Re: time impact of the delta signature

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On 23/02/2019 12:40, Graham Cobb wrote:
> On 23/02/2019 09:24, Denis Corbin wrote:
>> Of course an approximate value of root mathematical functions is 
>> sufficient to answer the need. And as infinint type is just an
>> integer type that has no limit (except the available memory
>> resources) and that the shift left and right operations are
>> already implemented, thus little remains to do to add the
>> approximate square, cube, ... root functions! :)
> 
> By the way, it also occurred to me that infinint exp2 can easily
> be optimised by a compile-time lookup table for the first N values
> (maybe 32 or 64) -- backed up by a generalised implementation for
> larger values (maybe based on knowing the internal structure of
> infinint instead of needing an actual shift loop).

that's effectively an option. Though such option hits the problem of
how big should be the size of the lookup table. Anyway, I will first
have to measure the execution cost of this optimized/approximate math
operation and see if it worth optimizing any further by a lookup
table, noted.

> 
> In reality it may be easiest using the rs_long_sqrt algorithm
> pointed out by andycapo for filesizes which do fit in a simple type
> and keeping the log/exp algorithm for cases where the
> multiplications in the> rs_long_sqrt algorithm would require
> expensive infinint arithmetic.

as filesize are stored as infinint, it would require testing the value
of the file size below which it would be possible to convert it to a
'simple type' then apply the rs_long_sqrt to it... I'm affraid the
conversion will ruine the optimization ... will here too have to
measure the cost of conversion against the gain rs_long_sqrt brings.

Thanks for these ideas
Denis

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