1 files changed, 123 insertions, 0 deletions
diff --git a/aoc-2020-gleam/src/days/day12.gleam b/aoc-2020-gleam/src/days/day12.gleam
new file mode 100644
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+++ b/aoc-2020-gleam/src/days/day12.gleam
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+import gleam/io
+import gleam/int
+import gleam/list
+import gleam/string as str
+import util/input_util
+import util/pos.{Pos}
+import util/dir.{Dir, East, North, South, West}
+
+type Instr {
+  MoveIn(dir: Dir, by: Int)
+  Turn(by: Int)
+  MoveForward(by: Int)
+}
+
+fn parse_instr(line: String) -> Instr {
+  let assert Ok(#(action, value)) = str.pop_grapheme(line)
+  let assert Ok(value) = int.parse(value)
+  case action {
+    "N" -> MoveIn(dir: North, by: value)
+    "E" -> MoveIn(dir: East, by: value)
+    "S" -> MoveIn(dir: South, by: value)
+    "W" -> MoveIn(dir: West, by: value)
+    "L" -> Turn(by: dir.degree_to_turn(-value))
+    "R" -> Turn(by: dir.degree_to_turn(value))
+    "F" -> MoveForward(by: value)
+  }
+}
+
+fn process_moves(
+  lines: List(String),
+  initial: a,
+  execute: fn(a, Instr) -> a,
+  locator: fn(a) -> Pos,
+) -> Int {
+  lines
+  |> list.map(with: parse_instr)
+  |> list.fold(from: initial, with: execute)
+  |> fn(s: a) { pos.manhattan_dist(from: pos.zero, to: locator(s)) }
+}
+
+type State1 {
+  State1(pos: Pos, dir: Dir)
+}
+
+const initial_state1 = State1(pos: pos.zero, dir: East)
+
+fn execute_instr1(prev: State1, instr: Instr) -> State1 {
+  case instr {
+    MoveIn(target, times) ->
+      State1(
+        ..prev,
+        pos: target
+        |> dir.offset
+        |> pos.mul(by: times)
+        |> pos.add(prev.pos),
+      )
+    Turn(times) ->
+      State1(
+        ..prev,
+        dir: prev.dir
+        |> dir.rotate_clockwise(by: times),
+      )
+    MoveForward(times) ->
+      State1(
+        ..prev,
+        pos: prev.dir
+        |> dir.offset
+        |> pos.mul(by: times)
+        |> pos.add(prev.pos),
+      )
+  }
+}
+
+fn part1(lines: List(String)) -> Int {
+  process_moves(lines, initial_state1, execute_instr1, fn(s) { s.pos })
+}
+
+type State2 {
+  State2(ship_pos: Pos, anchor_pos: Pos)
+}
+
+const initial_state2 = State2(ship_pos: pos.zero, anchor_pos: #(10, 1))
+
+fn execute_instr2(prev: State2, instr: Instr) -> State2 {
+  case instr {
+    MoveIn(target, times) ->
+      State2(
+        ..prev,
+        anchor_pos: target
+        |> dir.offset
+        |> pos.mul(by: times)
+        |> pos.add(prev.anchor_pos),
+      )
+    Turn(times) ->
+      State2(
+        ..prev,
+        anchor_pos: pos.rotate_around_origin(this: prev.anchor_pos, by: times),
+      )
+    MoveForward(times) ->
+      State2(
+        ..prev,
+        ship_pos: prev.anchor_pos
+        |> pos.mul(by: times)
+        |> pos.add(prev.ship_pos),
+      )
+  }
+}
+
+fn part2(lines: List(String)) -> Int {
+  process_moves(lines, initial_state2, execute_instr2, fn(s) { s.ship_pos })
+}
+
+pub fn main() -> Nil {
+  let test = input_util.read_lines("test12")
+  let assert 25 = part1(test)
+  let assert 286 = part2(test)
+
+  let input = input_util.read_lines("day12")
+  io.debug(part1(input))
+  io.debug(part2(input))
+
+  Nil
+}