1 files changed, 70 insertions, 0 deletions
diff --git a/aoc2023-gleam/src/day9/solve.gleam b/aoc2023-gleam/src/day9/solve.gleam
new file mode 100644
index 0000000..a2cc7ae
--- /dev/null
+++ b/aoc2023-gleam/src/day9/solve.gleam
@@ -0,0 +1,70 @@
+import adglent.{First, Second}
+import gleam/io
+import gleam/list
+import gleam/string
+import gleam/int
+
+fn parse(input: String, backwards backwards: Bool) -> List(List(Int)) {
+  use line <- list.map(string.split(input, "\n"))
+  use n_str <- list.map(maybe_backwards(string.split(line, " "), backwards))
+  let assert Ok(n) = int.parse(n_str)
+  n
+}
+
+fn maybe_backwards(xs: List(a), backwards: Bool) -> List(a) {
+  case backwards {
+    False -> list.reverse(xs)
+    True -> xs
+  }
+}
+
+fn is_constant(ns: List(Int)) -> Bool {
+  case list.unique(ns) {
+    [_] -> True
+    _ -> False
+  }
+}
+
+fn take_derivative(ns: List(Int)) -> List(Int) {
+  ns
+  |> list.window_by_2
+  |> list.map(fn(tup) { tup.0 - tup.1 })
+}
+
+fn extrapolate(ns: List(Int)) {
+  case is_constant(ns), ns {
+    True, [n, ..] -> n
+    False, [n, ..] -> n + extrapolate(take_derivative(ns))
+    _, _ -> panic as "list empty when it shouldn't be"
+  }
+}
+
+fn part(input: String, backwards backwards: Bool) {
+  input
+  |> parse(backwards: backwards)
+  |> list.fold(0, fn(acc, ns) { extrapolate(ns) + acc })
+  |> string.inspect
+}
+
+pub fn part1(input: String) {
+  part(input, backwards: False)
+}
+
+pub fn part2(input: String) {
+  part(input, backwards: True)
+}
+
+pub fn main() {
+  let assert Ok(part) = adglent.get_part()
+  let assert Ok(input) = adglent.get_input("9")
+  case part {
+    First ->
+      part1(input)
+      |> adglent.inspect
+      |> io.println
+    Second ->
+      part2(input)
+      |> adglent.inspect
+      |> io.println
+  }
+}