1 files changed, 108 insertions, 0 deletions

diff --git a/2024/src/day20.cpp b/2024/src/day20.cpp
new file mode 100644
index 0000000..42a2fba
--- /dev/null
+++ b/2024/src/day20.cpp
@@ -0,0 +1,108 @@
+#include <fstream>
+#include <iostream>
+#include <map>
+#include <queue>
+#include <set>
+#include <string>
+#include <tuple>
+#include <vector>
+using namespace std;
+
+using pos_type = tuple<size_t, size_t>;
+
+tuple<pos_type, pos_type, set<pos_type>>
+read_file(string_view filename)
+{
+        fstream       input{ filename };
+        set<pos_type> walls;
+        pos_type      start;
+        pos_type      end;
+
+        size_t yPos = 0;
+        for ( string line; getline(input, line); ) {
+                for ( size_t xPos = 0; xPos != line.size(); ++xPos ) {
+                        auto chr = line[xPos];
+                        if ( chr == '#' ) {
+                                walls.emplace(xPos, yPos);
+                        }
+                        else if ( chr == 'S' ) {
+                                start = { xPos, yPos };
+                        }
+                        else if ( chr == 'E' ) {
+                                end = { xPos, yPos };
+                        }
+                }
+                ++yPos;
+        }
+
+        return { start, end, walls };
+}
+
+map<pos_type, long>
+bfs(const tuple<pos_type, pos_type, set<pos_type>>& data)
+{
+        const auto& [start, end, walls] = data;
+
+        map<pos_type, long> path;
+
+        set<pos_type> seen;
+
+        queue<tuple<pos_type, long>> queue;
+        queue.push({ start, 0 });
+
+        while ( !queue.empty() ) {
+                auto [pos, dist] = queue.front();
+                queue.pop();
+
+                if ( seen.contains(pos) ) {
+                        continue;
+                }
+                seen.insert(pos);
+
+                path[pos] = dist;
+
+                if ( pos == end ) {
+                        break;
+                }
+
+                auto [x, y] = pos;
+                for ( const auto& new_pos: vector<pos_type>{ { x - 1, y }, { x, y - 1 }, { x + 1, y }, { x, y + 1 } } ) {
+                        if ( walls.contains(new_pos) ) {
+                                continue;
+                        }
+                        queue.push({ new_pos, dist + 1 });
+                }
+        }
+        return path;
+}
+
+void
+part1(const tuple<pos_type, pos_type, set<pos_type>>& data)
+{
+        auto path = bfs(data);
+
+        const auto& [start, end, walls] = data;
+
+        long count = 0;
+        for ( const auto& [pos, dist]: path ) {
+                auto [x, y] = pos;
+
+                for ( const auto& [dx, dy]: vector<pos_type>{ { -1, 0 }, { 0, -1 }, { 1, 0 }, { 0, 1 } } ) {
+                        if ( !walls.contains({ x + dx, y + dy }) ) {
+                                continue;
+                        }
+                        pos_type new_pos{ x + 2 * dx, y + 2 * dy };
+                        if ( path.contains(new_pos) && dist < path.at(new_pos) && path.at(new_pos) - dist >= 100 + 2 ) {
+                                ++count;
+                        }
+                }
+        }
+        cout << count << endl;
+}
+
+int
+main()
+{
+        auto data = read_file("data/day20.txt");
+        part1(data);
+}