requires: {#ExtensibleCollection. #Dictionary. #Method. #Set. #ExtensibleArray}.
provides: {#Grouping. #SequentialGrouping}.

collections addPrototype: #Grouping derivedFrom: {ExtensibleCollection}.
"Similar to a Bag, storing elements given into bins keyed by the result of
applying a block to each."
Grouping addSlot: #contents valued: Dictionary newEmpty.
Grouping addSlot: #attributeBlock valued: Method Identity.

g@(Grouping traits) newSize: n
[| newG |
  newG: g clone.
  newG contents: (g contents newSize: n).
  newG
].

c@(Collection traits) grouped
[c groupedBy: Method Identity].

c@(Collection traits) groupedBy: keyBlock
[| newG |
  newG: Grouping newEmpty.
  newG attributeBlock: keyBlock.
  newG addAll: c.
  newG
].

g@(Grouping traits) add: obj
[| key group |
  key: (g attributeBlock applyWith: obj).
  group: (g contents at: key ifAbsent:
    [g contents at: key put: Set newEmpty]).
  group include: obj.
  obj
].

g@(Grouping traits) remove: obj
[| key |
  key: (g attributeBlock applyWith: obj).
  group: (g contents at: key ifAbsent: [^ obj]).
  group remove: obj.
  group isEmpty ifTrue: [g contents removeKey: key].
  obj
].

collections addPrototype: #SequentialGrouping derivedFrom: {Grouping}.
"A Grouping that preserves the order of an original sequential ordering
(whether from a WriteStream on the Grouping or an actual Sequence)."

s@(Sequence traits) groupedBy: keyBlock
[| newG |
  newG: SequentialGrouping newEmpty.
  newG attributeBlock: keyBlock.
  newG addAll: c.
  newG
].

g@(Grouping traits) add: obj
[| key group |
  key: (g attributeBlock applyWith: obj).
  group: (g contents at: key ifAbsent:
    [g contents at: key put: ExtensibleArray newEmpty]).
  group include: obj.
  obj
].