I have a version of this inside of the monoid library buried in the Data.Ring.Semi.BitSet module: http://comonad.com/haskell/monoids/dist/doc/html/monoids/src/Data-Ring-Semi-BitSet.html#hwm
<http://comonad.com/haskell/monoids/dist/doc/html/monoids/src/Data-Ring-Semi-BitSet.html#hwm>To do any better by walking the raw Integer internals you need to know the 'finger' size for the GMP for your platform, which isn't possible to do portably. -Edward Kmett On Wed, Aug 26, 2009 at 10:42 AM, Uwe Hollerbach <uhollerb...@gmail.com>wrote: > Here's my version... maybe not as elegant as some, but it seems to > work. For base 2 (or 2^k), it's probably possible to make this even > more efficient by just walking along the integer as stored in memory, > but that difference probably won't show up until at least tens of > thousands of digits. > > Uwe > > ilogb :: Integer -> Integer -> Integer > ilogb b n | n < 0 = ilogb b (- n) > | n < b = 0 > | otherwise = (up 1) - 1 > where up a = if n < (b ^ a) > then bin (quot a 2) a > else up (2*a) > bin lo hi = if (hi - lo) <= 1 > then hi > else let av = quot (lo + hi) 2 > in if n < (b ^ av) > then bin lo av > else bin av hi > > numDigits n = 1 + ilogb 10 n > > [fire up ghci, load, etc] > > *Main> numDigits (10^1500 - 1) > 1500 > *Main> numDigits (10^1500) > 1501 > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
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