Day 2 / part 2

src/days/day02.gleam

···
   Range(start: String, end: String)
 }
 
+pub fn solve(input: String) -> Nil {
+  io.println("Day 2:")
+  io.println("Part 1: " <> part1(input))
+  io.println("Part 2: " <> part2(input))
+}
+
+pub fn part1(input: String) -> String {
+  parse_input(input)
+  |> list.flat_map(fn(range) {
+    find_invalid_ids_in_range(range, is_id_invalid_exactly_twice)
+  })
+  |> int.sum
+  |> int.to_string
+}
+
+pub fn part2(input: String) -> String {
+  parse_input(input)
+  |> list.flat_map(fn(range) {
+    find_invalid_ids_in_range(range, is_id_invalid_at_least_twice)
+  })
+  |> int.sum
+  |> int.to_string
+}
+
 pub fn parse_input(input: String) -> List(Range) {
   input
   |> string.trim
···
   })
 }
 
-fn is_id_invalid(id: String) -> Bool {
+fn find_invalid_ids_in_range(
+  range: Range,
+  validator: fn(String) -> Bool,
+) -> List(Int) {
+  let assert Ok(start) = int.parse(range.start)
+  let assert Ok(end) = int.parse(range.end)
+  list.range(start, end)
+  |> list.filter(fn(id) { validator(int.to_string(id)) })
+}
+
+fn is_id_invalid_exactly_twice(id: String) -> Bool {
   let len = string.length(id)
   case len % 2 == 0 {
     False -> False
···
   }
 }
 
-fn find_invalid_ids_in_range(range: Range) -> List(Int) {
-  let assert Ok(start) = int.parse(range.start)
-  let assert Ok(end) = int.parse(range.end)
-  list.range(start, end)
-  |> list.filter(fn(id) { is_id_invalid(int.to_string(id)) })
-}
-
-pub fn part1(input: String) -> String {
-  parse_input(input)
-  |> list.flat_map(find_invalid_ids_in_range)
-  |> int.sum
-  |> int.to_string
-}
-
-pub fn part2(_input: String) -> String {
-  // TODO: Implement part 2
-  "Not implemented"
-}
-
-pub fn solve(input: String) -> Nil {
-  io.println("Day 2:")
-  io.println("Part 1: " <> part1(input))
-  io.println("Part 2: " <> part2(input))
+fn is_id_invalid_at_least_twice(id: String) -> Bool {
+  let len = string.length(id)
+  list.range(2, len)
+  |> list.any(fn(divisor) {
+    case len % divisor == 0 {
+      False -> False
+      True -> {
+        let part_len = len / divisor
+        let first_part = string.slice(id, 0, part_len)
+        list.range(1, divisor - 1)
+        |> list.all(fn(i) {
+          string.slice(id, i * part_len, part_len) == first_part
+        })
+      }
+    }
+  })
 }

test/days/day02_test.gleam

···
 
 pub fn part1_test() {
   let assert Ok(input) = simplifile.read("inputs/day2.txt")
-  day02.part1(input) |> should.equal("TODO")
+  day02.part1(input) |> should.equal("54234399924")
 }
 
 pub fn part2_test() {
   let assert Ok(input) = simplifile.read("inputs/day2.txt")
-  day02.part2(input) |> should.equal("TODO")
+  day02.part2(input) |> should.equal("70187097315")
 }