Advent.linq

(int x, int y) Naive(int target, List<int> values)
{
	for (var i = 0; i < values.Count; ++i)
	{
		for (var j = i + 1; j < values.Count; ++j)
		{
			if (values[i] + values[j] == target)
			{
				return (values[i], values[j]);
			}
		}
	}
	throw new Exception($"Failed to find 2 numbers that sum to {target}.");
}

(int x, int y) Optimized(bool preSorted, int target, List<int> values)
{
	// Sorting is a requirement for this method.
	if (!preSorted)
	{
		values.Sort();
	}

	int iPtr = 0, jPtr = values.Count - 1;
	int MoveJPtr() => --jPtr >= 0 ? jPtr : throw new IndexOutOfRangeException($"{nameof(jPtr)} went out of range.");
	int MoveIPtr() => ++iPtr <= values.Count ? iPtr : throw new IndexOutOfRangeException($"{nameof(iPtr)} went out of range.");
	while (iPtr < jPtr)
	{
		// If we're too big, get smaller.
		while (values[iPtr] + values[jPtr] > target)
		{
			MoveJPtr();
		}
		while (values[iPtr] + values[jPtr] < target)
		{
			MoveIPtr();
		}

		if (values[iPtr] + values[jPtr] == target)
		{
			return (values[iPtr], values[jPtr]);
		}
	}
	throw new Exception($"Gave up at {values[iPtr]}, {values[jPtr]} - failed to find 2 numbers that sum to {target}.");
}

var input = File.ReadAllLines(@"C:\Code\Advent2020Scratchpad\Day1Part1Input.txt")
	.Select(int.Parse)
	.ToList();

var target = 2020;
try
{
	var sw = new Stopwatch();
	sw.Start();
	var result = Naive(target, input);
	sw.Stop();
	result.Dump($"Naive finished in {sw.ElapsedTicks} ticks");
}
catch (Exception ex)
{
	ex.Message.Dump("Naive failed.");
}

try
{
	var oddInput = input.Where(i => i % 2 == 1).ToList();
	oddInput.Sort();
	var sw = new Stopwatch();
	sw.Start();
	var result = Optimized(true, target, oddInput);
	sw.Stop();
	result.Dump($"Odd Optimized finished in {sw.ElapsedTicks} ticks");
}
catch (Exception ex)
{
	ex.Message.Dump("Odd Optimized failed.");
}

try
{
	var evenInput = input.Where(i => i % 2 == 0).ToList();
	evenInput.Sort();
	var sw = new Stopwatch();
	sw.Start();
	var result = Optimized(true, target, evenInput);
	sw.Stop();
	result.Dump($"Even Optimized finished in {sw.ElapsedTicks} ticks");
}
catch (Exception ex)
{
	ex.Message.Dump("Even Optimized failed.");
}

try
{
	var sw = new Stopwatch();
	sw.Start();
	var result = Optimized(false, target, input);
	sw.Stop();
	result.Dump($"Unsorted Optimized finished in {sw.ElapsedTicks} ticks");
}
catch (Exception ex)
{
	ex.Message.Dump("Unsorted Optimized failed.");
}