fix: Consistent probability sampler init (#1447)

* fix: Consistent probability sampler init

* Math is hard

* Update sdk_experimental/test/opentelemetry/sdk/trace/samplers/consistent_probability_based_test.rb

Co-authored-by: Sam <370182+plantfansam@users.noreply.github.com>

* Appease the Handler

---------

Co-authored-by: Sam <370182+plantfansam@users.noreply.github.com>

sdk_experimental/lib/opentelemetry/sdk/trace/samplers/consistent_probability_based.rb

@@ -25,7 +25,7 @@ def initialize(probability)
             end
 
             @p_floor = (Math.frexp(probability)[1] - 1).abs
-            @p_ceil = @p_floor + 1
+            @p_ceil = @p_floor - 1
             floor = Math.ldexp(1.0, -@p_floor)
             ceil = Math.ldexp(1.0, -@p_ceil)
             @p_ceil_probability = (probability - floor) / (ceil - floor)

sdk_experimental/test/opentelemetry/sdk/trace/samplers/consistent_probability_based_test.rb

@@ -62,4 +62,54 @@
 
     # TODO: statistical tests
   end
+
+  describe '#initialize' do
+    # Check that the internal state is initialized correctly. We cache the
+    # floor and ceil of the negative power-of-two of the provided probability,
+    # and the probability of the ceil. It should be possible to calculate the
+    # original probability from these.
+    #
+    # Note that these are negative power-of-two exponent values, so the ceiling
+    # is the smaller value.
+
+    it 'initializes correctly for power-of-two probabilities' do
+      p_values = 62.downto(0)
+      probabilities = p_values.map { |i| 2**-i }
+      p_values.zip(probabilities).each do |p, probability|
+        sampler = OpenTelemetry::SDK::Trace::Samplers::ConsistentProbabilityBased.new(probability)
+        _(sampler.instance_variable_get(:@p_floor)).must_equal(p)
+        _(sampler.instance_variable_get(:@p_ceil)).must_equal(p-1)
+        _(sampler.instance_variable_get(:@p_ceil_probability)).must_equal(0)
+      end
+    end
+
+    it 'initializes correctly for 0' do
+      sampler = OpenTelemetry::SDK::Trace::Samplers::ConsistentProbabilityBased.new(0)
+      _(sampler.instance_variable_get(:@p_floor)).must_equal(63)
+      _(sampler.instance_variable_get(:@p_ceil)).must_equal(0)
+      _(sampler.instance_variable_get(:@p_ceil_probability)).must_equal(0)
+    end
+
+    it 'initializes correctly when the probability is close to a negative power-of-two' do
+      sampler = OpenTelemetry::SDK::Trace::Samplers::ConsistentProbabilityBased.new(0.12)
+      _(sampler.instance_variable_get(:@p_floor)).must_equal(4) # 2**-4 = 0.0625
+      _(sampler.instance_variable_get(:@p_ceil)).must_equal(3) # 2**-3 = 0.125
+      # Note obvious skew here: 0.12 is closer to 0.125 than 0.0625, so the probability of choosing the ceil is higher.
+      _(sampler.instance_variable_get(:@p_ceil_probability)).must_be_within_epsilon(0.92) # 0.0575 / 0.0625
+    end
+
+    it 'initializes correctly for 0.1' do
+      sampler = OpenTelemetry::SDK::Trace::Samplers::ConsistentProbabilityBased.new(0.1)
+      _(sampler.instance_variable_get(:@p_floor)).must_equal(4) # 2**-4 = 0.0625
+      _(sampler.instance_variable_get(:@p_ceil)).must_equal(3) # 2**-3 = 0.125
+      _(sampler.instance_variable_get(:@p_ceil_probability)).must_be_within_epsilon(0.6) # 0.0375 / 0.0625
+      ceil_prob = sampler.instance_variable_get(:@p_ceil_probability)
+      floor_prob = 1 - ceil_prob
+      p_floor = sampler.instance_variable_get(:@p_floor)
+      p_ceil = sampler.instance_variable_get(:@p_ceil)
+      # Verify that the sampling probability encoded in each `p` adds up to the probability
+      # in the the initializer argument
+      _((2**-p_ceil * ceil_prob) + (2**-p_floor * floor_prob)).must_be_within_epsilon(0.1)
+    end
+  end
 end