1 files changed, 163 insertions, 0 deletions

diff --git a/src/day10.cpp b/src/day10.cpp
index b9d6355..6b0b28e 100644
--- a/src/day10.cpp
+++ b/src/day10.cpp
@@ -1,3 +1,4 @@
+#include <algorithm>
 #include <fstream>
 #include <iostream>
 #include <sstream>
@@ -137,8 +138,170 @@ part1()
         cout << max_steps / 2 << endl;
 }
 
+void
+part2()
+{
+        typedef tuple<size_t, size_t> pos_t;
+
+        const auto puzzle = read_file("data/day10.txt");
+
+        const auto find_start_pos = [&]() -> pos_t {
+                for ( size_t y = 0; y != puzzle.size(); ++y ) { // NOLINT
+                        auto x = puzzle[y].find('S');               // NOLINT
+                        if ( x != string::npos ) {
+                                return { x, y };
+                        }
+                }
+
+                return { 0, 0 };
+        };
+
+        const auto predict_direction = [&](pos_t& current, char& direction) -> bool {
+                auto x = get<0>(current); // NOLINT
+                auto y = get<1>(current); // NOLINT
+
+                if ( direction == 'S' ) {
+                        ++y;
+                        if ( y >= puzzle.size() ) {
+                                return false;
+                        }
+                        auto tile = puzzle[y][x];
+                        if ( tile != 'J' && tile != '|' && tile != 'L' ) {
+                                return false;
+                        }
+                        if ( tile == 'J' ) {
+                                direction = 'W';
+                        }
+                        else if ( tile == 'L' ) {
+                                direction = 'E';
+                        }
+                }
+                else if ( direction == 'N' ) {
+                        --y;
+                        if ( y >= puzzle.size() ) {
+                                return false;
+                        }
+                        auto tile = puzzle[y][x];
+                        if ( tile != '7' && tile != '|' && tile != 'F' ) {
+                                return false;
+                        }
+                        if ( tile == '7' ) {
+                                direction = 'W';
+                        }
+                        else if ( tile == 'F' ) {
+                                direction = 'E';
+                        }
+                }
+                else if ( direction == 'E' ) {
+                        ++x;
+                        if ( x >= puzzle[y].size() ) {
+                                return false;
+                        }
+                        auto tile = puzzle[y][x];
+                        if ( tile != 'J' && tile != '-' && tile != '7' ) {
+                                return false;
+                        }
+                        if ( tile == 'J' ) {
+                                direction = 'N';
+                        }
+                        else if ( tile == '7' ) {
+                                direction = 'S';
+                        }
+                }
+                else if ( direction == 'W' ) {
+                        --x;
+                        if ( x >= puzzle[y].size() ) {
+                                return false;
+                        }
+                        auto tile = puzzle[y][x];
+                        if ( tile != 'L' && tile != '-' && tile != 'F' ) {
+                                return false;
+                        }
+                        if ( tile == 'L' ) {
+                                direction = 'N';
+                        }
+                        else if ( tile == 'F' ) {
+                                direction = 'S';
+                        }
+                }
+                else {
+                        cerr << "invalid direction " << direction << ")!" << endl;
+                        return false;
+                }
+
+                current = { x, y };
+                return true;
+        };
+
+        vector<string> puzzle_copy(puzzle.size(), string(puzzle[0].size(), ' '));
+
+        const auto start_pos = find_start_pos();
+
+        for ( const auto direction: { 'N', 'S', 'E', 'W' } ) {
+                auto current_direction = direction;
+                auto current_pos       = start_pos;
+
+                for ( ;; ) {
+                        auto x = get<0>(current_pos); // NOLINT
+                        auto y = get<1>(current_pos); // NOLINT
+
+                        puzzle_copy[y][x] = puzzle[y][x];
+
+                        if ( !predict_direction(current_pos, current_direction) ) {
+                                break;
+                        }
+                }
+        }
+
+        function<bool(vector<string>&, pos_t)> flood_fill = [&](vector<string>& puzzle, pos_t position) -> bool {
+                auto x = get<0>(position); // NOLINT
+                auto y = get<1>(position); // NOLINT
+
+                if ( y >= puzzle.size() || x >= puzzle[0].size() ) {
+                        return false;
+                }
+
+                if ( puzzle[y][x] == ' ' ) {
+                        puzzle[y][x] = 'o';
+                        if ( !flood_fill(puzzle, { x, y + 1 }) ||
+                             !flood_fill(puzzle, { x, y - 1 }) ||
+                             !flood_fill(puzzle, { x + 1, y }) ||
+                             !flood_fill(puzzle, { x - 1, y }) ) {
+                                return false;
+                        }
+                }
+
+                return true;
+        };
+
+        for ( size_t y = 0; y != puzzle_copy.size(); ++y ) {
+                for ( size_t x = 0; x != puzzle_copy[y].size(); ++x ) {
+                        auto test_puzzle = puzzle_copy;
+
+                        if ( flood_fill(test_puzzle, { x, y }) ) {
+                                puzzle_copy = test_puzzle;
+                        }
+                }
+        }
+
+        int sum = 0;
+        for ( auto& line: puzzle_copy ) {
+                auto pipes = 0U;
+                for ( char chr: line ) {
+                        if ( chr == '|' || chr == 'L' || chr == 'J' ) {
+                                ++pipes;
+                        }
+                        if ( chr == 'o' && (pipes & 1U) == 1U ) {
+                                ++sum;
+                        }
+                }
+        }
+        cout << sum << endl;
+}
+
 int
 main()
 {
         part1();
+        part2();
 }