Note that the grading model changed at IOI'94 and beyond. Before IOI'94, a human evaluator would sit behind the competitor's computer and do the testing manually (enter test data through the keyboard, check the output on the screen). I don't know how the test cases were specified (may be written on a piece of paper) and how carefully they were applied. Starting at IOI'94 an automatic grading process has been used, with test input defined in files.
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