String: Fix a bug in calculating the number of significant digits for exact powers of 10

Author: Anthony-Nicholls

modules/juce_core/text/juce_String.cpp

@@ -2878,6 +2878,20 @@ class StringTests final : public UnitTest
             expectEquals (String::toDecimalStringWithSignificantFigures (2.8647,     6), String ("2.86470"));
 
             expectEquals (String::toDecimalStringWithSignificantFigures (-0.0000000000019, 1), String ("-0.000000000002"));
+
+            // Powers of 10
+
+            expectEquals (String::toDecimalStringWithSignificantFigures (       0.001, 7), String (       "0.001000000"));
+            expectEquals (String::toDecimalStringWithSignificantFigures (       0.01,  7), String (       "0.01000000"));
+            expectEquals (String::toDecimalStringWithSignificantFigures (       0.1,   7), String (       "0.1000000"));
+            expectEquals (String::toDecimalStringWithSignificantFigures (       1,     7), String (       "1.000000"));
+            expectEquals (String::toDecimalStringWithSignificantFigures (      10,     7), String (      "10.00000"));
+            expectEquals (String::toDecimalStringWithSignificantFigures (     100,     7), String (     "100.0000"));
+            expectEquals (String::toDecimalStringWithSignificantFigures (    1000,     7), String (    "1000.000"));
+            expectEquals (String::toDecimalStringWithSignificantFigures (   10000,     7), String (   "10000.00"));
+            expectEquals (String::toDecimalStringWithSignificantFigures (  100000,     7), String (  "100000.0"));
+            expectEquals (String::toDecimalStringWithSignificantFigures ( 1000000,     7), String ( "1000000"));
+            expectEquals (String::toDecimalStringWithSignificantFigures (10000000,     7), String ("10000000"));
         }
 
         beginTest ("Float trimming");

modules/juce_core/text/juce_String.h

@@ -1159,11 +1159,10 @@ class JUCE_API  String  final
             return "0";
         }
 
-        auto numDigitsBeforePoint = (int) std::ceil (std::log10 (number < 0 ? -number : number));
-
-        auto shift = numberOfSignificantFigures - numDigitsBeforePoint;
-        auto factor = std::pow (10.0, shift);
-        auto rounded = std::round (number * factor) / factor;
+        const auto numDigitsBeforePoint = (int) std::floor (std::log10 (std::abs (number)) + DecimalType (1));
+        const auto shift = numberOfSignificantFigures - numDigitsBeforePoint;
+        const auto factor = std::pow (10.0, shift);
+        const auto rounded = std::round (number * factor) / factor;
 
         std::stringstream ss;
         ss << std::fixed << std::setprecision (std::max (shift, 0)) << rounded;