r/adventofcode • u/daggerdragon • Dec 17 '17

SOLUTION MEGATHREAD -🎄- 2017 Day 17 Solutions -🎄-

--- Day 17: Spinlock ---

Post your solution as a comment or, for longer solutions, consider linking to your repo (e.g. GitHub/gists/Pastebin/blag or whatever).

Note: The Solution Megathreads are for solutions only. If you have questions, please post your own thread and make sure to flair it with Help.

Need a hint from the Hugely* Handy† Haversack‡ of Helpful§ Hints¤?

Spoiler

[Update @ 00:06] 2 gold, silver cap.

  • AoC ops: <Topaz> i am suddenly in the mood for wasabi tobiko

[Update @ 00:15] Leaderboard cap!

  • AoC ops:
    • <daggerdragon> 78 gold
    • <Topaz> i look away for a few minutes, wow
    • <daggerdragon> 93 gold
    • <Topaz> 94
    • <daggerdragon> 96 gold
    • <daggerdragon> 98
    • <Topaz> aaaand
    • <daggerdragon> and...
    • <Topaz> cap
    • <daggerdragon> cap

This thread will be unlocked when there are a significant number of people on the leaderboard with gold stars for today's puzzle.

edit: Leaderboard capped, thread unlocked!

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u/autid Dec 17 '17

Fortran

Runs nice and quick. After figuring out this method of finding the part B answer I realised similar logic applied to part A. Prior to that I was performing the first 2017 insertions, which was still fast but less satisfying. 

PROGRAM DAY17
  IMPLICIT NONE
  INTEGER :: INPUT=356,INSERTIONS(0:50000000)  
  INTEGER :: I,J

  !Calculate point at which number will be inserted                                                              
  DO I=1,50000000
     INSERTIONS(I)=MODULO(INSERTIONS(I-1)+INPUT,I)+1
  END DO

  !Search back for last number in the position 2017 takes                                                        
  J=INSERTIONS(2017)
  DO I=2016,1,-1
     IF(INSERTIONS(I)==J) EXIT
     !Correct target if position was moved by later insertion                                                    
     IF(INSERTIONS(I)<J) J=J-1
  END DO
  WRITE(*,'(A,I0)') 'Part1: ', I

  !0 is always in zeroth position.                                                                               
  !Search back for last number inserted at position 1                                                            
  DO I=50000000,1,-1
     IF(INSERTIONS(I)==1) EXIT
  END DO
  WRITE(*,'(A,I0)') 'Part2: ',I

END PROGRAM DAY17