Memory Reallocation

16 memory banks: a list with 16 items Each can hold any number of blocks: each item stores an integer Each cycle, the bank with the most blocks (if more than one, the first) empties its blocks and spreads them among several others: largest stored integer, at the first known location, save it's block count as n before making it 0, then increment the next n blocks by one Track each cycle with a counter After each cycle, store a stringified stamp of the bank's block counts As soon as a stamp is repeated, return the counter 

new RegExp(/\d+/g) Extract each digit from the puzzle input new Set() Create a unique set of values Set.add() Attempt to add a unique value to a Set Set.has() Check whether a Set contains some unique value Math.max() Determine the largest number among the banks Array.indexOf(max) Determine the location of the first or only largest number Array.join('|') Create a stringified stamp of the bank counts for (let i = 1; i <= count; i++) {} Reallocate one bank's blocks to several other banks Array[(location + i) % Array.length]++ Increment the next bank's block count by one, wrapping from last to first bank where necessary 

Extract each digit from the puzzle input and coerce to a number Set cycles at 0 Set stamps as an empty set Sub-routine: cycle Concatenate each number with a | character into a string Add that string to stamps Determine the largest number among the banks' blocks Determine the location of the first - or only - occurrence of that number Set count to the number Update the first - or only - occurrence of the largest number to 0 For i from 0 up to and including count Increment by 1 the number in banks at the location resulting from the remainder after following operation: The sum of the location of the first - or only - occurrence of the most recent largest number, and i Divided by the length of the list of banks (16) Increment cycles by 1 Main loop: Do as long as stamps does not have the stringified version of the latest state of each banks' blocks Perform a cycle Return cycles 

The difference between: - The number stored in cycles (Part 1's answer) - The only location in stamps of the stringified version of the first repeating state of banks 

 5 - 1 --- 4 

cycles - Array.from(stamps).indexOf(banks.join('|'))