ShareMyScreen/AdventOfCode/2023/04/Scratchpads/rdm
1sample=: {{)n
2Card 1: 41 48 83 86 17 | 83 86 6 31 17 9 48 53
3Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
4Card 3: 1 21 53 59 44 | 69 82 63 72 16 21 14 1
5Card 4: 41 92 73 84 69 | 59 84 76 51 58 5 54 83
6Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
7Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11
8}}
9day4=: ([fwrite&'2023-day4.txt') fread '~/Downloads/input.txt'
Here, we were working with "scratchcards" and for each line, we needed to know the quantity of numbers to the left of the | character which appeared in the list to its right.
10tallies=: {{ +/@e."1&>/ <@:>"1|:<@(0&".);._1@('|',]);._2 y}}
In other words:
tallies sample
4 2 2 1 0 0
It's perhaps worth noting that I'm using 0&". to parse each "half" of each line. In other words:
0 ". 'Card 1: 41 48 83 86 17 '
0 0 41 48 83 86 17
0 ". ' 83 86 6 31 17 9 48 53'
83 86 6 31 17 9 48 53
The extra zeros in the first "half" don't matter for my purposes, because zeros never appear in the other "half" of the line.
For part 1, I needed to discard 0s from the tallies, and sum a corresponding power of 2. There's a variety of good ways of doing this, but I opted for a slightly awkward brute force approach:
11part1=: +/@{{ 1 -:@-.~ 2 ^ tallies y }}
I probably should have used +/<.2^<:tallies y
For part 2, it was a bit more complicated - for each card we generated 1 extra copy of each of the n following cards where n was the tally from that card. In other words:
12part2=: {{
13 cop=. win=win=. tallies y
14 for_w. win do.
15 ndx=. w_index+1+i.w
16 cop=. cop ndx}~ (ndx{cop)+w_index{cop
17 end.
18 +/cop
19}}
Here, cop is the number of copies of each scratchcard - initially all 1s, and the tally for each scratchcard added 1 copy of each of that many of the following cards. Easier to read the code, maybe, than the english description.
But, it was pointed out to me that there's a much more concise approach:
12part2=: +/@{{ <.,%.y ((]#0:),1,[#_1:)"0 i.#y }}@tallies
It's probably best to sit down with an implementation of J and look at intermediate results here.
t ((]#0:),1,[#_1:)"0 i.#t=: tallies sample
1 _1 _1 _1 _1 0
0 1 _1 _1 0 0
0 0 1 _1 _1 0
0 0 0 1 _1 0
0 0 0 0 1 0
0 0 0 0 0 1
%.t ((]#0:),1,[#_1:)"0 i.#t=: tallies sample
1 1 2 4 7 0
0 1 1 2 3 0
0 0 1 1 2 0
0 0 0 1 1 0
0 0 0 0 1 0
0 0 0 0 0 1
But I'm still trying to think of a good way of describing the problem itself in english, ...
Note that what's happening here is vaguely related to the fibonacci sequence.
%.(=i.9)-1|.!.0(=i.9)+1|.!.0(=i.9)
1 1 2 3 5 8 13 21 34
0 1 1 2 3 5 8 13 21
0 0 1 1 2 3 5 8 13
0 0 0 1 1 2 3 5 8
0 0 0 0 1 1 2 3 5
0 0 0 0 0 1 1 2 3
0 0 0 0 0 0 1 1 2
0 0 0 0 0 0 0 1 1
0 0 0 0 0 0 0 0 1