From a1f44e58866d5b09570703da67c9111dec6fe1b1 Mon Sep 17 00:00:00 2001
From: Keenan Tims <ktims@demonware.net>
Date: Tue, 5 Dec 2023 19:47:45 -0800
Subject: [PATCH] day4: problem 2 solution

---
 4/src/main.rs | 89 +++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 86 insertions(+), 3 deletions(-)

diff --git a/4/src/main.rs b/4/src/main.rs
index 95a0f03..5af7175 100644
--- a/4/src/main.rs
+++ b/4/src/main.rs
@@ -77,6 +77,13 @@ impl Card {
             0
         }
     }
+    // Return the number of copies won
+    fn win_copies(&self) -> usize {
+        self.winning_numbers
+            .iter()
+            .filter(|win| self.our_numbers.contains(win))
+            .count()
+    }
 }
 
 impl From<&str> for Card {
@@ -127,11 +134,87 @@ fn problem1(input: InputIter) -> u64 {
         .map(|line| Card::from(line.unwrap().as_str()))
         .collect();
 
-
     cards.iter().map(|x| x.score()).sum()
 }
 
 // PROBLEM 2 solution
-fn problem2(input: InputIter) -> u64 {
-    0
+
+// --- Part Two ---
+
+// Just as you're about to report your findings to the Elf, one of you realizes that the
+// rules have actually been printed on the back of every card this whole time.
+
+// There's no such thing as "points". Instead, scratchcards only cause you to win more
+// scratchcards equal to the number of winning numbers you have.
+
+// Specifically, you win copies of the scratchcards below the winning card equal to the
+// number of matches. So, if card 10 were to have 5 matching numbers, you would win one
+// copy each of cards 11, 12, 13, 14, and 15.
+
+// Copies of scratchcards are scored like normal scratchcards and have the same card
+// number as the card they copied. So, if you win a copy of card 10 and it has 5
+// matching numbers, it would then win a copy of the same cards that the original card
+// 10 won: cards 11, 12, 13, 14, and 15. This process repeats until none of the copies
+// cause you to win any more cards. (Cards will never make you copy a card past the end
+// of the table.)
+
+// This time, the above example goes differently:
+
+// Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53 Card 2: 13 32 20 16 61 | 61 30 68 82
+// 17 32 24 19 Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1 Card 4: 41 92 73 84 69 |
+// 59 84 76 51 58  5 54 83 Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36 Card 6: 31
+// 18 13 56 72 | 74 77 10 23 35 67 36 11
+
+//     Card 1 has four matching numbers, so you win one copy each of the next four
+//     cards: cards 2, 3, 4, and 5. Your original card 2 has two matching numbers, so
+//     you win one copy each of cards 3 and 4. Your copy of card 2 also wins one copy
+//     each of cards 3 and 4. Your four instances of card 3 (one original and three
+//     copies) have two matching numbers, so you win four copies each of cards 4 and 5.
+//     Your eight instances of card 4 (one original and seven copies) have one matching
+//     number, so you win eight copies of card 5. Your fourteen instances of card 5 (one
+//     original and thirteen copies) have no matching numbers and win no more cards.
+//     Your one instance of card 6 (one original) has no matching numbers and wins no
+//     more cards.
+
+// Once all of the originals and copies have been processed, you end up with 1 instance
+// of card 1, 2 instances of card 2, 4 instances of card 3, 8 instances of card 4, 14
+// instances of card 5, and 1 instance of card 6. In total, this example pile of
+// scratchcards causes you to ultimately have 30 scratchcards!
+
+// Process all of the original and copied scratchcards until no more scratchcards are
+// won. Including the original set of scratchcards, how many total scratchcards do you
+// end up with?
+
+fn problem2_award_copies<'a>(orig_cards: &'a Vec<Card>, card: &Card) -> Vec<&'a Card> {
+    let win_copies = card.win_copies();
+    let mut new_cards = Vec::new();
+    if win_copies != 0 {
+        for ofs in 0..win_copies {
+            // card.id is 1-indexed, so would need to be card.id-1, but offset needs to
+            // start at 1, which offsets this
+            new_cards.push(&orig_cards[card.id + ofs]);
+        }
+    }
+
+    new_cards
+}
+
+fn problem2(input: InputIter) -> u64 {
+    let orig_cards: Vec<Card> = input
+        .map(|line| Card::from(line.unwrap().as_str()))
+        .collect();
+
+    let mut new_cards = Vec::new();
+    let mut won_cards = Vec::new();
+
+    for card in &orig_cards {
+        new_cards.append(&mut problem2_award_copies(&orig_cards, card))
+    }
+
+    while let Some(card) = new_cards.pop() {
+        won_cards.push(card);
+        new_cards.append(&mut problem2_award_copies(&orig_cards, card))
+    }
+
+    (won_cards.len() + orig_cards.len()) as u64
 }