From de440ef10e0f7c53d6176b5dfdba07b9ccf95e21 Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Sat, 9 Dec 2023 10:52:23 +0100
Subject: [PATCH] Implement 2023 day 9

---
 2023/src/day09.rs       | 80 ++++++++++++++++++++++++++++++++++++++---
 2023/src/samples/09.txt |  3 ++
 2 files changed, 79 insertions(+), 4 deletions(-)
 create mode 100644 2023/src/samples/09.txt

diff --git a/2023/src/day09.rs b/2023/src/day09.rs
index 7c1760f..4742663 100644
--- a/2023/src/day09.rs
+++ b/2023/src/day09.rs
@@ -1,7 +1,79 @@
-pub fn part1(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+use std::mem;
+
+use nom::bytes::complete::tag;
+use nom::multi::many1;
+use nom::multi::separated_list1;
+use nom::sequence::terminated;
+use nom::IResult;
+
+use crate::common::parse_input;
+
+fn parse_reports(i: &[u8]) -> IResult<&[u8], Vec<Vec<i32>>> {
+    many1(terminated(
+        separated_list1(tag(" "), nom::character::complete::i32),
+        tag("\n"),
+    ))(i)
 }
 
-pub fn part2(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+fn compute_next(report: &[i32]) -> i32 {
+    let mut deltas = Vec::new();
+
+    for w in report.windows(2) {
+        let mut delta = w[1] - w[0];
+
+        for prev_delta in &mut deltas {
+            let prev = mem::replace(prev_delta, delta);
+            delta = delta - prev;
+        }
+
+        if delta != 0 {
+            deltas.push(delta);
+        }
+    }
+
+    let next_step = deltas.drain(..).rev().fold(0, |c, d| c + d);
+
+    report.last().unwrap() + next_step
+}
+
+pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+    let reports = parse_input(input, parse_reports)?;
+    let result: i32 = reports.iter().map(|report| compute_next(&report)).sum();
+    Ok(result.to_string())
+}
+
+pub fn part2(input: &[u8]) -> anyhow::Result<String> {
+    let mut reports = parse_input(input, parse_reports)?;
+    let result: i32 = reports
+        .iter_mut()
+        .map(|report| {
+            report.reverse();
+            compute_next(&report)
+        })
+        .sum();
+    Ok(result.to_string())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/09.txt");
+
+    #[test]
+    fn samples_separate() {
+        assert_eq!(18, compute_next(&[0, 3, 6, 9, 12, 15]));
+        assert_eq!(28, compute_next(&[1, 3, 6, 10, 15, 21]));
+        assert_eq!(68, compute_next(&[10, 13, 16, 21, 30, 45]));
+    }
+
+    #[test]
+    fn sample_part1() {
+        assert_eq!("114", part1(SAMPLE).unwrap());
+    }
+
+    #[test]
+    fn sample_part2() {
+        assert_eq!("2", part2(SAMPLE).unwrap());
+    }
 }
diff --git a/2023/src/samples/09.txt b/2023/src/samples/09.txt
new file mode 100644
index 0000000..539a763
--- /dev/null
+++ b/2023/src/samples/09.txt
@@ -0,0 +1,3 @@
+0 3 6 9 12 15
+1 3 6 10 15 21
+10 13 16 21 30 45