2025 advent of code day 4

day_4.R

# day 4 2025
# first star
input <- do.call(rbind, strsplit(readLines("input.txt"),""))

# calculate distances from each matrix cell to all others
# extract each cell's neighboring values for each element- max distance is one
indices <- which(input == input, arr.ind = TRUE)
d <- apply(as.matrix(dist(indices, "maximum", diag = TRUE)), 1, function(i) input[i == 1]) 
names(d) <- input
d <- d[grep("@", names(d))] 

# get neighbors and filter out those with 4+ roll neighbors
neighbors <- lapply(d, function(x) sum(x == "@"))
length(neighbors[neighbors < 4])