day 8 puzzle 2

progress.yaml

@@ -13,4 +13,4 @@
 7:
   stars: 2
 8:
-  stars: 1
+  stars: 2

src/day8/puzzle2.typ

@@ -0,0 +1,65 @@
+#import "/src/utils.typ": *
+
+#let in-grid(w, h, x, y) = {
+  return 0 <= x and x < w and 0 <= y and y < h
+}
+
+#let get-antinodes(in-grid, x1, y1, x2, y2) = {
+  let (dx, dy) = (x2 - x1, y2 - y1)
+  let f = calc.gcd(dx, dy)
+  dx /= f
+  dy /= f
+
+  let walk(ox, oy, dx, dy) = {
+    let x = ox
+    let y = oy
+    let pos = ()
+    while in-grid(x, y) {
+      pos.push((x, y))
+      x += dx
+      y += dy
+    }
+    return pos
+  }
+
+  let antinodes = walk(x1, y1, dx, dy) + walk(x1, y1, -dx, -dy)
+
+  return antinodes
+}
+
+#let solve(input) = {
+  let by-freq = (:)
+  let antinodes = ()
+
+  let grid = input.split("\n").map(l => l.clusters())
+  let w = grid.first().len()
+  let h = grid.len()
+
+  let in-grid = in-grid.with(w, h)
+  let get-antinodes = get-antinodes.with(in-grid)
+
+  for y in range(h) {
+    for x in range(w) {
+      let c = grid.at(y).at(x)
+      if c == "." {
+        continue
+      }
+      if c not in by-freq {
+        by-freq.insert(c, ())
+      }
+
+      for (x2, y2) in by-freq.at(c) {
+        antinodes += get-antinodes(x, y, x2, y2)
+      }
+      by-freq.at(c).push((x, y))
+    }
+  }
+
+  return antinodes.dedup().len()
+}
+
+#show-puzzle(
+  8, 2,
+  solve,
+  example: 34
+)