From ce39d0a4d051d14071ef658c8d5cd86a8eb8b22f Mon Sep 17 00:00:00 2001
From: Codex instance <matt+codex@gretton-dann.org.uk>
Date: Thu, 30 Jul 2026 16:50:19 +0100
Subject: [PATCH] solver: check completion before probing square

next_pos() returns the grid end sentinel when a placement completes the board. Avoid passing that sentinel to largest_square(), which requires an in-range position and otherwise reads past the grid.

Tests: Debug, Release, ASan, and UBSan CTest suites

Refs: #14
---
 main.cc | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/main.cc b/main.cc
index c71cae1..55e69f0 100644
--- a/main.cc
+++ b/main.cc
@@ -297,10 +297,11 @@ namespace {
       sqs.push_back(sq);
 
       pos = grid.next_pos(pos + idx);
-      idx = grid.largest_square(pos, n);
 
       // Have we reached the end?  If so success!
       if (pos == grid.end()) { break; }
+
+      idx = grid.largest_square(pos, n);
     }
 
     return {length, sqs};