[otlLib] buildCoverage does not remove duplicate glyphs

[dscorbett]

If a class in a feature file includes duplicate glyphs, buildCoverage is called on a list that includes duplicate glyphs. buildCoverage sorts the input glyphs but does not remove duplicates.

>>> import fontTools.otlLib.builder
>>> fontTools.otlLib.builder.buildCoverage(['A', 'B', 'A'], {'A': 1, 'B': 2}).glyphs
['A', 'A', 'B']

The spec says “A Coverage table defines a unique index value, the Coverage Index, for each covered glyph”, but a glyph listed multiple times won’t have a unique index.

This can cause a problem when using the OpenType Sanitizer: if a CoverageFormat1 table lists more glyphs than are declared in 'maxp', OTS reports an error.

[jenskutilek]

It’s at least consistent with what Adobe’s makeotf does, which also doesn’t remove guplicates.

[anthrotype]

I'm actually not sure about this. Can @khaledhosny or @simoncozens take a look?

[simoncozens]

Related problem I've been wanting to fix for a while: feaLib allows empty classes, which it then hands to the builder, and the builder dies. Both issues should be caught earlier.

[khaledhosny]

I defer to @dscorbett expertise here, but it looks like the right thing to do in general.