Advent of Code

--- Day 6: Memory Reallocation ---

   A debugger program here is having an issue: it is trying to repair a memory
   reallocation routine, but it keeps getting stuck in an infinite loop.

   In this area, there are sixteen memory banks; each memory bank can hold any
   number of blocks. The goal of the reallocation routine is to balance the
   blocks between the memory banks.

   The reallocation routine operates in cycles. In each cycle, it finds the
   memory bank with the most blocks (ties won by the lowest-numbered memory
   bank) and redistributes those blocks among the banks. To do this, it removes
   all of the blocks from the selected bank, then moves to the next (by index)
   memory bank and inserts one of the blocks. It continues doing this until it
   runs out of blocks; if it reaches the last memory bank, it wraps around to
   the first one.

   The debugger would like to know how many redistributions can be done before
   a blocks-in-banks configuration is produced that has been seen before.

   For example, imagine a scenario with only four memory banks:

     • The banks start with 0, 2, 7, and 0 blocks. The third bank has the most
       blocks, so it is chosen for redistribution.

     • Starting with the next bank (the fourth bank) and then continuing to the
       first bank, the second bank, and so on, the 7 blocks are spread out over
       the memory banks. The fourth, first, and second banks get two blocks each,
       and the third bank gets one back. The final result looks like this: 2 4 1
       2.

     • Next, the second bank is chosen because it contains the most blocks
       (four). Because there are four memory banks, each gets one block. The
       result is: 3 1 2 3.

     • Now, there is a tie between the first and fourth memory banks, both of
       which have three blocks. The first bank wins the tie, and its three blocks
       are distributed evenly over the other three banks, leaving it with none: 0
       2 3 4.

     • The fourth bank is chosen, and its four blocks are distributed such that
       each of the four banks receives one: 1 3 4 1.

     • The third bank is chosen, and the same thing happens: 2 4 1 2.

   At this point, we've reached a state we've seen before: 2 4 1 2 was already
   seen. The infinite loop is detected after the fifth block redistribution
   cycle, and so the answer in this example is 5.

   Given the initial block counts in your puzzle input, how many redistribution
   cycles must be completed before a configuration is produced that has been
   seen before?

   Your puzzle answer was 12841.

--- Part Two ---

   Out of curiosity, the debugger would also like to know the size of the loop:
   starting from a state that has already been seen, how many block
   redistribution cycles must be performed before that same state is seen
   again?

   In the example above, 2 4 1 2 is seen again after four cycles, and so the
   answer in that example would be 4.

   How many cycles are in the infinite loop that arises from the configuration
   in your puzzle input?

   Your puzzle answer was 8038.

   Both parts of this puzzle are complete! They provide two gold stars: **

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