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, 6 insertions, 3 deletions

diff --git a/latin.h b/latin.h
index bb172ec..96a0480 100644
--- a/latin.h
+++ b/latin.h
@@ -61,10 +61,13 @@ int latin_solver_forcing(struct latin_solver *solver,
 /* --- Solver allocation --- */
 
 /* Fills in (and allocates members for) a latin_solver struct.
- * Will allocate members of snew, but not snew itself
+ * Will allocate members of solver, but not solver itself
  * (allowing 'struct latin_solver' to be the first element in a larger
- * struct, for example). */
-void latin_solver_alloc(struct latin_solver *solver, digit *grid, int o);
+ * struct, for example).
+ *
+ * latin_solver_alloc returns false if the digits already in the grid
+ * could not be legally placed. */
+bool latin_solver_alloc(struct latin_solver *solver, digit *grid, int o);
 void latin_solver_free(struct latin_solver *solver);
 
 /* Allocates scratch space (for _set and _forcing) */