AoC2020_20.jl

## Input #######################################################################

str = read(open("/home/chubbc/Dropbox/Prog/Advent/2020/20.txt"),String);
tiles = Dict{Int,Matrix{Char}}();
for rawtile∈split(str,"\n\n")
    n = parse(Int,rawtile[6:9]);
    tiles[n] = Matrix{Char}(undef,10,10);
    for x∈1:10, y∈1:10
        tiles[n][x,y] = rawtile[x+11y];
    end
end

## Part 1 ######################################################################

reorient(tile::Matrix{Char}, i::Int) =
    i<0 ? rotl90(reverse(tile,dims=1),i) : rotl90(tile,i);
matchUD(t1::Tuple{Int,Int}, t2::Tuple{Int,Int}) =
    (reorient(tiles[t1[1]],t1[2])[end,:]==reorient(tiles[t2[1]],t2[2])[1,:]);
matchLR(t1::Tuple{Int,Int}, t2::Tuple{Int,Int}) =
    (reorient(tiles[t1[1]],t1[2])[:,end]==reorient(tiles[t2[1]],t2[2])[:,1]);

function populate(x::Int, y::Int)
    for k∈setdiff(keys(tiles),first.(state)), i∈-4:3
        state[x,y] = (k,i);
        if (y==1||matchUD(state[x,y-1],state[x,y])) && (x==1||matchLR(state[x-1,y],state[x,y]))
            if (x,y)==size(state) || populate(mod1(x+1,size(state,1)), y+(x==size(state,1)))
                return true;
            end
        end
    end
    state[x,y] = (0,0);
    return false;
end

N = Int(√length(tiles));
state = fill((0,0),N,N);
populate(1, 1);
p1 = prod(first.(state[[1,end],[1,end]]));

## Part 2 ######################################################################

image = Matrix{Char}(undef,8*N,8*N);
for x∈1:N, y∈1:N
    t = reorient(tiles[state[x,y][1]], state[x,y][2]);
    image[(1:8).+8*(x-1),(1:8).+8*(y-1)] = permutedims(t[2:end-1,2:end-1]);
end

monster=
    "..................#." *
    "#....##....##....###" *
    ".#..#..#..#..#..#...";
monster = reshape(collect(monster),(20,3));

maxmons=0;
for i∈-4:3
    global maxmons, image, monster;
    image2 = reorient(image,i);
    mons = 0;
    for x∈1:size(image2,1)-size(monster,1)+1, y∈1:size(image2,2)-size(monster,2)+1
        found = true;
        for xx∈1:size(monster,1), yy∈1:size(monster,2)
            if monster[xx,yy]=='#' && image2[x+xx-1,y+yy-1]!='#'
                found = false;
            end
        end
        mons += found;
    end
    maxmons = max(maxmons,mons);
end
p2 = count(image.=='#')-maxmons*count(monster.=='#');

## Output ######################################################################

println((p1,p2));