TODO: Will golf it down some more tomorrow morning and add an explanation.
ć¡ # Split the input-string on its first character ('B')
# i.e. "BIDABIDIBIDOBIDABIDUBUBIDUBIDI"
# → ["IDA","IDI","IDO","IDA","IDU","U","IDU","IDI"]
Ç # Convert each character to it's ordinal value
# → [[73,68,65],[73,68,73],[73,68,79],[73,68,65],[73,68,85],85,[73,68,85],[73,68,73]]
1^ # XOR it by 1
# → [[72,69,64],[72,69,72],[72,69,78],[72,69,64],[72,69,84],84,[72,69,84],[72,69,72]]
9% # Take modulo-9
# → [[0,6,1],[0,6,0],[0,6,6],[0,6,1],[0,6,3],3,[0,6,3],[0,6,0]]
4% # Take Modulo-4
# → [[0,2,1],[0,2,0],[0,2,2],[0,2,1],[0,2,3],3,[0,2,3],[0,2,0]]
ε ] # Now map it to:
Dg≠i # If the length is not 1:
# i.e. [0,2,1] → 3 → 1 (truthy)
# i.e. 3 → 1 → 0 (falsey)
421øP # Multiply the first number by 4, second by 2, and third by 1
# i.e. [0,2,1] and [4,2,1] → [[0,4],[2,2],[1,1]] → [0,4,1]
€O # Then sum every inner list
# [[0,4,1],[0,4,0],[0,4,2],[0,4,1],[0,4,3],3,[0,4,3],[0,4,0]]
# → [5,4,6,5,7,3,7,4]
žvβ # Convert this list from base-16 to base-10
# → 1415934836
žzв # Convert this integer from base-10 to base-256
# → [84,101,115,116]
ç # Convert this number to ASCII characters
# → ["T","e","s","t"]
J # Join the characters together (and output implicitly)
# → "Test"
