aoc 2025, day 7, part 2

1 files changed, 34 insertions, 0 deletions

diff --git a/2025/src/day07.cpp b/2025/src/day07.cpp
index ca73d87..e902a43 100644
--- a/2025/src/day07.cpp
+++ b/2025/src/day07.cpp
@@ -1,6 +1,8 @@
 #include <filesystem>
 #include <fstream>
+#include <functional>
 #include <iostream>
+#include <map>
 #include <set>
 #include <string>
 #include <vector>
@@ -48,6 +50,37 @@ part1(const vector<string>& grid)
         cout << "Part 1: " << count << '\n';
 }
 
+void
+part2(const vector<string>& grid)
+{
+        using Pos = tuple<size_t, size_t>;
+
+        map<Pos, long>             cache;
+        function<long(const Pos&)> walk = [&](const Pos& pos) {
+                if ( cache.contains(pos) ) {
+                        return cache.at(pos);
+                }
+
+                const auto [col, row] = pos;
+
+                if ( row == grid.size() ) {
+                        return (cache[pos] = 1L);
+                }
+
+                if ( grid[row][col] == '^' ) {
+                        const auto left  = walk({ col - 1, row });
+                        const auto right = walk({ col + 1, row });
+
+                        return (cache[pos] = left + right);
+                }
+
+                return (cache[pos] = walk({ col, row + 1 }));
+        };
+
+        const auto col = grid[0].find('S');
+        cout << "Part 2: " << walk(Pos{ col, 0 }) << '\n';
+}
+
 } // namespace
 
 int
@@ -55,4 +88,5 @@ main()
 {
         auto grid = read_file("data/day07.txt");
         part1(grid);
+        part2(grid);
 }