Rename hint.txt and hint.text to README.md
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# Most ingenious puzzle solution
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Aidan Thornton
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United Kingdom
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makomk@lycos.co.uk
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## Judges' comments
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Are you puzzled about puzzles? This entry might puzzle you
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more while it puzzles out some puzzles: all in a puzzling way!
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Goto the source and notice the lack of functions. Jump
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to the switch statement. And if you think you can puzzle
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it out better, try solving insane1.sudoku all by yourself!
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### To build
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cc -O2 -o aidan aidan.c
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## Author's comments
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This program can solve a type of logic puzzle known as a Sudoku, and also
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generate new ones. (Apparently, the puzzle is known as Number Place in the US,
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but we in the UK got it via Japan, where it's quite common in magazines, etc.
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These things happen sometimes.)
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### Contents
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* build file: build.txt - you don't need this
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* program file: sudoku-sf.c - sudoku solver and generator
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* remarks file: sudoku-sf.txt - this file
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* info file: test-su.perl - optional test script (written for Perl5)
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* info file: eg-sudoku.tar - various assorted example sudokus
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**XXX - now the tar file includes the test script and the examples**
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Yes, the test suite really is 5 times larger than the actual program. You
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**did** say this was a parody of the software development process...
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> **IMPORTANT!**
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>
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> test.perl expects the sudoku-sf executable to be `./sudoku-sf`. If you rename
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> it, please change the definition of $sf_exe to the new name
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>
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> **IMPORTANT!**
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### Legal blurb
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This program comes with NO WARRANTY, not even that it'll do what I say it
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should. Any misfortunes that occur as a result of using this are not my fault.
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Go find some unpopular multinational to sue instead.
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This program is not copyrighted and is entirely my work. Distribute or modify
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it as you wish, though please give me credit.
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### What is a sudoku?
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(If you know, feel free to skip this section. Get more details from [the
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wikipedia entry])
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[the wikipedia entry]: http://en.wikipedia.org/wiki/Sudoku
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A sudoku is a type of logic puzzle. You are given a grid of 9x9 boxes, some
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of which contain digits. This is divided into nine 3x3 boxes, e.g. (from the
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Wikipedia article - fairly easy):
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. . . | 1 . . | 7 4 .
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. 5 . | . 9 . | . 3 2
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. . 6 | 7 . . | 9 . .
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------+-------+------
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4 . . | 8 . . | . . .
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. 2 . | . . . | . 1 .
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. . . | . . 9 | . . 5
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------+-------+------
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. . 4 | . . 7 | 3 . .
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7 3 . | . 2 . | . 6 .
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. 6 5 | . . 4 | . . .
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You have to fill in the remaining boxes so that each row, column, and smaller
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box contains all of the digits 1-9. That's all the information I needed to
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figure out how to solve them, but there's a useful tutorial on
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<http://www.sudoku.org.uk/>, which is roughly affiliated with the Daily
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Telegraph (a UK newspaper).
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Now, with modern computers this **can** be solved using brute force, e.g.:
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#include <stdio.h> /* sudoku-bfi.c */
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#define S(t) for(n=0;n<9;v[n++]=0);for(n=0;n<81;n++)if(i[n]){z=1<<i[n];if(v[t]\
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&z)for(;;){if(p<0)goto d;if(i[p]++<9)goto f;i[p]=0;p=s[p];}v[t]|=z;}
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int i[82],s[81],v[9],n,p,z;int main(void){for(n=0;n<81;){p=getchar()^48;if(p==
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30)p=0;if(p<10)i[n++]=p;}i[81]=0;p=-1;f:S(n%9)S(n/9)S(n%9/3+n/27*3)if(p>80)goto
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d;for(n=p;i[++p];);s[p]=n;i[p]=1;goto f;d:z=p<81;printf("\n%s!\n\n",z?"Fail":
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"Success");for(n=0;n<81;){p=i[n];printf("%c %s",(p?p|48:'.'),(++n%3?"":n%9?"| "
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:n%27?"\n":n%81?"\n------+-------+------\n":"\n\n"));}return z;}
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That isn't the approach I've used; it's slow (particularly in worst-case or
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nearly so scenarios), inelegant, and not a good starting point for sudoku
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generation. It's also much too easy to understand ;-) Instead, I've written a
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program which solves them in much the same way as I do, only a lot faster (and
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hopefully more reliably).
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Oh, and if you think that example sudoku is too easy, try this one (which my
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program generated):
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--- insane1.sudoku --
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. . 4 | . . . | . 5 6
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5 . . | . 7 2 | . . .
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. . 1 | . . . | 8 . .
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------+-------+------
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. . . | . . . | . . .
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. . . | 6 9 3 | . . 5
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. . . | . . . | 7 3 4
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------+-------+------
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. 5 . | 2 . 1 | 4 . 8
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3 . . | . . . | . . .
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. . . | . . . | . 6 1
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Be warned - it's evil! (I certainly haven't been able to solve it by hand. The
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brute-force program given above, sudoku-bfi.c, also has trouble - it took 66
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seconds to solve it - but probably for different reasons. That's the worst
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performance I've had from brute-force so far!)
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### Usage
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#### Building
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cc -o sudoku-bf sudoku-bf.c
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-- or, if using gcc, try --
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gcc -O2 -Wall -Wextra -ansi -pedantic-errors -o sudoku-sf sudoku-sf.c
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-- or, if using gcc and feeling lazy --
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sh build.txt
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#### Testing (optional)
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perl ./test-su.perl
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Note that this requires Perl 5 (I use 5.8.5, earlier versions untested) and is
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somewhat slow (should take under a minute on a fairly modern PC - try
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appending "-n 0" if you're in a hurry). I wrote it mostly for my own benefit,
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really.
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#### Solving
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./sudoku-sf < somefile.txt
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Input should be the numbers of each row in turn. Empty spaces can be
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represented as a period (.) or zero (0). Other characters are ignored, so you
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can cut-and-paste either example sudoku from above into a file and feed that
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in. The program has coped with every input I've thrown at it so far, including
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an empty grid.
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Output consists of the problem, then the solution, then a message of
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success/failure.
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`./sudoku-sf U` (capital U), may also be useful, or perhaps not. It only took
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an extra 15 bytes of (very simple) code to add, so it doesn't really matter to
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me. (It's the same as the normal mode for most input you're likely to feed in,
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though slightly slower. See if you can figure out what it does.)
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#### Generating
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./sudoku <seed>
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Output is a blank grid, then the solution, then the problem. There's no
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control over the difficulty. Sorry. However, all generated sudokus should have
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exactly one solution - if one doesn't, that's a bug.
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### Speed
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Fast enough. Solving and generating are practically instant on my 1Ghz Duron.
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Of course, there could be some cases which take longer...
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More precisely - 400 random sudokus are generated and solved in about 45
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seconds in tests on my PC (1Ghz Duron, gcc 3.4.1, -O2 optimisation - takes
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about 60 seconds with no optimisation). If that isn't fast enough, what on
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Earth are you doing with it?
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### Bugs
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None known - hopefully I've worked out all the major ones. Nothing has come up
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in testing since the last bugfix, but what does that mean?
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Tested on:
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- gcc 3.4.1-3mdk, Linux/x86
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*RETEST* Borland C 4.5, 16-bit DOS .exe (small memory model)
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I don't count the fact that it first prints a blank grid whenever asked to
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generate a sudoku as a bug - just a minor quirk. The fix is slightly awkward
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and would make the program slightly longer, so I haven't bothered - it's not
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important.
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### Missing Features
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* No control over the difficulty of generated sudokus - they vary from easy
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to hair-tugging near-impossibility. Solver beware. Seeing as apparently one
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program [took six years], I'm in no hurry.
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[took six years]: http://news.bbc.co.uk/1/hi/magazine/4469719.stm
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* Some sort of curses-based UI to let you solve interactively might be cool,
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but I don't feel like learning curses just for this. Besides, I spent enough
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of the time swearing as it is.
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* If you mistype a sudoku (e.g. from a paper/magazine) - and you will it
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can't help you figure out where the mistake is, sadly.
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### Portability
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Dependant on ASCII, requires that an int is at least 15 bits and a long at
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least 32. (No, that isn't a typo - I did say 15, not 16).
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Also requires that cpp can properly handle something like:
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#define foo(x,y) x y
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foo(bar,)
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Apparently, a few can't (ANSI C specifically doesn't require that this works,
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though it usually does). Unfortunately, I didn't discover this issue until too
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late. (It was mentioned in the notes for people modifying GCC in the "beware
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of obscure compiler limitations" section, so I'm figuring it's quite unusual.)
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### Compiler warnings
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Some when compiled with `gcc -Wall -Wextra -ansi -pedantic`:
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* The left-hand operand of a comma has no effect in two places
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* Parentheses are suggested around a + in an operand of & in the "i" macro
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definition.
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Borland C also spots some code in expressions which has no effect, and some
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uses of '=' where you'd expect a comparison operator.
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### Obfuscation
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Let's see now:
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* Squint-inducing variable naming
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* Slightly odd `#defines` (which will be cleaned up by cpp + a code
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beautifier, but see the next item)
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* No functions but main() - all code reuse via #defines, or by various other
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methods (usually somewhat icky ones). This wasn't deliberate - it just ended
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up that way somehow...
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* Gotos from everywhere to everywhere else
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* A slightly... interesting switch statement.
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* Generally odd flow of control (see above items)
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* Plenty of bit-twiddling
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* To save space, no 'A'-style char constants - hard-coded numbers are used.
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(There's one, but it isn't used to make things **clearer**)
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* `array[index]` notation, because:
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- a) it's traditional
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- b) take a look at how the s and N(I,l) macros are used
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* various other little things not worth mentioning
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So basically, just what you'd normally expect in an IOCCC entry
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