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part 2! thank god that one was quick
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2023/day09/part2.ts
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2023/day09/part2.ts
···
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export function answer(input: string): number {
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console.log(input);
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return 42;
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const seqs = input
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.split("\n")
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.map((l) => l.split(" ").map((x) => parseInt(x)));
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return seqs.reduce((sum, seq) => (sum += prevNum(seq)), 0);
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}
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function prevNum(seq: number[]): number {
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const seqs = [seq];
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const reduce = (s: number[]): number[] => {
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let [prev, ...rest] = s;
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return rest.map((x) => {
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const next = x - prev;
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prev = x;
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return next;
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});
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};
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while (!seqs[0].every((x) => x === 0)) {
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// console.log(reduce(seqs[0]));
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seqs.unshift(reduce(seqs[0]));
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}
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for (const [i, s] of seqs.entries()) {
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if (i === 0) continue;
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const prevSeq = seqs[i - 1];
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s.unshift(s[0] - prevSeq[0]);
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}
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return seq[0];
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}
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if (import.meta.main) {
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2023/day09/puzzle.md
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2023/day09/puzzle.md
···
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Analyze your OASIS report and extrapolate the next value for each history. *What is the sum of these extrapolated values?*
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To begin, [get your puzzle input](9/input).
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Your puzzle answer was `1641934234`.
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The first half of this puzzle is complete! It provides one gold star: \*
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\--- Part Two ---
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----------
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Of course, it would be nice to have *even more history* included in your report. Surely it's safe to just *extrapolate backwards* as well, right?
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For each history, repeat the process of finding differences until the sequence of differences is entirely zero. Then, rather than adding a zero to the end and filling in the next values of each previous sequence, you should instead add a zero to the *beginning* of your sequence of zeroes, then fill in new *first* values for each previous sequence.
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In particular, here is what the third example history looks like when extrapolating back in time:
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```
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5 10 13 16 21 30 45
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5 3 3 5 9 15
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-2 0 2 4 6
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2 2 2 2
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0 0 0
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```
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Adding the new values on the left side of each sequence from bottom to top eventually reveals the new left-most history value: `*5*`.
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Doing this for the remaining example data above results in previous values of `*-3*` for the first history and `*0*` for the second history. Adding all three new values together produces `*2*`.
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Analyze your OASIS report again, this time extrapolating the *previous* value for each history. *What is the sum of these extrapolated values?*
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Answer:
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You can also [Shareon [Twitter](https://twitter.com/intent/tweet?text=%22Mirage+Maintenance%22+%2D+Day+9+%2D+Advent+of+Code+2023&url=https%3A%2F%2Fadventofcode%2Ecom%2F2023%2Fday%2F9&related=ericwastl&hashtags=AdventOfCode) [Mastodon](javascript:void(0);)] this puzzle.
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Although it hasn't changed, you can still [get your puzzle input](9/input).
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You can also [Shareon [Twitter](https://twitter.com/intent/tweet?text=I%27ve+completed+Part+One+of+%22Mirage+Maintenance%22+%2D+Day+9+%2D+Advent+of+Code+2023&url=https%3A%2F%2Fadventofcode%2Ecom%2F2023%2Fday%2F9&related=ericwastl&hashtags=AdventOfCode) [Mastodon](javascript:void(0);)] this puzzle.
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2023/day09/test.ts
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2023/day09/test.ts
···
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assertEquals(p1.answer(examples), 114);
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});
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// Deno.test("part2", () => {
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// const examples = ["abc", "def"].join("\n");
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// assertEquals(p2.answer(examples), 42);
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// });
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Deno.test("part2", () => {
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const examples = [
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"0 3 6 9 12 15",
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"1 3 6 10 15 21",
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"10 13 16 21 30 45",
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].join("\n");
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assertEquals(p2.answer(examples), 2);
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});