2 files changed, 114 insertions, 1 deletions

diff --git a/makefile b/makefile
index 539cc3c..0ce0a6f 100644
--- a/makefile
+++ b/makefile
@@ -15,7 +15,8 @@ all: bin/day01 \
          bin/day13 \
          bin/day14 \
          bin/day15 \
-         bin/day16
+         bin/day16 \
+         bin/day17
 
 bin:
         mkdir $@
diff --git a/src/day17.cpp b/src/day17.cpp
new file mode 100644
index 0000000..4098bba
--- /dev/null
+++ b/src/day17.cpp
@@ -0,0 +1,112 @@
+#include <array>
+#include <fstream>
+#include <functional>
+#include <iostream>
+#include <queue>
+#include <set>
+#include <string>
+#include <vector>
+using namespace std;
+
+using puzzle_t = vector<vector<int>>;
+
+puzzle_t
+read_file(string_view filename)
+{
+        fstream  input{ filename };
+        puzzle_t data;
+
+        for ( string line; getline(input, line); ) {
+                puzzle_t::value_type numbers;
+                for ( auto chr: line ) {
+                        numbers.emplace_back(chr - '0');
+                }
+                data.emplace_back(numbers);
+        }
+
+        return data;
+}
+
+int
+solve(const puzzle_t& puzzle, size_t min_steps, size_t max_steps)
+{
+        using pos_t = tuple<size_t, size_t>;
+        using dir_t = tuple<size_t, size_t>;
+        using entry = tuple<int, pos_t, dir_t>;
+
+        static const array<pos_t, 4> directions = {
+                make_tuple(0, 1),
+                make_tuple(0, -1),
+                make_tuple(1, 0),
+                make_tuple(-1, 0)
+        };
+
+        priority_queue<entry, vector<entry>, greater<>> queue;
+
+        set<tuple<pos_t, dir_t>> seen;
+
+        const pos_t target = { puzzle.size() - 1, puzzle[0].size() - 1 };
+
+        queue.emplace(0, pos_t(0, 0), dir_t(0, 0));
+
+        while ( !queue.empty() ) {
+                const auto [heat, pos, dir] = queue.top();
+                queue.pop();
+
+                if ( pos == target ) {
+                        return heat;
+                }
+
+                const auto key = make_tuple(pos, dir);
+                if ( seen.contains(key) ) {
+                        continue;
+                }
+                seen.emplace(key);
+
+                const dir_t inv_dir = { -get<0>(dir), -get<1>(dir) };
+
+                for ( const auto& next_dir: directions ) {
+                        if ( next_dir == dir || next_dir == inv_dir ) {
+                                continue;
+                        }
+
+                        auto heat_so_far = heat;
+
+                        for ( size_t steps = 1; steps <= max_steps; ++steps ) {
+                                const pos_t next_pos = { get<0>(pos) + get<0>(next_dir) * steps,
+                                                             get<1>(pos) + get<1>(next_dir) * steps };
+
+                                if ( get<0>(next_pos) >= puzzle.size() || get<1>(next_pos) >= puzzle[0].size() ) {
+                                        continue;
+                                }
+
+                                heat_so_far += puzzle[get<0>(next_pos)][get<1>(next_pos)];
+
+                                if ( steps >= min_steps ) {
+                                        queue.emplace(heat_so_far, next_pos, next_dir);
+                                }
+                        }
+                }
+        }
+        return -1;
+}
+
+void
+part1(const puzzle_t& puzzle)
+{
+        cout << solve(puzzle, 1, 3) << endl;
+}
+
+void
+part2(const puzzle_t& puzzle)
+{
+        cout << solve(puzzle, 4, 10) << endl;
+}
+
+int
+main()
+{
+        const auto puzzle = read_file("data/day17.txt");
+        part1(puzzle);
+        part2(puzzle);
+}