Feature: grouping functions

[kitsuyui]

from dataclasses import dataclass
from richset import RichSet

@dataclass(frozen=True)
class Something:
    id: int
    name: str

richset = RichSet.from_list([
    Something(1, 'one'),
    Something(2, 'two'),
    Something(3, 'three'),
])

richset.group_by(lambda item: item.id % 2)  # => {1: RichSet(records=(Something(id=1, name='one'), Something(id=3, name='three'))), 0: RichSet(records=(Something(id=2, name='two'),))}
richset.size_of_group_by(lambda item: item.id % 2)  # => {1: 2, 0: 1}
richset.count_of_group_by(key=lambda item: item.id % 2, predicate=lambda item: item.name.startswith('t'))  # => {1: 1, 0: 1}
richset.aggregate_by(key=lambda r: r.id % 2, fn=lambda a, b: a + b.name, initial='')  # => {1: 'onethree', 0: 'two'}

Close: #11

[codecov-commenter]

Codecov Report

Merging #15 (dbd4562) into main (287a68f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #15   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          195       203    +8     
=========================================
+ Hits           195       203    +8     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
richset/__init__.py	100.00% <100.00%> (ø)

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