0,W) {i c=m[y][x]; putchar(c=='!'?
'#':c);if(x==W-1)printf("\n");}r 0;}

Obligatory (badly-)golfed version:

#include <time.h> //  Robert Nystrom
#define l for(y=u-1; y< u+h+2; y++)\
for  (x = t - 1; x < t + w + 2; x++)
a,b,d,e,f,h,j,k,n,u,t,w,x,y;m[40][80
];g(x){return rand()%x;};cave(s){w=g
(10)+5;h=g(6)+3;t=g(80-w-2)+1;u=g(40
-h-2)+1;l if(m[y][x]==14)return;d=0;
if(!s){l{k=x<t||x>t+w;n=y<u||y>u+h ;
if(k^n&&m[y][x]==3){d++;if(!g(d))e=x
,f=y;}}if(!d)return;}l{a=x<t||x>t+w;
b=y<u||y>u+h;m[y][x]="/$ "[a+b]-33;}
if(d)m[f][e]=g(2)?7:11;for(j=0;j++<(
s?1:g(6)+1);)m[g(h)+u][g(w)+t]=s?32:
g(4)?33+g(62):4;}main(){srand(time(j
=0));for(;j<999;j++)cave(!j);for(y=0
;y<40;puts(""),y++)for(x=0;x<80;x++)
putchar(32 + (a = m[y][x])+4*!~a) ;}

Putting the @ back in left as an exercise for the reader because I'm too lazy.
Note that this relies heavily on UB as you can see from the screenful of warnings.

I found a bug where rooms are placed corner-to-corner and the door is not traversable:

..#
..'##
###..

Quick and dirty workaround (added - 1):

let s = x < left || x > left + width - 1;
let t = y < top || y > top + height - 1;

On @grownseed .js version.

CoffeeScript https://codepen.io/samme/pen/gOppRwV

Haha, I also de-obfuscated this code before coming here and seeing that other people did it too!
I've had a lot of fun in understanding how this piece of code works :)

Here's my version: https://gist.github.com/paskozdilar/48d7532733ccd11144bb43fed953c334

conversion to calculang formulas; visualizations with animated evolution: https://declann.observablehq.cloud/calculang-dx-pattern-exampling/dungeon/dungeon

or https://fosstodon.org/@calculang/111961605363861853

truly beautiful, thank you for sharing this work of art