mgrettondann/partridge-cpp
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Optimize selection of next position.

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Grid::next_pos was starting its scan from the position after the current
 one.

This is inefficient as we know how large the square we've just placed is
 as we scan those positions.

This optimisation just skips that square when starting the scan.
This commit is contained in:
Matthew Gretton-Dann
2025-09-01 11:00:56 +02:00
parent 3095d38b8a
commit d7b1290379
1 changed files with 6 additions and 4 deletions
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10
main.cc
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@@ -140,14 +140,16 @@ struct Grid {
return b - x(pos);
}
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 {
/** Get the next position to check. n is the size of the square we just
* added.
*/
auto next_pos(Pos const &pos, int n) const noexcept -> Pos {
const auto p = x(pos) + n + y(pos) * length_;
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_;
@@ -176,7 +178,7 @@ auto find_solution_impl(Grid &grid, int n, Pos const &pos, Avail &avail_sqs) ->
auto const sq = Square(pos, idx);
--avail_sqs[idx];
grid.add(sq);
if (find_solution_impl(grid, n, grid.next_pos(pos), avail_sqs)) { return true; }
if (find_solution_impl(grid, n, grid.next_pos(pos, idx), avail_sqs)) { return true; }
++avail_sqs[idx];
grid.clear(sq);
}