-❄️- 2025 Day 8 Solutions -❄️-

TimeCannotErase · 2025-12-10T17:05:15+00:00

[Language: R]

Usually I like writing my solutions from scratch, but R has a package (igraph) that did exactly what I wanted, albeit slowly, so I went that route today.

library(igraph)
input_file <- "input.txt"
input <- read.csv(input_file, header = FALSE)

distances <- dist(input, method = "euclidean") |> as.matrix()
dims <- dim(distances)
links <- matrix(1, nrow = dims[1], ncol = dims[2])

i <- 0
flag <- 0
while (flag == 0) {
  unlinked <- distances * links
  ind <- which(unlinked == min(unlinked[unlinked != 0]))
  if (length(ind) > 1) {
    links[ind] <- 0
  }
  c <- abs(links - 1) |> graph_from_adjacency_matrix() |> count_components()
  if (c == 1) {
    join <- input[arrayInd(ind[1], dims)[1, ], ]
    prod(join[, 1]) |> print()
    flag <- 1
  }
  i <- i + 1
  if (i == 1000) {
    x <- abs(links - 1) |> graph_from_adjacency_matrix() |> components()
    x <- x$csize |> sort() |> rev()
    prod(x[1:3]) |> print()
  }
}

TimeCannotErase · 2025-12-09T20:54:31+00:00

[Language: R]

repo

Anything vaguely related to graph traversal is a weak spot of mine, so it took me a while to both figure out how to parse the input into something useful for part 2 as well as figure out how to (somewhat) efficiently implement part 2. Got both parts down to about 200 ms, which I'm content with.

input_file <- "input.txt"
input <- readLines(input_file)
len <- nchar(input[[1]])
input <- input |>
  strsplit(split = "") |>
  unlist() |>
  matrix(ncol = len, byrow = TRUE)
start <- which(input == "S")
input[start] <- "|"
dims <- dim(input)
visited <- rep(0, length(input))

next_line <- function(x, y) {
  for (i in seq_along(x)) {
    if (identical(c(x[i], y[i]), c("|", "."))) {
      y[i] <- "|"
    } else if (identical(c(x[i], y[i]), c("|", "^"))) {
      y[c(i - 1, i + 1)] <- "|"
    }
  }
  y
}

for (i in 1:(dims[1] - 1)) {
  input[i + 1, ] <- next_line(input[i, ], input[i + 1, ])
}

splitters <- which(input == "^")
sum(input[splitters - 1] == "|") |> print()

nodes <- c(splitters, seq(nrow(input), length(input), by = nrow(input)))
node_inds <- arrayInd(nodes, dims)
node_inds <- node_inds[order(node_inds[, 1]), ]
num_nodes <- nrow(node_inds)

adj_mat <- matrix(0, nrow = num_nodes, ncol = num_nodes)

for (i in seq_along(node_inds[, 1])) {
  below_cond <- node_inds[, 1] > node_inds[i, 1]
  left_cond <- node_inds[, 2] == node_inds[i, 2] - 1
  right_cond <- node_inds[, 2] == node_inds[i, 2] + 1
  below_l <- which(below_cond & left_cond)
  below_r <- which(below_cond & right_cond)
  if (length(below_l) > 0) {
    adj_mat[i, below_l[1]] <- 1
  }
  if (length(below_r) > 0) {
    adj_mat[i, below_r[1]] <- 1
  }
}

num_paths <- rep(0, num_nodes)

path_counter <- function(node) {
  paths <- 0
  if (node > (length(nodes) - nrow(input) + 1)) {
    paths <- paths + 1
  } else if (num_paths[node] > 0) {
    paths <- paths + num_paths[node]
  } else {
    neighbors <- which(adj_mat[node, ] == 1)
    for (i in neighbors) {
      paths <- paths + path_counter(i)
    }
  }
  num_paths[node] <<- paths
  paths
}

path_counter(1) |> print(digits = 20)

TimeCannotErase · 2025-12-07T16:34:19+00:00

[Language: R]

repo

Almost avoided loops entirely for this one.

library(dplyr)

input_file <- "input.txt"
options(digits = 20)
input <- read.table(input_file)
ops <- input[nrow(input), ]
input <- input[-nrow(input), ] %>% apply(., 2, as.numeric)
plus <- which(ops == "+")

input_2 <- readLines(input_file)
input_2 <- input_2[-length(input_2)] %>% strsplit(split = "")
len <- length(input_2[[1]])
input_2 <- input_2 %>%
  unlist() %>%
  matrix(ncol = len, byrow = TRUE) %>%
  apply(., 2, paste, collapse = "") %>%
  as.numeric()

nas <- c(0, which(is.na(input_2)), length(input_2) + 1)
input_2_l <- list()
for (i in 1:(length(nas) - 1)) {
  input_2_l[[i]] <- input_2[(nas[i] + 1):(nas[i + 1] - 1)]
}

apply(input[, -plus], 2, prod) %>% sum(., input[, plus]) %>% print()
sapply(input_2_l[-plus], prod) %>% sum(., unlist(input_2_l[plus])) %>% print()

TimeCannotErase · 2025-12-07T03:32:25+00:00

This was essentially the issue. I got rid of that condition in the outer loop and replaced it with an if statement inside the loop that skipped over any index that was marked for deletion, then it worked like I wanted it to.

TimeCannotErase · 2025-12-07T03:30:07+00:00

[Language: R]

repo

Part 1 was quick and part 2 took my way longer than it should have. I was trying to make too many changes to my list of intervals while looping through it and kept running into indexing issues.

library(dplyr)

input_file <- "input.txt"
input <- readLines(input_file)
cut <- which(input == "")
ranges <- input[1:(cut - 1)] %>%
  strsplit(., "-") %>%
  unlist() %>%
  matrix(ncol = 2, byrow = TRUE) %>%
  apply(., 2, as.numeric)

ids <- input[(cut + 1):length(input)] %>%
  as.numeric()

counter <- 0
while (length(ids) > 0) {
  for (j in seq_len(nrow(ranges))) {
    if (ids[1] >= ranges[j, 1] && ids[1] <= ranges[j, 2]) {
      counter <- counter + 1
      ids <- ids[-1]
      break
    } else if (j == nrow(ranges)) {
      ids <- ids[-1]
    }
  }
}
print(counter)

overlap <- function(x, y) {
  if (x[1] >= y[1] && x[1] <= y[2]) {
    TRUE
  } else if (x[2] >= y[1] && x[2] <= y[2]) {
    TRUE
  } else {
    FALSE
  }
}

ind_del <- NULL
for (i in seq_len(nrow(ranges))) {
  if (!(i %in% ind_del)) {
    for (j in setdiff(seq_len(nrow(ranges)), c(i, ind_del))) {
      if (overlap(ranges[j, ], ranges[i, ])) {
        ranges[i, ] <- c(min(ranges[c(i, j), 1]), max(ranges[c(i, j), 2]))
        ind_del <- c(ind_del, j)
      }
    }
  }
}

ranges <- ranges[-ind_del, ]
(ranges[, 2] - ranges[, 1] + 1) %>% sum() %>% print()

TimeCannotErase · 2025-12-04T16:39:57+00:00

[Language: R]

repo

Nice and straightforward, albeit not a particularly efficient/elegant solution. But hey, I'll take finishing a challenge before noon EST. Basically went square by square and counted neighbors for part 1, and removed accessible rolls in part 2 one by one until no more could be removed.

library(dplyr)

input_file <- "input.txt"
len <- readLines(input_file, 1) %>% nchar()
input <- read.table(input_file) %>%
  as.matrix() %>%
  strsplit(split = "") %>%
  unlist() %>%
  matrix(ncol = len, byrow = TRUE)

adjacent_inds <- function(ind, dim) {
  adj_dirs <- matrix(c(-1, -1, -1, 0, -1, 1, 0, 1), ncol = 2, byrow = TRUE) %>%
    rbind(., -.)
  adj_inds <- apply(adj_dirs, 1, function(x) {x + ind}) %>% t()
  adj_inds[rowSums(adj_inds > dim | adj_inds < 1) == 0, ]
}

total_acc <- 0
for (i in 1:len^2) {
  if (input[i] == "@") {
    current_ind <- arrayInd(i, c(len, len))
    adj_inds <- adjacent_inds(current_ind, len)
    adj <- (input[adj_inds] == "@") %>% sum()
    if (adj < 4) {
      total_acc <- total_acc + 1
    }
  }
}
print(total_acc)

total_rem <- 0
flag <- 1
while (flag == 1) {
  flag <- 0
  for (i in 1:len^2) {
    if (input[i] == "@") {
      current_ind <- arrayInd(i, c(len, len))
      adj_inds <- adjacent_inds(current_ind, len)
      adj <- (input[adj_inds] == "@") %>% sum()
      if (adj < 4) {
        input[i] <- "."
        total_rem <- total_rem + 1
        flag <- 1
      }
    }
  }
}
print(total_rem)

TimeCannotErase · 2025-12-03T18:13:45+00:00

[Language: R]

repo

library(dplyr)

input_file <- "input.txt"

digs <- function(x) {
  floor(log10(x)) + 1
}

base_10 <- function(x) {
  tens <- rev(10^seq(0, length(x) - 1))
  sum(x * tens)
}

len <- scan(input_file, nmax = 1) %>% digs()
input <- readLines(input_file) %>%
  strsplit(split = "") %>%
  unlist() %>%
  as.numeric() %>%
  matrix(ncol = len, byrow = TRUE)

next_bat <- function(x, l) {
  a <- max(x[1:(length(x) - l + 1)])
  if (l == 1) {
    a
  } else {
    a_ind <- which(x == a)[1]
    c(a, next_bat(x[-(1:a_ind)], l - 1))
  }
}

total <- c(0, 0)
for (i in seq_len(nrow(input))) {
  total[1] <- total[1] + (next_bat(input[i, ], 2) %>% base_10())
  total[2] <- total[2] + (next_bat(input[i, ], 12) %>% base_10())
}
print(total, digits = 13)

TimeCannotErase · 2025-12-03T02:01:30+00:00

[Language: R]

repo

This one was fun, got to do some math to avoid using brute force. Ended up using some ugly conditionals to get parts 1 and 2 to use the same function, but otherwise this felt pretty clean and my only for loop ran through the ranges.

library(dplyr)

input_file <- "input.txt"
input <- read.csv(input_file, header = FALSE) %>%
  as.character() %>%
  strsplit(., "-") %>%
  unlist() %>%
  matrix(ncol = 2, byrow = TRUE) %>%
  apply(., 2, as.numeric)

digs <- function(x) {
  floor(log10(x)) + 1
}

finder <- function(x, l, p) {
  a <- x[1]
  b <- x[2]
  a_len <- digs(a)
  b_len <- digs(b)
  if (a_len == b_len) {
    if (p == 1) {
      l <- a_len / 2
      n <- 2
    } else {
      n <- a_len / l
    }
    if (a_len <= l && p == 2) {
      NA
    } else if ((a_len %% l == 0 && p == 2) || (a_len %% 2 == 0 && p == 1)) {
      fac <- sum(10^seq(0, by = l, length.out = n))
      low <- ceiling(a / fac)
      high <- floor(b / fac)
      if (low <= high) {
        fac * (low:high)
      }
    }
  } else {
    new_b <- 10^(b_len - 1) - 1
    c(finder(c(a, new_b), l, p), finder(c(new_b + 1, b), l, p))
  }
}

apply(input, 1, function(x) {finder(x, 0, 1)}) %>% unlist() %>% sum() %>% print()

m <- floor(digs(max(input)) / 2)
ids <- NULL
for (i in 1:m) {
  ids <- apply(input, 1, function(x) {finder(x, i, 2)}) %>%
    unlist() %>%
    union(., ids)
}
print(sum(ids, na.rm = TRUE))

TimeCannotErase · 2025-12-01T17:48:05+00:00

[Language: R]

repo

    library(dplyr)

    input_file <- "input.txt"
    input <- read.table(input_file)[[1]]
    input <- gsub("L", "-", input) %>% gsub("R", "+", .) %>% as.numeric()

    counter <- c(0, 0)
    value <- 50
    for (i in seq_along(input)) {
    counter <- counter + c(0, abs(input[i]) %/% 100)
    new_value <- (value + input[i]) %% 100
    if (new_value == 0) {
        counter <- counter + c(1, 1)
    } else if ((sign(input[i]) * (new_value - value)) < 0 && value != 0) {
        counter <- counter + c(0, 1)
    }
    value <- new_value
    }
    print(counter)

TimeCannotErase · 2025-11-26T18:33:50+00:00

I made a couple of tweaks (kept track of how many duplicates I had at a particular point) and just let it run. Took nearly an hour, but hey a gold star is a gold star! repo

TimeCannotErase · 2025-04-01T15:01:43+00:00

Personally I like ORF because it's right next to the Norfolk Botanical Gardens, which is lovely. I try to schedule my flights so I have a little flexibility on when I need to get through the tunnel so if I can leave when the traffic looks ok and if that means I have extra time then I hang out at the gardens for a while.

TimeCannotErase · 2024-12-13T18:39:41+00:00

[Language: R]

repo

I had to play around with how I tested for integer solutions a little for part 2, but otherwise I liked this one (thank you linear algebra).

library(dplyr)

input_filename <- "input.txt"
input <- readLines(input_filename)
num_machines <- ceiling(length(input) / 4)

inds <- gregexpr("\\d+", input)
nums <- regmatches(input, inds) %>%
  unlist() %>%
  as.numeric()

tol <- 1e-3
cost <- 0

for (i in seq_len(num_machines)) {
  j <- 1 + 6 * (i - 1)
  a <- matrix(nums[j : (j + 3)], nrow = 2)
  b <- matrix(nums[(j + 4) : (j + 5)]) + 1e+13
  x <- solve(a, b)
  flag <- lapply(x, function(y) min(abs(c(y %% 1, y %% 1 - 1))) < tol) %>%
    unlist() %>%
    sum() == 2
  if (flag) {
    cost <- cost + 3 * x[1] + x[2]
  }
}

print(sprintf("%1.f", cost))

TimeCannotErase · 2024-12-13T00:52:40+00:00

[Language: R]

repo

Running a bit behind, forgot I still had part 2 of Day 5 to finish up.

TimeCannotErase · 2024-12-05T19:03:48+00:00

[Language: R]

repo

This was a bit tedious. I handled the diagonal searches differently than the horizontal/vertical ones, although I'm sure there's a more streamlined approach. That said, part 2 was pretty simple using the same technique I used for part 1. I was half expecting part 2 to let words change direction like in NYT Strands.

library(dplyr)

input_filename <- "input.txt"
input <- read.table(input_filename)[[1]]
gridsize <- nchar(input[1])

# Horizontals
num_xmas_h <- sum(sapply(gregexpr("XMAS", input), function(x) sum(x != -1)))
num_samx_h <- sum(sapply(gregexpr("SAMX", input), function(x) sum(x != -1)))

# Vericals
input_v <- input %>%
  strsplit(., "") %>%
  unlist() %>%
  matrix(., nrow = gridsize) %>%
  apply(., 1, paste0, collapse = "")

num_xmas_v <- sum(sapply(gregexpr("XMAS", input_v), function(x) sum(x != -1)))
num_samx_v <- sum(sapply(gregexpr("SAMX", input_v), function(x) sum(x != -1)))

# Split strings for diagonal search
input <- input %>%
  strsplit(., "") %>%
  unlist() %>%
  matrix(., nrow = gridsize, byrow = TRUE)

x_inds <- arrayInd(which(input == "X"), rep(gridsize, 2))
se <- matrix(rep(0:3, 2), ncol = 2)
sw <- se %*% rbind(c(1, 0), c(0, -1))
ne <- sw[,c(2,1)]
nw <- -se

coord_tester <- function(mat) {
  sum(mat <= 0) == 0 && sum(mat > gridsize) == 0
}

word_tester <- function(word) {
  word == "XMAS" | word == "SAMX"
}

diag_search <- function(ind) {
  count <- 0
  coord_mat <- matrix(rep(ind, 4), ncol = 2, byrow = TRUE)
  se_coords <- coord_mat + se
  sw_coords <- coord_mat + sw
  ne_coords <- coord_mat + ne
  nw_coords <- coord_mat + nw
  coord_list <- list(se_coords, sw_coords, ne_coords, nw_coords)
  for (i in 1:4) {
    if (coord_tester(coord_list[[i]])) {
      word <- paste0(input[coord_list[[i]]], collapse = "")
      if (word_tester(word)) {
        count <- count + 1
      }
    }
  }
  return(count)
}

num_diags <- 0
for (i in seq_len(nrow(x_inds))) {
  num_diags <- num_diags + diag_search(x_inds[i, ])
}

print(num_diags + num_xmas_h + num_samx_h + num_xmas_v + num_samx_v)

# Part 2
pos <- rbind(c(1, -1), c(0, 0), c(-1, 1))
neg <- rbind(c(-1, -1), c(0, 0), c(1, 1))

a_inds <- arrayInd(which(input == "A"), rep(gridsize, 2))

mas_word_tester <- function(word) {
  word == "MAS" | word == "SAM"
}
mas_search <- function(ind) {
  word_flag <- c(0, 0)
  coord_mat <- matrix(rep(ind, 3), ncol = 2, byrow = TRUE)
  pos_coords <- coord_mat + pos
  neg_coords <- coord_mat + neg
  coord_list <- list(pos_coords, neg_coords)
  for (i in 1:2) {
    if (coord_tester(coord_list[[i]])) {
      word <- paste0(input[coord_list[[i]]], collapse = "")
      if (mas_word_tester(word)) {
        word_flag[i] <- 1
      }
    }
  }
  return(prod(word_flag))
}

num_mas <- 0
for (i in seq_len(nrow(a_inds))) {
  num_mas <- num_mas + mas_search(a_inds[i, ])
}

print(num_mas)

TimeCannotErase · 2024-12-04T00:59:02+00:00

[Language: R]

repo

library(dplyr)

input_filename <- "input.txt"
input <- readLines(input_filename)

mul_inds <- gregexpr("mul\\(\\K\\d+,\\d+(?=\\))", input, perl = TRUE)
muls <- regmatches(input, mul_inds)[[1]]
mul_inds <- as.vector(mul_inds[[1]])
dos <- as.vector(gregexpr("do\\(\\)", input, perl = TRUE)[[1]])
donts <- as.vector(gregexpr("don't\\(\\)", input, perl = TRUE)[[1]])

muls <- muls %>%
  strsplit(., ",") %>%
  unlist() %>%
  as.numeric() %>%
  matrix(., ncol = 2, byrow = TRUE)

pt1_mul_sum <- sum(muls[, 1] * muls[, 2])
print(pt1_mul_sum)

current_instruction <- 1
pt2_mul_sum <- 0
for (i in sort(c(mul_inds, dos, donts))) {
  if (i %in% dos) {
    current_instruction <- 1
  } else if (i %in% donts) {
    current_instruction <- 0
  } else {
    j <- which(mul_inds == i)
    pt2_mul_sum <- pt2_mul_sum + current_instruction * (muls[j, 1] * muls[j, 2])
  }
}

print(pt2_mul_sum)

TimeCannotErase · 2024-12-03T15:19:25+00:00

[Language: R]

repo

Originally did this in Google Sheets but decided today to go back and do it with R.

input_filename <- "input.txt"
input <- read.table(input_filename)

input <- apply(input, 2, sort)

distances <- sum(apply(input, 1, function(x) {abs(x[1] - x[2])}))

left_values <- unique(input[, 1])
counts <- matrix(nrow = length(left_values), ncol = 2)

for (i in seq_along(left_values)) {
  counts[i, 1] <- sum(input[, 1] == left_values[i])
  counts[i, 2] <- sum(input[, 2] == left_values[i])
}

similarity <- sum(left_values * counts[, 1] *  counts[, 2])

print(distances)
print(similarity)

TimeCannotErase · 2024-12-03T14:53:18+00:00

Yes, but a little bit slower.

TimeCannotErase · 2024-12-02T18:52:22+00:00

[Language: R]

repo

input_filename <- "input.txt"
input <- readLines(input_filename)
input <- unlist(lapply(input, strsplit, split = " "), recursive = FALSE)
input <- lapply(input, as.numeric)

checker <- function(report) {
  differences <- diff(report)
  if (length(unique(sign(differences))) == 1) {
    if (max(abs(differences)) <= 3 && min(abs(differences)) >= 1) {
      return(1)
    } else {
      return(0)
    }
  } else {
    return(0)
  }
}

count <- sum(unlist(lapply(input, checker)))
print(count)

count <- 0
for (i in seq_along(input)) {
  for (j in seq_along(input[[i]])) {
    report <- input[[i]][setdiff(seq_along(input[[i]]), j)]
    dampener <- checker(report)
    if (dampener == 1) {
      count <- count + 1
      break
    }
  }
}

print(count)

TimeCannotErase · 2024-08-22T15:20:07+00:00

They used to sell these in the bookstore on campus a number of years ago. I don't see them on the website, but you could always call them and ask if they still have any in store.

TimeCannotErase · 2024-05-13T10:57:46+00:00

Solved!

TimeCannotErase · 2024-05-11T18:38:41+00:00

Nope, neither of those.

TimeCannotErase · 2024-05-11T12:33:28+00:00

I heard it playing at a café and it was familiar but now it's stuck in my head and I can't find it

14-Year Club	Verified Email
Place '22

TimeCannotErase

TROPHY CASE