Hi Greg,

All good. I think you described it fine. The question I had was more on what the big picture was. Why do it at all?

And yes, there’d be a way to manipulate the bits to manually construct it, but in most languages, that’d be pretty ugly anyway.

Regards,

Greg

Dr Greg Low

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From: Greg Keogh <gfkeogh@gmail.com>
Sent: Tuesday, 5 July 2022 8:58 AM
To: Dr Greg Low <greg@sqldownunder.com>
Cc: ozDotNet <ozdotnet@ozdotnet.com>
Subject: Re: 53-bit double

I might have missed it earlier Greg but was the actual problem that this helps with? I was intrigued by the underlying problem.

I didn't express myself clearly originally. I was trying to convert a 64-bit random integer into a double and guarantee that all possible 2^53 floating values in the range 0 to 1 could result. Someone suggested that weird bit of earlier incomprehensible code, but it turns out a simple shift-and-multiply does that trick.

There are lots of modern PRNGs that generate 64-bits, like the xoroshift** that is being used in the latest Random class. But how do you convert 64-bits "perfectly" into doubles? I finally confirmed a correct way.

I'm still wondering if there is a way of manually constructing an IEEE 754 number from the raw bits, but it makes my head hurt.

Cheers, GK