"""
This function:
- finds all positions in string1 where string2 occurs.
- returns the total number of such positions, and the list
  of the positions.

Note: overlapping occurrences of string2 are NOT discarded.

For example, if string1 = "abcIOIOIOIOIOabc" and string2 is "IOIOIO",
the program will report that string2 appears 3 times in string1.
"""

def find_substrings(string1, string2):
    # temp_string starts off equal to string1, and becomes a more and more
    # truncated version of string1. Every time we find string2 in temp_string,
    # we truncate the start of temp_string up to (and including) the first
    # character of the occurrence of string2.
    temp_string = string1
    count = 0
    positions = []
    adjustment = 0

    # go through the first string, looking for occurrences of the second string
    while True:
        # find next position
        position = temp_string.find(string2)
        if position == -1:
            break
        
        count = count + 1
        positions.append(position + adjustment)

        # truncate the start of temp_string, to strip off the
        # occurrence of string2 that we just found
        temp_string = temp_string[position+1:]

        # adjustment remembers how many total characters we have truncated
        # from string1 to obtain the current version of temp_string
        # this is important so that we can convert the position of string2
        # in temp_string into the corresponding position of string2 in string1.
        adjustment = adjustment + position + 1

    return [count, positions]