BigRational: fix pow to support negative integer exponents

src/FSharp/BigRational.fs

@@ -115,7 +115,8 @@ type BigRationalLarge (p : BigInteger, q : BigInteger) =
     //
     static member Pow (num : BigRationalLarge, n : int) =
         // p,q powers still coprime
-        BigRationalLarge (BigInteger.Pow (num.Numerator, n), BigInteger.Pow (num.Denominator, n))
+        if n < 0 then BigRationalLarge.Normalize (BigInteger.Pow (num.Denominator, -n), BigInteger.Pow (num.Numerator, -n))
+        else BigRationalLarge (BigInteger.Pow (num.Numerator, n), BigInteger.Pow (num.Denominator, n))
 
     //
     static member FromBigInteger z =
@@ -414,14 +415,18 @@ type BigRational =
     /// Returns the multiplicative inverse of a rational number
     static member Reciprocal (n) =
         match n with
-        | Z z -> Q (BigRationalLarge.Create (BigInteger.One, z))
-        | Q q -> Q (BigRationalLarge.Reciprocal q)
+        | Z z ->
+            Q (BigRationalLarge.Create (BigInteger.One, z))
+        | Q q ->
+            Q (BigRationalLarge.Reciprocal q)
 
     /// Return the result of raising the given rational number to the given power
     static member Pow (n, i : int) =
         match n with
-        | Z z ->
+        | Z z when i > 0 ->
             Z (BigInteger.Pow (z, i))
+        | Z z ->
+            Q (BigRationalLarge.Pow (BigRationalLarge.FromBigInteger z, i))
         | Q q ->
             Q (BigRationalLarge.Pow (q, i))
 

src/FSharpUnitTests/BigRationalTests.fs

@@ -286,10 +286,15 @@ type public BigRationalTests() =
         test1All  "Abs"         (BigRational.Abs)     (fun (p,q) -> (abs p,abs q)) vector1s
         testR1All "Sign"        (fun (x:BigRational) -> x.Sign)    (fun (p,q) -> check "NonZeroDenom" (sign q <> 0I) true; (sign p * sign q) |> int32) vector1s
 
-        // Test: PowN
-        test1All  "PowN(x,2)"   (fun x -> BigRational.PowN(x,2))   (fun (p,q) -> (p*p,q*q)) vector1s
-        test1All  "PowN(x,1)"   (fun x -> BigRational.PowN(x,1))   (fun (p,q) -> (p,q)) vector1s
-        test1All  "PowN(x,0)"   (fun x -> BigRational.PowN(x,0))   (fun (p,q) -> (1I,1I)) vector1s
+        // Test: Pow
+        test1All  "Pow(x,2)"   (fun x -> BigRational.Pow(x,2))   (fun (p,q) -> (p*p,q*q)) vector1s
+        test1All  "Pow(x,1)"   (fun x -> BigRational.Pow(x,1))   (fun (p,q) -> (p,q)) vector1s
+        test1All  "Pow(x,0)"   (fun x -> BigRational.Pow(x,0))   (fun (p,q) -> (1I,1I)) vector1s
+        test1All  "Pow(x,-1)"   (fun x -> BigRational.Pow(x,-1))   (fun (p,q) -> (q,p)) (vector1s |> List.filter (fun (p,_) -> p <> 0I))
+        test1All  "Pow(x,-2)"   (fun x -> BigRational.Pow(x,-2))   (fun (p,q) -> (q*q,p*p)) (vector1s |> List.filter (fun (p,_) -> p <> 0I))
+
+        testR1One  "Pow(0,-1)"  (fun x -> throws (fun () -> BigRational.PowN(x,-1))) (fun (p,q) -> true) (0I, -1I)
+        testR1One  "Pow(0,-2)"  (fun x -> throws (fun () -> BigRational.PowN(x,-2))) (fun (p,q) -> true) (0I, -2I)
 
         // MatteoT: moved to numbersVS2008\test.ml
         //test1All  "PowN(x,200)" (fun x -> BigRational.PowN(x,200)) (fun (p,q) -> (BigInteger.Pow(p,200I),BigInteger.Pow(q,200I))) vector1s
@@ -298,10 +303,6 @@ 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: PowN with negative powers - expect exception
-        testR1All  "PowN(x,-1)"  (fun x -> throws (fun () -> BigRational.PowN(x,-1))) (fun (p,q) -> true) vector1s
-        testR1All  "PowN(x,-4)"  (fun x -> throws (fun () -> BigRational.PowN(x,-4))) (fun (p,q) -> true) vector1s
-
 
 
 [<TestFixture>]
@@ -387,8 +388,10 @@ type BigNumType() =
     member this.Pow() =
         Assert.AreEqual(bignum.Pow(100N,2), 10000N)
         Assert.AreEqual(bignum.Pow(-3N,3),  -27N)
+        Assert.AreEqual(bignum.Pow(2N,-2), 1N/4N)
+        Assert.AreEqual(bignum.Pow(2N/3N,-2), 9N/4N)
         Assert.AreEqual(bignum.Pow(g_zero,2147483647), 0N)
-        Assert.AreEqual(bignum.Pow(g_normal,0),        1N)
+        Assert.AreEqual(bignum.Pow(g_normal,0), 1N)
         ()
 
     [<Test>]