export Data.IntSet.Internal.lowestBitSet, highestBitSet, revNat

[jwaldmann]

While working on #470 I noticed this in the source of Data.IntSet.Internal:

  For lowestBitSet we use wordsize-dependant implementation based on
  multiplication and DeBrujn indeces, which was proposed by Edward Kmett
  <http://haskell.org/pipermail/libraries/2011-September/016749.html>

  The core of this implementation is fast indexOfTheOnlyBit,
  which is given a Nat with exactly one bit set, and returns
  its index.

  Lot of effort was put in these implementations, please benchmark carefully
  before changing this code.

Don't these belong in Data.Bits then, so they can be re-used? This was already suggested in
https://mail.haskell.org/pipermail/libraries/2011-September/016755.html
https://mail.haskell.org/pipermail/libraries/2011-September/016756.html

[jwaldmann]

Ah, it's in https://hackage.haskell.org/package/containers-0.6.2.1/docs/Utils-Containers-Internal-BitUtil.html

[jwaldmann]

But is it really? It has highestBitMask but not lowestBitSet etc.

[jwaldmann]

not needed, as GHC.Exts has clt, ctz and so on https://hackage.haskell.org/package/base-4.12.0.0/docs/GHC-Exts.html#v:clz-35-