Initial C++ version of Partridge solver

This is almost a complete copy of the OCaml version

About twice as fast.
This commit is contained in:
2025-08-31 17:04:59 +01:00
commit 2423f39c57
8 changed files with 299 additions and 0 deletions

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cmake-build*/

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partridge_cpp

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cmake_minimum_required(VERSION 4.0)
project(partridge_cpp)
set(CMAKE_CXX_STANDARD 20)
add_executable(partridge_cpp
main.cc)

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//
// Created by Matthew Gretton-Dann on 31/08/2025.
//
#include <cassert>
#include <utility>
#include <string>
#include <string_view>
#include <vector>
#include <iostream>
using Pos = std::pair<int, int>;
auto x(Pos const& p) noexcept -> int { return p.first; }
auto y(Pos const& p) noexcept -> int { return p.second; }
struct Square {
Square(Pos const& pos, int length) noexcept : pos_(pos), length_(length) {}
Square(Square const& other) noexcept = default;
Square(Square&& other) noexcept = default;
Square& operator=(Square const& other) noexcept = default;
Square& operator=(Square&& other) noexcept = default;
~Square() noexcept = default;
auto x() const noexcept -> int { return ::x(pos_); }
auto y() const noexcept -> int { return ::y(pos_); }
auto length() const noexcept -> int { return length_; }
Pos pos_;
int length_;
};
struct OverlappingSquares { Pos pos_; };
struct Grid {
Grid(int length) : grid_(length * length, '.'), length_(length) {}
Grid(Grid const& other) = default;
Grid(Grid&& other) noexcept = default;
Grid& operator=(Grid const& other) = default;
Grid& operator=(Grid&& other) noexcept = default;
~Grid() noexcept = default;
auto length() const noexcept -> int { return length_; }
auto set(int x, int y, char c) noexcept -> void {
assert (x < length_);
assert (y < length_);
assert (grid_[x + y * length_] != c);
grid_[x + y * length_] = c;
}
auto get(int x, int y) const noexcept -> char {
assert (x < length_);
assert (y < length_);
return grid_[x + y * length_];
}
auto add(Square const& sq) -> void {
switch (sq.length()) {
case 1: set(sq.x(), sq.y(), '*'); break;
case 2: set(sq.x(), sq.y(), '+');
set(sq.x() + 1, sq.y(), '+');set(sq.x(), sq.y() + 1, '+');set(sq.x() + 1, sq.y() + 1, '+');
break;
default: {
auto n = sq.length();
set(sq.x(), sq.y(), '+');
set(sq.x() + n - 1, sq.y(), '+');
set(sq.x(), sq.y() + n -1, '+');
set(sq.x() + n - 1, sq.y() + n -1, '+');
for (int i = 1; i < n -1; ++i) {
set(sq.x() + i, sq.y(), '-');
set(sq.x() + i, sq.y() + n - 1, '-');
set(sq.x(), sq.y() + i, '|');
for (int j = 1; j < n -1; ++j) {
set (sq.x() + j, sq.y() + i, ' ');
}
set(sq.x() + n - 1, sq.y() + i, '|');
}
int i = sq.x() + n - 1;
while (n != 0) {
set(--i, sq.y() + 1, '0' + (n % 10));
n /= 10;
}
}
}
}
auto clear(Square const& sq) {
for (auto i = sq.x(); i < sq.x() + sq.length(); ++i) {
for (auto j = sq.y(); j < sq.y() + sq.length(); ++j) {
set(i, j, '.');
}
}
}
auto output() const {
for (auto i = 0; i < length_; ++i) {
std::cout << grid_.substr(i * length_, length_) << '\n';
}
}
auto fits(Square const& sq) const noexcept -> bool {
assert (sq.x() < length_);
assert (sq.y() < length_);
if (sq.x() + sq.length() > length_) { return false; }
else if (sq.y() + sq.length() > length_) { return false; }
else {
auto pos = grid_.begin() + sq.x() + sq.y() * length_;
std::string_view v(pos, pos + sq.length());
return v.find_first_not_of('.') == std::string_view::npos;
}
}
auto next_pos(Pos const& pos) const noexcept -> Pos {
if (const auto p = x(pos) + 1 + y(pos) * length_; p >= grid_.size()) { return std::make_pair(0, length_); }
else {
const auto next_p = grid_.find('.', p);
if (next_p == std::string::npos) {
return std::make_pair(0, length_);
}
return std::make_pair(next_p % length_, next_p / length_);
}
}
std::string grid_;
int length_;
};
auto triangle_num(int n) { return (n * (n + 1)) / 2; }
using Avail = std::vector<int>;
auto find_solution_impl(Grid& grid, int n, Pos const& pos, int idx, Avail& avail_sqs)-> bool {
auto const sq = Square(pos, idx);
if (x(pos) == 0 && y(pos) == grid.length()) { return true; }
if (idx == 0) { return false; }
if (avail_sqs[idx] == 0) { return find_solution_impl(grid, n, pos, idx - 1, avail_sqs); }
if (grid.fits(sq)) {
--avail_sqs[idx];
grid.add(sq);
auto next_pos = grid.next_pos(pos);
auto const result = find_solution_impl(grid, n, next_pos, n, avail_sqs);
++avail_sqs[idx];
if (result) { return true; }
grid.clear(sq);
return find_solution_impl(grid, n, pos, idx - 1, avail_sqs);
}
return find_solution_impl(grid, n, pos, idx - 1, avail_sqs);
}
auto find_solution(int n) -> Grid{
auto length = triangle_num(n);
Grid grid(length);
Avail avail_sqs;
for (auto i = 0; i <= n; ++i) { avail_sqs.push_back(i); }
find_solution_impl(grid, n, std::make_pair(0, 0), n, avail_sqs);
return grid;
}
int main(int argc, char** argv)
{
auto n = (argc == 1) ? 9 : atoi(argv[1]);
auto grid = find_solution(n);
std::cout << "Partridge problem " << n << " side length " << grid.length() << '\n';
grid.output();
return 0;
}