Most alphabetic

Marcin Ciura
Twitter: @mciura

Judges' comments:

To use:


./ en | grep .. | ./prog aeiouvwxyz


./ en | grep .. | ./prog aeiouhjklmnvwxyz

./ en | grep .. | ./prog mrjocktvquizphdbagsfewlynx




Selected Judges Remarks:

A few letters, one at a time, with no repeats. How many different ways can this be done?

While a quick brown fox might jump over the lazy dog, it has too many repeat letters to allow this entry to repeat. It is better that Mr Jock, TV quiz PhD, bags few lynx.

Speaking of jumping, can you rewrite the code to remote all of the goto jumps in this code?

Author’s comments:

What is this?

This entry reads from standard input a list of words in any language written with an alphabet and outputs perfect pangrams composed of these words. A perfect pangram is a series of words that contains every letter of the alphabet exactly once.

Example execution:

$ grep .. /usr/share/dict/words | ./prog abcdefghijklmnopqrstuvwxyz
qualm fjord wiz pyx vs beg kc nth
quartz jinx vs fed kc womb glyph
quid jamb fez vs pyx kc growl nth
quiz fjord pyx vs bag kc mewl nth
quiz fjord pyx vs gab kc mewl nth
quiz fjord pyx vs gem bawl kc nth
quiz fjord pyx vs meg bawl kc nth
quiz fjord vex bawl kc gyp ms nth
quiz jamb flex vs kc gyp word nth
quiz jamb pyx vs kc flog drew nth
quiz jamb pyx vs kc frog lewd nth
quiz jamb pyx vs kc frog weld nth
quiz jamb pyx vs kc golf drew nth
quiz jamb pyx vs kc grew fold nth
quiz jamb pyx vs kc grow nth fled
quiz jamb pyx vs kc growl fed nth

Rearranging the words into more or less meaningful expressions is the user’s duty.

This entry implements Algorithm X (Exact cover via dancing links) from Section in fascicle 5C of The Art of Computer Programming by Donald E. Knuth. As of June 2019, you can download an incomplete draft of the fascicle from Knuth’s website.


prog LETTERS [N] reads allowed words from standard input and writes perfect pangrams with at most N words composed of LETTERS to standard output, one per line. If N is not given, the number of words in the pangrams is unlimited. The characters in LETTERS must not repeat. The length of LETTERS must not exceed 97 characters.

The exit status is 0 if no error occurred, 1 if the input word list is too long, and 2 if the command-line arguments are missing.

Helper scripts LANGUAGE_CODE outputs a sorted list of unique words in a given language, one per line. It requires aspell with a dictionary for LANGUAGE_CODE. LETTERS [N] works similarly to prog LETTERS [N] except that it merges anagrams before passing them to prog and expands them in prog’s output. This way, it finishes the job faster than prog when there are millions of perfect pangrams. It requires a UTF-8 locale.,,,,, and output German, English, French, Italian, Polish, and Russian perfect pangrams, respectively. They require a UTF-8 locale.

Miscellaneous remarks

I dare submit this entry to categories algorithms, internationalization (I have not found active use of <wchar.h> among the programs that won IOCCC in the past), and obscure bugs (see the questions about Linux core dumps below).

To a casual eye, this entry may look similar to 2005 klausler. However, prog is internationalized and way faster. On my machine, ./klausler abcdefghijklmnopqrstuvwxyz printed only 3,965 perfect pangrams until I killed it after 144 hours running, while the equivalent tr A-Z a-z < /usr/share/dict/words | egrep -v '^[^ais]$' | ./prog abcdefghijklmnopqrstuvwxyz | wc -l finished in 62 minutes, reporting 1,640,569,315 perfect pangrams, and the example from section What is this? finished in 1.35 seconds. On the other hand, prog can only output series of words with nonrepeating characters, unlike klausler.

In contrast to prog.orig.c, prog.c does not call fflush(stdout) after outputting each line, thus running faster. I am grateful to Witold Jarnicki for suggesting this change. The original prog finished the example above in 100 minutes instead of 62 minutes.

If your word list contains meaningless one-letter words, pipe grep .. [FILE] through prog to eliminate the pangrams that contain them.

According to prog and aspell, some languages, e.g. Spanish, have no perfect pangrams.

Why does yes | ./prog y hang on Linux? Change static int to int and static wchar_t to wchar_t, and recompile prog. Why does yes | ./prog y dump core now? Change the line #define P 9<<23^1 to #define P 9<<23 and recompile prog. Why does yes | ./prog y exit gracefully?

Answer: Nf bs Whar 2019, guhf znavsrfg ohtf va tyvop 2.22–2.29: 1 naq 2.

The source code uses goto three times. While it could easily get by without goto M and goto H, I challenge the adherents of structured programming to refactor goto T, which jumps back into a nested if block.

With the supplied Makefile, both gcc and clang compile prog.c without warnings in C11 and C99 mode. For a clean compilation with gcc -std=c90, add -Wno-format to CSILENCE in Makefile.

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