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, 8 insertions, 7 deletions

diff --git a/unfinished/group.c b/unfinished/group.c
index e72e3ce..2eca7c4 100644
--- a/unfinished/group.c
+++ b/unfinished/group.c
@@ -580,13 +580,11 @@ static int solver(const game_params *params, digit *grid, int maxdiff)
     int w = params->w;
     int ret;
     struct latin_solver solver;
+
 #ifdef STANDALONE_SOLVER
     char *p, text[100], *names[50];
     int i;
-#endif
 
-    latin_solver_alloc(&solver, grid, w);
-#ifdef STANDALONE_SOLVER
     for (i = 0, p = text; i < w; i++) {
 	names[i] = p;
 	*p++ = TOCHAR(i+1, params->id);
@@ -595,10 +593,13 @@ static int solver(const game_params *params, digit *grid, int maxdiff)
     solver.names = names;
 #endif
 
-    ret = latin_solver_main(&solver, maxdiff,
-			    DIFF_TRIVIAL, DIFF_HARD, DIFF_EXTREME,
-			    DIFF_EXTREME, DIFF_UNREASONABLE,
-			    group_solvers, group_valid, NULL, NULL, NULL);
+    if (latin_solver_alloc(&solver, grid, w))
+        ret = latin_solver_main(&solver, maxdiff,
+                                DIFF_TRIVIAL, DIFF_HARD, DIFF_EXTREME,
+                                DIFF_EXTREME, DIFF_UNREASONABLE,
+                                group_solvers, group_valid, NULL, NULL, NULL);
+    else
+        ret = diff_impossible;
 
     latin_solver_free(&solver);