// TODO: I've shamelessly copied this off Reddit because i had no idea how to solve it.
// I need to come back to this and actually solve this myself.

/*using var sr = new StreamReader("./day05.txt");
var input = sr.ReadToEnd().Split("\n").Select(x => x.Trim());
foreach (var line in input)
{
    var nums = line.Split(", ->".ToArray(), StringSplitOptions.RemoveEmptyEntries).Select(byte.Parse).ToArray();
    var start = new Vector2(nums[0], nums[1]);
    var end = new Vector2(nums[2], nums[3]);
}*/

using var sr = new StreamReader("../inputs/day05.txt");
var input = sr.ReadToEnd();

Console.WriteLine(GetIntersectionCount(ParseLines(input, skipDiagonals: true)));
Console.WriteLine(GetIntersectionCount(ParseLines(input, skipDiagonals: false)));

private static int GetIntersectionCount(IEnumerable<IEnumerable<Vec2>> lines)
{
    // build a grid with iterating over all points, counting intersections:
    var grid = new Dictionary<Vec2, int>();
    foreach (var pt in lines.SelectMany(pt => pt))
    {
        grid[pt] = grid.GetValueOrDefault(pt, 0) + 1;
    }

    return grid.Count(kvp => kvp.Value > 1);
}

private static IEnumerable<IEnumerable<Vec2>> ParseLines(string input, bool skipDiagonals) =>
    from line in input.Split("\n")
    // parse the numbers first:
    let ns = (
        from st in line.Split(", ->".ToArray(), StringSplitOptions.RemoveEmptyEntries)
        select int.Parse(st)
    ).ToArray()

    // line properties:
    let start = new Vec2(ns[0], ns[1])
    let end = new Vec2(ns[2], ns[3])
    let displacement = new Vec2(end.x - start.x, end.y - start.y)
    let length = 1 + Math.Max(Math.Abs(displacement.x), Math.Abs(displacement.y))
    let dir = new Vec2(Math.Sign(displacement.x), Math.Sign(displacement.y))

    // represent lines with a set of points:
    let points =
        from i in Enumerable.Range(0, length)
        select new Vec2(start.x + i * dir.x, start.y + i * dir.y)

    where !skipDiagonals || dir.x == 0 || dir.y == 0 // skip diagonals in part 1

    select points;

    record Vec2(int x, int y);