Fix FormattingHelpers.CountDigits(UInt128) and add more tests for Int128/UInt128.ToString (#74536)

Co-authored-by: Tanner Gooding <[email protected]>

Author: github-actions[bot]

src/libraries/System.Private.CoreLib/src/System/Buffers/Text/FormattingHelpers.CountDigits.cs

@@ -14,22 +14,37 @@ public static int CountDigits(UInt128 value)
         {
             ulong upper = value.Upper;
 
-            if (upper < 5)
+            // 1e19 is    8AC7_2304_89E8_0000
+            // 1e20 is  5_6BC7_5E2D_6310_0000
+            // 1e21 is 36_35C9_ADC5_DEA0_0000
+
+            if (upper == 0)
             {
+                // We have less than 64-bits, so just return the lower count
                 return CountDigits(value.Lower);
             }
 
-            int digits = 19;
+            // We have more than 1e19, so we have at least 20 digits
+            int digits = 20;
 
             if (upper > 5)
             {
-                digits++;
+                // ((2^128) - 1) / 1e20 < 34_02_823_669_209_384_635 which
+                // is 18.5318 digits, meaning the result definitely fits
+                // into 64-bits and we only need to add the lower digit count
+
                 value /= new UInt128(0x5, 0x6BC7_5E2D_6310_0000); // value /= 1e20
+                Debug.Assert(value.Upper == 0);
+
                 digits += CountDigits(value.Lower);
             }
-            else if (value.Lower >= 0x6BC75E2D63100000)
+            else if ((upper == 5) && (value.Lower >= 0x6BC75E2D63100000))
             {
+                // We're greater than 1e20, but definitely less than 1e21
+                // so we have exactly 21 digits
+
                 digits++;
+                Debug.Assert(digits == 21);
             }
 
             return digits;

src/libraries/System.Runtime/tests/System/Int128Tests.cs

@@ -116,6 +116,12 @@ public static IEnumerable<object[]> ToString_TestData()
                     yield return new object[] { (Int128)(-4567), defaultSpecifier, defaultFormat, "-4567" };
                     yield return new object[] { (Int128)0, defaultSpecifier, defaultFormat, "0" };
                     yield return new object[] { (Int128)4567, defaultSpecifier, defaultFormat, "4567" };
+                    yield return new object[] { new Int128(0x0000_0000_0000_0001, 0x0000_0000_0000_0003), defaultSpecifier, defaultFormat, "18446744073709551619" };
+                    yield return new object[] { new Int128(0x0000_0000_0000_0001, 0x0000_0000_0000_000A), defaultSpecifier, defaultFormat, "18446744073709551626" };
+                    yield return new object[] { new Int128(0x0000_0000_0000_0005, 0x0000_0000_0000_0001), defaultSpecifier, defaultFormat, "92233720368547758081" };
+                    yield return new object[] { new Int128(0x0000_0000_0000_0005, 0x6BC7_5E2D_6310_0000), defaultSpecifier, defaultFormat, "100000000000000000000" };
+                    yield return new object[] { new Int128(0x0000_0000_0000_0036, 0x35C9_ADC5_DEA0_0000), defaultSpecifier, defaultFormat, "1000000000000000000000" };
+                    yield return new object[] { new Int128(0x0013_4261_72C7_4D82, 0x2B87_8FE8_0000_0000), defaultSpecifier, defaultFormat, "100000000000000000000000000000000000" };
                     yield return new object[] { Int128.MaxValue, defaultSpecifier, defaultFormat, "170141183460469231731687303715884105727" };
                 }
 

src/libraries/System.Runtime/tests/System/UInt128Tests.cs

@@ -101,6 +101,14 @@ public static IEnumerable<object[]> ToString_TestData()
                 {
                     yield return new object[] { (UInt128)0, defaultSpecifier, defaultFormat, "0" };
                     yield return new object[] { (UInt128)4567, defaultSpecifier, defaultFormat, "4567" };
+                    yield return new object[] { new UInt128(0x0000_0000_0000_0001, 0x0000_0000_0000_0003), defaultSpecifier, defaultFormat, "18446744073709551619" };
+                    yield return new object[] { new UInt128(0x0000_0000_0000_0001, 0x0000_0000_0000_000A), defaultSpecifier, defaultFormat, "18446744073709551626" };
+                    yield return new object[] { new UInt128(0x0000_0000_0000_0005, 0x0000_0000_0000_0001), defaultSpecifier, defaultFormat, "92233720368547758081" };
+                    yield return new object[] { new UInt128(0x0000_0000_0000_0005, 0x6BC7_5E2D_6310_0000), defaultSpecifier, defaultFormat, "100000000000000000000" };
+                    yield return new object[] { new UInt128(0x0000_0000_0000_0036, 0x35C9_ADC5_DEA0_0000), defaultSpecifier, defaultFormat, "1000000000000000000000" };
+                    yield return new object[] { new UInt128(0x0013_4261_72C7_4D82, 0x2B87_8FE8_0000_0000), defaultSpecifier, defaultFormat, "100000000000000000000000000000000000" };
+                    yield return new object[] { new UInt128(0x7FFF_FFFF_FFFF_FFFF, 0xFFFF_FFFF_FFFF_FFFF), defaultSpecifier, defaultFormat, "170141183460469231731687303715884105727" };
+                    yield return new object[] { new UInt128(0x8000_0000_0000_0000, 0x0000_0000_0000_0000), defaultSpecifier, defaultFormat, "170141183460469231731687303715884105728" };
                     yield return new object[] { UInt128.MaxValue, defaultSpecifier, defaultFormat, "340282366920938463463374607431768211455" };
                 }