F#: BigRational.FromDecimal #716

src/FSharp.Tests/BigRationalTests.fs

@@ -300,6 +300,27 @@ type public BigRationalTests() =
         //let powers = [0I .. 100I]
         //powers |> List.iter (fun i -> test1All  "PowN(x,i)" (fun x -> BigRational.PowN(x,int i)) (fun (p,q) -> (BigInteger.Pow(p,i),BigInteger.Pow(q,i))) vector1s)
 
+    [<Test>]
+    member this.FromDecimal() =
+        let pairs =
+            [
+                1M, 1N
+                100000000000000M, 100000000000000N
+                10000000000000000000000000000M, 10000000000000000000000000000N
+                100000000000000.00000000000000M, 100000000000000N
+                1.0000000000000000000000000000M, 1N
+                123456789M, 123456789N
+                0.123456789M, 123456789N/1000000000N
+                0.000000000123456789M, 123456789N/1000000000000000000N
+                0.000000000000000000123456789M, 123456789N/1000000000000000000000000000N
+                4294967295M, 4294967295N
+                18446744073709551615M, 18446744073709551615N
+                Decimal.MaxValue, 79228162514264337593543950335N
+                Decimal.MinValue, -79228162514264337593543950335N
+                -7.9228162514264337593543950335M, -79228162514264337593543950335N/10000000000000000000000000000N
+            ]
+        for (decimal, rational) in pairs do
+            Assert.AreEqual(rational, BigRational.FromDecimal(decimal))
 
 
 [<TestFixture>]

src/FSharp/BigRational.fs

@@ -276,6 +276,20 @@ type BigRational =
     static member FromBigIntFraction (numerator: BigInteger, denominator: BigInteger) =
         Q (BigRationalLarge.Create (numerator, denominator))
 
+    // See https://docs.microsoft.com/en-us/dotnet/api/system.decimal.getbits
+    static member FromDecimal(n : decimal) =
+        let parts = Decimal.GetBits(n)
+        assert(parts.Length = 4)
+        let toBigInt = uint32 >> bigint
+        let lo =  (toBigInt parts.[0]) <<<  0
+        let mid = (toBigInt parts.[1]) <<< 32
+        let hi =  (toBigInt parts.[2]) <<< 64
+        let sign = if (parts.[3] &&& 0x80000000) = 0 then 1I else -1I
+        let scale = (parts.[3] >>> 16) &&& 0x7F
+        BigRational.FromBigIntFraction(
+            sign * (lo + mid + hi),
+            BigInteger.Pow(10I, scale))
+
     /// Get zero as a rational number
     static member Zero =
         BigRational.FromInt 0

src/FSharp/BigRational.fsi

@@ -62,6 +62,8 @@ type BigRational =
     static member FromIntFraction : int * int -> BigRational
     static member FromBigIntFraction : BigInteger * BigInteger -> BigRational
 
+    static member FromDecimal : decimal -> BigRational
+
     static member ToDouble: BigRational -> float
     static member ToBigInt: BigRational -> BigInteger
     static member ToInt32 : BigRational -> int