const std = @import("std");

// https://stackoverflow.com/a/328122
fn isBetween(a: Point, b: Point, c: Point) bool {
    const crossproduct = (c.y - a.y) * (b.x - a.x) - (c.x - a.x) * (b.y - a.y);

    // compare versus epsilon
    if (std.math.absFloat(crossproduct) > std.math.f64_epsilon) return false;

    const dotproduct = (c.x - a.x) * (b.x - a.x) + (c.y - a.y) * (b.y - a.y);
    if (dotproduct < 0.0) return false;

    const squaredlengthba = (b.x - a.x) * (b.x - a.x) + (b.y - a.y) * (b.y - a.y);
    if (dotproduct > squaredlengthba) return false;

    return true;
}

const Point = struct {
    x: f64,
    y: f64,

    fn eql(self: Point, other: Point) bool {
        return self.x == other.x and self.y == other.y;
    }
};

const Asteroid = struct {
    p: Point,
    visible: usize = 0,
};

const Asteroids = std.ArrayList(Asteroid);
const input = @embedFile("../input");

fn part1(asteroids: Asteroids) void {
    for (asteroids.toSlice()) |*a| {
        for (asteroids.toSlice()) |b| {
            if (a.p.eql(b.p)) continue;

            // Check every asteroid to see if they fall between A and B
            var blocked: bool = false;
            for (asteroids.toSlice()) |c| {
                if (a.p.eql(c.p) or b.p.eql(c.p)) continue;

                //var dist = distToSegment(a.p, b.p, c.p);
                if (isBetween(a.p, b.p, c.p)) {
                    blocked = true;
                    break;
                }
            }

            // A can see B, so add it to A's score
            if (!blocked) a.visible += 1;
        }
    }

    var best: usize = 0;
    for (asteroids.toSlice()) |asteroid| {
        if (asteroid.visible > best) best = asteroid.visible;
    }

    std.debug.warn("Day 10, Part 1: {}\n", best);
}

pub fn main() anyerror!void {
    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
    defer arena.deinit();

    const alloc = &arena.allocator;

    var asteroids = Asteroids.init(alloc);
    var lines = std.mem.separate(input, "\n");
    var y: usize = 0;

    while (lines.next()) |line| {
        for (line) |byte, x| {
            if (byte == '#') {
                const px = @intToFloat(f64, x) + 0.5;
                const py = @intToFloat(f64, y) + 0.5;
                try asteroids.append(Asteroid{ .p = Point{ .x = px, .y = py } });
            }
        }

        y += 1;
    }

    part1(asteroids);
}