Most Useful Program: Albert van der Horst S P&A R&C Oranjestraat 8 3511 RA UTRECHT The Netherlands Judges' comments: For a canned example: make albert albert 1234567890123456789 We were impressed with the speed at which it was able to factor arbitrarily large numbers consisting of factors that fit into a long. Selected notes from the author: The Obfuscated version of the Horst algorithm. This program will factor unlimited length numbers and print the factors in ascending order. Numbers of one digit (e.g. 8) are rejected without notice. It quits as soon as there is at most one factor left, but that factor will not be shown. It accomplishes this efficiently, without resorting to division or multiplication, until the candidate factor no longer fits in a signed long. The nicest way is to rename the program into e.g. 4294967297 if you want to factor Fermat's 4th number. Then just run it. Or you may type "prog " A nice one is also (30 ones) albert 111111111111111111111111111111 Apart from the foregoing there are no special execution instructions. To customize the program into a factorizer of a fixed number, use cc albert.c -o 4294967297 or some such. There are no data files used, and it is not possible to feed input via stdin. I think this program is a nice example of algorithmic obfuscation. Several times two similar algorithms are merged into one. Then you need quite some tricks to get it running, such as long jumps through recursive subroutines. I felt like a sword smith, welding the sword very long, folding it again to the proper length, keep on hammering till it is tight, then fold again. Always keeping it at the proper red hot temperature, but not too hot lest the hardness fades. The strict naming conventions for subroutines did not make things much clearer after all, but it was not supposed to. I would like to draw attention to the robustness of the program with respect to error handling and the nice stopping criterion. The esthetic appeal of some lines is at the expense of clearness, I apologize. Running the program through the c-beautifier reveals nothing, it will only destroy some of the lay out. Running the program through lint shows the usual remarks for a K&R program. Defeating this through casts does not make a program cleaner in my opinion. Here are some hints, but they may not be too helpful. 1. The Horst algorithm is described in the Hobby Computer Club Newsletter, year 82, part 4, a real Dutch treat. Assembler, c and Forth version have been around for some years. 2. Does fractal programming exist after all? 3. You remember the ToomCook algorithm in Knuth? It uses iteration instead of recursion and is quite jumpy. This program shares these disadvantages in a modified form. 4. The Conversion is to be found in Knuth, not so the Observation. The Observation: "if it ends in a zero, it is divisible by ten" Copyright (c) 1992, Landon Curt Noll & Larry Bassel. All Rights Reserved. Permission for personal, educational or non-profit use is granted provided this this copyright and notice are included in its entirety and remains unaltered. All other uses must receive prior permission in writing from both Landon Curt Noll and Larry Bassel.