Day 4: Ceres Search
Megathread guidelines
- Keep top level comments as only solutions, if you want to say something other than a solution put it in a new post. (replies to comments can be whatever)
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FAQ
- What is this?: Here is a post with a large amount of details: https://programming.dev/post/6637268
- Where do I participate?: https://adventofcode.com/
- Is there a leaderboard for the community?: We have a programming.dev leaderboard with the info on how to join in this post: https://programming.dev/post/6631465
J
Unsurprisingly this is the kind of problem that J is really good at. The dyadic case (table) of the adverb
/is doing all the heavy lifting here: it makes a higher rank tensor by traversing items of the specified rank on each side and combining them according to the remaining frame of each side’s shape. The hard part is arranging the arguments so that your resulting matrix has its axes in the correct order.data_file_name =: '4.data' NB. cutopen yields boxed lines, so unbox them and ravel items to make a letter matrix grid =: ,. > cutopen fread data_file_name NB. pad the grid on every side with #'XMAS' - 1 spaces hpadded_grid =: ((' ' & ,) @: (, & ' '))"1 grid padded_grid =: (3 1 $ ' ') , hpadded_grid , (3 1 $ ' ') NB. traversal vectors directions =: 8 2 $ 1 0 1 1 0 1 _1 1 _1 0 _1 _1 0 _1 1 _1 NB. rpos cpos matches rdir cdir if the string starting at rpos cpos in NB. direction rdir cdir is the string we want matches =: 4 : 0 */ ,'XMAS' -: padded_grid {~ <"1 x +"1 y *"1 0 i. 4 )"1 positions =: (3 + i. 0 { $ grid) ,"0/ (3 + i. 1 { $ grid) result1 =: +/, positions matches/ directions NB. pairs of traversal vectors x_directions =: 4 2 2 $ 1 1 _1 1 1 1 1 _1 _1 _1 _1 1 _1 _1 1 _1 NB. rpos cpos x_matches 2 2 $ rdir1 cdir1 rdir2 cdir2 if there is an 'A' at NB. rpos cpos and the string in each of dir1 and dir2 centered at rpos cpos NB. is the string we want x_matches =: 4 : 0 NB. (2 2 $ rdir1 cdir1 rdir2 cdir2) *"1 0/ (_1 + i.3) yields a matrix NB. 2 3 $ (_1 * dir1) , (0 * dir1) , (1 * dir1) followed by the same for dir2 */ ,'MAS' -:"1 padded_grid {~ <"1 x +"1 y *"1 0/ _1 + i. 3 )"1 2 result2 =: +/, positions x_matches/ x_directions