| HN Mirror

One of the smallest implementations is my heavily obfuscated https://www.ioccc.org/2012/tromp/ :

           Int L[A],m,b,*D=A,
            *c,*a=L,C,*U=L,u;s
             (_){u--&&s(a=*a);}
              char*B,I,O;S(){b=b
               --?b:m|read(0,&I,1
                )-1;return~I>>b&1;
                 }k(l,u){for(;l<=u;
                  U-L<A?*U++=46^l++[
                   "-,&,,/.--/,:-,'/"
                   ".-,-,,/.-,*,//..,"
                  ]:exit(5));}p(Int*m){
                 return!*U?*m=S()?U++,!S
                ()?m[1]=p(++U),2:3:1,p(U)
               :S()?U+=2:p(U[1]++),U-m;}x(
              c){k(7*!b,9);*U++=b&&S();c&&x
             (b);}d(Int*l){--l[1]||d(l[d(*l),
            *l=B,B=l,2]);}main(e){for(k(10,33
           ),a[4]-=m=e-2&7,a[23]=p(U),b=0;;e-2
          ?e?e-3?s(D=a),C=a  [3],++1[a=a[2]],d(
         D):c?D=c,c=*D,*D=    a,a=D:exit(L[C+1])
        :C--<23?C=u+m&1?O      =O+O|C&1,9:write(m
       ||(O=C+28),&O,1)+        1:(S(),x(0<b++?k(0,
      6),U[-5]=96:0)):(          D=B?B:calloc(4,X))
     ?B=*D,*D=c,c=D,D[            2]=a,a[++D[1]]++,D
    [3]=++C+u:exit(6)              )e=L[C++],u=L[C];}

while a less obfuscated and highly performant implementation https://github.com/tromp/AIT/blob/master/uni.c based on combinatory graph reduction takes 446 lines.