Format fix + unrot13 text in 1992/gson/README.md
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# Most Humorous Output
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Andreas Gustafsson\
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Helsinki University of Technology\
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Arentikuja 1 D 305\
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00410 Helsinki\
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Finland
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@@ -69,16 +66,16 @@ The name of the game:
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AG is short for either Anagram Generator or simply AnaGram. It might also be
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construed to mean Alphabet Game, and by pure coincidence it happens to be the
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author's initials.
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\
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\
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### What it does
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\
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AG takes one or more words as arguments, and tries to find anagrams of those
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words, i.e. words or sentences containing exactly the same letters.
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### How to use it
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\
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To run AG, you need a dictionary file consisting of distinct words in the
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natural language of your choice, one word on each line. If your machine doesn't
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have one already, you can make your own dictionary by concatenating a few
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@@ -102,7 +99,7 @@ limit can be changed using a numeric command line option, as in
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`./ag -4 international obfuscated c code contest </usr/dict/words`.
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### Bugs
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\
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- There is no error checking.
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- Standard input must be seekable, so you can't pipe the dictionary into AG.
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- The input sentence and each line in the dictionary may contain at most 32
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@@ -115,8 +112,8 @@ less.
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### Obfuscatory notes
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\
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As you can see, AG takes advantage of the new '92 whitespace rules to
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As you can see, AG takes advantage of the new '92 whitespace rules' to
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achieve a clear, readable, self-documenting layout. The identifiers
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have been chosen in a way appropriate for an alphabet game, and common
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sources of bugs such as goto statements and malloc/free have been
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@@ -124,135 +121,139 @@ eliminated. As AG also refrains from abusing the preprocessor, it
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doesn't really have much to offer in terms of "surface obfuscation".
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Instead, it tries to achieve both its speed and its obscurity through a
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careful choice of algorithms. Some of the finer points of those
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algorithms are outlined in the rot-13 encoded spoiler below.
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algorithms are outlined in the spoiler below.
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### How it works: (ROT13 to read)
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Here follows a description of some of the data structures and
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algorithms used by AG. It is by no means complete, but it may help
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you get an idea about the general principles.
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--
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Internally, AG represents words and sentences as arrays of 32
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4-bit integer elements. Each element represents the number of
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times a letter occurs in the word/sentence. There are 32 elements
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because 32 is a convenient power of two larger than the number of
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letters in most western alphabets, and the elements are 4 bits
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each because the same letter is unlikely to occur more than 15
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times in a practical anagram generation problem.
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These `32*4`-bit arrays are actually stored in memory in a
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"bit-transposed" format, as arrays of four "long" values. It is
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assumed that a "long" is at least 32 bits. The first 4-bit letter
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count is formed by the least significant (2^0) bit in each of the
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four longs, the next one is formed by the 2^1 bits, etc.
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This storage format makes it possible to add or subtract two such
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vectors of 32 4-bit values in parallel by simulating a set of 32
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binary full adders in software using bitwise logical operations.
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E.g., all the LSB:s of the result are formed in parallel by taking
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the exclusive OR of the LSB:s in each summand, and 32 carry bits
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are formed in parallel in a similar way using a logical AND.
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Thus, 32 independent 4-bit additions can be performed by just four
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iterations of a loop containing some 32-bit bitwise logical
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operations, but no arithmetic operations other than those implied
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by array indexing.
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Subtraction works similarly, and in fact AG only implements
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subtraction directly, handling addition by means of the identity
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`a+b = a-(0-b)`.
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In addition to this `32*4`-bit representation, AG also forms a so-called
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"signature" that is the bitwise OR of the four longs, which is
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equivalent to saying that the signature of a word contains a logical 1
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in the bit positions corresponding to letters occurring at least once
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in that word.
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The first thing AG does is to construct a lookup table of 256
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longs, one for each 8-bit character value. The entry for a
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character will be zero if that character doesn't appear in the
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sentence given on the command line, or it will have a single bit
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set if the character does appear in the sentence. By adding
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together the bit masks for all the letters in the input sentence
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using the transpose addition method described above, AG forms the
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`32*4` bit array representation of the input sentence.
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The next action performed is reading the dictionary. Those words that
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contain letters not in the input sentence are immediately discarded.
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Words containing the right letters but in excessive numbers are
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eliminated in a separate check involving the `32*4` bit array.
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The remaining words, which will be referred to as "candidate words",
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are stored in `32*4`-bit representation, together with their signatures\
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and offsets into the dictionary file so that the plain-text version of
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a word can later be found for printing. This information is kept in a
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local "struct" in the dictionary-reading function, and memory is
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allocated for each candidate word simply by making another recursive
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call to that function.
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Each struct so allocated is linked into a fixed-size hash table of
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4096 entries indexed by the 12 low bits of the word's signature.\
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When the dictionary-reading function encounters end-of-file, all the
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candidate words have been stored in nested activation records on the
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stack, accessible through the hash table.
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Generating the anagrams is then done by traversing the hash table and
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subtracting the letters of each word in the hash table from the
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"current sentence", which initially is the sentence given on the
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command line.
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The subtraction is performed in parallel on the 4-bit letter counts
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as described above, and if all 32 results are zero, an anagram has
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been found. If the result is negative for one or more of the letters
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(as indicated by one or more "1" in a vector of 32 borrow bits
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returned by the subtraction routine), the word did not match the
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current sentence and is ignored. Finally, if the result contained
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only non-negative letter counts, we have found a partial anagram:\
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a word containing some, but not all, of the letters in the current
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sentence. In this case we recursively try to find an anagram of the
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remaining letters. The depth of the recursion is limited to the
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maximum number of words in the anagram, as specified by the user.
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When the deepest recursion level has been reached, an optimization can
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be applied: because no further recursion will be done, there is no
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need to look for partial anagrams, and therefore AG only needs to
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check for words that contain exactly the same letters as the current
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sentence. Those words can be found simply by indexing the hash table
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with the signature of the current sentence.
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Even when not on the deepest recursion level, AG generally avoids
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examining all the entries of the hash table. The idea is that we are
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not interested in hash buckets whose words contain any letters not
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in the current sentence; these buckets are exactly those whose index
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has a logical one in a bit position where the signature of the current
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sentence has a zero. Put another way, we want to loop through only
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those hash bucket indices "i" that contain zeroes in all the bit
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positions where the signature "s" of the current sentence contains
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a zero; this can be expressed in C as (i & ~s == 0).
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It is possible to loop through all such numbers in an efficient way by
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taking advantage of certain properties of binary arithmetic: by
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forcing the bits corresponding to zeroes in "s" to ones, we can make
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the carries generated in incrementing "i" propagate straight across
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those bits that should remain zero. For example, the following
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program prints all those 16-bit integers that contain zeroes in all
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even bit positions:
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```c
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main(){int i=0,s=0xAAAA;do{printf("%04x\t",i);}while(i=((i|~s)+1)&s);}
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```
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AG uses a similar method but works in the opposite direction, finding
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the next lower value with zeroes in given bit positions by propagating
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borrows across those bits. Some additional adjustments are made
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to the hash table index when initiating a recursive search, using
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similar bit-twiddling techniques.
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Whenever an anagram has been found, it is printed by traversing a
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linked list formed by structs in the activation records of the
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recursive invocations of the search function, seeking to the beginning
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of the word matched by that invocation, and copying the characters of
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the word directly from standard input to standard output.
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Urer sbyybjf n qrfpevcgvba bs fbzr bs gur qngn fgehpgherf naq
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nytbevguzf hfrq ol NT. Vg vf ol ab zrnaf pbzcyrgr, ohg vg znl uryc
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lbh trg na vqrn nobhg gur trareny cevapvcyrf.
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\
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--
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\
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Vagreanyyl, NT ercerfragf jbeqf naq fragraprf nf neenlf bs 32
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4-ovg vagrtre ryrzragf. Rnpu ryrzrag ercerfragf gur ahzore bs
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gvzrf n yrggre bpphef va gur jbeq/fragrapr. Gurer ner 32 ryrzragf
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orpnhfr 32 vf n pbairavrag cbjre bs gjb ynetre guna gur ahzore bs
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yrggref va zbfg jrfgrea nycunorgf, naq gur ryrzragf ner 4 ovgf
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rnpu orpnhfr gur fnzr yrggre vf hayvxryl gb bpphe zber guna 15
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gvzrf va n cenpgvpny nantenz trarengvba ceboyrz.
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\
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Gurfr 32*4-ovg neenlf ner npghnyyl fgberq va zrzbel va n
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"ovg-genafcbfrq" sbezng, nf neenlf bs sbhe "ybat" inyhrf. Vg vf
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nffhzrq gung n "ybat" vf ng yrnfg 32 ovgf. Gur svefg 4-ovg yrggre
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pbhag vf sbezrq ol gur yrnfg fvtavsvpnag (2^0) ovg va rnpu bs gur
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sbhe ybatf, gur arkg bar vf sbezrq ol gur 2^1 ovgf, rgp.
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\
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Guvf fgbentr sbezng znxrf vg cbffvoyr gb nqq be fhogenpg gjb fhpu
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irpgbef bs 32 4-ovg inyhrf va cnenyyry ol fvzhyngvat n frg bs 32
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ovanel shyy nqqref va fbsgjner hfvat ovgjvfr ybtvpny bcrengvbaf.
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R.t., nyy gur YFO:f bs gur erfhyg ner sbezrq va cnenyyry ol gnxvat
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gur rkpyhfvir BE bs gur YFO:f va rnpu fhzznaq, naq 32 pneel ovgf
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ner sbezrq va cnenyyry va n fvzvyne jnl hfvat n ybtvpny NAQ.
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Guhf, 32 vaqrcraqrag 4-ovg nqqvgvbaf pna or cresbezrq ol whfg sbhe
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vgrengvbaf bs n ybbc pbagnvavat fbzr 32-ovg ovgjvfr ybtvpny
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bcrengvbaf, ohg ab nevguzrgvp bcrengvbaf bgure guna gubfr vzcyvrq
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ol neenl vaqrkvat.
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\
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Fhogenpgvba jbexf fvzvyneyl, naq va snpg NT bayl vzcyrzragf
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fhogenpgvba qverpgyl, unaqyvat nqqvgvba ol zrnaf bs gur vqragvgl
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n+o = n-(0-o).
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\
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Va nqqvgvba gb guvf 32*4-ovg ercerfragngvba, NT nyfb sbezf n fb-pnyyrq
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"fvtangher" gung vf gur ovgjvfr BE bs gur sbhe ybatf, juvpu vf
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rdhvinyrag gb fnlvat gung gur fvtangher bs n jbeq pbagnvaf n ybtvpny 1
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va gur ovg cbfvgvbaf pbeerfcbaqvat gb yrggref bppheevat ng yrnfg bapr
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va gung jbeq.
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\
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Gur svefg guvat NT qbrf vf gb pbafgehpg n ybbxhc gnoyr bs 256
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ybatf, bar sbe rnpu 8-ovg punenpgre inyhr. Gur ragel sbe n
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punenpgre jvyy or mreb vs gung punenpgre qbrfa'g nccrne va gur
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fragrapr tvira ba gur pbzznaq yvar, be vg jvyy unir n fvatyr ovg
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frg vs gur punenpgre qbrf nccrne va gur fragrapr. Ol nqqvat
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gbtrgure gur ovg znfxf sbe nyy gur yrggref va gur vachg fragrapr
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hfvat gur genafcbfr nqqvgvba zrgubq qrfpevorq nobir, NT sbezf gur
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32*4 ovg neenl ercerfragngvba bs gur vachg fragrapr.
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\
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Gur arkg npgvba cresbezrq vf ernqvat gur qvpgvbanel. Gubfr jbeqf gung
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pbagnva yrggref abg va gur vachg fragrapr ner vzzrqvngryl qvfpneqrq.
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Jbeqf pbagnvavat gur evtug yrggref ohg va rkprffvir ahzoref ner
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ryvzvangrq va n frcnengr purpx vaibyivat gur 32*4 ovg neenl.
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\
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Gur erznvavat jbeqf, juvpu jvyy or ersreerq gb nf "pnaqvqngr jbeqf",
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ner fgberq va 32*4-ovg ercerfragngvba, gbtrgure jvgu gurve fvtangherf\
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naq bssfrgf vagb gur qvpgvbanel svyr fb gung gur cynva-grkg irefvba bs
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n jbeq pna yngre or sbhaq sbe cevagvat. Guvf vasbezngvba vf xrcg va n
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ybpny "fgehpg" va gur qvpgvbanel-ernqvat shapgvba, naq zrzbel vf
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nyybpngrq sbe rnpu pnaqvqngr jbeq fvzcyl ol znxvat nabgure erphefvir
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pnyy gb gung shapgvba.
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\
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Rnpu fgehpg fb nyybpngrq vf yvaxrq vagb n svkrq-fvmr unfu gnoyr bs
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4096 ragevrf vaqrkrq ol gur 12 ybj ovgf bs gur jbeq'f fvtangher.\
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Jura gur qvpgvbanel-ernqvat shapgvba rapbhagref raq-bs-svyr, nyy gur
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pnaqvqngr jbeqf unir orra fgberq va arfgrq npgvingvba erpbeqf ba gur
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fgnpx, npprffvoyr guebhtu gur unfu gnoyr.
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\
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Trarengvat gur nantenzf vf gura qbar ol genirefvat gur unfu gnoyr naq
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fhogenpgvat gur yrggref bs rnpu jbeq va gur unfu gnoyr sebz gur
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"pheerag fragrapr", juvpu vavgvnyyl vf gur fragrapr tvira ba gur
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pbzznaq yvar.
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\
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Gur fhogenpgvba vf cresbezrq va cnenyyry ba gur 4-ovg yrggre pbhagf
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nf qrfpevorq nobir, naq vs nyy 32 erfhygf ner mreb, na nantenz unf
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orra sbhaq. Vs gur erfhyg vf artngvir sbe bar be zber bs gur yrggref
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(nf vaqvpngrq ol bar be zber "1" va n irpgbe bs 32 obeebj ovgf
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erghearq ol gur fhogenpgvba ebhgvar), gur jbeq qvq abg zngpu gur
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pheerag fragrapr naq vf vtaberq. Svanyyl, vs gur erfhyg pbagnvarq
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bayl abaartngvir yrggre pbhagf, jr unir sbhaq n cnegvny nantenz:\
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n jbeq pbagnvavat fbzr, ohg abg nyy, bs gur yrggref va gur pheerag
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fragrapr. Va guvf pnfr jr erphefviryl gel gb svaq na nantenz bs gur
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erznvavat yrggref. Gur qrcgu bs gur erphefvba vf yvzvgrq gb gur
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znkvzhz ahzore bs jbeqf va gur nantenz, nf fcrpvsvrq ol gur hfre.
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\
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Jura gur qrrcrfg erphefvba yriry unf orra ernpurq, na bcgvzvmngvba pna
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or nccyvrq: orpnhfr ab shegure erphefvba jvyy or qbar, gurer vf ab
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arrq gb ybbx sbe cnegvny nantenzf, naq gurersber NT bayl arrqf gb
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purpx sbe jbeqf gung pbagnva rknpgyl gur fnzr yrggref nf gur pheerag
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fragrapr. Gubfr jbeqf pna or sbhaq fvzcyl ol vaqrkvat gur unfu gnoyr
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jvgu gur fvtangher bs gur pheerag fragrapr.
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\
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Rira jura abg ba gur qrrcrfg erphefvba yriry, NT trarenyyl nibvqf
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rknzvavat nyy gur ragevrf bs gur unfu gnoyr. Gur vqrn vf gung jr ner
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abg vagrerfgrq va unfu ohpxrgf jubfr jbeqf pbagnva nal yrggref abg
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va gur pheerag fragrapr; gurfr ohpxrgf ner rknpgyl gubfr jubfr vaqrk
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unf n ybtvpny bar va n ovg cbfvgvba jurer gur fvtangher bs gur pheerag
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fragrapr unf n mreb. Chg nabgure jnl, jr jnag gb ybbc guebhtu bayl
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gubfr unfu ohpxrg vaqvprf "v" gung pbagnva mrebrf va nyy gur ovg
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cbfvgvbaf jurer gur fvtangher "f" bs gur pheerag fragrapr pbagnvaf
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n mreb; guvf pna or rkcerffrq va P nf (v & ~f == 0).
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\
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Vg vf cbffvoyr gb ybbc guebhtu nyy fhpu ahzoref va na rssvpvrag jnl ol
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gnxvat nqinagntr bs pregnva cebcregvrf bs ovanel nevguzrgvp: ol
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sbepvat gur ovgf pbeerfcbaqvat gb mrebrf va "f" gb barf, jr pna znxr
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gur pneevrf trarengrq va vaperzragvat "v" cebcntngr fgenvtug npebff
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gubfr ovgf gung fubhyq erznva mreb. Sbe rknzcyr, gur sbyybjvat
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cebtenz cevagf nyy gubfr 16-ovg vagrtref gung pbagnva mrebrf va nyy
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rira ovg cbfvgvbaf:
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\
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znva(){vag v=0,f=0kNNNN;qb{cevags("%04k\g",v);}juvyr(v=((v|~f)+1)&f);}
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\
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NT hfrf n fvzvyne zrgubq ohg jbexf va gur bccbfvgr qverpgvba, svaqvat
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gur arkg ybjre inyhr jvgu mrebrf va tvira ovg cbfvgvbaf ol cebcntngvat
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obeebjf npebff gubfr ovgf. Fbzr nqqvgvbany nqwhfgzragf ner znqr
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gb gur unfu gnoyr vaqrk jura vavgvngvat n erphefvir frnepu, hfvat
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fvzvyne ovg-gjvqqyvat grpuavdhrf.
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\
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Jurarire na nantenz unf orra sbhaq, vg vf cevagrq ol genirefvat n
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yvaxrq yvfg sbezrq ol fgehpgf va gur npgvingvba erpbeqf bs gur
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erphefvir vaibpngvbaf bs gur frnepu shapgvba, frrxvat gb gur ortvaavat
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bs gur jbeq zngpurq ol gung vaibpngvba, naq pbclvat gur punenpgref bs
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gur jbeq qverpgyl sebz fgnaqneq vachg gb fgnaqneq bhgchg.
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## Copyright and CC BY-SA 4.0 License:
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Reference in new issue
Block a user