[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

calc-eval and lsh/logand of large numbers

From: Sam Halliday
Subject: calc-eval and lsh/logand of large numbers
Date: Mon, 27 Oct 2014 15:58:37 -0700 (PDT)
User-agent: G2/1.0

Hi all,

I was playing with calc-eval to get a feel for the arbitrary precision support 
and decided to see if it could be used to represent a BitSet. If you're not 
familiar with the concept, imagine a list of bits and if the N'th bit is true 
then the set contains the integer N. It's reasonably efficient at storing 
arbitrary collections of integers: memory requirement is specified by the 
maximum value rather than the number of entries in the set.

This looked promising:

  (require 'calc)
  (defmath bitIndex (i)
    (lsh 1 i))

  (calc-eval "bitIndex(10)") ; "1024"

but when I went to a number that would be shifted beyond the 64 bit length of 
an integer it all fell apart

  (calc-eval "bitIndex(64)") ; "0"

I would have expected this to return "18446744073709551616".

Is this a known limitation or have I simply not understood something?

BTW, if anybody is interested I was planning on doing something like the 
following. If you have a cleaner way of doing this, please let me know as I'll 
be interested to hear about it. The reason I'm doing this is because I am 
writing an S-Exp serialisation layer in Scala and this notation works well for 
serialising Scala's BitSet's... I was curious to see if I could get it to work 
in elisp:

  (defmath bitsetAnd (bitset i)
    (logand (lsh 1 i) bitset))
  (defun bitsetContains (bitset i)
    (<= 1 (calc-eval "bitsetAnd($, $$)" 'raw bitset i)))

  (bitsetContains "10#3" 0) ; 't
  (bitsetContains "10#3" 1) ; 't
  (bitsetContains "10#3" 2) ; nil
  (bitsetContains "16#10000000000000000" 64) ; nil
  (bitsetContains "16#10000000000000001" 0) ; t
  (bitsetContains "16#10000000000000001" 64) ; t WRONG
  (bitsetContains "16#10000000000000002" 1) ; t
  (bitsetContains "16#10000000000000002" 64) ; t WRONG
  (bitsetContains "16#10000000000000002" 0) ; nil

Best regards,

reply via email to

[Prev in Thread] Current Thread [Next in Thread]