From 92c985cbd4ea388c6d1ba9dbe05e0248f2f40dac Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Sat, 9 Dec 2023 19:10:01 +0100
Subject: [PATCH] Unify part 1 and 2 better

---
 2023/src/day09.rs | 21 +++++++++------------
 1 file changed, 9 insertions(+), 12 deletions(-)

diff --git a/2023/src/day09.rs b/2023/src/day09.rs
index 4742663..d7e00e0 100644
--- a/2023/src/day09.rs
+++ b/2023/src/day09.rs
@@ -15,12 +15,11 @@ fn parse_reports(i: &[u8]) -> IResult<&[u8], Vec<Vec<i32>>> {
     ))(i)
 }
 
-fn compute_next(report: &[i32]) -> i32 {
+fn compute_next<'a>(report: impl IntoIterator<Item = &'a i32>) -> i32 {
     let mut deltas = Vec::new();
 
-    for w in report.windows(2) {
-        let mut delta = w[1] - w[0];
-
+    for &entry in report {
+        let mut delta = entry;
         for prev_delta in &mut deltas {
             let prev = mem::replace(prev_delta, delta);
             delta = delta - prev;
@@ -31,14 +30,15 @@ fn compute_next(report: &[i32]) -> i32 {
         }
     }
 
-    let next_step = deltas.drain(..).rev().fold(0, |c, d| c + d);
-
-    report.last().unwrap() + next_step
+    deltas.drain(..).rev().fold(0, |c, d| c + d)
 }
 
 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();
+    let result: i32 = reports
+        .iter()
+        .map(|report| compute_next(report.iter()))
+        .sum();
     Ok(result.to_string())
 }
 
@@ -46,10 +46,7 @@ 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)
-        })
+        .map(|report| compute_next(report.iter().rev()))
         .sum();
     Ok(result.to_string())
 }