Use std::optional to represent unset a/b options.

This is more self-documenting than a sentinel value of zero.

Author: LTLA

include/umappp/Options.hpp

@@ -68,17 +68,17 @@ struct Options {
      * Positive value for the \f$a\f$ parameter for the fuzzy set membership strength calculations.
      * Larger values yield a sharper decay in membership strength with increasing distance between observations.
      *
-     * If this or `Options::b` is set to zero, a suitable value for this parameter is automatically determined from `Options::spread` and `Options::min_dist`.
+     * If this or `Options::b` are unset, a suitable value for this parameter is automatically determined from `Options::spread` and `Options::min_dist`.
      */
-    double a = 0;
+    std::optional<double> a;
 
     /**
      * Value in \f$(0, 1)\f$ for the \f$b\f$ parameter for the fuzzy set membership strength calculations.
      * Larger values yield an earlier decay in membership strength with increasing distance between observations.
      *
-     * If this or `Options::a` is set to zero, a suitable value for this parameter is automatically determined from `Options::spread` and `Options::min_dist`.
+     * If this or `Options::a` are unset, a suitable value for this parameter is automatically determined from `Options::spread` and `Options::min_dist`.
      */
-    double b = 0;
+    std::optional<double> b;
 
     /**
      * Modifier for the repulsive force.

include/umappp/Status.hpp

@@ -104,8 +104,8 @@ class Status {
                 my_num_dim,
                 embedding,
                 my_epochs,
-                my_options.a,
-                my_options.b,
+                *(my_options.a),
+                *(my_options.b),
                 my_options.repulsion_strength,
                 my_options.learning_rate,
                 my_engine,
@@ -116,8 +116,8 @@ class Status {
                 my_num_dim,
                 embedding,
                 my_epochs,
-                my_options.a,
-                my_options.b,
+                *(my_options.a),
+                *(my_options.b),
                 my_options.repulsion_strength,
                 my_options.learning_rate,
                 my_engine,

include/umappp/initialize.hpp

@@ -107,7 +107,7 @@ Status<Index_, Float_> initialize(NeighborList<Index_, Float_> x, const std::siz
     }
 
     // Finding a good a/b pair.
-    if (options.a <= 0 || options.b <= 0) {
+    if (!options.a.has_value() || !options.b.has_value()) {
         const auto found = internal::find_ab(options.spread, options.min_dist);
         options.a = found.first;
         options.b = found.second;