Common: refactor the deadtime processing in the 1st-order digital filter

modules/common/filters/digital_filter_coefficients.cc

@@ -56,13 +56,13 @@ void LpFirstOrderCoefficients(const double ts, const double settling_time,
     return;
   }
 
-  const size_t k_d = static_cast<int>(dead_time / ts);
+  const size_t k_d = static_cast<size_t>(dead_time / ts);
   double a_term;
 
   denominators->clear();
   numerators->clear();
   denominators->reserve(2);
-  numerators->reserve(k_d);  // size depends on dead-time
+  numerators->reserve(k_d + 1);  // size depends on dead-time
 
   if (settling_time == 0.0) {
     a_term = 0.0;
@@ -72,7 +72,7 @@ void LpFirstOrderCoefficients(const double ts, const double settling_time,
 
   denominators->push_back(1.0);
   denominators->push_back(-a_term);
-  numerators->insert(numerators->end(), k_d - 1, 0.0);
+  numerators->insert(numerators->end(), k_d, 0.0);
   numerators->push_back(1 - a_term);
 }
 

modules/common/filters/digital_filter_coefficients_test.cc

@@ -44,7 +44,7 @@ TEST_F(DigitalFilterCoefficientsTest, LpfCoefficients) {
 TEST_F(DigitalFilterCoefficientsTest, LpFirstOrderCoefficients) {
   double ts = 0.01;
   double settling_time = 0.005;
-  double dead_time = 0.05;
+  double dead_time = 0.04;
   std::vector<double> den;
   std::vector<double> num;
   LpFirstOrderCoefficients(ts, settling_time, dead_time, &den, &num);
@@ -54,6 +54,12 @@ TEST_F(DigitalFilterCoefficientsTest, LpFirstOrderCoefficients) {
   EXPECT_DOUBLE_EQ(num[0], 0.0);
   EXPECT_DOUBLE_EQ(num[1], 0.0);
   EXPECT_NEAR(num[4], 1 - 0.13533, 0.01);
+  dead_time = 0.0;
+  LpFirstOrderCoefficients(ts, settling_time, dead_time, &den, &num);
+  EXPECT_EQ(den.size(), 2);
+  EXPECT_EQ(num.size(), 1);
+  EXPECT_NEAR(den[1], -0.13533, 0.01);
+  EXPECT_NEAR(num[0], 1 - 0.13533, 0.01);
 }
 
 }  // namespace common