Added 2024 days 12 & 13

2024/R/day12.qmd

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+---
+title: "Day 12"
+date: 2024-12-12
+author:
+  name: https://adventofcode.com/2024/day/12
+  url: https://adventofcode.com/2024/day/12
+---
+
+## Setup
+
+```{r setup}
+# Libraries
+library(tidyverse)
+library(igraph)
+
+# Read input from file into a data frame
+input <- read_table("../input/day12.txt", col_names = "chr") |> 
+  mutate(
+    row = row_number(),
+    chr = str_split(chr, "")
+  ) |> 
+  unnest(chr) |> 
+  mutate(col = row_number(), .by = row) |> 
+  mutate(idx = row_number(), .before = everything())
+```
+
+## Part 1
+
+Format the input as a graph, with edges connecting neighbors of the same type:
+
+```{r}
+
+# Flag neighboring characters of the same value that border one other
+edges_wide <- input |> 
+  mutate(v = case_when(row + 1 == lead(row) ~ lead(idx)), .by = c(chr, col)) |> 
+  mutate(h = case_when(col + 1 == lead(col) ~ lead(idx)), .by = c(chr, row))
+
+edges_long <- edges_wide |> 
+  pivot_longer(
+    c(v, h), 
+    names_to = NULL, 
+    values_to = "target", 
+    values_drop_na = TRUE
+  )
+
+# Format neighbors as a list of edges and add to add a graph
+g <- edges_long |> 
+  transmute(
+    edge_id = row_number(),
+    src = idx, 
+    target
+  ) |> 
+  pivot_longer(c(src, target)) |> 
+  arrange(edge_id, value) |> 
+  pull(value) |> 
+  make_graph(n = nrow(input), directed = FALSE)
+
+V(g)$name <- 1:nrow(input)
+
+# Separate out the resulting graph into sub-graphs of innerconnected regions
+dg <- decompose(g)
+
+```
+
+Compute the perimeter, area, and cost of each subgraph then sum the total:
+
+```{r}
+dg |> 
+  map_int(\(subgraph) {
+    perim <- sum(4 - degree(subgraph))
+    area  <- gorder(subgraph)
+    perim * area
+  }) |> 
+  sum()
+```
+
+## Part 2
+
+Used a hint from reddit: the number of corners is equal to the number of sides.
+
+A plot can have a convex corner or a concave corner.
+
+-   A cell has a convex corner for each pair of adjacent borders
+-   A cell has a concave corner if it has two adjacent cells of its same group, but its diagonal cell between the two has a different group.
+
+```{r}
+
+# Get original row/column input and join on the group output from the graph
+groups <- left_join(
+  input,
+  imap_dfr(dg, \(g, grp_idx) tibble(grp = grp_idx, idx = V(g)$name)),
+  join_by(idx)
+) |> 
+  select(idx, grp, row, col)
+
+# For each of a cell's neighbors, flag if they're in the same group
+neighbors <- groups |> 
+  # Get group number of each adjacent cell (N/S/E/W)
+  left_join(transmute(groups, n = grp, row = row + 1, col), join_by(row, col)) |> 
+  left_join(transmute(groups, w = grp, col = col + 1, row), join_by(row, col)) |> 
+  left_join(transmute(groups, s = grp, row = row - 1, col), join_by(row, col)) |> 
+  left_join(transmute(groups, e = grp, col = col - 1, row), join_by(row, col)) |> 
+  # Get group number of each diagonal cell (NW/NE/SW/SE)
+  left_join(transmute(groups, nw = grp, row = row + 1, col = col + 1), join_by(row, col)) |> 
+  left_join(transmute(groups, ne = grp, row = row + 1, col = col - 1), join_by(row, col)) |> 
+  left_join(transmute(groups, sw = grp, row = row - 1, col = col + 1), join_by(row, col)) |> 
+  left_join(transmute(groups, se = grp, row = row - 1, col = col - 1), join_by(row, col)) |> 
+  select(-c(row, col)) |> 
+  # Compare group numbers of adjacent/diagonal cells to the current cell
+  mutate(across(c(n, w, s, e, nw, ne, sw, se), ~ replace_na(.x == grp, FALSE)))
+
+# Compute total number of concave/convex corners for each cell
+corners <- neighbors |> 
+  mutate(
+    convex = (!n & !w) + (!s & !w) + (!s & !e) + (!n & !e),
+    concave = (n & w & !nw) + (s & w & !sw) + (s & e & !se) + (n & e & !ne)
+  )
+
+```
+
+Total the number of corners per group and multiply by the group's area to get the total cost:
+
+```{r}
+corners |> 
+  summarize(
+    area = n(),
+    num_sides = sum(convex + concave), 
+    .by = grp
+  ) |> 
+  mutate(cost = area * num_sides) |> 
+  pull(cost) |> 
+  sum()
+
+```
+
+
+
+
+

2024/R/day13.qmd

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+---
+title: "Day 13"
+date: 2024-12-13
+author:
+  name: https://adventofcode.com/2024/day/13
+  url: https://adventofcode.com/2024/day/13
+---
+
+## Setup
+
+```{r setup}
+# Libraries
+library(tidyverse)
+library(unglue)
+
+# Read input from file
+input <- read_lines("../input/day13.txt", skip_empty_rows = TRUE)
+```
+
+## Part 1
+
+Extract numerical values from input text:
+```{r}
+df <- input |> 
+  unglue_data(c(
+    "Button {button}: X+{x=\\d+}, Y+{y=\\d+}",
+    "{button}: X={x=\\d+}, Y={y=\\d+}"
+  )) |> 
+  mutate(
+    machine_id = floor((row_number() - 1) / 3),
+    across(c(x, y), parse_number),
+    .before = everything()
+  ) |> 
+  pivot_wider(names_from = button, values_from = c(x, y))
+```
+
+
+Define a function to convert numeric equation input and output token counts:
+
+```{r}
+
+compute_tokens <- function(df) {
+  
+  # Convert each machine's properties into a system of equations and solve.
+  soln <- df |> 
+    nest(coeff = c(x_A, x_B, y_A, y_B)) |> 
+    nest(intercept = c(x_Prize, y_Prize)) |> 
+    mutate(
+      coeff = map(coeff, ~ matrix(as.numeric(.x), nrow = 2, byrow = TRUE)),
+      intercept = map(intercept, as.numeric),
+      soln = map2(
+        coeff, 
+        intercept, 
+        ~ solve(.x, .y) |> 
+          set_names("A", "B") |> 
+          as_tibble_row()
+      )
+    ) |> 
+    unnest(soln) |> 
+    select(machine_id, A, B)
+  
+  
+  # Check that the solution is two whole numbers, then sum the token cost
+  soln |> 
+    mutate(
+      across(
+        c(A, B), 
+        ~ near(.x, abs(round(.x)), tol = 0.001), 
+        .names = "{.col}_valid"
+      ),
+      win = A_valid & B_valid,
+      tokens = if_else(win, 3 * A + B, 0)
+    ) |> 
+    pull(tokens) |> 
+    sum()
+}
+
+```
+
+Run function on puzzle input: 
+
+```{r}
+compute_tokens(df)
+```
+
+
+## Part 2
+
+Add 10000000000000 to each prize intercept and re-compute:
+
+```{r}
+
+df |> 
+  mutate(across(c(x_Prize, y_Prize), ~ .x + 10000000000000)) |> 
+  compute_tokens() |> 
+  format(scientific = FALSE)
+
+```
+