From 825f08d01e2691980ed4604992d4058dcc5dbbde Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Thu, 19 Dec 2019 20:57:28 +0100
Subject: [PATCH] Implement day 19 part 2.

---
 2019/src/day19.cpp | 79 ++++++++++++++++++++++++++++++++--------------
 1 file changed, 55 insertions(+), 24 deletions(-)

diff --git a/2019/src/day19.cpp b/2019/src/day19.cpp
index e1ee29a..15eea3c 100644
--- a/2019/src/day19.cpp
+++ b/2019/src/day19.cpp
@@ -1,5 +1,6 @@
 #include <iostream>
 #include <cassert>
+#include <queue>
 #include "days.hpp"
 #include "utils.hpp"
 
@@ -17,42 +18,72 @@ namespace {
 
         return output_buffer.front();
     }
+
+    class Beam {
+    private:
+        aoc2019::IntCodeComputer computer;
+        std::int64_t last_width = 1;
+        std::int64_t last_start = 0;
+        std::int64_t y = 0;
+
+    public:
+        Beam(std::istream &input) : computer(input) {};
+
+        std::pair<std::int64_t, std::int64_t> next() {
+            auto x = last_start;
+
+            while (!bounds_check(computer, x, y)) {
+                ++x;
+            }
+
+            last_start = x;
+            x += last_width - 1;
+
+            while (bounds_check(computer, x, y)) {
+                ++x;
+            }
+
+            last_width = x - last_start;
+            ++y;
+
+            return {last_start, last_width};
+        }
+    };
 }
 
 void aoc2019::day19_part1(std::istream &input, std::ostream &output) {
-    IntCodeComputer computer(input);
+    Beam beam(input);
 
     std::int64_t covered = 0;
-    int last_width = 1;
-    int last_start = 0;
     for (std::int64_t y = 0; y < 50; ++y) {
-        auto x = last_start;
+        const auto[start, width] = beam.next();
 
-        while (!bounds_check(computer, x, y) && x < 50) {
-            ++x;
-        }
+        if (start >= 50) break;
 
-        if (x == 50) break;
-
-        last_start = x;
-        x += last_width - 1;
-
-        if (!bounds_check(computer, x, y)) {
-            std::cerr << x << "," << y << "," << covered << std::endl;
-            throw std::logic_error("Assumption false");
-        }
-
-        while (bounds_check(computer, x, y) && x < 50) {
-            ++x;
-        }
-
-        last_width = x - last_start;
-        covered += last_width;
+        covered += std::min(50 - start, width);
     }
 
     output << covered << std::endl;
 }
 
 void aoc2019::day19_part2(std::istream &input, std::ostream &output) {
-	output << "Not implemented\n";
+    Beam beam(input);
+    std::queue<std::int64_t> beam_ends;
+
+    constexpr std::int64_t DIMENSION = 100;
+
+    for (std::int64_t y = 0; true; ++y) {
+        const auto[start, width] = beam.next();
+
+        beam_ends.push(start + width);
+        if (beam_ends.size() == DIMENSION) {
+            auto end = beam_ends.front();
+            if (end - start >= DIMENSION) {
+                auto result = start * 10000 + y - DIMENSION + 1;
+                output << result << std::endl;
+                return;
+            }
+            beam_ends.pop();
+        }
+    }
 }