day3 part 2

day1/README.md

@@ -52,7 +52,7 @@ fourth Elf).
 
 Find the Elf carrying the most Calories. How many total Calories is that Elf carrying?
 
-## --- Part Two ---
+## Part Two
 
 By the time you calculate the answer to the Elves' question, they've already realized that the Elf carrying the most
 Calories of food might eventually run out of snacks.

day3/README.md

@@ -7,7 +7,8 @@ Each rucksack has two large **compartments**. All items of a given type are mean
 compartments. The Elf that did the packing failed to follow this rule for exactly one item type per rucksack.
 
 The Elves have made a list of all the items currently in each rucksack (your puzzle input), but they need your help
-finding the errors. Every item type is identified by a single lowercase or uppercase letter (that is, `a` and `A` refer to
+finding the errors. Every item type is identified by a single lowercase or uppercase letter (that is, `a` and `A` refer
+to
 different types of items).
 
 The list of items for each rucksack is given as characters all on a single line. A given rucksack always has the same
@@ -46,3 +47,48 @@ L), 42 (P), 22 (v), 20 (t), and 19 (s); the sum of these is `157`.
 
 Find the item type that appears in both compartments of each rucksack. **What is the sum of the priorities of those item
 types?**
+
+## Part Two
+
+As you finish identifying the misplaced items, the Elves come to you with another issue.
+
+For safety, the Elves are divided into groups of three. Every Elf carries a badge that identifies their group. For
+efficiency, within each group of three Elves, the badge is the **only item type carried by all three Elves**. That is,
+if a
+group's badge is item type B, then all three Elves will have item type B somewhere in their rucksack, and at most two of
+the Elves will be carrying any other item type.
+
+The problem is that someone forgot to put this year's updated authenticity sticker on the badges. All the badges need
+to be pulled out of the rucksacks so the new authenticity stickers can be attached.
+
+Additionally, nobody wrote down which item type corresponds to each group's badges. The only way to tell which item type
+is the right one is by finding the one item type that is **common between all three Elves** in each group.
+
+Every set of three lines in your list corresponds to a single group, but each group can have a different badge item
+type. So, in the above example, the first group's rucksacks are the first three lines:
+
+```
+vJrwpWtwJgWrhcsFMMfFFhFp
+jqHRNqRjqzjGDLGLrsFMfFZSrLrFZsSL
+PmmdzqPrVvPwwTWBwg
+```
+
+And the second group's rucksacks are the next three lines:
+
+```
+wMqvLMZHhHMvwLHjbvcjnnSBnvTQFn
+ttgJtRGJQctTZtZT
+CrZsJsPPZsGzwwsLwLmpwMDw
+```
+
+In the first group, the only item type that appears in all three rucksacks is lowercase `r`; this must be their badges.
+In
+the second group, their badge item type must be `Z`.
+
+Priorities for these items must still be found to organize the sticker attachment efforts: here, they are 18 (`r`) for
+the
+first group and 52 (`Z`) for the second group. The sum of these is **70**.
+
+Find the item type that corresponds to the badges of each three-Elf group.\
+**What is the sum of the priorities of those
+item types?**

day3/main.py

@@ -29,7 +29,26 @@ def read_data() -> list[str]:
         return [line.strip() for line in f.readlines()]
 
 
+def priority_shared_items_sum_2(data: list[str]) -> int:
+    badges = []
+    i = 0
+    while i < len(data):
+        group = [set(line) for line in data[i: i + 3]]
+        badge = find_intersection(group)
+        badges.append(ascii_letters.index(badge) + 1)
+        i += 3
+    return sum(badges)
+
+
+def find_intersection(group: list[set[str]]) -> str:
+    return group[0].intersection(group[1]).intersection(group[2]).pop()
+
+
 if __name__ == "__main__":
     dataset = read_data()
+
     res = priority_shared_items_sum_1(dataset)
     assert res == 157, f"{res} is not the right value"
+
+    res = priority_shared_items_sum_2(dataset)
+    assert res == 70, f"{res} is not the right value"