Make width further restrict definition of 'deep' variables

Compared to using 'seen_widest' in the previous commit, this better encapsulate a diamond-shaped but
BDD that gets wide early. The 'deep' variables are then symmetrically only the ones as deep as the
ones at the top (measured in number of nodes, not number of levels).

src/adiar/internal/algorithms/quantify.h

@@ -1426,11 +1426,10 @@ namespace adiar::internal
 
     level_info_stream<false /* top-down */> lis(dd);
 
-    const size_t third_dd_size = res.dd_size / 3;
+    size_t shallow_threshold = res.dd_size / 3;
 
-    size_t nodes_above         = 0u;
-    size_t nodes_below         = res.dd_size;
-    bool seen_widest           = false;
+    size_t nodes_above       = 0u;
+    size_t nodes_below       = res.dd_size;
 
     while (lis.can_pull()) {
       const level_info li = lis.pull();
@@ -1440,8 +1439,8 @@ namespace adiar::internal
       if (pred(li.label()) == Policy::quantify_onset) {
         res.quant_all_vars += 1u;
         res.quant_all_size += li.width();
-        res.quant_deep_vars += seen_widest && nodes_below < third_dd_size;
-        res.quant_shallow_vars += !seen_widest && nodes_above <= third_dd_size;
+        res.quant_deep_vars += nodes_below < shallow_threshold;
+        res.quant_shallow_vars += nodes_above <= shallow_threshold;
 
         { // Deepest variable (always updated due to top-down direction).
           res.deepest_var.level       = li.level();
@@ -1456,8 +1455,10 @@ namespace adiar::internal
         if (li.width() < res.narrowest_var.width) { res.narrowest_var = res.deepest_var; }
       }
 
+      if (li.width() == res.dd_width) {
+        shallow_threshold = std::min(shallow_threshold, nodes_above);
+      }
       nodes_above += li.width();
-      seen_widest |= li.width() == res.dd_width;
     }
     return res;
   }