2024 advent of code day 6

day_6.R

# first star
input <- as.matrix(read.fwf("input.txt", widths = rep(1, 130), comment.char = ""))

# pad to indicate exit
#input <- rbind("E", cbind("E", input, "E"), "E") 

starting <- as.vector(which(input == "^", arr.ind = TRUE)[1,])

# function moves n,m position in matrix
step <- function(direction, mode){
  coords <- list(c(-1, 0), c(1, 0), c(0, 1), c(0, -1))
  mode == "unblocked"
  if(mode == "unblocked"){
    switch(direction, "^" = coords[[1]], "v" = coords[[2]], ">" = coords[[3]], "<" = coords[[4]])
    }
  else {mode == "blocked"
    switch(direction, "^" = ">", "v" = "<", ">" = "v", "<" = "^")
    }
}

# log steps in list
steps <- list(starting)

# starting from up
direction <- "^"

# move, log steps until blocked, then change mode to new move
while(starting[1] > 1 & starting[1] < 130 & starting[2] > 1 & starting[2] < 130){
  next_step <- starting + step(direction, "unblocked")
  if(input[next_step[1], next_step[2]] != "#"){
    steps[[length(steps) + 1]] <- paste(starting, collapse = ",")
    starting <- starting + step(direction, "unblocked")
  } 
  else{
    steps[[length(steps) + 1]] <- paste(starting, collapse = ",")
    direction <- step(direction, "blocked")
  }
}

length(steps[which(!duplicated(steps))]) # count not-retraced steps beyond start