From e81f3f6ab7234cc59bc3e5084d8d9779fb3ebb16 Mon Sep 17 00:00:00 2001
From: Nick Thomas <me@ur.gs>
Date: Tue, 10 Dec 2019 21:14:32 +0000
Subject: [PATCH] Day 10.1

---
 10/build.zig    | 14 ++++++++
 10/input        | 25 ++++++++++++++
 10/src/main.zig | 89 +++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 128 insertions(+)
 create mode 100644 10/build.zig
 create mode 100644 10/input
 create mode 100644 10/src/main.zig

diff --git a/10/build.zig b/10/build.zig
new file mode 100644
index 0000000..6edf1ed
--- /dev/null
+++ b/10/build.zig
@@ -0,0 +1,14 @@
+const Builder = @import("std").build.Builder;
+
+pub fn build(b: *Builder) void {
+    const mode = b.standardReleaseOptions();
+    const exe = b.addExecutable("10", "src/main.zig");
+    exe.setBuildMode(mode);
+    exe.install();
+
+    const run_cmd = exe.run();
+    run_cmd.step.dependOn(b.getInstallStep());
+
+    const run_step = b.step("run", "Run the app");
+    run_step.dependOn(&run_cmd.step);
+}
diff --git a/10/input b/10/input
new file mode 100644
index 0000000..c3f66aa
--- /dev/null
+++ b/10/input
@@ -0,0 +1,25 @@
+#..#.#.#.######..#.#...##
+##.#..#.#..##.#..######.#
+.#.##.#..##..#.#.####.#..
+.#..##.#.#..#.#...#...#.#
+#...###.##.##..##...#..#.
+##..#.#.#.###...#.##..#.#
+###.###.#.##.##....#####.
+.#####.#.#...#..#####..#.
+.#.##...#.#...#####.##...
+######.#..##.#..#.#.#....
+###.##.#######....##.#..#
+.####.##..#.##.#.#.##...#
+##...##.######..##..#.###
+...###...#..#...#.###..#.
+.#####...##..#..#####.###
+.#####..#.#######.###.##.
+#...###.####.##.##.#.##.#
+.#.#.#.#.#.##.#..#.#..###
+##.#.####.###....###..##.
+#..##.#....#..#..#.#..#.#
+##..#..#...#..##..####..#
+....#.....##..#.##.#...##
+.##..#.#..##..##.#..##..#
+.##..#####....#####.#.#.#
+#..#..#..##...#..#.#.#.##
diff --git a/10/src/main.zig b/10/src/main.zig
new file mode 100644
index 0000000..f5e6584
--- /dev/null
+++ b/10/src/main.zig
@@ -0,0 +1,89 @@
+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);
+}