From 3c682b31e96cc362c3d257b3ecf9af1b19a82a47 Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Tue, 24 Dec 2019 16:38:36 +0100
Subject: [PATCH] Implement day 24 part 1.

---
 2019/inputs/24.txt            |  5 +++
 2019/src/day24.cpp            | 68 +++++++++++++++++++++++++++++++++--
 2019/tests/samples/24-1-1.in  |  5 +++
 2019/tests/samples/24-1-1.out |  1 +
 4 files changed, 77 insertions(+), 2 deletions(-)
 create mode 100644 2019/inputs/24.txt
 create mode 100644 2019/tests/samples/24-1-1.in
 create mode 100644 2019/tests/samples/24-1-1.out

diff --git a/2019/inputs/24.txt b/2019/inputs/24.txt
new file mode 100644
index 0000000..63749fd
--- /dev/null
+++ b/2019/inputs/24.txt
@@ -0,0 +1,5 @@
+###..
+.##..
+#....
+##..#
+.###.
diff --git a/2019/src/day24.cpp b/2019/src/day24.cpp
index 632ed30..efec840 100644
--- a/2019/src/day24.cpp
+++ b/2019/src/day24.cpp
@@ -1,10 +1,74 @@
 #include <iostream>
+#include <vector>
+#include <algorithm>
+#include <set>
 #include "days.hpp"
 
+namespace {
+    using field_t = std::array<std::array<bool, 5>, 5>;
+
+    field_t read_input(std::istream &input) {
+        std::string buffer;
+        field_t map;
+
+        int y = 0;
+
+        while (std::getline(input, buffer)) {
+            auto &row = map[y++];
+
+            std::transform(buffer.begin(), buffer.end(), row.begin(), [](char c) { return c == '#'; });
+        }
+
+        return map;
+    }
+
+    void next_gen(const field_t &source, field_t &sink) {
+        for (int y = 0; y < source.size(); ++y) {
+            for (int x = 0; x < source[y].size(); ++x) {
+                int neighbours = source[y][x] ? -1 : 0;
+                for (int dy = -1; dy <= 1; ++dy) {
+                    if (dy + y < 0 || dy + y >= source.size()) {
+                        continue;
+                    }
+                    for (int dx = -1; dx <= 1; ++dx) {
+                        if (dx + x < 0 || dx + x >= source[y].size() || dx * dy) {
+                            continue;
+                        }
+                        neighbours += source[y + dy][x + dx];
+                    }
+                }
+
+                sink[y][x] = neighbours == 1 || (!source[y][x] && neighbours == 2);
+            }
+        }
+    }
+}
+
 void aoc2019::day24_part1(std::istream &input, std::ostream &output) {
-	output << "Not implemented\n";
+    auto map = read_input(input);
+    auto copy = map;
+
+    std::set<field_t> seen;
+    do {
+        seen.insert(map);
+        next_gen(map, copy);
+        std::swap(map, copy);
+    } while (!seen.count(map));
+
+    unsigned int pow = 1;
+    unsigned int diversity = 0;
+    for (auto &row : map) {
+        for (auto b : row) {
+            if (b) {
+                diversity += pow;
+            }
+
+            pow <<= 1u;
+        }
+    }
+    output << diversity << std::endl;
 }
 
 void aoc2019::day24_part2(std::istream &input, std::ostream &output) {
-	output << "Not implemented\n";
+    output << "Not implemented\n";
 }
diff --git a/2019/tests/samples/24-1-1.in b/2019/tests/samples/24-1-1.in
new file mode 100644
index 0000000..704a112
--- /dev/null
+++ b/2019/tests/samples/24-1-1.in
@@ -0,0 +1,5 @@
+....#
+#..#.
+#..##
+..#..
+#....
diff --git a/2019/tests/samples/24-1-1.out b/2019/tests/samples/24-1-1.out
new file mode 100644
index 0000000..39f6036
--- /dev/null
+++ b/2019/tests/samples/24-1-1.out
@@ -0,0 +1 @@
+2129920