Day 1 Part 1

day_1/src/day_1.gleam

@@ -1,5 +1,78 @@
+import gleam/int
 import gleam/io
+import gleam/list
+import gleam/result
+import gleam/string
+import simplifile
+
+pub type Rotation {
+  Left(Int)
+  Right(Int)
+}
+
+pub fn rotate(pos: Int, rot: Rotation) -> Int {
+  case rot {
+    Left(r) ->
+      case pos, r {
+        p, r if p < r -> 100 - { r - p }
+        p, r -> p - r
+      }
+    Right(r) -> { pos + r } % 100
+  }
+}
+
+pub fn rotate_step(hist: List(Int), rot: Rotation) -> List(Int) {
+  case hist {
+    [] -> [rotate(50, rot), 50]
+    [p, ..] -> [rotate(p, rot), ..hist]
+  }
+}
+
+pub fn count_zeros(l: List(Int)) -> Int {
+  list.filter(l, fn(x) { x == 0 }) |> list.length
+}
+
+pub fn line_to_rotation(l: String) -> Result(Rotation, Nil) {
+  let first_char = string.first(l)
+  let numeral =
+    string.drop_start(l, up_to: 1) |> int.parse |> result.map(fn(n) { n % 100 })
+  result.map(first_char, fn(c) {
+    case c {
+      "L" -> result.map(numeral, Left)
+      "R" -> result.map(numeral, Right)
+      _ -> Error(Nil)
+    }
+  })
+  |> result.flatten
+}
+
+pub fn read_line(file: String) -> Result(List(String), simplifile.FileError) {
+  simplifile.read(file) |> result.map(fn(c) { string.split(c, on: "\n") })
+}
+
+pub fn file_to_rotations(file: String) -> Result(List(Rotation), Nil) {
+  read_line(file)
+  |> result.map_error(fn(_) { Nil })
+  |> result.map(fn(ls) {
+    list.map(ls, line_to_rotation)
+    |> list.filter_map(fn(a) { a })
+  })
+}
+
+pub fn apply_rotations(rots: List(Rotation)) -> List(Int) {
+  list.fold(rots, [], rotate_step)
+}
+
+pub fn rotations_file_to_zeros(file: String) -> Result(Int, Nil) {
+  file_to_rotations(file)
+  |> result.map(apply_rotations)
+  |> result.map(count_zeros)
+}
 
 pub fn main() -> Nil {
-  io.println("Hello from day_1!")
+  let _ =
+    rotations_file_to_zeros("input.txt")
+    |> result.unwrap(-1)
+    |> int.to_string
+    |> io.println
 }

day_1/src/good_test.txt

@@ -0,0 +1,2 @@
+hello
+world