Most lazy SKIer

Yusuke Endoh

Judges' comments:

To build:


This builds endoh1, echo, hello, and tac.

To run:

./endoh1 [file.lazy]


./endoh1 hello.lazy
echo Hello | ./echo 
./tac < endoh1.c

Be patient with the last one.

Selected Judges Remarks:

We liked this entry because it can serve as a standalone program as well as an include file; and, as usual, because its input data is at least as obfuscated as the program itself.

This entry can be considered an abuse of the GCC’s optimizer; it takes GCC about 4x longer to compile it with -O3 than without, compared to clang’s 2x.

Author’s comments:


$ less prog.c
$ gcc -o prog prog.c
$ ./prog

… Isn’t there any more to it?

Yes, of course.

This is a tool for C programmers to play the SKI combinator calculus, especially, Lazy K.

This program will shine when it is used as a library. For example, hello.lazy is a “Hello world” program written in Lazy K.

$ lazy hello.lazy
Hello, world!

where lazy is a reference implementation of Lazy K. At the same time, hello.lazy is a valid C program that uses prog.c as a library.

$ gcc -o hello -xc hello.lazy
$ ./hello
Hello, world!

In other words, this program is a kind of polyglot.

The usage is simple: you just have to wrap Lazy K program with #include "prog.c". Note that # is a comment in Lazy K.


… is inherent in SKI combinator calculus :-)

In addition, it uses various hacks to parse SKI code as C, and to satisfy IOCCC’s size rule.

These led to the good obfuscation. See Spoiler section in detail, if you need.


This program supports only “Combinator-calculus style notation” of Lazy K. “Unlambda style” and “Iota and Jot” style are not supported. Also, it requires a space between identifiers. In short, use (S K) instead of (SK), “`sk”, **i*i*i*ii*i*i*ii, or 11111100011100.

Huge memory may be required to compile the program (about 300 MB on my machine).

In addition, there are some limitations (and workarounds) mentioned in Spoiler section. But I think it wouldn’t matter only when you run the attached programs.


I confirmed that the program successfully worked with the following compilers.

Recent compilers with -Wall -W -Wextra -pedantic say nothing.

gcc -Wall -W -Wextra -pedantic prog.c
clang -Wall -W -Wextra -pedantic prog.c
tcc -Wall -W -Wextra -pedantic prog.c

I think it will work on almost all platforms. I confirmed:

To check whether the program is specified on the command line or included from another source file, the program uses a predefined macro __INCLUDE_LEVEL__. It is a gcc extention, and also supported by clang. If your compiler does not support it, you cannot use the program as a library. But you can compile and run it as a standalone program, at least. In fact, tcc does not support the macro, but does work.

Spoiler (rot13)

Gur jubyr cebtenz vf vagrecergrq ol znpeb rkcnafvba. Sbe rknzcyr, F (X V) vf genafyngrq gb n abezny P pbqr, (f)((x)(v)), be fvzcyl f(x(v)). Guvf rkcerffvba ergheaf na nofgenpg flagnk gerr, naq cebt.p rinyhngrf vg. Guvf cebtenz hfrf irel fvzcyr “grez erjevgvat” nccebnpu sbe rinyhngvat FXV pbzovangbe pnyphyhf. Gur erjevgvat ehyrf ner fubja va gur funcr bs gur pbqr.

Nohfr bs shapgvba cbvagref

Pbafvqre n frdhrapr bs shapgvba nccyvpngvbaf va P:


Jung glcr fubhyq f unir? Hasbeghangryl, P qbrf abg cebivqr n “erphefvir glcr”, fhpu nf glcrqrs s (*s)();. Fb, V hfrq n urniyl arfgrq shapgvba cbvagre glcr:

glcrqrs ibvq *(*(*(*...(*(s))()...)())())

Guvf yvzvgf ubj znal nethzragf bar shapgvba pna or pbafrphgviryl nccyvrq gb. Ohg lbh pna vapernfr gur ahzore ol gjrnxvat gur qrsvavgvba bs znpeb c.

Pbqr qhcyvpngvba ol znpebf

Arkg, jr arrq gb rapbqr “pybfherf”. N pybfher vf n shapgvba gbtrgure jvgu na raivebazrag juvpu vf n ersrerapr gb aba-ybpny inevnoyr.

Va guvf pnfr, jr arrq fbzrguvat gung:

Ubjrire, ab glcr va P vf pnyynoyr naq unf n ersrerapr ng gur fnzr gvzr. (Vg vf srnfvoyr ol qlanzvp pbqr trarengvba, ohg vg vf sne sebz cbegnoyr.)

Fb, V nqqerffrq guvf vffhr ol trarengvat znal shapgvba qrsvavgvbaf fgngvpnyyl ol (no)hfvat znpebf:

ibvq *k1; ibvq s1(ibvq *l) { erghea nccyl(k1, l); }
ibvq *k2; ibvq s2(ibvq *l) { erghea nccyl(k2, l); }
ibvq *k3; ibvq s3(ibvq *l) { erghea nccyl(k3, l); }
ibvq *(u[]) = { s1, s2, s3, ... }

naq ol nyybgvat rnpu bs gurz jura n pybfher vf arrqrq.

Guvf yrnqf gb nabgure yvzvgngvba: gur ahzore bs cer-qrsvarq pybfherf yvzvgf ubj znal vaqragvsvref (F X V) bar cebtenz pna hfr. Ohg lbh pna vapernfr guvf ahzore ol gjrnxvat gur qrsvavgvba bs znpeb N O P naq Q. (Abgr gung pybfherf ner nyybggrq bayl jura cnefvat; nsgre gur rinyhngvba fgnegf, “bhg bs pybfher” pnaabg bpphe.)

Fubeg pbeqvat

Guvf znetva vf gbb aneebj gb pbagnva n qrgnvyrq rkcynangvba. Vafgrnq, V whfg nfx lbh bar dhrfgvba. Pna lbh gryy jung i f[]={0,0,f+6,f+2,f+4,f,f+3,f+5,f+1}; vf? V sbhaq guvf ol hfvat FZG fbyire.

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