ref: 1e6e3a815eb67a0d0d369fd0971cf9f3fd9fbf9a
parent: 59ac8a69af1330a6ea482154b05bc23ec33b594d
author: Simon Tatham <anakin@pobox.com>
date: Thu Apr 11 15:39:03 EDT 2019
Dominosa: be more careful about >= Hard layout. Now we don't just ensure that alloc_try_hard arranged a confounder for every domino; we also make sure that the full Basic-mode solver can't place even a single domino with certainty.
--- a/dominosa.c
+++ b/dominosa.c
@@ -1916,9 +1916,45 @@
if (!alloc_try_unique(as, rs))
continue;
} else {- /* Try to arrange that there is no easy starting point */
+ /*
+ * For Hard puzzles and above, we'd like there not to be
+ * any easy toehold to start with.
+ *
+ * Mostly, that's arranged by alloc_try_hard, which will
+ * ensure that no domino starts off with only one
+ * potential placement. But a few other deductions
+ * possible at Basic level can still sneak through the
+ * cracks - for example, if the only two placements of one
+ * domino overlap in a square, and you therefore rule out
+ * some other domino that can use that square, you might
+ * then find that _that_ domino now has only one
+ * placement, and you've made a start.
+ *
+ * Of course, the main difficulty-level check will still
+ * guarantee that you have to do a harder deduction
+ * _somewhere_ in the grid. But it's more elegant if
+ * there's nowhere obvious to get started at all.
+ */
+ int di;
+ bool ok;
+
if (!alloc_try_hard(as, rs))
continue;
+
+ solver_setup_grid(sc, as->numbers);
+ if (run_solver(sc, DIFF_BASIC) < 2)
+ continue;
+
+ ok = true;
+ for (di = 0; di < sc->dc; di++)
+ if (sc->dominoes[di].nplacements <= 1) {+ ok = false;
+ break;
+ }
+
+ if (!ok) {+ continue;
+ }
}
if (diff != DIFF_AMBIGUOUS) {