From 7f166a5075c8cc08af9e405002a31f9757f782f5 Mon Sep 17 00:00:00 2001
From: Wesley Moore <wes@wezm.net>
Date: Sat, 9 Dec 2023 16:05:00 +1000
Subject: [PATCH] 2023: Day 9, part 2

---
 2023/day9/src/main.rs | 46 ++++++++++++++++++++++++++++++++-----------
 1 file changed, 34 insertions(+), 12 deletions(-)

diff --git a/2023/day9/src/main.rs b/2023/day9/src/main.rs
index 04ff99c..9737598 100644
--- a/2023/day9/src/main.rs
+++ b/2023/day9/src/main.rs
@@ -11,15 +11,19 @@ fn main() -> Result<(), BoxError> {
         .next()
         .ok_or("missing input file path")?;
     let file = BufReader::new(File::open(input_path)?);
-    let mut sum = 0;
+    let mut part1 = 0;
+    let mut part2 = 0;
     for line in file.lines() {
         let line = line?;
         let history = parse_line(&line)?;
         let value = calculate_next_value(&history);
-        sum += value;
+        let prev_value = calculate_prev_value(&history);
+        part1 += value;
+        part2 += prev_value;
     }
 
-    println!("Part 1: {sum}");
+    println!("Part 1: {part1}");
+    println!("Part 2: {part2}");
 
     Ok(())
 }
@@ -31,15 +35,7 @@ fn parse_line(line: &str) -> Result<Vec<i32>, ParseIntError> {
 }
 
 fn calculate_next_value(history: &[i32]) -> i32 {
-    let x: Vec<_> = history
-        .windows(2)
-        .map(|window| {
-            let &[a, b] = window else {
-                unreachable!("window is not a pair")
-            };
-            b - a
-        })
-        .collect();
+    let x = window_differences(history);
 
     // See if they're all the same, if so then the next value is the same
     let first = x[0];
@@ -50,3 +46,29 @@ fn calculate_next_value(history: &[i32]) -> i32 {
         calculate_next_value(&x)
     }
 }
+
+fn calculate_prev_value(history: &[i32]) -> i32 {
+    let x = window_differences(history);
+
+    // See if they're all the same, if so then the next value is the same
+    let same = x[0];
+    let first = history.first().unwrap();
+    first
+        - if x.iter().all(|val| *val == same) {
+            same
+        } else {
+            calculate_prev_value(&x)
+        }
+}
+
+fn window_differences(values: &[i32]) -> Vec<i32> {
+    values
+        .windows(2)
+        .map(|window| {
+            let &[a, b] = window else {
+                unreachable!("window is not a pair")
+            };
+            b - a
+        })
+        .collect()
+}