From f1445b754426df8b0863438fd0106c16bf629bbf Mon Sep 17 00:00:00 2001
From: Matthew Gretton-Dann <matt@gretton-dann.org.uk>
Date: Thu, 4 Sep 2025 11:12:51 +0200
Subject: [PATCH] Make next_pos iterate over indices.

---
 main.cc | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/main.cc b/main.cc
index 20b51ae..f92ab2a 100644
--- a/main.cc
+++ b/main.cc
@@ -145,6 +145,7 @@ namespace {
 
     /** Get grid length */
     auto length() const noexcept -> size_t { return length_; }
+    auto end() const noexcept -> size_t { return grid_.size(); }
 
     auto pos_x(Pos const& pos) const noexcept -> size_t { return pos % length_; }
     auto pos_y(Pos const& pos) const noexcept -> size_t { return pos / length_; }
@@ -195,14 +196,14 @@ namespace {
       return ye - pos_y(pos);
     }
 
-    /** Get the next position to check.  n is the size of the square we just
-     *  added.
+    /** Get the next position to check starting at pos.
+     *
+     * Returns grid_.length() if no more positions avaialble.
      */
-    auto next_pos(Pos const &pos, size_t n) const noexcept -> Pos {
-      auto const b = grid_.begin() + pos_x(pos) + n + pos_y(pos) * length_;
+    auto next_pos(Pos const &pos) const noexcept -> Pos {
+      auto const b = grid_.begin() + pos;
       auto const p = std::find(b, grid_.end(), empty);
-      auto const v = p - grid_.begin();
-      return v;
+      return p - grid_.begin();
     }
 
   private:
@@ -296,11 +297,11 @@ namespace {
       grid.add(sq);
       sqs.push_back(sq);
 
-      pos = grid.next_pos(pos, idx);
+      pos = grid.next_pos(pos + idx);
       idx = grid.largest_square(pos, n);
 
       // Have we reached the end?  If so success!
-      if (grid.pos_x(pos) == 0 && grid.pos_y(pos) == grid.length()) { break; }
+      if (pos == grid.end()) { break; }
     }
 
     return Results(length, sqs);