Snippet:

        static void Part1(string data)
        {
            var result = ParseInput(data)
                .Select(history => ProcessHistory(history, g => g.Length - 1, (a, b) => a + b))
                .Sum();

            Console.WriteLine(result);
        }

        static void Part2(string data)
        {
            var result = ParseInput(data)
                .Select(history => ProcessHistory(history, g => 0, (a, b) => a - b))
                .Sum();

            Console.WriteLine(result);
        }

        static int ProcessHistory(
            int[] history,
            Func guessIndex,
            Func collateGuess)
        {
            bool allZeros = true;

            var diffs = new int[history.Length - 1];
            for (int i = 0; i < diffs.Length; i++)
            {
                var diff = history[i + 1] - history[i];
                diffs[i] = diff;
                allZeros = allZeros && (diff == 0);
            }

            var guess = history[guessIndex(history)];

            if (!allZeros)
            {
                guess = collateGuess(
                    guess,
                    ProcessHistory(diffs, guessIndex, collateGuess));
            }

            return guess;
        }