Finish day 13

3 files changed, 135 insertions, 1 deletions

diff --git a/aoc-2020-gleam/src/days/day10.gleam b/aoc-2020-gleam/src/days/day10.gleam
index b53000b..86c0137 100644
--- a/aoc-2020-gleam/src/days/day10.gleam
+++ b/aoc-2020-gleam/src/days/day10.gleam
@@ -49,7 +49,7 @@ fn arrangements(number: Int, adapters: List(Int), cache: Cache(Int, Int)) -> Int
 
 fn part2(numbers: List(Int)) -> Int {
   let adapters = process_adapters(numbers)
-  let device_joltage = resx.assert_unwrap(list.last(adapters))
+  let assert Ok(device_joltage) = list.last(adapters)
   use cache <- cache.create()
   arrangements(device_joltage, adapters, cache)
 }
diff --git a/aoc-2020-gleam/src/days/day13.gleam b/aoc-2020-gleam/src/days/day13.gleam
new file mode 100644
index 0000000..c95cd42
--- /dev/null
+++ b/aoc-2020-gleam/src/days/day13.gleam
@@ -0,0 +1,113 @@
+import gleam/io
+import gleam/int
+import gleam/list
+import gleam/pair
+import gleam/string as str
+import gleam/iterator as iter
+import ext/intx
+import ext/resultx as resx
+import ext/iteratorx as iterx
+import util/input_util
+
+type Bus {
+  Any
+  Line(Int)
+}
+
+type Problem {
+  Problem(timestamp: Int, buses: List(Bus))
+}
+
+fn parse_problem(input: String) -> Problem {
+  let assert Ok(#(timestamp, buses)) =
+    input
+    |> str.trim
+    |> str.split_once(on: "\n")
+  let assert Ok(timestamp) = int.parse(timestamp)
+  let buses =
+    buses
+    |> str.split(on: ",")
+    |> list.map(with: fn(b) {
+      case b == "x" {
+        True -> Any
+        False ->
+          b
+          |> int.parse
+          |> resx.assert_unwrap
+          |> Line
+      }
+    })
+  Problem(timestamp, buses)
+}
+
+fn part1(input: String) -> Int {
+  let problem = parse_problem(input)
+
+  problem.buses
+  |> list.filter_map(with: fn(b) {
+    case b {
+      Line(line) -> Ok(line)
+      Any -> Error(Nil)
+    }
+  })
+  |> list.map(with: fn(b) {
+    #(b, b * intx.ceil_divide(problem.timestamp, by: b) - problem.timestamp)
+  })
+  |> list.reduce(with: fn(acc, cur) {
+    case cur.1 < acc.1 {
+      True -> cur
+      False -> acc
+    }
+  })
+  |> resx.assert_unwrap
+  |> fn(res: #(Int, Int)) { res.0 * res.1 }
+}
+
+fn part2(input: String) -> Int {
+  let problem = parse_problem(input)
+
+  let buses =
+    problem.buses
+    |> iter.from_list
+    |> iter.index
+    |> iter.flat_map(fn(entry) {
+      let #(i, b) = entry
+      case b {
+        Line(line) -> iter.single(#(line, i))
+        Any -> iter.empty()
+      }
+    })
+    |> iter.to_list
+
+  let assert [#(timestamp, _), ..] = buses
+
+  buses
+  |> list.fold(
+    from: #(timestamp, timestamp),
+    with: fn(prev, entry) {
+      let #(timestamp, period) = prev
+      let #(id, i) = entry
+
+      let assert Ok(timestamp) =
+        timestamp
+        |> iterx.unfold_infinitely(with: int.add(_, period))
+        |> iter.find(one_that: fn(t) { { t + i } % id == 0 })
+      let period = intx.lcm(period, id)
+
+      #(timestamp, period)
+    },
+  )
+  |> pair.first
+}
+
+pub fn main() -> Nil {
+  let test = input_util.read_text("test13")
+  let assert 295 = part1(test)
+  let assert 1_068_781 = part2(test)
+
+  let input = input_util.read_text("day13")
+  io.debug(part1(input))
+  io.debug(part2(input))
+
+  Nil
+}
diff --git a/aoc-2020-gleam/src/ext/intx.gleam b/aoc-2020-gleam/src/ext/intx.gleam
index 9f06850..077b4f2 100644
--- a/aoc-2020-gleam/src/ext/intx.gleam
+++ b/aoc-2020-gleam/src/ext/intx.gleam
@@ -1,3 +1,24 @@
+import gleam/int
+import gleam/order.{Eq, Gt, Lt}
+
 pub fn is_between(number: Int, min: Int, and max: Int) {
   min <= number && number <= max
 }
+
+pub fn ceil_divide(dividend: Int, by divisor: Int) -> Int {
+  { dividend + divisor - 1 } / divisor
+}
+
+fn gcd(a: Int, b: Int) -> Int {
+  case b == 0 {
+    True -> a
+    False -> gcd(b, a % b)
+  }
+}
+
+pub fn lcm(a: Int, b: Int) -> Int {
+  case int.compare(a, b) {
+    Gt | Eq -> { a / gcd(a, b) } * b
+    Lt -> { b / gcd(a, b) } * a
+  }
+}