Refactor cleanChild()

[rvagg]

Closes: https://github.com/ipfs/go-hamt-ipld/issues/56

I've tried to push cleanChild() around in the various ways I was suspecting it might be flawed, but couldn't make it misbehave, so I'm going to call it good. Added more testing for CHAMP compaction along with a printHamt() that can show you the kinds of shapes you have.

(edit: note that the below examples are using identity hash with a bitWidth of 8 so each character is treated as an index at one level, so AAA1 and AAA2 collide for 3 levels, it's a useful way to force perverse structures without having to deal with the annoying randomness of a hash function).

So in this test we do things like:

‣ bafy2bzaceaabidljpjqxkb3bwz2kymapl6w6zazcaehtwh55d3solznpudfji:                                                                                     
  ⇶ [ AAAAAA11, AAAAAA12, AAAAAA21 ]                                                                                                                  

overflows when we add another colliding element to:

‣ bafy2bzacec2e7uxlmpn5bzfjfhrjbvgkgyfbi76k43epdijp626i3baaao436:                                                                                     
  ‣ bafy2bzacebgcvdzkq4glgp2kl65oyz65w2qb6ptbzjwxgbvnw2hwzs25bgeaw:                                                                                   
    ‣ bafy2bzacect7ahd4hdy24w7jfrttzgx5mh264lzobxdq5dyisqeelol6dia5o:                                                                                 
      ‣ bafy2bzacebxdmdcvhpnpsiia632eiov45yynuijl3faw7zzfzbupg4tombhnu:                                                                               
        ‣ bafy2bzacecrptawydwevr4rt3w4mbvuvh6wteh2w3kplwohkzu64zip5lslc2:                                                                             
          ‣ bafy2bzacedxu2dpomrsnqgj45pbkgyxrc4nujwfgjrwlpm3nkjvfomhkmqang:                                                                           
            ‣ bafy2bzaceadtlk6fe4pishrnyakhlz32iiakebraxudz34r4o5um3sgof3azs:                                                                         
              ⇶ [ AAAAAA11, AAAAAA12 ]                                                                                                                
              ⇶ [ AAAAAA21, AAAAAA22 ]                                                                                                                

and then back again.

It also tests a mid-way state where collisions happen at half that tree height with a collection of elements that collide in another way:

‣ bafy2bzaceacaykqvidugqydd2lskm7ajgdionqzgh22odxcmtfo6jkj6legqe:
  ‣ bafy2bzacear4ozwsojhwrjc3yroecudxdchelx74k2h6xlgzwtjsrpgo7sz3o:
    ‣ bafy2bzacec4owi2jruh53fve4vn5tyzg4yw7lfmrsuvshdrlsk4rdaa3nnbga:
      ‣ bafy2bzacedvt2qjfegib2k7bzfj4oxe4nxy46amtfwmybhhqjv4vs56xlv6gq:
        ‣ bafy2bzacecknjra3gxqdsaydwjwhgkh3pvun4kphn3xgpm6ek36mg3kzf3zrc:
          ⇶ [ AAA11AA ]
          ⇶ [ AAA12AA ]
          ⇶ [ AAA13AA ]
          ⇶ [ AAA14AA ]
        ‣ bafy2bzacecrptawydwevr4rt3w4mbvuvh6wteh2w3kplwohkzu64zip5lslc2:
          ‣ bafy2bzacedxu2dpomrsnqgj45pbkgyxrc4nujwfgjrwlpm3nkjvfomhkmqang:
            ‣ bafy2bzaceadtlk6fe4pishrnyakhlz32iiakebraxudz34r4o5um3sgof3azs:
              ⇶ [ AAAAAA11, AAAAAA12 ]
              ⇶ [ AAAAAA21, AAAAAA22 ]

and rolls that back out to see if it behaves properly. And it does.

I've refactored cleanChild() to a state that I think has more clarity about the various states it can be in and reasons it has to take action or not. I think this more closely matches the CHAMP semantics if you want to read the paper and see those semantics in this code. Maybe that's subjective, but I think I've reduced the number of branches here which should in itself clear up some confusion.

[ianopolous]

I would recommend fuzz testing this as well (generate random trees, add and remove a mapping and check the root cid hasn't changed). We found that useful after @rvagg pointed out the same problem in our champ implementation in Peergos.

[warpfork]

I'm leaving a couple of blobs of notes that I made as I both reviewed this and wandered around the codebase at large -- so if they're not topical to you, just mark 'em resolved. :)

Love the very explicit way the TestFillAndCollapse function steps through things. It talks its way through a lot of the scenarios I felt I was still looking for as I took stock of the other existing tests.

[rvagg]

rebased to master and autosquashed

[warpfork]

I don't know why I balked on giving this a thumbs-up earlier, but looking at it more, the tests again look pretty comprehensive, so it seems likely that this is pretty solid.