4 files changed, 111 insertions(+), 1 deletions(-)

M Cargo.lock
M Cargo.toml
A day_09/Cargo.toml
A day_09/src/main.rs

@@ 59,3 59,10 @@ version = "1.0.0"
 dependencies = [
  "common",
 ]
+
+[[package]]
+name = "day_09"
+version = "1.0.0"
+dependencies = [
+ "common",
+]

@@ 8,7 8,8 @@ members = [
     "day_05",
     "day_06",
     "day_07",
-    "day_08"
+    "day_08",
+    "day_09"
 ]
 
 [profile.release]

@@ 0,0 1,12 @@
+[package]
+name = "day_09"
+version = "1.0.0"
+description = "Day 9 - Advent of Code 2020"
+authors = ["Jan Baudisch <jan@baudisch.xyz>"]
+license = "GPL-3.0-or-later"
+readme = "../README.md"
+edition = "2018"
+workspace = ".."
+
+[dependencies]
+common = { path = "../common" }

@@ 0,0 1,90 @@
+use common::input;
+
+fn main() {
+    let data = input::read_lines()
+        .iter()
+        .map(|x| u64::from_str_radix(x, 10).unwrap())
+        .collect::<Vec<u64>>();
+
+    let invalid = find_invalid(&data, 25);
+
+    println!("[PART ONE] first invalid number: {}", invalid);
+
+    println!(
+        "[PART TWO] sum for invalid number: {}",
+        find_sum(&data, invalid)
+    );
+}
+
+fn find_invalid(data: &[u64], preamble: usize) -> u64 {
+    for (i, number) in data.iter().skip(preamble).enumerate() {
+        let mut flag = false;
+        let end = i + preamble;
+
+        'outer: for (a_i, a) in data[i..end].iter().enumerate() {
+            for (b_i, b) in data[i..end].iter().enumerate() {
+                if a_i != b_i && a + b == *number {
+                    flag = true;
+                    break 'outer;
+                }
+            }
+        }
+
+        if !flag {
+            return *number;
+        }
+    }
+
+    panic!("Unable to find invalid number!");
+}
+
+fn find_sum(data: &[u64], number: u64) -> u64 {
+    for (index, _) in data.iter().enumerate() {
+        let mut other = data.iter().skip(index);
+        let mut sum = Vec::new();
+
+        loop {
+            if let Some(next) = other.next() {
+                sum.push(*next);
+            } else {
+                break;
+            }
+
+            let current_sum = sum.iter().sum::<u64>();
+
+            if current_sum > number {
+                break;
+            }
+
+            if current_sum == number {
+                sum.sort_unstable();
+                return sum[0] + sum.pop().unwrap();
+            }
+        }
+    }
+
+    panic!("Unable to find sum!");
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{find_invalid, find_sum};
+
+    fn generate_data() -> Vec<u64> {
+        vec![
+            35, 20, 15, 25, 47, 40, 62, 55, 65, 95, 102, 117, 150, 182, 127, 219, 299, 277, 309,
+            576,
+        ]
+    }
+
+    #[test]
+    fn part_one() {
+        assert_eq!(find_invalid(&generate_data(), 5), 127);
+    }
+
+    #[test]
+    fn part_two() {
+        let data = generate_data();
+        assert_eq!(find_sum(&data, find_invalid(&data, 5)), 62);
+    }
+}