aoc2023/day-9/two.go

package main

// previous returns the previous value in the sequence
func previous(nums []int) int {
	diffs := []int{}
	for i := 1; i < len(nums); i++ {
		diffs = append(diffs, nums[i]-nums[i-1])
	}

	if allEqual(diffs) {
		return nums[0] - diffs[0]
	}
	return nums[0] - previous(diffs)
}

// two returns the sum of all previous values
func two(lines []string) int {
	sum := 0
	for _, line := range lines {
		nums := convertToInts(line)
		sum += previous(nums)
	}

	return sum
}