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#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);
}
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