do day 7

input/2025-7.sample.txt

@@ -0,0 +1,16 @@
+.......S.......
+...............
+.......^.......
+...............
+......^.^......
+...............
+.....^.^.^.....
+...............
+....^.^...^....
+...............
+...^.^...^.^...
+...............
+..^...^.....^..
+...............
+.^.^.^.^.^...^.
+...............

src/day7.cpp

@@ -0,0 +1,100 @@
+// :autocmd BufWritePost *.cpp :silent exec "!script/async-build 7"
+#include <cstdint>
+#include <fstream>
+#include <iostream>
+#include <map>
+#include <numeric>
+#include <set>
+#include <string>
+#include <vector>
+
+using Grid = std::vector<std::string>;
+using Location = std::tuple<size_t, size_t>;
+
+int64_t part_1(const Grid &grid);
+int64_t part_2(const Grid &grid);
+
+int main() {
+  Grid grid;
+  std::ifstream file{"input/2025-7.txt"};
+  std::string line;
+  while (std::getline(file, line)) {
+    grid.emplace_back(line);
+  }
+  std::cout << "Part 1: " << part_1(grid) << std::endl;
+  std::cout << "Part 2: " << part_2(grid) << std::endl;
+  return 0;
+}
+
+char at_loc(const Grid &grid, size_t row, size_t col) {
+  if (row >= 0 && row < grid.size() && col >= 0 && col < grid[row].size()) {
+    return grid[row][col];
+  } else {
+    return '.';
+  }
+}
+
+Location find_start(const Grid &grid) {
+  for (int64_t row{0}; row < grid.size(); ++row) {
+    for (int64_t col{0}; col < grid[row].length(); ++col) {
+      if (grid[row][col] == 'S') {
+        return Location{row, col};
+      }
+    }
+  }
+  throw "unreachable";
+}
+
+int64_t part_1(const Grid &grid) {
+  int64_t ans{0};
+  Location start = find_start(grid);
+  const auto [start_row, start_col]{start};
+  std::set<size_t> cols{};
+  cols.insert(start_col);
+  for (size_t row{start_row}; row < grid.size(); ++row) {
+    std::set<size_t> next_cols{};
+    for (auto col : cols) {
+      if (at_loc(grid, row, col) == '^') {
+        ans += 1;
+        next_cols.insert(col + 1);
+        next_cols.insert(col - 1);
+      } else {
+        next_cols.insert(col);
+      }
+    }
+    cols = next_cols;
+  }
+  return ans;
+}
+
+void insert_or_increment(std::map<size_t, int64_t> &cols, size_t col,
+                    int64_t value) {
+  auto iter = cols.find(col);
+  if (iter == cols.end()) {
+    cols.insert({col, value});
+  } else {
+    iter->second += value;
+  }
+}
+
+int64_t part_2(const Grid &grid) {
+  Location start = find_start(grid);
+  const auto [start_row, start_col]{start};
+  std::map<size_t, int64_t> cols;
+  cols.insert(std::make_pair(start_col, 1));
+  for (size_t row{start_row}; row < grid.size(); ++row) {
+    std::map<size_t, int64_t> next_cols;
+    for (auto [col, cnt] : cols) {
+      if (at_loc(grid, row, col) == '^') {
+        insert_or_increment(next_cols, col - 1, cnt);
+        insert_or_increment(next_cols, col + 1, cnt);
+      } else {
+        insert_or_increment(next_cols, col, cnt);
+      }
+    }
+    cols = std::move(next_cols);
+  }
+  return std::accumulate(
+      cols.begin(), cols.end(), 0ll,
+      [](int64_t acc, auto entry) { return acc + entry.second; });
+}