Implement toBiggestFloat

Fixes Feature request: toBiggestInt nim-lang#34
rotu · Jul 26, 2022 · b38ff2b · b38ff2b
1 parent d843ecf
commit b38ff2b
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diff --git a/src/bigints.nim b/src/bigints.nim
@@ -1,6 +1,6 @@
 ## Arbitrary precision integers.
 
-import std/[algorithm, bitops, math, options]
+import std/[algorithm, bitops, fenv, math, options]
 
 type
  BigInt* = object
@@ -1140,6 +1140,11 @@ func fastLog2*(a: BigInt): int =
  return -1
  bitops.fastLog2(a.limbs[^1]) + 32*(a.limbs.high)
 
+func toBiggestFloat*(x: BigInt): BiggestFloat =
+ let l = mantissaDigits(BiggestFloat)+1
+ let shift = max(fastLog2(x) - l, 0)
+ let mantissa = BiggestFloat(toInt[BiggestInt](x shr shift).get())
+ result = mantissa * pow(2.0, BiggestFloat(shift))
 
 func invmod*(a, modulus: BigInt): BigInt =
  ## Compute the modular inverse of `a` modulo `modulus`.

diff --git a/tests/tbigints.nim b/tests/tbigints.nim
@@ -786,6 +786,16 @@ proc main() =
  doAssert pred(a, 3) == initBigInt(4)
  doAssert succ(a, 3) == initBigInt(10)
 
+ block: # to float
+ doAssert toBiggestFloat(initBigInt("0")) == 0.0
+ doAssert toBiggestFloat(initBigInt("1")) == 1.0
+ doAssert toBiggestFloat(initBigInt("-1")) == -1.0
+ doAssert toBiggestFloat(initBigInt(BiggestInt.high)) == BiggestInt.high.toBiggestFloat
+ doAssert toBiggestFloat(initBigInt(BiggestInt.low)) == BiggestInt.low.toBiggestFloat
+ doAssert toBiggestFloat(initBigInt(BiggestInt.high) * initBigInt(4)) == BiggestInt.high.toBiggestFloat * 4.0
+ doAssert toBiggestFloat(initBigInt(BiggestInt.low) * initBigInt(4)) == BiggestInt.low.toBiggestFloat * 4.0
+ doAssert toBiggestFloat(initBigInt("17976931348623157") * initBigInt(10).pow(292)) == 17976931348623157e292
+
 
 static: main()
 main()