[WIP] Implement dict to receive Object as key

[HyeockJinKim]

Implement dict struct that takes Object as key
store key and value in array together so that dict is ordered

Issue #118

[codecov-io]

Codecov Report

Merging #119 into master will decrease coverage by 0.65%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master    #119      +/-   ##
=========================================
- Coverage   72.85%   72.2%   -0.66%     
=========================================
  Files          60      60              
  Lines       11949   12057     +108     
=========================================
  Hits         8706    8706              
- Misses       2709    2815     +106     
- Partials      534     536       +2

Impacted Files	Coverage Δ
py/dict.go	36.65% <0%> (-35.03%)	⬇️

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[HyeockJinKim]

Since Hash is not implemented properly in gpython, I'm going to implement a dict that takes an Object as a key first.

So, even if an object representing the same value comes in as key, the value is not found correctly.

This can break the test code, so instead of getting rid of StringDict, I will fully implement Dict and replace StringDict with Dict.

[ncw]

I put some notes inline...

Making a Dict is quite a dififcult thing to do (probably why I didn't do it originally!)

Perhaps the thing to do to start with would be to make one with a simple internal structure which is just

type keyValue {
    key Object
    value Object
}

type Dict struct {
    kvs []keyValue
}

And make that work. Yes it won't be very efficient if there are lots of keys, but it should enable us to get the interface right, then we can work on optimizing it.

I note that in go1.14 the go team will expose the internal hashing function which would make implementing this much easier.

[ncw]

I don't think this is the right representation alas...

Putting the Object as the key means that you are relying on how Go compares interfaces. It does this with a shallow compare. An interface is basically

• pointer to type
• pointer to data

so comparing two interfaces just compares the pointer to type and pointer to data - it doesn't compare the data.

This means that you could insert two objects eg py.String("hello") and py.String("hello") and these would both get inserted into the dictionary.

I think what we want as the key is

• python type
• hash of object

And the values need to be a []Object (because different objects can have the same hash).

[HyeockJinKim]

There is no hash implementation, so I've got Object as key now.

We only receive hash values as key values. so I'm going to get int as the key.
What do you think about this way?

type Dict struct {
	keys  map[int]int  // int key

[ncw]

I think maybe we should try the really simple version first, then we can make it more efficient later

type dictEntry struct {
    key Object
    value Object
}
type Dict struct {
    entries []dictEntry 
}

[sbinet]

@ncw, to be nit-picky, the way py.String is currently implemented, I believe this would work.
see:
https://play.golang.org/p/bDvOEmkim6r

(ie: py.String just contains values and no pointers.)

but ok with going with the KISS version first.

[ncw]

Indeed! The problem comes when the py.Object interface is satisfied with a pointer: https://play.golang.org/p/vf4dGmh0Gb4