day 9 solutions

Signed-off-by: Stephen Waits <swaits@amazon.com>

src/day09.rs

@@ -0,0 +1,51 @@
+// ==== day 9
+
+use crate::utils;
+use anyhow::{anyhow, Result};
+
+fn is_bad(data: &Vec<i64>, n: usize, i: usize) -> Result<i64> {
+    let prev_n = &data[i - n..i];
+    let found =
+        utils::combinations(2, prev_n.len()).any(|c| data[i] == prev_n[c[0]] + prev_n[c[1]]);
+    if !found {
+        return Ok(data[i]);
+    } else {
+        Err(anyhow!("no result found"))
+    }
+}
+
+fn find_first_bad(data: &Vec<i64>, n: usize, start: usize) -> Result<i64> {
+    for i in start..data.len() {
+        let ret = is_bad(data, n, i);
+        if ret.is_ok() {
+            return ret;
+        }
+    }
+    Err(anyhow!("no result found"))
+}
+
+fn find_contiguous_set(data: &Vec<i64>, x: i64) -> Result<i64> {
+    for set_size in 2..data.len() - 1 {
+        for i in 0..data.len() - set_size {
+            if data[i..i + set_size].iter().sum::<i64>() == x {
+                // TODO: this can be optimized by using a rolling window, but for now this is fast enough
+                let min = data[i..i + set_size].iter().min().unwrap();
+                let max = data[i..i + set_size].iter().max().unwrap();
+                return Ok(min + max);
+            }
+        }
+    }
+    Err(anyhow!("no result found"))
+}
+
+pub fn run() -> Result<(i64, i64)> {
+    let data = utils::read_i64s("data/input-09.txt")?;
+    let answers = utils::read_i64s("data/output-09.txt")?;
+
+    let p1 = find_first_bad(&data, 25, 25)?;
+    let p2 = find_contiguous_set(&data, 756008079)?;
+
+    assert_eq!(p1, answers[0]);
+    assert_eq!(p2, answers[1]);
+    Ok((p1, p2))
+}

src/main.rs

@@ -8,6 +8,7 @@ mod day05;
 mod day06;
 mod day07;
 mod day08;
+mod day09;
 mod utils;
 
 // TODO
@@ -29,5 +30,6 @@ fn main() -> Result<()> {
     println!("Day 06: {:?}", day06::run()?);
     println!("Day 07: {:?}", day07::run()?);
     println!("Day 08: {:?}", day08::run()?);
+    println!("Day 09: {:?}", day09::run()?);
     Ok(())
 }