fix: bound_mean_cumn returns correct value if only one present

lib/tdigest/tdigest.rb

@@ -3,6 +3,7 @@
 
 module TDigest
   class TDigest
+    attr_accessor :centroids
     def initialize(delta = 0.01, k = 25, cx = 1.1)
       @delta = delta
       @k = k
@@ -33,10 +34,10 @@ def bound_mean_cumn(cumn)
           last_c = v
         end
       end
-      lower = bounds[0]
-      upper = bounds[1]
+      # If still no results, pick lagging value if any
+      bounds << last_c if bounds.empty? && !last_c.nil?
 
-      [lower, upper]
+      bounds
     end
 
     def compress!

test/tdigest_test.rb

@@ -1,15 +1,22 @@
 require 'test_helper'
-require 'benchmark'
 
 class TDigestTest < Minitest::Test
   def test_that_it_has_a_version_number
     refute_nil ::TDigest::VERSION
   end
 
-  def test_percentile_edge_case
-    tdigest = ::TDigest::TDigest.new
-    tdigest.push(60, 100)
-    pct = tdigest.percentile(0.90) # This should not crash
-    assert_equal nil, pct
+  describe '#percentile' do
+    it 'returns nil if empty' do
+      tdigest = ::TDigest::TDigest.new
+      tdigest.percentile(0.90).must_be_nil # This should not crash
+    end
+
+    describe 'with only signle value' do
+      it 'returns the value' do
+        tdigest = ::TDigest::TDigest.new
+        tdigest.push(60, 100)
+        tdigest.percentile(0.90).must_equal 60
+      end
+    end
   end
 end