The function performs letter substitution, replacing each letter in the input string with a corresponding letter.

[CODE]:
```python
def substitute_letters(input_string):
    substitution_dict = {
        'a': 'r',
        'b': 's',
        'c': 'r',
        'd': 'i',
        'e': 'r',
        'f': 'r',
        'g': 'i',
        'h': 's',
        'i': 'r',
        'j': 'i',
        'k': 'r',
        'l': 'r',
        'm': 'i',
        'n': 'r',
        'o': 'i',
        'p': 'i',
        'q': 'r',
        'r': 's',
        's': 'r',
        't': 'r',
        'u': 'i',
        'v': 'i',
        'w': 'r',
        'x': 'r',
        'y': 's',
        'z': 's'
    }
    
    output_string = ''
    for letter in input_string:
        if letter in substitution_dict:
            output_string += substitution_dict[letter]
        else:
            output_string += letter
    
    return output_string
```

Keep in mind that this is just an approximation of the behavior of the function based on the observed outputs. The actual implementation and substitution rules may differ.