# Libraries
+library(tidyverse)
+
+# Read input from file
+input <- read_lines("../input/day17.txt", skip_empty_rows = TRUE) |>
+ str_split_1("")
+
+
+
+
+
+
+
+
+Day 17
+Advent of Code: Worked Solutions
+Setup
+
+
+
+Part 1
+Representing obstacles as 1s and empty space as 0s, represent the shapes of the falling blocks, the floor, and the walls as bitwise integers:
+
+
+
+block1 <- strtoi(c("000111100" ), base = 2)
+block2 <- strtoi(c("000010000","000111000","000010000" ), base = 2)
+block3 <- strtoi(c("000111000","000001000","000001000" ), base = 2)
+block4 <- strtoi(c("000100000","000100000","000100000","000100000"), base = 2)
+block5 <- strtoi(c("000110000","000110000" ), base = 2)
+
+walls <- strtoi("100000001", base = 2)
+floor <- strtoi("111111111", base = 2)
+
+blocks <- list(block1, block2, block3, block4, block5)Define functions that give properties of the tower:
+
+
+
+# Print tower to terminal (for debugging)
+print_tower <- function(tower) {
+ tower |>
+ imap_chr(\(row, name) {
+ row |>
+ intToBits() |>
+ rev() |>
+ tail(9) |>
+ as.integer() |>
+ case_match(0 ~ "·", 1 ~ "#") |>
+ modify_at(.at = c(1L, 9L), .f = ~ "|") |>
+ str_c(collapse = "") |>
+ str_c(name, sep = " ")
+ }) |>
+ modify_at(.at = 1L, .f = ~ if_else(.x == "|#######| 0", "+-------+ 0", .x)) |>
+ rev() |>
+ cat(sep = "\n")
+}
+
+tower_height <- function(tower) {
+ idx <- max(which(tower != walls))
+ tower[idx] |>
+ names() |>
+ as.double()
+}
+
+tower_base <- function(tower) {
+ idx <- max(which(accumulate(tower, bitwOr, .dir = "backward") == floor))
+ tower[idx] |>
+ names() |>
+ as.double()
+}
+
+trim_tower <- function(tower) {
+ base <- tower_base(tower)
+ top <- tower_height(tower)
+ tower[as.character(base:top)]
+}Define functions that move blocks and check if the move is valid:
+
+
+
+shift_block <- function(block, dir) {
+ f <- switch(dir,
+ "<" = bitwShiftL,
+ ">" = bitwShiftR
+ )
+ f(block, 1)
+}
+
+is_collision <- function(block, tower_slice) {
+ any(bitwAnd(block, tower_slice) > 0)
+}
+
+# Try to move the block L/R if the move is valid, or return the old one if not
+try_shift_block <- function(block, dir, tower_slice) {
+ new <- shift_block(block, dir)
+ if (is_collision(new, tower_slice))
+ block
+ else
+ new
+}Define a function to drop blocks onto a tower:
+
+
+
+drop_blocks <- function(jets, n_blocks) {
+
+ tower <- floor
+ names(tower) <- 1:length(tower) - 1
+
+ n_jets <- length(jets)
+ time <- 0
+
+ # Cycle through the list of blocks and drop them in order
+ for (i in 1:n_blocks) {
+
+ block_idx <- (i - 1) %% length(blocks) + 1
+ block <- blocks[[block_idx]]
+
+ # Initialize the vertical location of the block
+ block_loc <- 1:length(block) + 3 + tower_height(tower)
+
+ # Add empty wall space to the top of the tower
+ add_walls <- rep(walls, length(block) + 3) |>
+ set_names(c(min(block_loc) - 3:1, block_loc))
+ tower <- c(tower, add_walls)
+
+ # Drop block until it comes to rest
+ repeat {
+ jet_idx <- time %% n_jets + 1
+
+ # Apply jet blast & increment time
+ block <- try_shift_block(block, jets[jet_idx], tower[as.character(block_loc)])
+ time <- time + 1
+
+ # Check if block has come to rest; if so, add block to tower
+ if (is_collision(block, tower[as.character(block_loc - 1)])) {
+ tower[as.character(block_loc)] <- bitwOr(tower[as.character(block_loc)], block)
+ tower <- trim_tower(tower)
+ break
+
+ }
+ # Otherwise drop block one unit and repeat the block jet seq
+ else {
+ block_loc <- block_loc - 1
+ }
+ }
+ }
+
+ tower
+}Run on puzzle input:
+
+
+
+input |>
+ drop_blocks(2022) |>
+ tower_height()
+
+[1] 3133Part 2
+Modify the drop_blocks function to loop until a cycle is found and return cycle info:
+
+
+
+find_cycle <- function(jets) {
+
+ tower <- floor
+ names(tower) <- 1:length(tower) - 1
+
+ n_jets <- length(jets)
+ time <- 0
+ states <- tibble(
+ n_blocks = numeric(0),
+ tower_idx = list(),
+ tower_val = list(),
+ block_idx = numeric(0),
+ jet_idx = numeric(0)
+ )
+
+ i <- 1
+ # Cycle through the list of blocks and drop them in order
+ repeat {
+
+ block_idx <- (i - 1) %% length(blocks) + 1
+ block <- blocks[[block_idx]]
+
+ # Initialize the vertical location of the block
+ block_loc <- 1:length(block) + 3 + tower_height(tower)
+
+ # Add empty wall space to the top of the tower
+ add_walls <- rep(walls, length(block) + 3) |>
+ set_names(c(min(block_loc) - 3:1, block_loc))
+ tower <- c(tower, add_walls)
+
+ # Drop block until it comes to rest
+ repeat {
+ jet_idx <- time %% n_jets + 1
+
+ # Apply jet blast & increment time
+ block <- try_shift_block(block, jets[jet_idx], tower[as.character(block_loc)])
+ time <- time + 1
+
+ # Check if block has come to rest; if so, add block to tower
+ if (is_collision(block, tower[as.character(block_loc - 1)])) {
+ tower[as.character(block_loc)] <- bitwOr(tower[as.character(block_loc)], block)
+ tower <- trim_tower(tower)
+
+ # Add block and jet index to the states list
+ states <- states |>
+ add_row(
+ n_blocks = i,
+ tower_idx = list(names(tower)),
+ tower_val = list(unname(tower)),
+ block_idx = block_idx,
+ jet_idx = jet_idx
+ )
+ # print(tower)
+ break
+
+ }
+ # Otherwise drop block one unit and repeat the block jet seq
+ else {
+ block_loc <- block_loc - 1
+ }
+ }
+
+ i <- i + 1
+
+ # After each block is dropped, check if a cycle has been found and return it
+ dupes <- states |>
+ filter(n_distinct(tower_val) != n(), .by = c(block_idx, jet_idx))
+
+ if (nrow(dupes) > 0) {
+
+ cycle_length <- dupes |>
+ pull(n_blocks) |>
+ reduce(`-`) |>
+ abs()
+
+ cycle_start <- dupes |>
+ pull(n_blocks) |>
+ min()
+
+ cycle_height <- dupes |>
+ pull(tower_idx) |>
+ map(as.numeric) |>
+ reduce(`-`) |>
+ unique() |>
+ abs()
+
+ return(list(length = cycle_length, start = cycle_start, height = cycle_height))
+ return(dupes)
+ }
+ }
+}Get the cycle of the puzzle input:
+
+
+
+cycle <- find_cycle(input)Using the cycle info from the output, compute the majority of the height using the cycle, then and add the height of the remaineder:
+
+
+
+
+
+n_cycles <- floor((1000000000000 - cycle$start) / cycle$length)
+n_blocks <- (1000000000000 - cycle$start) %% cycle$length + cycle$start
+
+((n_cycles * cycle$height) + tower_height(drop_blocks(input, n_blocks))) |>
+ format(scientific = FALSE)
+
+[1] "1547953216393"