evacuate in hashmap: where to put when migrate data from oldbucket to bucket

[lizhi...@gmail.com]

when evacuate oldbucket, go will migrate the data to a new position. 

and i found it used X and Y as two bucket where Xi equal the old index in h.oldbuckets and Yi equal Xi plus newbit

suppose the length of oldbuckets is 8 ， and the hash of a key is 13 (just a example), then it will be put in the 5. Now, the buckets grow to 16,  it should be put in the 13. I think.

but i found the code below.  

https://github.com/golang/go/blob/6732fcc06df713fc737cee5c5860bad87599bc6d/src/runtime/hashmap.go#L1115

useX will be true, if so, the 13 will be put in the 5 still, then i can't understand it. Did i miss something ? 

go 1.9

[Ian Lance Taylor]

When a bucket is split into two new buckets, X and Y, each value has
to go to either X or Y. The decision is made by looking at the hash.
That is the code you are looking at.

Ian

[keith....@gmail.com]

On Thursday, March 15, 2018 at 9:46:34 AM UTC-7, Ian Lance Taylor wrote:

On Thu, Mar 15, 2018 at 1:34 AM,  <lizhi...@gmail.com> wrote:
>
> when evacuate oldbucket, go will migrate the data to a new position.
>
> and i found it used X and Y as two bucket where Xi equal the old index in
> h.oldbuckets and Yi equal Xi plus newbit
>
> suppose the length of oldbuckets is 8 ， and the hash of a key is 13 (just a
> example), then it will be put in the 5. Now, the buckets grow to 16,  it
> should be put in the 13. I think.

Correct.

 

>
> but i found the code below.
>
> https://github.com/golang/go/blob/6732fcc06df713fc737cee5c5860bad87599bc6d/src/runtime/hashmap.go#L1115
>
>
>
>
>
> useX will be true, if so, the 13 will be put in the 5 still, then i can't
> understand it. Did i miss something ?

useX will be false because 13 & 8 == 0 is false.

[lizhi...@gmail.com]

thanks for your correction ... i think i lost my mind some how. 

but can you tell me how the formula works? Or give me a hint that i can search 

[lizhi...@gmail.com]

I  think i have got it, thank you all