Day 7 part 1 solution.

2020/src/day7.rs

@@ -28,6 +28,33 @@
 //!
 //! How many bag colors can eventually contain at least one shiny gold bag? (The list of rules is quite long; make sure you get all of it.)
 //!
+//!
+//! --- Part Two ---
+//! It's getting pretty expensive to fly these days - not because of ticket prices, but because of the ridiculous number of bags you need to buy!
+//!
+//! Consider again your shiny gold bag and the rules from the above example:
+//!
+//! faded blue bags contain 0 other bags.
+//! dotted black bags contain 0 other bags.
+//! vibrant plum bags contain 11 other bags: 5 faded blue bags and 6 dotted black bags.
+//! dark olive bags contain 7 other bags: 3 faded blue bags and 4 dotted black bags.
+//! So, a single shiny gold bag must contain 1 dark olive bag (and the 7 bags within it) plus 2 vibrant plum bags (and the 11 bags within each of those): 1 + 1*7 + 2 + 2*11 = 32 bags!
+//!
+//! Of course, the actual rules have a small chance of going several levels deeper than this example; be sure to count all of the bags, even if the nesting becomes topologically impractical!
+//!
+//! Here's another example:
+//!
+//! shiny gold bags contain 2 dark red bags.
+//! dark red bags contain 2 dark orange bags.
+//! dark orange bags contain 2 dark yellow bags.
+//! dark yellow bags contain 2 dark green bags.
+//! dark green bags contain 2 dark blue bags.
+//! dark blue bags contain 2 dark violet bags.
+//! dark violet bags contain no other bags.
+//! In this example, a single shiny gold bag must contain 126 other bags.
+//!
+//! How many individual bags are required inside your single shiny gold bag?
+
 use std::collections::HashMap;
 use std::collections::HashSet;
 
@@ -39,6 +66,7 @@ type Color = String;
 struct Node {
     color: Color,
     parents: Vec<Color>,
+    children: Vec<(usize, Color)>,
 }
 
 #[derive(Debug, Default)]
@@ -53,12 +81,7 @@ impl Graph {
             0 | 1 => panic!(format!("line '{}' fails assumptions", line)),
             _ => {
                 let parent_color = parts[0].to_string();
-                // Get or create this parent color
-                let _ = self.nodes.entry(parent_color.clone()).or_insert(Node {
-                    color: parent_color.clone(),
-                    parents: Vec::new(),
-                });
-
+                let mut children = Vec::new();
                 if parts[1] != "no other bags." {
                     for chunk in parts[1].split(' ').collect::<Vec<_>>().chunks(4) {
                         // [0] quantity
@@ -67,12 +90,22 @@ impl Graph {
                         // [3] bag/bags[,.]
                         let color = format!("{} {}", chunk[1], chunk[2]);
                         let c = self.nodes.entry(color.clone()).or_insert(Node {
-                            color,
+                            color: color.clone(),
                             parents: Vec::new(),
+                            children: Vec::new(),
                         });
                         c.parents.push(parent_color.clone());
+                        let count = chunk[0].parse::<usize>().expect("couldn't parse bag count");
+                        children.push((count, color.clone()));
                     }
                 }
+                // Get or create this parent color
+                let mut p = self.nodes.entry(parent_color.clone()).or_insert(Node {
+                    color: parent_color.clone(),
+                    parents: Vec::new(),
+                    children: Vec::new(),
+                });
+                p.children = children;
             }
         }
     }
@@ -94,6 +127,22 @@ impl Graph {
         });
         set
     }
+
+    fn bag_count(&self, color: &Color) -> usize {
+        let n = self.nodes.get(color).expect("Couldn't find node");
+        if n.children.is_empty() {
+            // No children.
+            return 0;
+        } else {
+            // Number of children bags and multiple the number of child bags by the transitive
+            // closure of the child's sub bags.
+            n.children
+                .iter()
+                // Return the number of sub
+                .map(|(cnt, color)| cnt + cnt * self.bag_count(color))
+                .sum()
+        }
+    }
 }
 
 #[aoc_generator(day7)]
@@ -105,14 +154,26 @@ fn parse(input: &str) -> Graph {
 
 #[aoc(day7, part1)]
 fn solution1(g: &Graph) -> usize {
-    g.top_level(&"shiny gold".to_string()).len()
+    let answer = g.top_level(&"shiny gold".to_string()).len();
+
+    /*
+    // Ensure we don't break part 1 while working on part 2.
+    let correct_answer = 222;
+    assert_eq!(answer, correct_answer);
+    */
+    answer
+}
+
+#[aoc(day7, part2)]
+fn solution2(g: &Graph) -> usize {
+    g.bag_count(&"shiny gold".to_string())
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
-    const INPUT: &'static str = r#"light red bags contain 1 bright white bag, 2 muted yellow bags.
+    const INPUT1: &'static str = r#"light red bags contain 1 bright white bag, 2 muted yellow bags.
 dark orange bags contain 3 bright white bags, 4 muted yellow bags.
 bright white bags contain 1 shiny gold bag.
 muted yellow bags contain 2 shiny gold bags, 9 faded blue bags.
@@ -124,6 +185,20 @@ dotted black bags contain no other bags."#;
 
     #[test]
     fn part1() {
-        assert_eq!(solution1(&parse(INPUT)), 4);
+        assert_eq!(solution1(&parse(INPUT1)), 4);
+    }
+
+    const INPUT2: &'static str = r#"shiny gold bags contain 2 dark red bags.
+dark red bags contain 2 dark orange bags.
+dark orange bags contain 2 dark yellow bags.
+dark yellow bags contain 2 dark green bags.
+dark green bags contain 2 dark blue bags.
+dark blue bags contain 2 dark violet bags.
+dark violet bags contain no other bags."#;
+
+    #[test]
+    fn part2() {
+        assert_eq!(solution2(&parse(INPUT1)), 32);
+        assert_eq!(solution2(&parse(INPUT2)), 126);
     }
 }