🚀 go-pugleaf

In article <4fsv1e$2u7@solutions.solon.com> sl14@crux5.cit.cornell.edu
(S. Lee) writes: 
<snip>
   :Picture a table like the following:
   :
   :  A B C D E F G ... Z
   :A A B C D E F G ... Z
   :B B C D E F G H ... A
   :C C D E F G H I ... B
   :: : : : : : : : ::: :
   :Z Z A B C D E F ... Y
   :
   :The students were supposed to match each character in a plaintext string
   :with each character in a key string to produce the ciphertext, as an
   :exercise to string handling.  Of 21 students, none realized that the
   :table can be computed.  Many actually typed in the lookup table, one
   :even read it from a file.  I tend to blame this phenomenon on TV and
   :debuggers ;)

   First, you don't need a lookup table.  The function is just S1[i] - S2[i]
   + 'A'.  I catch it when I read the table, but I already know how to
   handle strings.

Careful. S1[i] < S2[i] is possible.

Careful. Why assume ASCII? If S1[i] is guaranteed larger than S2[i],

  " ABCDEFGHIJKLMNOPQRSTUVWXYZ"[ (unsigned)(S1[i] - S2[i]) <= 26?
                                 (S1[i]-S2[i]+1) : 0 ]

makes no further assumption. The obfuscation can ofcourse be trivially
removed.

Cheers
Tanmoy
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