Day 09b

README.md

@@ -1,4 +1,4 @@
-![AoC](https://img.shields.io/badge/AoC%20%E2%AD%90-12-yellow)
+![AoC](https://img.shields.io/badge/AoC%20%E2%AD%90-14-yellow)
 [![CI](https://github.com/lpenz/adventofcode2023/workflows/CI/badge.svg)](https://github.com/lpenz/adventofcode2023/actions)
 [![coveralls](https://coveralls.io/repos/github/lpenz/adventofcode2023/badge.svg?branch=main)](https://coveralls.io/github/lpenz/adventofcode2023?branch=main)
 

day09/src/bin/day09a.rs

@@ -7,10 +7,6 @@ use std::io::{stdin, BufRead};
 
 use day09::*;
 
-fn diffs(nums: &[i64]) -> Vec<i64> {
-    nums.par_windows(2).map(|l| l[1] - l[0]).collect()
-}
-
 fn calc_next(nums: &Vec<i64>) -> i64 {
     let mut lastsum = nums[nums.len() - 1];
     let mut currdiff = diffs(nums);

day09/src/bin/day09b.rs

@@ -0,0 +1,37 @@
+// Copyright (C) 2023 Leandro Lisboa Penz <lpenz@lpenz.org>
+// This file is subject to the terms and conditions defined in
+// file 'LICENSE', which is part of this source code package.
+
+use rayon::prelude::*;
+use std::io::{stdin, BufRead};
+
+use day09::*;
+
+fn calc_prev(nums: &[i64]) -> i64 {
+    let mut mult = -1;
+    let mut result = nums[0];
+    let mut currdiff = diffs(nums);
+    while !currdiff.par_iter().all(|n| n == &0) {
+        result += mult * currdiff[0];
+        mult *= -1;
+        currdiff = diffs(&currdiff);
+    }
+    result
+}
+
+fn process(bufin: impl BufRead) -> Result<i64> {
+    let input = parser::parse(bufin)?;
+    Ok(input.par_iter().map(|v| calc_prev(v)).sum())
+}
+
+#[test]
+fn test() -> Result<()> {
+    assert_eq!(process(EXAMPLE.as_bytes())?, 2);
+    Ok(())
+}
+
+fn main() -> Result<()> {
+    color_eyre::install()?;
+    println!("{}", process(stdin().lock())?);
+    Ok(())
+}

day09/src/lib.rs

@@ -3,6 +3,7 @@
 // file 'LICENSE', which is part of this source code package.
 
 pub use color_eyre::{eyre::eyre, Result};
+use rayon::prelude::*;
 
 pub const EXAMPLE: &str = "0 3 6 9 12 15
 1 3 6 10 15 21
@@ -25,6 +26,10 @@ pub mod parser {
     }
 }
 
+pub fn diffs(nums: &[i64]) -> Vec<i64> {
+    nums.par_windows(2).map(|l| l[1] - l[0]).collect()
+}
+
 #[test]
 fn test() -> Result<()> {
     assert_eq!(parser::parse(EXAMPLE.as_bytes())?.len(), 3);