From b146637ad2f8bdacf35743ef851356efdb58958e Mon Sep 17 00:00:00 2001
From: Lars Martens <lars@martens.red>
Date: Sat, 9 Dec 2023 12:58:33 +0100
Subject: [PATCH] Improve day 9

---
 src/bin/09.rs | 43 +++++++++++++++++++------------------------
 src/lib.rs    |  2 +-
 2 files changed, 20 insertions(+), 25 deletions(-)

diff --git a/src/bin/09.rs b/src/bin/09.rs
index dfab902..c29fa26 100644
--- a/src/bin/09.rs
+++ b/src/bin/09.rs
@@ -4,51 +4,46 @@ const INPUT: &str = include_str!("../../input/09");
 const TEST_INPUT: &str = include_str!("../../input/09-test");
 
 fn main() {
-    let (test1, test2) = solve(TEST_INPUT);
-    let (part1, part2) = solve(INPUT);
-
-    assert_eq!(test1, 114, "Part 1");
-    println!("Part 1: {}", part1);
-    assert_eq!(test2, 2, "Part 2");
-    println!("Part 2: {}", part2);
+    assert_eq!(part1(TEST_INPUT), 114, "Part 1");
+    println!("Part 1: {}", part1(INPUT));
+    assert_eq!(part2(TEST_INPUT), 2, "Part 2");
+    println!("Part 2: {}", part2(INPUT));
 }
 
-fn solve(input: &str) -> (i64, i64) {
-    let (forward, backward): (Vec<i64>, Vec<i64>) = input
+fn part1(input: &str) -> i64 {
+    input.lines().map(parse_ws_separated).map(extrapolate).sum()
+}
+
+fn part2(input: &str) -> i64 {
+    input
         .lines()
         .map(parse_ws_separated)
+        .map(|numbers| numbers.rev())
         .map(extrapolate)
-        .unzip();
-    (forward.into_iter().sum(), backward.into_iter().sum())
+        .sum()
 }
 
-/// Extrapolate (forward, backward).
-fn extrapolate(mut input: impl Iterator<Item = i64>) -> (i64, i64) {
+fn extrapolate(mut input: impl Iterator<Item = i64>) -> i64 {
     let start = input.next().unwrap();
 
     // We do not need to store all values,
-    // only the first and last difference values are important.
-    let mut forward = vec![start];
-    let mut backward = vec![start];
+    // only the difference values are important.
+    let mut differences = vec![start];
 
     // Build the list of differences
     for next in input {
         let mut propagate_diff = next;
-        for diff in &mut forward {
+        for diff in &mut differences {
             let previous = *diff;
             *diff = propagate_diff;
             propagate_diff -= previous;
         }
-        let double_0 = *forward.last().unwrap() == 0 && propagate_diff == 0;
+        let double_0 = *differences.last().unwrap() == 0 && propagate_diff == 0;
         if !double_0 {
-            forward.push(propagate_diff);
-            backward.push(propagate_diff);
+            differences.push(propagate_diff);
         }
     }
 
     // Extrapolate
-    let forward = forward.into_iter().sum();
-    let backward = backward.into_iter().rev().fold(0, |a, b| b - a);
-
-    (forward, backward)
+    differences.into_iter().sum()
 }
diff --git a/src/lib.rs b/src/lib.rs
index 6673bea..d312850 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -7,7 +7,7 @@ use std::{
 /// Parse a whitespace separated list of things.
 ///
 /// Panics on parse error.
-pub fn parse_ws_separated<T>(s: &str) -> impl Iterator<Item = T> + '_
+pub fn parse_ws_separated<T>(s: &str) -> impl DoubleEndedIterator<Item = T> + '_
 where
     T: FromStr,
     <T as FromStr>::Err: Debug,