Internet Problem Solving Contest

• Correct output – C1
• Correct output – C2

As the problem statement suggested, the input contains 80-column punch cards and the encoding used is EBCDIC.

You can read about this encoding at http://www.cs.uiowa.edu/~jones/cards/codes.html, and at http://www.kloth.net/services/cardpunch.php you can find a service that allows you to create your very own punched card images.

However, getting to the correct answer required not only decoding the information, but also a deeper knowledge of times long past.

Easy data set

In the easy input, three of the cards make sense. They contain the following text:

However, the third card does not contain any useful information – and this should immediately be suspicious.

The third card is a lace card (see http://en.wikipedia.org/wiki/Lace_card). These cards usually caused the reader to jam. This is where the formulation “(most probably)” from the problem statement comes to play. If we took these four cards and feed them into a card reader, the most probable outcome would be that it reads the first two cards and then gets jammed on the third one.

Hence the text it read is just

giving us the answer 49698866.

Hard data set

When we decode the cards in the hard input, we get a program in Fortran:

Again, this program hides several nasty surprises. Getting a Fortran compiler (such as gfortran) and running it is much better than trying to understand it.

The tricks include:

• Columns 73 to 80 are ignored. The reason: when programs were entered on punch cards, these columns often contained serial numbers of cards. Hence row 3 only reads COUNT = COUNT + 4.
• For backwards compatibility, there are multiple ways how to make comments. Line 7 shows the modern one (a line starting with a *), line 4 is the original one (a line starting with a C in the first column). Hence line 4 is ignored.
• If there is a character in column 6, it means that this row is a continuation of previous one. Additionally, Fortran ignores whitespace. Hence lines 5 and 6 together are equivalent to the single statement COUNT = COUNT + 477 * 47.

Thus this program outputs 22470.