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Thank you all for your responses. I'm still digesting some of your notes, but I do see now that the scope of my little investigation was too narrow, that's why I wasn't able to find failing cases. (And as I haven't found those I was wondering how that can be...)

Fortunately my program would be used for statistical purposes, and nowhere near to the petabyte (2**50) file size range, so it'll avoid the real problems.

It is still interesting what would I do if I should care about the inaccuracy. Probably something like this sub I've written since my OP:

use bigint; { my @lookup = (1); # 2, 4, 8, ... sub int_log2 { my $n = shift; return if $n < 1; my $exp = 0; # this still could be optimized by starting at # $exp = min( $#lookup, $some_educated_guess_using_floating_point_ +approximation ) while ( $lookup[ $exp++ ] <= $n ) { $lookup[$exp] = 2 * $lookup[ $exp - 1 ] if not exists $lookup[$e +xp]; } return $exp - 2; } } for ( 1 .. 2**10 ) { print int_log2(2**$_-1), ' ', int_log2(2**$_), ' ', int_log2(2**$_+1 +), "\n"; }

This completes under 100sec on my several year old laptop.

In reply to Re: May I be bitten by floating point arithmetic in the following restricted case? by rubasov
in thread May I be bitten by floating point arithmetic in the following restricted case? by rubasov

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