You come across a field of hydrothermal vents on the ocean floor! These vents constantly produce large, opaque clouds, so it would be best to avoid them if possible.

They tend to form in lines; the submarine helpfully produces a list of nearby lines of vents (your puzzle input) for you to review. For example:

Read the full puzzle.

using System;
using System.Collections.Generic;
using System.Linq;

namespace AdventOfCode.Y2021.Day05;

[ProblemName("Hydrothermal Venture")]
class Solution : Solver {

    public object PartOne(string input) => GetIntersections(ParseLines(input, skipDiagonals: true)).Count();
    public object PartTwo(string input) => GetIntersections(ParseLines(input, skipDiagonals: false)).Count();

    IEnumerable<Vec2> GetIntersections(IEnumerable<IEnumerable<Vec2>> lines) => 
        // group all the points and return the intersections:
        lines.SelectMany(pt => pt).GroupBy(pt => pt).Where(g => g.Count() > 1).Select(g => g.Key);
    
    IEnumerable<IEnumerable<Vec2>> ParseLines(string input, bool skipDiagonals) =>
        from line in input.Split("\n")
        // parse out 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)

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

        select points;
}

record Vec2(int x, int y);

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