Convert logarithmic values when calculating a hypothesis [#18]

lib/pocketsphinx/api/pocketsphinx.rb

@@ -6,12 +6,16 @@ module Pocketsphinx
 
       typedef :pointer, :decoder
       typedef :pointer, :configuration
+      typedef :pointer, :logmath
 
       # Allows expect(API::Pocketsphinx).to receive(:ps_init) in JRuby specs
       def self.ps_init(*args)
         ps_init_private(*args)
       end
 
+      attach_function :ps_get_logmath, [:decoder], :logmath
+      attach_function :logmath_exp, [:logmath, :int], :double
+
       attach_function :ps_init_private, :ps_init, [:configuration], :decoder
       attach_function :ps_reinit, [:decoder, :configuration], :int
       attach_function :ps_default_search_args, [:pointer], :void

lib/pocketsphinx/decoder.rb

@@ -123,8 +123,8 @@ def hypothesis
 
       hypothesis.nil? ? nil : Hypothesis.new(
         hypothesis,
-        mp_path_score.get_int32(0),
-        posterior_prob
+        log_prob_to_linear(mp_path_score.get_int32(0)),
+        log_prob_to_linear(posterior_prob)
       )
     end
 
@@ -211,5 +211,11 @@ def post_init_decoder
         configuration.post_init_decoder(self)
       end
     end
+
+    # Convert logarithmic probability to linear floating point
+    def log_prob_to_linear(log_prob)
+      logmath = ps_api.ps_get_logmath(ps_decoder)
+      ps_api.logmath_exp(logmath, log_prob)
+    end
   end
 end

spec/decoder_spec.rb

@@ -143,7 +143,7 @@
   describe '#hypothesis' do
     it 'calls libpocketsphinx' do
       expect(ps_api)
-        .to receive(:ps_get_hyp) do |ps_decoder, mp_path_score|
+        .to receive(:ps_get_hyp).ordered do |ps_decoder, mp_path_score|
           expect(ps_decoder).to eq(subject.ps_decoder)
           expect(mp_path_score).to be_a(FFI::MemoryPointer)
 
@@ -152,17 +152,17 @@
           "Hypothesis"
         end
 
-      expect(ps_api)
-        .to receive(:ps_get_prob) do |ps_decoder|
-          expect(ps_decoder).to eq(subject.ps_decoder)
-          1
-        end
+      expect(ps_api).to receive(:ps_get_prob).with(subject.ps_decoder).ordered.and_return(1)
+      expect(ps_api).to receive(:ps_get_logmath).with(subject.ps_decoder).ordered.and_return(:logmath)
+      expect(ps_api).to receive(:logmath_exp).with(:logmath, 20).ordered.and_return(0.5)
+      expect(ps_api).to receive(:ps_get_logmath).with(subject.ps_decoder).ordered.and_return(:logmath)
+      expect(ps_api).to receive(:logmath_exp).with(:logmath, 1).ordered.and_return(0.4)
 
       hypothesis = subject.hypothesis
 
       expect(hypothesis).to eq("Hypothesis")
-      expect(hypothesis.path_score).to eq(20)
-      expect(hypothesis.posterior_prob).to eq(1)
+      expect(hypothesis.path_score).to eq(0.5)
+      expect(hypothesis.posterior_prob).to eq(0.4)
     end
   end
 

spec/integration/decoder_spec.rb

@@ -20,8 +20,8 @@
       subject.decode File.open('spec/assets/audio/goforward.raw', 'rb')
       expect(subject.hypothesis).to eq("go forward ten meters")
 
-      expect(subject.hypothesis.path_score).to eq(-7636)
-      expect(subject.hypothesis.posterior_prob).to eq(-54501)
+      expect(subject.hypothesis.path_score).to eq(0.4659446069409511)
+      expect(subject.hypothesis.posterior_prob).to eq(0.004293161767851012)
     end
 
     # FIXME: This test illustrates a current issue discussed in: