From 4c268eae6066846d17823d838370b4344d60dbe9 Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Tue, 10 Dec 2019 18:44:39 +0100
Subject: [PATCH] Implement day 10 part 2.

---
 2019/src/day10.cpp | 89 ++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 74 insertions(+), 15 deletions(-)

diff --git a/2019/src/day10.cpp b/2019/src/day10.cpp
index 1b76f1e..33bf1d0 100644
--- a/2019/src/day10.cpp
+++ b/2019/src/day10.cpp
@@ -1,13 +1,15 @@
+#include <algorithm>
 #include <iostream>
 #include <numeric>
 #include <unordered_set>
+#include <cmath>
 #include "days.hpp"
 #include "point.hpp"
 
 namespace {
     typedef aoc2019::Point<int, 2> point_t;
 
-    std::vector<point_t> read_points(std::istream& input) {
+    std::vector<point_t> read_points(std::istream &input) {
         std::vector<point_t> result;
 
         int y = 0;
@@ -27,31 +29,88 @@ namespace {
     point_t simplify(point_t x) {
         auto gcd = std::abs(std::gcd(x[0], x[1]));
         if (gcd > 1) {
-            return { x[0] / gcd, x[1] / gcd };
+            return {x[0] / gcd, x[1] / gcd};
         }
 
         return x;
     }
+
+    std::pair<std::size_t, std::size_t> part1(const std::vector<point_t> &points) {
+        std::size_t best = 0;
+        std::size_t best_index = 0;
+
+        for (std::size_t i = 0; i < points.size(); ++i) {
+            std::unordered_set<point_t> visible;
+            const auto point = points[i];
+
+            for (auto asteroid : points) {
+                if (asteroid == point) continue;
+                visible.insert(simplify(asteroid - point));
+            }
+
+            if (visible.size() > best) {
+                best = visible.size();
+                best_index = i;
+            }
+
+            best = std::max(visible.size(), best);
+        }
+
+        return {best, best_index};
+    }
 }
 
 void aoc2019::day10_part1(std::istream &input, std::ostream &output) {
     const auto points = read_points(input);
-    std::size_t best = 0;
 
-    for (auto point : points) {
-        std::unordered_set<point_t> visible;
+    auto[best, _] = part1(points);
 
-        for (auto asteroid : points) {
-            if (asteroid == point) continue;
-            visible.insert(simplify(asteroid - point));
-        }
-
-        best = std::max(visible.size(), best);
-    }
-
-	output << best << std::endl;
+    output << best << std::endl;
 }
 
 void aoc2019::day10_part2(std::istream &input, std::ostream &output) {
-	output << "Not implemented\n";
+    const auto points = read_points(input);
+    const auto[_, base] = part1(points);
+    const auto base_point = points[base];
+
+    std::unordered_map<point_t, std::vector<point_t>> angle_points;
+
+    for (auto point : points) {
+        if (point == base_point) continue;
+        auto diff = point - base_point;
+
+        angle_points[simplify(diff)].push_back(diff);
+    }
+
+    std::vector<std::pair<float, point_t>> angles;
+
+    for (auto &entry : angle_points) {
+        angles.emplace_back(std::atan2(entry.first[1], entry.first[0]), entry.first);
+        // Sort entries in descending order of distance so we can pop_back() them
+        std::sort(entry.second.begin(), entry.second.end(), [](auto a, auto b) { return a.l1() > b.l1(); });
+    }
+
+    std::sort(angles.begin(), angles.end(), std::greater<>{});
+
+    const auto starting_point = std::make_pair(float(0.5 * M_PI),
+                                               point_t{std::numeric_limits<int>::max(),
+                                                       std::numeric_limits<int>::max()});
+
+    auto it = std::lower_bound(angles.begin(), angles.end(), starting_point, std::greater<>{});
+
+    for (int hits = 0; hits < 199; ++hits) {
+        angle_points[it->second].pop_back();
+
+        // Advance it to the next asteroid we can hit.
+        while (angle_points[it->second].empty()) {
+            ++it;
+            if (it == angles.end()) {
+                it = angles.begin();
+            }
+        }
+    }
+
+    auto final_asteroid = angle_points[it->second].back() + base_point;
+
+    output << final_asteroid[0] * 100 + final_asteroid[1] << std::endl;
 }