Complete day 9

day-9/src/main.rs

@@ -0,0 +1,134 @@
+use std::fs;
+use std::num::ParseIntError;
+
+use itertools::{Itertools, MinMaxResult};
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let input = fs::read_to_string("input")?;
+    let xmas = parse_xmas(&input)?;
+
+    // Part 1
+    let result = find_not_summed(&xmas, 25)?;
+    println!("{}", result);
+
+    // Part 2
+    let result = find_weakness(&xmas, result)?;
+    println!("{}", result);
+
+    Ok(())
+}
+
+fn parse_xmas(input: &str) -> Result<Vec<u64>, ParseIntError> {
+    input.lines().map(|line| line.parse()).collect()
+}
+
+fn find_weakness(xmas: &Vec<u64>, key: u64) -> Result<u64, &str> {
+    // Now we use the key to find the weakness, by getting all
+    // possible sub-sumbs of the list of numbers, and finding the one
+    // that happens to match our key
+    let weak_window = (2..xmas.len())
+        .find_map(|window_size| {
+            xmas.windows(window_size)
+                .find(|window| window.iter().sum::<u64>() == key)
+        })
+        .ok_or("Could not find a weakness")?;
+
+    if let MinMaxResult::MinMax(min, max) = weak_window.iter().minmax() {
+        Ok(min + max)
+    } else {
+        unreachable!("We must have at least two elements due to the window size")
+    }
+}
+
+fn find_not_summed(xmas: &Vec<u64>, window_size: usize) -> Result<u64, &str> {
+    xmas.windows(window_size + 1)
+        .find_map(|window| {
+            if let [preamble @ .., number] = window {
+                // If none of the number combinations in the preamble
+                // sum to the final number, we've got the key!
+                if !preamble
+                    .iter()
+                    .combinations(2)
+                    .any(|combination| combination.into_iter().sum::<u64>() == *number)
+                {
+                    Some(*number)
+                } else {
+                    None
+                }
+            } else {
+                unreachable!("The windows must all have a length of at least 1");
+            }
+        })
+        .ok_or("Could not find the entry")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use indoc::indoc;
+
+    #[test]
+    fn test_simple() -> Result<(), Box<dyn std::error::Error>> {
+        let input = indoc!(
+            "35
+             20
+             15
+             25
+             47
+             40
+             62
+             55
+             65
+             95
+             102
+             117
+             150
+             182
+             127
+             219
+             299
+             277
+             309
+             576"
+        );
+
+        let xmas = parse_xmas(input)?;
+        let result = find_not_summed(&xmas, 5)?;
+
+        assert_eq!(result, 127);
+        Ok(())
+    }
+
+    #[test]
+    fn test_simple2() -> Result<(), Box<dyn std::error::Error>> {
+        let input = indoc!(
+            "35
+             20
+             15
+             25
+             47
+             40
+             62
+             55
+             65
+             95
+             102
+             117
+             150
+             182
+             127
+             219
+             299
+             277
+             309
+             576"
+        );
+
+        let xmas = parse_xmas(input)?;
+        let key = find_not_summed(&xmas, 5)?;
+        let result = find_weakness(&xmas, key)?;
+
+        assert_eq!(result, 62);
+        Ok(())
+    }
+}