4 files changed, 144 insertions, 0 deletions
diff --git a/aoc2023-other/day-09/day-09.rkt b/aoc2023-other/day-09/day-09.rkt
new file mode 100644
index 0000000..c632150
--- /dev/null
+++ b/aoc2023-other/day-09/day-09.rkt
@@ -0,0 +1,22 @@
+#lang racket
+
+(require advent-of-code
+         threading)
+
+(define histories
+  (for/list ([raw-history (in-lines (open-aoc-input (find-session) 2023 9 #:cache #true))])
+    (~>> raw-history string-split (map string->number))))
+
+(define (constant? xs) (= 1 (length (remove-duplicates xs))))
+
+(define/match (derivative xs)
+  [((list a b)) (list (- b a))]
+  [((list* a b _)) (cons (- b a) (derivative (rest xs)))])
+
+(define (extrapolate xs) (if (constant? xs) (car xs) (+ (last xs) (extrapolate (derivative xs)))))
+
+;; part 1
+(~>> histories (map extrapolate) (apply +))
+
+;; part 2
+(~>> histories (map (λ~> reverse extrapolate)) (apply +))
diff --git a/aoc2023/src/day9/.gitignore b/aoc2023/src/day9/.gitignore
new file mode 100644
index 0000000..ae40cea
--- /dev/null
+++ b/aoc2023/src/day9/.gitignore
@@ -0,0 +1 @@
+input.txt
+\ No newline at end of file
diff --git a/aoc2023/src/day9/solve.gleam b/aoc2023/src/day9/solve.gleam
new file mode 100644
index 0000000..ce061ae
--- /dev/null
+++ b/aoc2023/src/day9/solve.gleam
@@ -0,0 +1,69 @@
+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))
+  }
+}
+
+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
+  }
+}
diff --git a/aoc2023/test/day9/day9_test.gleam b/aoc2023/test/day9/day9_test.gleam
new file mode 100644
index 0000000..84fd3ba
--- /dev/null
+++ b/aoc2023/test/day9/day9_test.gleam
@@ -0,0 +1,52 @@
+import gleam/list
+import showtime/tests/should
+import adglent.{type Example, Example}
+import day9/solve
+
+type Problem1AnswerType =
+  String
+
+type Problem2AnswerType =
+  String
+
+/// Add examples for part 1 here:
+/// ```gleam
+///const part1_examples: List(Example(Problem1AnswerType)) = [Example("some input", "")]
+/// ```
+const part1_examples: List(Example(Problem1AnswerType)) = [
+  Example(
+    "0 3 6 9 12 15
+1 3 6 10 15 21
+10 13 16 21 30 45",
+    "114",
+  ),
+]
+
+/// Add examples for part 2 here:
+/// ```gleam
+///const part2_examples: List(Example(Problem2AnswerType)) = [Example("some input", "")]
+/// ```
+const part2_examples: List(Example(Problem2AnswerType)) = [
+  Example(
+    "0 3 6 9 12 15
+1 3 6 10 15 21
+10 13 16 21 30 45",
+    "2",
+  ),
+]
+
+pub fn part1_test() {
+  part1_examples
+  |> should.not_equal([])
+  use example <- list.map(part1_examples)
+  solve.part1(example.input)
+  |> should.equal(example.answer)
+}
+
+pub fn part2_test() {
+  part2_examples
+  |> should.not_equal([])
+  use example <- list.map(part2_examples)
+  solve.part2(example.input)
+  |> should.equal(example.answer)
+}