Merge pull request #59 from romellem/2017/day-9

2017 - Day 9

Author: romellem

2017/9/README.md

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+# Answers
+
+| Part 1  | Part 2 |
+|---------|--------|
+| `10050` | `4482` |
+
+## --- Day 9: Stream Processing ---
+
+A large stream blocks your path. According to the locals, it's not safe to cross the stream at the moment because it's full of _garbage_. You look down at the stream; rather than water, you discover that it's a _stream of characters_.
+
+You sit for a while and record part of the stream (your puzzle input). The characters represent _groups_ - sequences that begin with `{` and end with `}`. Within a group, there are zero or more other things, separated by commas: either another _group_ or _garbage_. Since groups can contain other groups, a `}` only closes the _most-recently-opened unclosed group_ - that is, they are nestable. Your puzzle input represents a single, large group which itself contains many smaller ones.
+
+Sometimes, instead of a group, you will find _garbage_. Garbage begins with `<` and ends with `>`. Between those angle brackets, almost any character can appear, including `{` and `}`. _Within_ garbage, `<` has no special meaning.
+
+In a futile attempt to clean up the garbage, some program has _canceled_ some of the characters within it using `!`: inside garbage, _any_ character that comes after `!` should be _ignored_, including `<`, `>`, and even another `!`.
+
+You don't see any characters that deviate from these rules. Outside garbage, you only find well-formed groups, and garbage always terminates according to the rules above.
+
+Here are some self-contained pieces of garbage:
+
+*   `<>`, empty garbage.
+*   `<random characters>`, garbage containing random characters.
+*   `<<<<>`, because the extra `<` are ignored.
+*   `<{!>}>`, because the first `>` is canceled.
+*   `<!!>`, because the second `!` is canceled, allowing the `>` to terminate the garbage.
+*   `<!!!>>`, because the second `!` and the first `>` are canceled.
+*   `<{o"i!a,<{i<a>`, which ends at the first `>`.
+
+Here are some examples of whole streams and the number of groups they contain:
+
+*   `{}`, `1` group.
+*   `{{{}}}`, `3` groups.
+*   `{{},{}}`, also `3` groups.
+*   `{{{},{},{{}}}}`, `6` groups.
+*   `{<{},{},{{}}>}`, `1` group (which itself contains garbage).
+*   `{<a>,<a>,<a>,<a>}`, `1` group.
+*   `{{<a>},{<a>},{<a>},{<a>}}`, `5` groups.
+*   `{{<!>},{<!>},{<!>},{<a>}}`, `2` groups (since all but the last `>` are canceled).
+
+Your goal is to find the total score for all groups in your input. Each group is assigned a _score_ which is one more than the score of the group that immediately contains it. (The outermost group gets a score of `1`.)
+
+*   `{}`, score of `1`.
+*   `{{{}}}`, score of `1 + 2 + 3 = 6`.
+*   `{{},{}}`, score of `1 + 2 + 2 = 5`.
+*   `{{{},{},{{}}}}`, score of `1 + 2 + 3 + 3 + 3 + 4 = 16`.
+*   `{<a>,<a>,<a>,<a>}`, score of `1`.
+*   `{{<ab>},{<ab>},{<ab>},{<ab>}}`, score of `1 + 2 + 2 + 2 + 2 = 9`.
+*   `{{<!!>},{<!!>},{<!!>},{<!!>}}`, score of `1 + 2 + 2 + 2 + 2 = 9`.
+*   `{{<a!>},{<a!>},{<a!>},{<ab>}}`, score of `1 + 2 = 3`.
+
+_What is the total score_ for all groups in your input?
+
+-----------------
+
+## --- Part Two ---
+
+Now, you're ready to remove the garbage.
+
+To prove you've removed it, you need to count all of the characters within the garbage. The leading and trailing `<` and `>` don't count, nor do any canceled characters or the `!` doing the canceling.
+
+*   `<>`, `0` characters.
+*   `<random characters>`, `17` characters.
+*   `<<<<>`, `3` characters.
+*   `<{!>}>`, `2` characters.
+*   `<!!>`, `0` characters.
+*   `<!!!>>`, `0` characters.
+*   `<{o"i!a,<{i<a>`, `10` characters.
+
+_How many non-canceled characters are within the garbage_ in your puzzle input?