375 lines
10 KiB
C++
375 lines
10 KiB
C++
#include <algorithm>
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#include <cassert>
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#include <cstdint>
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#include <cstdlib>
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#include <iostream>
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#include <string>
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#include <utility>
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#include <vector>
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using Int = std::int64_t;
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using NumVec = std::vector<Int>;
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using Pos = std::pair<std::size_t, std::size_t>; // (x, y) pair
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[[nodiscard]] constexpr auto operator==(Pos const& lhs, Pos const& rhs) -> bool
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{
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return lhs.first == rhs.first && lhs.second == rhs.second;
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}
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struct Grid;
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auto operator<<(std::ostream& os, Grid const& grid) -> std::ostream&;
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struct Grid
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{
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void push_line(std::string const& line)
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{
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if (auto x{line.find('S')}; x != std::string::npos) {
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assert(animal_.first == std::string::npos);
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animal_.first = x;
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animal_.second = grid_.size();
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}
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grid_.push_back(line);
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}
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[[nodiscard]] auto animal_pos() const -> Pos { return animal_; }
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[[nodiscard]] auto width() const -> std::size_t { return grid_.front().size(); }
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[[nodiscard]] auto height() const -> std::size_t { return grid_.size(); }
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[[nodiscard]] auto at(Pos const& pos) const -> char { return grid_[pos.second][pos.first]; }
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void set(Pos const& pos, char c) { grid_[pos.second][pos.first] = c; }
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[[nodiscard]] auto walk_pipes(Pos const& start, Pos pos) const -> std::pair<Pos, std::size_t>
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{
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auto last{start};
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std::size_t steps{0};
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while (true) {
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auto const [x, y] = pos;
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auto const c{grid_[y][x]};
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if (c == '.' || c == 'S') { break; }
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if (start == pos) { break; }
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if (c == '|' || c == 'L' || c == 'J') {
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if (Pos const next_pos{x, y - 1}; y > 0 && next_pos != last) {
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last = pos;
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pos = next_pos;
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++steps;
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continue;
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}
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}
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if (c == '|' || c == 'F' || c == '7') {
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if (Pos const next_pos{x, y + 1}; y < height() - 1 && next_pos != last) {
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last = pos;
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pos = next_pos;
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++steps;
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continue;
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}
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}
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if (c == '-' || c == '7' || c == 'J') {
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if (Pos const next_pos{x - 1, y}; x > 0 && next_pos != last) {
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last = pos;
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pos = next_pos;
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++steps;
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continue;
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}
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}
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if (c == '-' || c == 'L' || c == 'F') {
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if (Pos const next_pos{x + 1, y}; x < width() - 1 && next_pos != last) {
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last = pos;
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pos = next_pos;
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++steps;
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continue;
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}
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}
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std::abort();
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}
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return std::make_pair(pos, steps);
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}
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void hightlight_pipes(Pos const& start)
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{
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auto last{start};
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auto pos{start};
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auto const c{at(start)};
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if (c == '|' || c == 'L' || c == 'J') { pos.second -= 1; }
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else if (c == 'F' || c == '7') { pos.second += 1; }
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else if (c == '-') { pos.first += 1; }
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else { std::abort(); }
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set(start, c == '|' ? 'v' : '>');
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while (start != pos) {
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auto const [x, y] = pos;
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auto const c{grid_[y][x]};
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if (c == '.') { std::abort(); }
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grid_[y][x] = c == '|' ? 'v' : '>';
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if (c == '|' || c == 'L' || c == 'J') {
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if (Pos const next_pos{x, y - 1}; y > 0 && next_pos != last) {
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last = pos;
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pos = next_pos;
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continue;
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}
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}
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if (c == '|' || c == 'F' || c == '7') {
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if (Pos const next_pos{x, y + 1}; y < height() - 1 && next_pos != last) {
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last = pos;
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pos = next_pos;
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continue;
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}
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}
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if (c == '-' || c == '7' || c == 'J') {
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if (Pos const next_pos{x - 1, y}; x > 0 && next_pos != last) {
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last = pos;
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pos = next_pos;
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continue;
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}
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}
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if (c == '-' || c == 'L' || c == 'F') {
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if (Pos const next_pos{x + 1, y}; x < width() - 1 && next_pos != last) {
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last = pos;
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pos = next_pos;
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continue;
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}
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}
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std::abort();
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}
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}
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auto flood_fill()
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{
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/* Ensure every character is either an edge, or a '.' for outside, maybe inside, and a '*'
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* for the initial starting locations.
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*/
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for (auto& row : grid_) {
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/* We know we're inside if we've passed an odd number of vertical walls and no corners/
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* horizontal walls.
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*/
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bool inside = false;
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bool definite = true;
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for (auto& c : row) {
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if (c == 'v') { inside = !inside; }
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else if (c == '>') { definite = false; }
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else if (inside && definite) {
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c = '*';
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}
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else { c = '.'; }
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}
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}
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/* Very simple algorithm, that just iterates over all squares until we don't change anything */
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bool again{true};
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while (again) {
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again = false;
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for (std::size_t y{0}; y != height(); ++y) {
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for (std::size_t x{0}; x != width(); ++x) {
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if (grid_[y][x] != '.') { continue; }
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if ((y > 0 && grid_[y - 1][x] == '*') || (x > 0 && grid_[y][x - 1] == '*') || (
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y < height() - 1 && grid_[y + 1][x] == '*') || (
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x < width() - 1 && grid_[y][x + 1] == '*')) {
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grid_[y][x] = '*';
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again = true;
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}
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}
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}
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}
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}
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/* Shrink the grid by a factor of two in all directions. We take the even places to be the new
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* entry in the new grid.
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*
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* So grid.enlarge().shrink() should produce the same grid as grid.
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*/
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[[nodiscard]] auto shrink() const -> Grid
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{
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Grid new_grid;
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for (std::size_t y{0}; y < height(); y += 2) {
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std::string line;
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for (std::size_t x{0}; x < width(); x += 2) {
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line += grid_[y][x];
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}
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new_grid.push_line(line);
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}
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return new_grid;
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}
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/* Enlarge the grid by a factor of two in each direction.
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*
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* The contents of grid_[y][x] are mapped to grid_[y * 2][x * 2] and the appropriate additional
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* characters are added.
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*/
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[[nodiscard]] auto enlarge() const -> Grid
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{
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Grid new_grid;
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for (auto const& row : grid_) {
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std::string line1;
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std::string line2;
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for (auto const c : row) {
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if (c == '.') {
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line1 += "..";
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line2 += "..";
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}
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if (c == '-') {
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line1 += "--";
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line2 += "..";
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}
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if (c == '7') {
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line1 += "7.";
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line2 += "|.";
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}
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if (c == 'F') {
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line1 += "F-";
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line2 += "|.";
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}
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if (c == '|') {
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line1 += "|.";
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line2 += "|.";
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}
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if (c == 'J') {
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line1 += "J.";
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line2 += "..";
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}
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if (c == 'L') {
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line1 += "L-";
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line2 += "..";
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}
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}
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new_grid.push_line(line1);
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new_grid.push_line(line2);
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}
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return new_grid;
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}
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[[nodiscard]] auto count_interior() const -> std::size_t
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{
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std::size_t interior{0};
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for (auto const& row : grid_) {
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interior += std::ranges::count_if(row, [](char const c) { return c == '*'; });
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}
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return interior;
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}
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friend auto operator<<(std::ostream& os, Grid const& grid) -> std::ostream&;
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private:
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std::vector<std::string> grid_;
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Pos animal_{std::string::npos, std::string::npos};
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};
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auto operator<<(std::ostream& os, Grid const& grid) -> std::ostream&
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{
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for (auto const& row : grid.grid_) {
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os << row << '\n';
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}
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return os;
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}
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auto main() -> int try {
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std::string line;
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Grid grid;
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while (std::getline(std::cin, line)) {
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grid.push_line(line);
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}
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auto animal = grid.animal_pos();
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/* Find the loop in both directions as this tells us what the animal pipe piece should be. */
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bool east{false};
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bool south{false};
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bool west{false};
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bool north{false};
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if (animal.second > 0) {
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Pos const next_pos{animal.first, animal.second - 1};
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if (auto const c{grid.at(next_pos)}; c == '|' || c == 'F' || c == '7') {
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auto [end_pos, length] = grid.walk_pipes(animal, next_pos);
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if (end_pos == animal) {
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std::cout << "Started going north ended back at animal: " << length / 2 + 1 << '\n';
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north = true;
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}
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}
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}
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if (animal.first > 0) {
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Pos const next_pos{animal.first - 1, animal.second};
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if (auto const c{grid.at(next_pos)}; c == '-' || c == 'F' || c == 'L') {
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auto [end_pos, length] = grid.walk_pipes(animal, next_pos);
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if (end_pos == animal) {
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std::cout << "Started going west ended back at animal: " << length / 2 + 1 << '\n';
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west = true;
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}
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}
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}
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if (animal.second < grid.height() - 1) {
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Pos const next_pos{animal.first, animal.second + 1};
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if (auto const c{grid.at(next_pos)}; c == '|' || c == 'J' || c == 'L') {
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auto [end_pos, length] = grid.walk_pipes(animal, next_pos);
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if (end_pos == animal) {
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std::cout << "Started going south ended back at animal: " << length / 2 + 1 << '\n';
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south = true;
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}
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}
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}
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if (animal.first < grid.width() - 1) {
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Pos const next_pos{animal.first + 1, animal.second};
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if (auto const c{grid.at(next_pos)}; c == '-' || c == 'J' || c == '7') {
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auto [end_pos, length] = grid.walk_pipes(animal, next_pos);
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if (end_pos == animal) {
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std::cout << "Started going east ended back at animal: " << length / 2 + 1 << '\n';
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east = true;
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}
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}
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}
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/* Replace animal with pipe piece. */
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if (north && south) { grid.set(animal, '|'); }
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else if (east && west) { grid.set(animal, '-'); }
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else if (north && east) { grid.set(animal, 'L'); }
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else if (north && west) { grid.set(animal, 'J'); }
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else if (south && east) { grid.set(animal, 'F'); }
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else if (south && west) { grid.set(animal, '7'); }
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else {
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std::abort();
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}
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/* To ensure that the interior is stongly connected we double the size of the grid in both
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* directions. This makes sure that there is always a . between two pipe edges.
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*
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* This then means we can do a very simple flood fill to find the interior.
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*
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* We then shrink the grid again and to get the interior count we want.
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*/
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auto big_grid{grid.enlarge()};
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// Mark the pipes we want to work with using > & v characters
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big_grid.hightlight_pipes(Pos{animal.first * 2, animal.second * 2});
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std::cout << "Before flood fill:\n" << big_grid;
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// Flood fill the interior with * characters
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big_grid.flood_fill();
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std::cout << "After flood fill:\n" << big_grid;
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// Shrink
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auto const small_grid = big_grid.shrink();
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// Count the number of * in the interior.
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auto const interior_count = small_grid.count_interior();
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std::cout << "Reconstituted grid\n" << small_grid;
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std::cout << "Interior count: " << interior_count << "\n";
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return EXIT_SUCCESS;;
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}
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catch (...) {
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std::cerr << "Uncaught exception.\n";
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return EXIT_FAILURE;
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}
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