/*
--- Part Two ---

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?

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.

In particular, here is what the third example history looks like when extrapolating back in time:

```
5  10  13  16  21  30  45
  5   3   3   5   9  15
   -2   0   2   4   6
      2   2   2   2
        0   0   0
```

Adding the new values on the left side of each sequence from bottom to top eventually reveals the
new left-most history value: 5.

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.

Analyze your OASIS report again, this time extrapolating the previous value for each history. What
is the sum of these extrapolated values?
*/

package com.curtislb.adventofcode.year2023.day09.part2

import com.curtislb.adventofcode.common.io.mapLines
import com.curtislb.adventofcode.common.parse.parseInts
import com.curtislb.adventofcode.year2023.day09.oasis.OasisReport
import java.nio.file.Path
import java.nio.file.Paths

/**
 * Returns the solution to the puzzle for 2023, day 9, part 2.
 *
 * @param inputPath The path to the input file for this puzzle.
 */
fun solve(inputPath: Path = Paths.get("..", "input", "input.txt")): Int {
    val reports = inputPath.toFile().mapLines { OasisReport(it.parseInts()) }
    return reports.sumOf { it.extrapolatePrevious() }
}

fun main() {
    println(solve())
}
