1 files changed, 216 insertions, 0 deletions

diff --git a/2023/src/day10.cpp b/2023/src/day10.cpp
new file mode 100644
index 0000000..264e111
--- /dev/null
+++ b/2023/src/day10.cpp
@@ -0,0 +1,216 @@
+#include <algorithm>
+#include <fstream>
+#include <iostream>
+#include <sstream>
+#include <string>
+#include <vector>
+using namespace std;
+
+using pos_t    = tuple<size_t, size_t>;
+using puzzle_t = vector<string>;
+
+puzzle_t
+read_file(string_view filename)
+{
+        fstream  input{ filename };
+        puzzle_t data;
+
+        for ( string line; getline(input, line); ) {
+                data.emplace_back(line);
+        }
+
+        return data;
+}
+
+pos_t
+find_start_pos(const puzzle_t& puzzle)
+{
+        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 };
+};
+
+bool
+predict_direction(const puzzle_t& puzzle, pos_t& current, char& direction)
+{
+        auto [x, y] = current;
+
+        if ( direction == 'S' ) {
+                ++y;
+                if ( y >= puzzle.size() ) {
+                        return false;
+                }
+                const 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;
+                }
+                const 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;
+                }
+                const 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;
+                }
+                const 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;
+};
+
+void
+part1(const puzzle_t& puzzle)
+{
+        const auto start_pos = find_start_pos(puzzle);
+
+        auto max_steps = 0;
+
+        for ( const auto direction: { 'N', 'S', 'E', 'W' } ) {
+                auto current_direction = direction;
+                auto current_pos       = start_pos;
+
+                auto steps = 0;
+                for ( ;; ) {
+                        ++steps;
+
+                        if ( !predict_direction(puzzle, current_pos, current_direction) ) {
+                                break;
+                        }
+                }
+
+                max_steps = max(max_steps, steps);
+        }
+
+        cout << max_steps / 2 << endl;
+}
+
+bool
+flood_fill(puzzle_t& puzzle, size_t x, size_t y)
+{
+        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;
+};
+
+void
+part2(const puzzle_t& puzzle_original)
+{
+        puzzle_t puzzle{ puzzle_original.size(), string(puzzle_original[0].size(), ' ') };
+
+        const auto start_pos = find_start_pos(puzzle_original);
+
+        for ( const auto direction: { 'N', 'S', 'E', 'W' } ) {
+                auto current_direction = direction;
+                auto current_pos       = start_pos;
+
+                for ( ;; ) {
+                        auto [x, y] = current_pos;
+
+                        puzzle[y][x] = puzzle_original[y][x];
+
+                        if ( !predict_direction(puzzle_original, current_pos, current_direction) ) {
+                                break;
+                        }
+                }
+        }
+
+        for ( size_t y = 0; y != puzzle.size(); ++y ) {        // NOLINT
+                for ( size_t x = 0; x != puzzle[y].size(); ++x ) { // NOLINT
+                        auto test_puzzle{ puzzle };
+
+                        if ( flood_fill(test_puzzle, x, y) ) {
+                                puzzle = test_puzzle;
+                        }
+                }
+        }
+
+        auto sum = 0;
+        for ( auto& line: puzzle ) {
+                auto pipes = 0U;
+                for ( auto chr: line ) {
+                        if ( chr == '|' || chr == 'L' || chr == 'J' ) {
+                                ++pipes;
+                        }
+                        if ( chr == 'o' && (pipes & 1U) == 1U ) {
+                                ++sum;
+                        }
+                }
+        }
+
+        cout << sum << endl;
+}
+
+int
+main()
+{
+        const auto puzzle = read_file("data/day10.txt");
+        part1(puzzle);
+        part2(puzzle);
+}