aoc2023day9.cs

static class Day9
{
    public static void Run()
    {
        var lines = Input.Split('\n', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);

        long total = 0;
        foreach (var line in lines)
        {
            // for part 2 just reverse this array
            var nums = line.Split(' ').Select(int.Parse).ToArray();
            
            var iters = 0;
            while (iters < nums.Length)
            {
                iters += 1;
                var allZeros = true;
                for (int i = 0; i < nums.Length - iters; i++)
                {
                    nums[i] = nums[i + 1] - nums[i];
                    if (nums[i] != 0)
                    {
                        allZeros = false;
                    }
                }


                if (allZeros)
                {
                    break;
                }
            }

            long next = 0;
            for (int i = nums.Length - 1 - iters; i < nums.Length; i++)
            {
                next += nums[i];
            }
            total += next;
        }

        Console.WriteLine("Total: {0}", total);
    }

    const string Input =
"""
0 3 6 9 12 15
1 3 6 10 15 21
10 13 16 21 30 45
""";
}