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authorBryan Brattlof <hello@bryanbrattlof.com>2020-12-13 17:38:50 -0500
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day 6 of Advent of Code 2020
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+Day 6: Custom Customs
+#####################
+
+As your flight approaches the regional airport where you'll switch to a much
+larger plane, customs declaration forms are distributed to the passengers.
+
+The form asks a series of 26 yes-or-no questions marked a through z. All you
+need to do is identify the questions for which anyone in your group answers
+"yes". Since your group is just you, this doesn't take very long.
+
+However, the person sitting next to you seems to be experiencing a language
+barrier and asks if you can help. For each of the people in their group, you
+write down the questions for which they answer "yes", one per line. For example:
+
+.. code-block::
+
+ abcx
+ abcy
+ abcz
+
+In this group, there are 6 questions to which anyone answered "yes": a, b, c, x,
+y, and z. (Duplicate answers to the same question don't count extra; each
+question counts at most once.)
+
+Another group asks for your help, then another, and eventually you've collected
+answers from every group on the plane (your puzzle input). Each group's answers
+are separated by a blank line, and within each group, each person's answers are
+on a single line. For example:
+
+.. code-block::
+
+ abc
+
+ a
+ b
+ c
+
+ ab
+ ac
+
+ a
+ a
+ a
+ a
+
+ b
+
+This list represents answers from five groups:
+
+- The first group contains one person who answered "yes" to 3 questions: a, b,
+ and c.
+- The second group contains three people; combined, they answered "yes" to 3
+ questions: a, b, and c.
+- The third group contains two people; combined, they answered "yes" to 3
+ questions: a, b, and c.
+- The fourth group contains four people; combined, they answered "yes" to only
+ 1 question, a.
+- The last group contains one person who answered "yes" to only 1 question, b.
+
+In this example, the sum of these counts is 3 + 3 + 3 + 1 + 1 = 11.
+
+For each group, count the number of questions to which anyone answered "yes".
+What is the sum of those counts?
+
+Your puzzle answer was 6430.
+
+The first half of this puzzle is complete! It provides one gold star: *
+
+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:
+
+.. code-block::
+
+ 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?
+
+Your puzzle answer was 3125.
+
+Both parts of this puzzle are complete! They provide two gold stars: **