On Thursday 25 August 2011, 19:57:37, Andrew Coppin wrote:
Quoting the Haddock documentation for Data.Bits.bitSize:
"Return the number of bits in the type of the argument. The actual value of the argument is ignored. The function bitSize is undefined for types that do not have a fixed bitsize, like Integer."
Does anybody else think it would be *far* more useful if bitSize applied to an Integer would tell you how many bits that particular Integer is using? Especially given that it can vary?
I'm not sure about that.
Is there a way to actually determine how many bits are in an Integer?
Sure. The exact method depends on what result you want and which integer-* package you use. You have to handle 0 and negative n first (how to treat negative n is not obvious). Then, for n > 0, f you want 'index of highest set bit' (+1), (1 +) integerLogBase 2 n does the trick (integerLogBase is available from GHC.Float, as of 7.2.1, it's decently fast). If you want 'WORD_SIZE_IN_BITS * number of words used', you could use the above and round up to the next multiple of WORD_SIZE_IN_BITS, or you could make use of the representation of Integers; with integer-gmp, data Integer = S# Int# | J# Int# ByteArray# so usedWords (S# _) = 1 usedWords (J# s# _) = I# s# (nota bene, I don't think it's positively guaranteed that the highest order word in a (J# _ _) is nonzero, so usedWords could give a higher answer than rounding up from integerLogBase 2 n).