Use float-epsilon calculation in Sabre best scores (#9117)

This should not meaningfully affect any small-scale outputs, but at
larger scales, there are floating-point instabilities that can make the
"best swaps" set miss swaps that are logically at the same score as
others.  This does not (cannot) fully stabilise us against all minor
floating-point rounding errors in calculations; the order of iteration
through the swaps can still technically change the swap set, for example
if three swaps with relative differences `{0, 0.8, 1.6} * BEST_EPSILON`
are encountered in a different orders.  The float epsilon is chosen to
be significantly larger than the actual magnitude of fluctuations, and
significantly smaller than the changes caused by all the actual
components of scoring, in order to mitigate this effect.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

src/sabre_swap/mod.rs

@@ -43,6 +43,8 @@ use neighbor_table::NeighborTable;
 use sabre_dag::SabreDAG;
 use swap_map::SwapMap;
 
+const BEST_EPSILON: f64 = 1e-10; // Epsilon used in minimum-score calculations.
+
 const EXTENDED_SET_SIZE: usize = 20; // Size of lookahead window.
 const DECAY_RATE: f64 = 0.001; // Decay coefficient for penalizing serial swaps.
 const DECAY_RESET_INTERVAL: u8 = 5; // How often to reset all decay rates to 1.
@@ -522,11 +524,11 @@ fn choose_best_swap(
                         + EXTENDED_SET_WEIGHT * extended_set.score(swap, layout, dist))
             }
         };
-        if score < min_score {
+        if score < min_score - BEST_EPSILON {
             min_score = score;
             best_swaps.clear();
             best_swaps.push(swap);
-        } else if score == min_score {
+        } else if (score - min_score).abs() < BEST_EPSILON {
             best_swaps.push(swap);
         }
     }