Regards #109: Now correctly rounding down the computed base-10 exponent. CAVEAT: This exposes an issue with printing values having exponent -308 (the minimum exponent for double's); and - we see some divergence from perfect corrtness with higher precision values.

Author: eyalroz

src/printf/printf.c

@@ -765,6 +765,15 @@ static void print_decimal_number(output_gadget_t* output, double number, printf_
   print_broken_up_decimal(value_, output, precision, width, flags, buf, len);
 }
 
+// A floor function - but one which only works for numbers whose
+// floor value is representable by an int.
+static int bastardized_floor(double x)
+{
+  if (x >= 0) { return (int) x; }
+  int n = (int) x;
+  return ( ((double) n) == x ) ? n : n-1;
+}
+
 #if PRINTF_SUPPORT_EXPONENTIAL_SPECIFIERS
 // internal ftoa variant for exponential floating-point type, contributed by Martijn Jasperse <[email protected]>
 static void print_exponential_number(output_gadget_t* output, double number, printf_size_t precision, printf_size_t width, printf_flags_t flags, char* buf, printf_size_t len)
@@ -791,9 +800,11 @@ static void print_exponential_number(output_gadget_t* output, double number, pri
 	  // drop the exponent, so conv.F comes into the range [1,2)
       conv.U = (conv.U & (( (double_uint_t)(1) << DOUBLE_STORED_MANTISSA_BITS) - 1U)) | ((double_uint_t) DOUBLE_BASE_EXPONENT << DOUBLE_STORED_MANTISSA_BITS);
       // now approximate log10 from the log2 integer part and an expansion of ln around 1.5
-      exp10 = (int)(0.1760912590558 + exp2 * 0.301029995663981 + (conv.F - 1.5) * 0.289529654602168);
+      double exp10_ = (0.1760912590558 + exp2 * 0.301029995663981 + (conv.F - 1.5) * 0.289529654602168);
+
+      exp10 = bastardized_floor(exp10_);
       // now we want to compute 10^exp10 but we want to be sure it won't overflow
-      exp2 = (int)(exp10 * 3.321928094887362 + 0.5);
+      exp2 = bastardized_floor(exp10 * 3.321928094887362 + 0.5);
       const double z  = exp10 * 2.302585092994046 - exp2 * 0.6931471805599453;
       const double z2 = z * z;
       conv.U = ((double_uint_t)(exp2) + DOUBLE_BASE_EXPONENT) << DOUBLE_STORED_MANTISSA_BITS;

test/test_suite.cpp

@@ -873,6 +873,30 @@ TEST_CASE("floating-point specifiers with 31-32 bit integer values", "[]" ) {
 }
 #endif
 
+#if PRINTF_SUPPORT_EXPONENTIAL_SPECIFIERS
+TEST_CASE("tiny floating-point values", "[]" ) {
+  char buffer[100];
+//  boltzman_s_constant = 1.380651569e-23;
+
+  PRINTING_CHECK("1e-23",                  ==, sprintf_, buffer, "%.0e",  1.380651569e-23);
+  PRINTING_CHECK("1.4e-23",                ==, sprintf_, buffer, "%.1e",  1.380651569e-23);
+  PRINTING_CHECK("1.38e-23",               ==, sprintf_, buffer, "%.2e",  1.380651569e-23);
+  PRINTING_CHECK("1.381e-23",              ==, sprintf_, buffer, "%.3e",  1.380651569e-23);
+  PRINTING_CHECK("1.3807e-23",             ==, sprintf_, buffer, "%.4e",  1.380651569e-23);
+  PRINTING_CHECK("1.38065e-23",            ==, sprintf_, buffer, "%.5e",  1.380651569e-23);
+  PRINTING_CHECK("1.380652e-23",           ==, sprintf_, buffer, "%.6e",  1.380651569e-23);
+  PRINTING_CHECK("1.3806516e-23",          ==, sprintf_, buffer, "%.7e",  1.380651569e-23);
+  PRINTING_CHECK("1.38065157e-23",         ==, sprintf_, buffer, "%.8e",  1.380651569e-23);
+  PRINTING_CHECK("1.380651569e-23",        ==, sprintf_, buffer, "%.9e",  1.380651569e-23);
+  PRINTING_CHECK("1.3806515690e-23",       ==, sprintf_, buffer, "%.10e", 1.380651569e-23);
+  PRINTING_CHECK("1.38065156900e-23",      ==, sprintf_, buffer, "%.11e", 1.380651569e-23);
+  PRINTING_CHECK("1.380651569000e-23",     ==, sprintf_, buffer, "%.12e", 1.380651569e-23);
+  PRINTING_CHECK("1.3806515690000e-23",    ==, sprintf_, buffer, "%.13e", 1.380651569e-23);
+  PRINTING_CHECK("1.38065156900000e-23",   ==, sprintf_, buffer, "%.14e", 1.380651569e-23);
+  PRINTING_CHECK("1.380651569000000e-23",  ==, sprintf_, buffer, "%.15e", 1.380651569e-23);
+  PRINTING_CHECK("1.3806515690000000e-23", ==, sprintf_, buffer, "%.16e", 1.380651569e-23);
+}
+#endif
 
 #if PRINTF_SUPPORT_DECIMAL_SPECIFIERS
 TEST_CASE("fallback from decimal to exponential", "[]" ) {