latin_solver_alloc: handle clashing numbers in input grid.

In the setup phase of the centralised latin.c solver, we start by
going over the input grid containing already-placed clue numbers, and
calling latin_solver_place to enter each on into the solver's data
structure. This has the side effect of ruling out each number from the
rest of the row and column, and _also_ checking by assertion that the
number being placed is not ruled out.

Those are a bad combination, because it means that if you give an
obviously inconsistent input grid to latin_solver_alloc (e.g. with two
identical numbers in a row already), it will fail an assertion. In
that situation, you want the solver run as a whole to return
diff_impossible so that the error is reported cleanly.

This assertion failure could be provoked by giving either Towers or
Group a manually-constructed game description inconsistent in that
way, and hitting Solve. Worse, it could be provoked during live play
in Unequal, by filling in a number clashing with a clue and then
pressing 'h' to get hints.

1 files changed, 16 insertions, 14 deletions

diff --git a/unequal.c b/unequal.c
index ed03c0b..f2328b6 100644
--- a/unequal.c
+++ b/unequal.c
@@ -890,13 +890,14 @@ static int solver_state(game_state *state, int maxdiff)
     struct latin_solver solver;
     int diff;
 
-    latin_solver_alloc(&solver, state->nums, state->order);
-
-    diff = latin_solver_main(&solver, maxdiff,
-			     DIFF_LATIN, DIFF_SET, DIFF_EXTREME,
-			     DIFF_EXTREME, DIFF_RECURSIVE,
-			     unequal_solvers, unequal_valid, ctx,
-                             clone_ctx, free_ctx);
+    if (!latin_solver_alloc(&solver, state->nums, state->order))
+        diff = latin_solver_main(&solver, maxdiff,
+                                 DIFF_LATIN, DIFF_SET, DIFF_EXTREME,
+                                 DIFF_EXTREME, DIFF_RECURSIVE,
+                                 unequal_solvers, unequal_valid, ctx,
+                                 clone_ctx, free_ctx);
+    else
+        diff = DIFF_IMPOSSIBLE;
 
     memcpy(state->hints, solver.cube, state->order*state->order*state->order);
 
@@ -2256,13 +2257,14 @@ static int solve(game_params *p, char *desc, int debug)
     solver_show_working = debug;
     game_debug(state);
 
-    latin_solver_alloc(&solver, state->nums, state->order);
-
-    diff = latin_solver_main(&solver, DIFF_RECURSIVE,
-			     DIFF_LATIN, DIFF_SET, DIFF_EXTREME,
-			     DIFF_EXTREME, DIFF_RECURSIVE,
-			     unequal_solvers, unequal_valid, ctx,
-                             clone_ctx, free_ctx);
+    if (latin_solver_alloc(&solver, state->nums, state->order))
+        diff = latin_solver_main(&solver, DIFF_RECURSIVE,
+                                 DIFF_LATIN, DIFF_SET, DIFF_EXTREME,
+                                 DIFF_EXTREME, DIFF_RECURSIVE,
+                                 unequal_solvers, unequal_valid, ctx,
+                                 clone_ctx, free_ctx);
+    else
+        diff = DIFF_IMPOSSIBLE;
 
     free_ctx(ctx);