2023: day 13 part 1 complete

2023/input/day13/example1.txt

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+#.##..##.
+..#.##.#.
+##......#
+##......#
+..#.##.#.
+..##..##.
+#.#.##.#.
+
+#...##..#
+#....#..#
+..##..###
+#####.##.
+#####.##.
+..##..###
+#....#..#

2023/src/day13.cpp

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+/******************************************************************************
+ * File:        day13.cpp
+ *
+ * Author:      Eric T. Johnson (yut23)
+ * Created:     2023-12-13
+ *****************************************************************************/
+
+#include "day13.hpp"
+#include "lib.hpp"  // for parse_args, DEBUG
+#include <iostream> // for cout, cerr
+
+void print_grid(std::ostream &os, const aoc::day13::Grid &grid) {
+    for (const auto &line : grid) {
+        os << line << "\n";
+    }
+}
+
+int main(int argc, char **argv) {
+    std::ifstream infile = aoc::parse_args(argc, argv);
+
+    auto grids = aoc::day13::read_grids(infile);
+
+    int part_1 = 0;
+    for (const auto &grid : grids) {
+        part_1 += 100 * aoc::day13::find_reflection(grid);
+        part_1 += aoc::day13::find_reflection(aoc::day13::transpose(grid));
+        if constexpr (aoc::DEBUG) {
+            print_grid(std::cerr, grid);
+            std::cerr << "\n";
+            print_grid(std::cerr, aoc::day13::transpose(grid));
+            std::cerr << "\n\n";
+        }
+    }
+
+    std::cout << part_1 << "\n";
+
+    return 0;
+}

2023/src/day13.hpp

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+/******************************************************************************
+ * File:        day13.hpp
+ *
+ * Author:      Eric T. Johnson (yut23)
+ * Created:     2023-12-13
+ *****************************************************************************/
+
+#include "lib.hpp"
+#include <cassert>  // for assert
+#include <cstddef>  // for size_t
+#include <iostream> // for istream
+#include <string>   // for string, getline
+#include <vector>   // for vector
+
+namespace aoc::day13 {
+
+using Grid = std::vector<std::string>;
+
+Grid transpose(const Grid &grid) {
+    if (grid.size() == 0) {
+        return {};
+    }
+    Grid new_grid(grid[0].size());
+    for (const std::string &row : grid) {
+        for (std::size_t i = 0; i < row.size(); ++i) {
+            new_grid[i].push_back(row[i]);
+        }
+    }
+    return new_grid;
+}
+
+/// returns 0 if no reflection is found
+std::size_t find_reflection(const Grid &grid) {
+    // just check every row to see if it's a line of reflection
+    // if this is a bottleneck, I'll improve it later
+    for (std::size_t i = 1; i < grid.size(); ++i) {
+        bool is_reflection = true;
+        for (int j = 0; i - j > 0 && i + j < grid.size(); ++j) {
+            if (grid[i - j - 1] != grid[i + j]) {
+                is_reflection = false;
+                break;
+            }
+        }
+        if (is_reflection) {
+            return i;
+        }
+    }
+    return 0;
+}
+
+std::vector<Grid> read_grids(std::istream &is) {
+    std::vector<Grid> grids(1);
+    std::string line;
+    Grid curr_grid;
+    while (std::getline(is, line)) {
+        if (line.empty()) {
+            grids.emplace_back();
+            continue;
+        } else {
+            grids.back().push_back(std::move(line));
+        }
+    }
+    return grids;
+}
+
+} // namespace aoc::day13