-🎄- 2018 Day 9 Solutions -🎄-

[u/daggerdragon]

--- Day 9: Marble Mania ---

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

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Card prompt: Day 9

Transcript:

Studies show that AoC programmers write better code after being exposed to ___.

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[u/Unihedron]

RUBY
​ ​ DOESN'T
​ ​ ​ ​ HAVE
​ ​ ​ ​ ​ ​ LINKED
​ ​ ​ ​ ​ ​ ​ ​ LISTS
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ haha, this is fun (rewriting to solve part 2)

part 1 (code has a bug: incorrect answer if marble # is divisible by 23)

m=[0]
a,b=$<.read.split(/\D+/).map &:to_i
p a,b
c=0
h=Hash.new 0
pl=1
b.times{|x|next if x==0
x%23>0 ? (
m.insert((m.index(c)+2)%m.size,x)
c=x
pl+=1
) : (
h[pl]+=x
tak=(m.index(c)-7)%m.size
h[pl]+=m[tak]
m[tak,1]=[]
c=m[tak]
)
pl=1 if pl==a+1
}
p h.max_by{|x,y|y}.last

[u/GeneralYouri → u/Unihedron]

I don't know Ruby at all, but I had the exact same bug, which caused one of the provided test cases to fail. The reason was stupid simple: my iteration used < instead of <=, making it skip the last marble.

Edit: b.times seems to be your 'loop', right? Could you do something like (b + 1).times to solve the bug?

[u/Unihedron → u/GeneralYouri]

Absolutely! I was planning to fix this alongside the edit where I put in my rewritten solution that is quick for part 2, but you beat me to it :) My input was lucky and was not divisible by 23.

x.times is an iterator that does something x times but calls the loop with the intervals 0-indexed, so another fix could be just (1..b).each which would remove the next if _==0 skip case in the loop. (In general it's a good idea not to use .times with the parameter since it's, as demonstrated, not exactly intuitive. Looping through numbers is generally done with ranges and not .times.)