The object [of Decode] is to figure out the computer's secret code in as few guesses as possible. The program gives you information about the accuracy of each of your guesses. By carefully selecting your guesses to make use of the information you have, you can determine what the secret code must be in a surprisingly small number of guesses. Five or six is usually enough.
Decode was written by Tom Rugg and Phil Feldman and published as type-in source code for TRS-80 BASIC in 32 BASIC Programs for the TRS-80 (Level II) Computer (0-918398-27-4) published by Dilithium Press, Portland, OR, in 1980.