Test that an infinite coordinate does not overflow the chain buffer

An infinite coordinate puts both infinity and NaN in the distance matrix, so
the chain can keep extending without finding a mutual pair. Walking past the
end of its buffer raised instead of reporting the merges it could not make.

Author: RicardoSantos-99

test/scholar/cluster/hierarchical_test.exs

@@ -401,6 +401,34 @@ defmodule Scholar.Cluster.HierarchicalTest do
       assert Nx.to_number(model.sizes[-1]) == 0
     end
 
+    test "an infinite coordinate does not run the chain past the end of its buffer" do
+      # An infinite coordinate puts both :infinity and :nan in the distance matrix, since
+      # subtracting infinities gives NaN. The chain can then keep extending without ever
+      # finding a mutual pair, and walking past the end of its own buffer raises instead
+      # of reporting the merges it could not make.
+      x = Nx.tensor([[0.0, 0.0], [1.0, 1.0], [2.0, 2.0], [:infinity, 0.0]])
+
+      model = Hierarchical.fit(x, linkage: :complete)
+
+      assert model.num_points == 4
+      assert Nx.shape(model.clades) == {3, 2}
+
+      # Whatever it did merge is still a well formed tree: no clade merged twice, and no
+      # row naming a clade that is not born yet.
+      made =
+        Nx.to_flat_list(model.clades)
+        |> Enum.chunk_every(2)
+        |> Enum.zip(Nx.to_flat_list(model.sizes))
+        |> Enum.reject(fn {_pair, size} -> size == 0 end)
+
+      children = Enum.flat_map(made, fn {pair, _size} -> pair end)
+      assert children == Enum.uniq(children)
+
+      assert Enum.all?(Enum.with_index(made), fn {{[a, b], _size}, k} ->
+               a < model.num_points + k and b < model.num_points + k
+             end)
+    end
+
     test "a NaN coordinate still makes every merge" do
       # NaN alone does not stall the loop: argmin keeps picking a mutual pair, so every
       # merge is made. The dissimilarities really are NaN here, since the distances are,