day 15 puzzle 2

progress.yaml

@@ -27,4 +27,4 @@
 14:
   stars: 2
 15:
-  stars: 1
+  stars: 2

src/day15/puzzle2.typ

@@ -0,0 +1,123 @@
+#import "/src/utils.typ": *
+
+#let WALL = "#"
+#let BOX = "O"
+#let BOX-L = "["
+#let BOX-R = "]"
+#let BOT = "@"
+#let EMPTY = "."
+
+#let offsets = (
+  "^": (0, -1),
+  "<": (-1, 0),
+  "v": (0, 1),
+  ">": (1, 0)
+)
+
+#let compute-value(grid) = {
+  let total = 0
+  for (y, row) in grid.enumerate() {
+    for (x, type) in row.enumerate() {
+      if type == BOX-L {
+        total += y * 100 + x
+      }
+    }
+  }
+  return total
+}
+
+#let can-move(grid, w, h, x, y, dx, dy) = {
+  let (x2, y2) = (x + dx, y + dy)
+  let type = grid.at(y2).at(x2)
+  if type == EMPTY {
+    return true
+  }
+  if type == WALL {
+    return false
+  }
+
+  // Horizontal move
+  if dy == 0 {
+    return can-move(grid, w, h, x2, y2, dx, dy)
+  }
+  
+  let lx = if type == BOX-L {x2} else {x2 - 1}
+
+  return (
+    can-move(grid, w, h, lx, y2, dx, dy) and
+    can-move(grid, w, h, lx + 1, y2, dx, dy)
+  )
+}
+
+#let do-move(grid, w, h, x, y, dx, dy) = {
+  let type = grid.at(y).at(x)
+  let (x2, y2) = (x + dx, y + dy)
+  let type2 = grid.at(y2).at(x2)
+  
+  if type2 in (BOX-L, BOX-R) {
+    grid = do-move(grid, w, h, x2, y2, dx, dy)
+    // Vertical move
+    if dx == 0 {
+      let x3 = if type2 == BOX-L {x2 + 1} else {x2 - 1}
+      grid = do-move(grid, w, h, x3, y2, dx, dy)
+    }
+    type2 = EMPTY
+  }
+  if type2 == EMPTY {
+    grid.at(y2).at(x2) = type
+    grid.at(y).at(x) = EMPTY
+    return grid
+  }
+  if type2 == WALL {
+    panic()
+  }
+  return grid
+}
+
+#let solve(input) = {
+  let (grid-data, move-data) = input.split("\n\n")
+  let grid = grid-data.split("\n").map(r => r.clusters())
+  
+  let w = grid.first().len()
+  let h = grid.len()
+  let (w2, h2) = (w * 2, h)
+  let bot-pos = none
+  let grid2 = ((EMPTY,) * w2,) * h2
+
+  for y in range(h) {
+    for x in range(w) {
+      let type = grid.at(y).at(x)
+      if type == BOT {
+        bot-pos = (x*2, y)
+        type = EMPTY
+        continue
+      }
+      grid2.at(y).at(x*2) = if type == BOX {BOX-L} else {type}
+      grid2.at(y).at(x*2 + 1) = if type == BOX {BOX-R} else {type}
+    }
+  }
+  let (bot-x, bot-y) = bot-pos
+
+  let moves = move-data.replace("\n", "").clusters()
+  for (move-i, move) in moves.enumerate() {
+    let (dx, dy) = offsets.at(move)
+    let (x2, y2) = (bot-x + dx, bot-y + dy)
+
+    if not can-move(grid2, w2, h2, bot-x, bot-y, dx, dy) {
+      continue
+    }
+
+    grid2 = do-move(grid2, w2, h2, bot-x, bot-y, dx, dy)
+    (bot-x, bot-y) = (x2, y2)
+  }
+
+  return compute-value(grid2)
+}
+
+#show-puzzle(
+  15, 2,
+  solve,
+  example: (
+    "2": 9021
+  )
+)