2024 advent of code day 4

day_4.R

# first star
input <- as.matrix(read.fwf("input.txt", widths = rep(1, 140)))

finder <- function(x) {x <- paste(x, collapse = "")
  lengths(regmatches(x, gregexpr("XMAS", x))) +
  lengths(regmatches(x, gregexpr("SAMX", x)))
}

# tally horizontal & vertical matches
h_finds <- apply(input, 1, finder)
v_finds <- apply(input, 2, finder)

# for diagonals index letter matrix by n,m value and split into list elements to search R-L and L-R 
idx <- row(input) - col(input)
r_l_diag <- lapply(split(input, idx), function(x) paste(x, collapse = ""))
l_r_diag <- lapply(split(input[nrow(input):1, ], idx), function(x) paste(x, collapse = ""))

r_l_finds <- lapply(r_l_diag, finder)
l_r_finds <- lapply(l_r_diag, finder)

d_finds <- sum(mapply("+", r_l_finds, l_r_finds))

sum(h_finds)+sum(v_finds)+d_finds


# second star
# index As, check for only two Ms and two Ss in diagonal
a <- which(input == input, arr.ind = TRUE)
d <- as.matrix(dist(a, "maximum", diag = TRUE))

# extract neighboring values for each element
# extract where max distance is one
d <- apply(d, 1, function(i) input[i == 1])
names(d) <- input
d <- d[grep("A", names(d))]  

# get diagonal neighbors only
neighbors <- lapply(d, `[`, c(1,3,6,8))
neighbors <- lapply(neighbors, function(x) paste(x, collapse = ""))
neighbors <- neighbors[grep("MMSS|SMSM|SSMM|MSMS", neighbors)]  
length(neighbors)