From 91f1c1b609605ba85ddeab12ae92c34b28df38c2 Mon Sep 17 00:00:00 2001
From: Xiretza <xiretza@xiretza.xyz>
Date: Sat, 9 Dec 2023 12:43:14 +0000
Subject: [PATCH] 2023 day9/rust: add solution

---
 2023/day9/rust/Cargo.toml  |  10 ++++
 2023/day9/rust/src/main.rs | 102 +++++++++++++++++++++++++++++++++++++
 Cargo.lock                 |   8 +++
 Cargo.toml                 |   1 +
 4 files changed, 121 insertions(+)
 create mode 100644 2023/day9/rust/Cargo.toml
 create mode 100644 2023/day9/rust/src/main.rs

diff --git a/2023/day9/rust/Cargo.toml b/2023/day9/rust/Cargo.toml
new file mode 100644
index 0000000..21b826f
--- /dev/null
+++ b/2023/day9/rust/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "rust_2023_09"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+aoc = { path = "../../../common/rust" }
+itertools = "0.12.0"
diff --git a/2023/day9/rust/src/main.rs b/2023/day9/rust/src/main.rs
new file mode 100644
index 0000000..b7e88fb
--- /dev/null
+++ b/2023/day9/rust/src/main.rs
@@ -0,0 +1,102 @@
+#![warn(clippy::pedantic)]
+
+use std::{collections::VecDeque, io::stdin};
+
+use itertools::Itertools;
+
+type Value = isize;
+
+fn make_derivative(xs: &[Value]) -> Option<VecDeque<Value>> {
+    if xs.iter().all_equal() {
+        None
+    } else {
+        Some(xs.iter().tuple_windows().map(|(a, b)| b - a).collect())
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+enum Direction {
+    Forward,
+    Backward,
+}
+
+#[derive(Debug)]
+struct History {
+    values: VecDeque<Value>,
+    derivatives: Vec<VecDeque<Value>>,
+}
+
+impl History {
+    pub fn new(values: VecDeque<Value>) -> Self {
+        let derivatives = std::iter::successors(Some(values.clone()), |prev| {
+            let (s1, s2) = prev.as_slices();
+            assert!(s2.is_empty());
+            make_derivative(s1)
+        })
+        .skip(1)
+        .collect();
+
+        Self {
+            values,
+            derivatives,
+        }
+    }
+
+    pub fn expand(&mut self, direction: Direction) {
+        Self::expand_values(&mut self.values, &mut self.derivatives, direction);
+    }
+
+    fn expand_values(
+        values: &mut VecDeque<Value>,
+        derivatives: &mut [VecDeque<Value>],
+        direction: Direction,
+    ) {
+        if !derivatives.is_empty() {
+            let (next_values, next_derivatives) = derivatives.split_first_mut().unwrap();
+            Self::expand_values(next_values, next_derivatives, direction);
+        }
+
+        match direction {
+            Direction::Forward => {
+                let derivative = derivatives.first().map_or(0, |d| *d.back().unwrap());
+                values.push_back(*values.back().unwrap() + derivative);
+            }
+            Direction::Backward => {
+                let derivative = derivatives.first().map_or(0, |d| *d.front().unwrap());
+                values.push_front(*values.front().unwrap() - derivative);
+            }
+        }
+    }
+}
+
+fn main() {
+    let mut histories: VecDeque<_> = stdin()
+        .lines()
+        .map(Result::unwrap)
+        .map(|l| History::new(l.split(' ').map(|n| n.parse().unwrap()).collect()))
+        .collect();
+
+    for history in &mut histories {
+        history.expand(Direction::Forward);
+    }
+
+    println!(
+        "{}",
+        histories
+            .iter()
+            .map(|h| h.values.back().unwrap())
+            .sum::<Value>()
+    );
+
+    for history in &mut histories {
+        history.expand(Direction::Backward);
+    }
+
+    println!(
+        "{}",
+        histories
+            .iter()
+            .map(|h| h.values.front().unwrap())
+            .sum::<Value>()
+    );
+}
diff --git a/Cargo.lock b/Cargo.lock
index b08ecf3..b014d6c 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -679,6 +679,14 @@ dependencies = [
  "petgraph",
 ]
 
+[[package]]
+name = "rust_2023_09"
+version = "0.1.0"
+dependencies = [
+ "aoc",
+ "itertools 0.12.0",
+]
+
 [[package]]
 name = "rustversion"
 version = "1.0.14"
diff --git a/Cargo.toml b/Cargo.toml
index 96c1b7c..961b037 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -34,4 +34,5 @@ members = [
 	"2023/day6/rust",
 	"2023/day7/rust",
 	"2023/day8/rust",
+	"2023/day9/rust",
 ]