Add handling of double values with just integer part

- Now at caller.

Author: josesimoes

src/CLR/CorLib/corlib_native_System_Number.cpp

@@ -1,4 +1,4 @@
-//
+//
 // Copyright (c) .NET Foundation and Contributors
 // Portions Copyright (c) Microsoft Corporation.  All rights reserved.
 // See LICENSE file in the project root for full license information.
@@ -361,11 +361,22 @@ int Library_corlib_native_System_Number::Format_G(
             defaultPrecision = 9;
             break;
         case DATATYPE_R8:
+        {
             // from .NET documentation:
             // When used with a Double value, the "G17" format specifier ensures that the original Double value
             // successfully round-trips.
             defaultPrecision = 17;
+
+            CLR_DOUBLE_TEMP_CAST number = (CLR_DOUBLE_TEMP_CAST)value->NumericByRef().r8;
+
+            // check if number is an integer
+            if (number == (CLR_INT64_TEMP_CAST)number)
+            {
+                // this is an integer, set precision to a value achievable by the library
+                defaultPrecision = 15;
+            }
             break;
+        }
         default:
             break;
     }

src/CLR/Helpers/nanoprintf/nanoprintf.c

@@ -1,4 +1,4 @@
-//
+//
 // Copyright (c) .NET Foundation and Contributors
 // Portions Copyright (c) 2006 - 2021 Skirrid Systems. All rights reserved.
 // See LICENSE file in the project root for full license information.
@@ -93,7 +93,7 @@ Floating point
   #define DP_LIMIT      310
   #define MAX_POWER     256
 // [NF_CHANGE]
-  #define FLOAT_DIGITS  18
+  #define FLOAT_DIGITS  19
 // [END_NF_CHANGE]
 #else
   #define DP_LIMIT      40
@@ -271,6 +271,8 @@ static char *format_float(double number, flt_width_t ndigits, flt_width_t width,
              * to the answer in the fastest time, with the minimum number of
              * operations to introduce rounding errors.
              */
+
+            // Normalise the number such that it lies in the range 1 <= n < 10.
             // First make small numbers bigger.
 
             i = 0;