Fix rounding error in stod on very small float numbers

rapidsai · Apr 15, 2022 · f6f246c · f6f246c
1 parent f7c35d5
commit f6f246c
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Showing 2 changed files with 12 additions and 3 deletions.
diff --git a/cpp/src/strings/convert/convert_floats.cu b/cpp/src/strings/convert/convert_floats.cu
@@ -130,10 +130,17 @@ __device__ inline double stod(string_view const& d_str)
   else if (exp_ten < std::numeric_limits<double>::min_exponent10)
     return double{0};
 
+  double base = sign * static_cast<double>(digits);
+
   exp_ten += 1 - num_digits;
-  // exp10() is faster than pow(10.0,exp_ten)
+  // extra floating-point division needed only in extreme range (e-287 - e-307)
+  // where num_digits may push exp_ten below min_exponent10 (e-307)
+  if (exp_ten < std::numeric_limits<double>::min_exponent10) {
+    base = base / exp10(static_cast<double>(num_digits - 1));
+    exp_ten += num_digits - 1;  // adjust exponent
+  }
+
   double const exponent = exp10(static_cast<double>(std::abs(exp_ten)));
-  double const base     = sign * static_cast<double>(digits);
   return exp_ten < 0 ? base / exponent : base * exponent;
 }
 

diff --git a/cpp/tests/strings/floats_tests.cpp b/cpp/tests/strings/floats_tests.cpp
@@ -125,11 +125,13 @@ TEST_F(StringsConvertTest, FromFloats32)
 
 TEST_F(StringsConvertTest, ToFloats64)
 {
+  // clang-format off
   std::vector<const char*> h_strings{
     "1234",   nullptr,    "-876",     "543.2",         "-0.12",   ".25",
     "-.002",  "",         "-0.0",     "1.28e256",      "NaN",     "abc123",
     "123abc", "456e",     "-1.78e+5", "-122.33644782", "12e+309", "1.7976931348623159E308",
-    "-Inf",   "-INFINITY"};
+    "-Inf",   "-INFINITY", "1.0",     "1.7976931348623157e+308",  "1.7976931348623157e-307"};
+  // clang-format on
   cudf::test::strings_column_wrapper strings(
     h_strings.begin(),
     h_strings.end(),