Day 3 part 2

Author: timocp

src/adventofcode2024.gleam

@@ -34,6 +34,7 @@ fn run(n: Int) -> Nil {
   io.println(case n {
     1 -> day1.part2(input) |> int.to_string
     2 -> day2.part2(input) |> int.to_string
+    3 -> day3.part2(input) |> int.to_string
     _ -> "(not implemented)"
   })
 }

src/day3.gleam

@@ -18,3 +18,54 @@ pub fn part1(input: String) -> Int {
   |> list.reduce(fn(acc, v) { acc + v })
   |> result.unwrap(0)
 }
+
+pub fn part2(input: String) -> Int {
+  input |> parse_input |> process(True)
+}
+
+fn process(input: List(Token), doing: Bool) -> Int {
+  let #(head, tail) = list.split(input, 1)
+  case list.first(head) {
+    Ok(token) ->
+      case token {
+        Do -> process(tail, True)
+        Dont -> process(tail, False)
+        Mul(a, b) ->
+          process(tail, doing)
+          + case doing {
+            True -> a * b
+            False -> 0
+          }
+      }
+    Error(_) -> 0
+  }
+}
+
+type Token {
+  Mul(a: Int, b: Int)
+  Do
+  Dont
+}
+
+fn parse_input(input: String) -> List(Token) {
+  let assert Ok(re) =
+    regexp.from_string("mul\\((\\d+),(\\d+)\\)|do\\(\\)|don't\\(\\)")
+  regexp.scan(re, input)
+  |> list.map(fn(match) {
+    case match.content {
+      "do()" -> Do
+      "don't()" -> Dont
+      _ -> {
+        // unwrap are safe due to regex, these are mul\(\d+,\d+\)
+        let values =
+          match.submatches
+          |> list.map(fn(submatch) { option.unwrap(submatch, "") })
+          |> list.filter_map(int.parse)
+        Mul(
+          values |> list.first |> result.unwrap(0),
+          values |> list.drop(1) |> list.first |> result.unwrap(0),
+        )
+      }
+    }
+  })
+}

test/day3_test.gleam

@@ -9,6 +9,12 @@ pub fn main() {
 
 const example1 = "xmul(2,4)%&mul[3,7]!@^do_not_mul(5,5)+mul(32,64]then(mul(11,8)mul(8,5))\n"
 
+const example2 = "xmul(2,4)&mul[3,7]!^don't()_mul(5,5)+mul(32,64](mul(11,8)undo()?mul(8,5))\n"
+
 pub fn part1_test() {
   day3.part1(example1) |> should.equal(161)
 }
+
+pub fn part2_test() {
+  day3.part2(example2) |> should.equal(48)
+}