[erlang-questions] data structure like skips list in Erlang

[Vans S]

Either im completely missing the point or this has been in core erlang for a long time.

ets ordered_set.

> * allows custom compare function to order the data in a specific order

Setup a key tuple instead of custom compare function.  If you need to sort by values pull those out into the key tuple.

> * allows the user to iterate forward (next) and backward (prev) the data>

ets:next|prev
> Has someone already written a lib that offers such datastructure? Or maybe we can add a `prev(Key) function to gb_tree? Any idea i welcome :)

Only part that this does not fit is the requirement for a custom compare function.  If the custom compare is very complex, itl be very expensive regardless and wont scale.  If the custom compare is simple, a tuple key should replace it. 

    On Monday, October 9, 2017 6:24 AM, Frank Muller <frank.muller.erl@REDACTED> wrote:
 

 Benoit,
It’s called “Judy Array” and Erlang bindings do exist:https://github.com/knutin/judy

You can search for next/previous elements and do stuffs no other hashing libs support.
/Frank

I’m didn’t find anything that fit my need. There are some btree libs but that doesn’t fit my needs. Also a tree is not that fast for memory imo. I’m thinking to build a red-black tree lib that support iterations for it. Red-black trees look simple enough to be build as a pure functional data structure. Still contemplating though.

Benoit

> On 9 Oct 2017, at 09:52, Billy Svensson <billy.svensson@REDACTED> wrote:
>
> I've needed this in the past too. Couldn't find any lib that could do
> it so I just ported the floor and ceiling methods of the TreeMap class
> in Java to an Erlang module that operated on a gb_tree.
> It's a bit ugly though since you have to work on the "hidden" data
> structure of the gb_trees module, so would be great if this could be a
> part of the standard module instead.
>
> Not sure I still have that code, but could try to find it if you want.
> You would probably need to test it a bit better though since I only
> used it for a small hobby project.
>
> On Sun, Oct 8, 2017 at 8:21 PM, Benoit Chesneau <bchesneau@REDACTED> wrote:
>> hmm i forgot to mention i need to also lookup for the data so not sure if a
>> zipper is designed for it.
>>
>> I will have a loom kn the libs anyway, thanks for the links :)
>>
>> - benoît
>>
>> On 8 Oct 2017, at 19:33, Dmitry Kolesnikov <dmkolesnikov@REDACTED> wrote:
>>
>> Hello,
>>
>> It looks like you are looking for data structure called zipper. I’ve not
>> used any of these but there is at least two versions of zippers for Erlang.
>>
>> https://github.com/ferd/zippers
>> https://github.com/inaka/zipper
>>
>> - Dmitry
>>
>>
>> On 8 Oct 2017, at 20.14, Benoit Chesneau <bchesneau@REDACTED> wrote:
>>
>> Hi all,
>>
>> I’m looking for a datastructure like a skiplist to maintain an ordered set
>> of Key/Values. It has to have the following properties:
>>
>> * allows custom compare function to order the data in a specific order
>> * allows the user to iterate forward (next) and backward (prev) the data
>>
>> Has someone already written a lib that offers such datastructure? Or maybe
>> we can add a `prev(Key) function to gb_tree? Any idea i welcome :)
>>
>> - benoit
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