day 19 puzzle 2

progress.yaml

@@ -35,4 +35,4 @@
 18:
   stars: 2
 19:
-  stars: 1
+  stars: 2

src/day19/puzzle2.typ

@@ -0,0 +1,46 @@
+#import "/src/utils.typ": *
+
+#let solve(input) = {
+  let (towels, targets) = input.split("\n\n")
+
+  towels = towels.split(", ")
+  let by-initial = (:)
+  for towel in towels {
+    let initial = towel.first()
+    if initial not in by-initial {
+      by-initial.insert(initial, ())
+    }
+    by-initial.at(initial).push(towel)
+  }
+
+  let count(target) = {
+    let initial = target.first()
+    if initial not in by-initial {
+      return 0
+    }
+    let cnt = 0
+    for towel in by-initial.at(initial) {
+      if towel == target {
+        cnt += 1
+      } else if target.starts-with(towel) {
+        cnt += count(target.slice(towel.len()))
+      }
+    }
+
+    return cnt
+  }
+
+  let total = 0
+  for target in targets.split("\n") {
+    total += count(target)
+  }
+  return total
+}
+
+#show-puzzle(
+  19, 2,
+  solve,
+  example: 16,
+  only-example: true
+)
+#show-result(632423618484345)