For 64 bit word size: https://stackoverflow.com/questions/11376288/fast -computing-of-log2-for-64-bit-integers/11398748#11398748
This can be derived from examples in https://graphics.stanford.edu/~sea nder/bithacks.html which gives the 32 bit version. However, the math/bits package gives you Len, which does the same thing: https://golang.org/pkg/math/bits/ On Sun, 2018-04-08 at 08:45 -0700, Darko Luketic wrote: > Yeah I'm not a math wiz and the older I get the harder I find maths. > But sometimes you can't evade maths in programming. > > So here's my question for math wizards. > Essentially, I'm trying to write an url shortener. > And I found this package I agree with (don't re-invent the wheel) > https://github.com/neptulon/shortid > > It base64-encodes a random number of [bits that need to be dividable > by 8] > length. > Dividable by 8 because 1 char = 8 bits. > > It generates an input+1/8 length result. > > What I like is, get a count of all links in the db, easy. > But now the problem. > > 8bit = 64^2 possible results without overlapping., 2 characters > (therefore > 64^2) > 16bit = 64^3 etc > up to 64bit with would equal an uint64 value > > My problem is computing the formula. > if link_count > number_bits/2 { > number_bits++ > } > > but how do I find out the bits? > and make it future proof (yes even with the size of uint64) > aka I must not define ranges e.g. >=0 <=(64^2)/2 > because I'm afraid the value would overflow in Go. (64^9) > > I could > exponent := 2 // starts at 2 because bits = (exponent-1)*8 > result := link_count / (64^exponent) > if result < 1.0 { > number_bits = exponent-1 > if result < 0.5 { > good_to_generate = true > } else { > exponent++ > } > } > > but it would still probably overflow > > However... MongoDB (my backing database), at least the mgo driver > returns > an int, which on 64bit machines is int64. > So there's my limit. Let's hope "int128" will be enough. > > I guess I answered my own question. I'll still post it, because it > took > some time to write. > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
