### Modules
from collections import defaultdict

### Constants
FIRST_PREFIX = 'public.kern1.'
SECOND_PREFIX = 'public.kern2.'

### Functions
def flatKerning(aFont):
    flatK = defaultdict(int)
    flatK.setdefault(0)

    for pair, correction in aFont.kerning.items():
        first, second = pair

        # both groups
        if first.startswith(FIRST_PREFIX) and second.startswith(SECOND_PREFIX):
            for firstGlyph in aFont.groups[first]:
                for secondGlyph in aFont.groups[second]:
                    flatK[(firstGlyph, secondGlyph)] = correction

        # first group, second glyph
        elif first.startswith(FIRST_PREFIX):
            for firstGlyph in aFont.groups[first]:
                flatK[(firstGlyph, second)] = correction

        # first glyph, second group
        elif second.startswith(SECOND_PREFIX):
            for secondGlyph in aFont.groups[second]:
                flatK[(first, secondGlyph)] = correction

        # first glyph, second glyph
        else:
            flatK[(first, second)] = correction

    return flatK