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

CMakeLists.txt

@@ -15,4 +15,5 @@ if(BUILD_TESTING)
   add_test(NAME construction COMMAND partridge_tests construction)
   add_test(NAME rendering COMMAND partridge_tests rendering)
   add_test(NAME solver-small COMMAND partridge_tests solver-small)
+  add_test(NAME solver-completion COMMAND partridge_tests solver-completion)
 endif()

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};

tests/tests.cc

@@ -280,6 +280,19 @@ namespace {
         "result adapter did not convert encoded placement coordinates");
     return failures;
   }
+
+  auto test_solver_completion() -> int {
+    int failures = 0;
+    for (auto const order: std::array<std::uint64_t, 2>{1, 8}) {
+      auto const solution = find_solution(order);
+      auto const validation = validate(order, solution);
+      failures += expect(
+          validation.valid(),
+          "solver returned an invalid order-" + std::to_string(order) +
+              " solution:\n" + validation.text());
+    }
+    return failures;
+  }
 }
 
 int main(int argc, char **argv) {
@@ -301,6 +314,9 @@ int main(int argc, char **argv) {
   if (test == "solver-small") {
     return test_small_solver();
   }
+  if (test == "solver-completion") {
+    return test_solver_completion();
+  }
   std::cerr << "unknown test: " << test << '\n';
   return 2;
 }