Day 6 part 2.

2020/src/day6.rs

@@ -38,6 +38,38 @@
 //!
 //! For each group, count the number of questions to which anyone answered "yes". What is the sum of those counts?
 //!
+//! --- Part Two ---
+//! As you finish the last group's customs declaration, you notice that you misread one word in the instructions:
+//!
+//! You don't need to identify the questions to which anyone answered "yes"; you need to identify the questions to which everyone answered "yes"!
+//!
+//! Using the same example as above:
+//!
+//! abc
+//!
+//! a
+//! b
+//! c
+//!
+//! ab
+//! ac
+//!
+//! a
+//! a
+//! a
+//! a
+//!
+//! b
+//! This list represents answers from five groups:
+//!
+//! In the first group, everyone (all 1 person) answered "yes" to 3 questions: a, b, and c.
+//! In the second group, there is no question to which everyone answered "yes".
+//! In the third group, everyone answered yes to only 1 question, a. Since some people did not answer "yes" to b or c, they don't count.
+//! In the fourth group, everyone answered yes to only 1 question, a.
+//! In the fifth group, everyone (all 1 person) answered "yes" to 1 question, b.
+//! In this example, the sum of these counts is 3 + 0 + 1 + 1 + 1 = 6.
+//!
+//! For each group, count the number of questions to which everyone answered "yes". What is the sum of those counts?
 
 use std::collections::HashSet;
 
@@ -52,6 +84,24 @@ fn solution1(input: &str) -> usize {
         .sum()
 }
 
+#[aoc(day6, part2)]
+fn solution2(input: &str) -> usize {
+    input
+        .split("\n\n")
+        .map(|group| {
+            let sets = group
+                .split('\n')
+                .map(|p| p.chars().collect::<HashSet<_>>())
+                .collect::<Vec<_>>();
+            // Find letters common to all sets int this group.
+            sets.iter().fold(sets.first().unwrap().clone(), |acc, s| {
+                acc.intersection(s).cloned().collect()
+            })
+        })
+        .map(|set| set.len())
+        .sum()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -76,4 +126,9 @@ b"#;
     fn part1() {
         assert_eq!(solution1(INPUT), 11);
     }
+
+    #[test]
+    fn part2() {
+        assert_eq!(solution2(INPUT), 6);
+    }
 }