76 lines
3.1 KiB
Markdown
76 lines
3.1 KiB
Markdown
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## \-\-- Day 9: Stream Processing \-\--
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A large stream blocks your path. According to the locals, it\'s not safe
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to [cross the
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stream]{title="\"Don't cross the streams!\", they yell, even though there's only one. They seem to think they're hilarious."}
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at the moment because it\'s full of *garbage*. You look down at the
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stream; rather than water, you discover that it\'s a *stream of
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characters*.
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You sit for a while and record part of the stream (your puzzle input).
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The characters represent *groups* - sequences that begin with `{` and
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end with `}`. Within a group, there are zero or more other things,
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separated by commas: either another *group* or *garbage*. Since groups
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can contain other groups, a `}` only closes the *most-recently-opened
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unclosed group* - that is, they are nestable. Your puzzle input
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represents a single, large group which itself contains many smaller
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ones.
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Sometimes, instead of a group, you will find *garbage*. Garbage begins
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with `<` and ends with `>`. Between those angle brackets, almost any
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character can appear, including `{` and `}`. *Within* garbage, `<` has
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no special meaning.
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In a futile attempt to clean up the garbage, some program has *canceled*
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some of the characters within it using `!`: inside garbage, *any*
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character that comes after `!` should be *ignored*, including `<`, `>`,
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and even another `!`.
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You don\'t see any characters that deviate from these rules. Outside
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garbage, you only find well-formed groups, and garbage always terminates
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according to the rules above.
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Here are some self-contained pieces of garbage:
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- `<>`, empty garbage.
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- `<random characters>`, garbage containing random characters.
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- `<<<<>`, because the extra `<` are ignored.
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- `<{!>}>`, because the first `>` is canceled.
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- `<!!>`, because the second `!` is canceled, allowing the `>` to
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terminate the garbage.
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- `<!!!>>`, because the second `!` and the first `>` are canceled.
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- `<{o"i!a,<{i<a>`, which ends at the first `>`.
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Here are some examples of whole streams and the number of groups they
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contain:
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- `{}`, `1` group.
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- `{{{}}}`, `3` groups.
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- `{{},{}}`, also `3` groups.
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- `{{{},{},{{}}}}`, `6` groups.
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- `{<{},{},{{}}>}`, `1` group (which itself contains garbage).
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- `{<a>,<a>,<a>,<a>}`, `1` group.
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- `{{<a>},{<a>},{<a>},{<a>}}`, `5` groups.
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- `{{<!>},{<!>},{<!>},{<a>}}`, `2` groups (since all but the last `>`
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are canceled).
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Your goal is to find the total score for all groups in your input. Each
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group is assigned a *score* which is one more than the score of the
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group that immediately contains it. (The outermost group gets a score of
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`1`.)
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- `{}`, score of `1`.
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- `{{{}}}`, score of `1 + 2 + 3 = 6`.
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- `{{},{}}`, score of `1 + 2 + 2 = 5`.
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- `{{{},{},{{}}}}`, score of `1 + 2 + 3 + 3 + 3 + 4 = 16`.
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- `{<a>,<a>,<a>,<a>}`, score of `1`.
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- `{{<ab>},{<ab>},{<ab>},{<ab>}}`, score of `1 + 2 + 2 + 2 + 2 = 9`.
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- `{{<!!>},{<!!>},{<!!>},{<!!>}}`, score of `1 + 2 + 2 + 2 + 2 = 9`.
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- `{{<a!>},{<a!>},{<a!>},{<ab>}}`, score of `1 + 2 = 3`.
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*What is the total score* for all groups in your input?
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To begin, [get your puzzle input](9/input).
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Answer:
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