Avoid an allocation in BigInteger.Parse("-2147483648")

dotnet · Jul 10, 2024 · e451786 · e451786
1 parent 812cb93
commit e451786
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Showing 2 changed files with 59 additions and 5 deletions.
diff --git a/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigInteger.cs b/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigInteger.cs
@@ -511,16 +511,23 @@ internal BigInteger(ReadOnlySpan<uint> value, bool negative)
             {
                 this = default;
             }
-            else if (value.Length == 1 && value[0] < kuMaskHighBit)
+            else if (value.Length == 1)
             {
-                // Values like (Int32.MaxValue+1) are stored as "0x80000000" and as such cannot be packed into _sign
-                _sign = negative ? -(int)value[0] : (int)value[0];
-                _bits = null;
-                if (_sign == int.MinValue)
+                if (value[0] < kuMaskHighBit)
+                {
+                    _sign = negative ? -(int)value[0] : (int)value[0];
+                    _bits = null;
+                }
+                else if (negative && value[0] == kuMaskHighBit)
                 {
                     // Although Int32.MinValue fits in _sign, we represent this case differently for negate
                     this = s_bnMinInt;
                 }
+                else
+                {
+                    _sign = negative ? -1 : +1;
+                    _bits = [value[0]];
+                }
             }
             else
             {

diff --git a/src/libraries/System.Runtime.Numerics/tests/BigInteger/parse.cs b/src/libraries/System.Runtime.Numerics/tests/BigInteger/parse.cs
@@ -114,6 +114,53 @@ public static void Parse_EmptySubspan_Fails()
             Assert.Equal(0, result);
         }
 
+        [Fact]
+        public void Parse_Small()
+        {
+            BigInteger result;
+
+            Assert.True(BigInteger.TryParse("0", out result));
+            Assert.Equal(0, result);
+
+            Assert.True(BigInteger.TryParse("1", out result));
+            Assert.Equal(1, result);
+
+            Assert.True(BigInteger.TryParse("-1", out result));
+            Assert.Equal(-1, result);
+
+            Assert.True(BigInteger.TryParse("-2147483647", out result));
+            Assert.Equal(-2147483647, result);
+
+            Assert.True(BigInteger.TryParse("2147483647", out result));
+            Assert.Equal(2147483647, result);
+
+            Assert.True(BigInteger.TryParse("-2147483648", out result));
+            Assert.Equal(-2147483648, result);
+
+            Assert.True(BigInteger.TryParse("2147483648", out result));
+            Assert.Equal(2147483648, result);
+
+
+            Assert.True(BigInteger.TryParse("-9223372036854775807", out result));
+            Assert.Equal(-9223372036854775807, result);
+
+            Assert.True(BigInteger.TryParse("9223372036854775807", out result));
+            Assert.Equal(9223372036854775807, result);
+
+            Assert.True(BigInteger.TryParse("-9223372036854775808", out result));
+            Assert.Equal(-9223372036854775808, result);
+
+            Assert.True(BigInteger.TryParse("9223372036854775808", out result));
+            Assert.Equal(Int128.Parse("9223372036854775808"), result);
+
+
+            Assert.True(BigInteger.TryParse(Int128.MinValue.ToString(), out result));
+            Assert.Equal(Int128.MinValue, result);
+
+            Assert.True(BigInteger.TryParse(Int128.MaxValue.ToString(), out result));
+            Assert.Equal(Int128.MaxValue, result);
+        }
+
         [Fact]
         public void Parse_Hex32Bits()
         {