Don't throw an error if the required number of neighbors is too high.

This is none of our business - if the user requests a certain perplexity
for a dataset, then we'll respect it and proceed with compute.

include/qdtsne/initialize.hpp

@@ -37,6 +37,7 @@
 
 #include <vector>
 #include <stdexcept>
+#include <algorithm>
 
 #include "Status.hpp"
 #include "Options.hpp"
@@ -108,12 +109,9 @@ Status<num_dim_, Index_, Float_> initialize(NeighborList<Index_, Float_> neighbo
  */
 template<int num_dim_, typename Dim_, typename Index_, typename Float_>
 Status<num_dim_, Index_, Float_> initialize(const knncolle::Prebuilt<Dim_, Index_, Float_>& prebuilt, const Options& options) { 
-    const Index_ K = perplexity_to_k(options.perplexity);
-    Index_ N = prebuilt.num_observations();
-    if (K >= N) {
-        throw std::runtime_error("number of observations should be greater than 3 * perplexity");
-    }
-
+    // TODO: replace max_neighbors with knncolle::cap_nearest_neighbors().
+    const int max_neighbors = std::max(1, static_cast<int>(prebuilt.num_observations())) - 1;
+    const int K = std::min(perplexity_to_k(options.perplexity), max_neighbors); 
     auto neighbors = find_nearest_neighbors(prebuilt, K, options.num_threads);
     return internal::initialize<num_dim_>(std::move(neighbors), options.perplexity, options);
 }