[HarfBuzz] Overwrite certain font characters

[Kelvin Ma]

Knockout overwrites most whitespace characters to a hardcoded width and
special codepoint since most fonts can’t be trusted to properly construct
or even contain these characters.

## Codepoints ##

nonbreaking_spaces = {
            '\u00A0': -30, # nbsp
            '\u2007': -36, # figure
            '\u202F': -40, # narrow nbsp
            }

breaking_spaces = {
            '\u2003': -31, # em
            '\u2002': -32, # en
            '\u2004': -33, # 1/3
            '\u2005': -34, # 1/4
            '\u2006': -35, # 1/6

            '\u2008': -37, # punctuation
            '\u2009': -38, # thin
            '\u200A': -39, # hair

            '\u205F': -41, # math med
            }


## Widths (as fraction of em) ##

space = self.get_advance(self.character_index(' '), True)/UPM
fig = self.get_advance(self.character_index('0'), True)/UPM
punc = self.get_advance(self.character_index(','), True)/UPM

self._widths = {
        None: 0,
        ' ': space,
        '\t': 0,
        '\u00A0': space,
        '\u2003': 1, # em
        '\u2002': 0.5, # en
        '\u2004': 1/3, # 1/3
        '\u2005': 0.25, # 1/4
        '\u2006': 1/6, # 1/6
        '\u2007': fig, # figure
        '\u2008': punc, # punctuation
        '\u2009': 0.2, # thin
        '\u200A': 0.1, # hair
        '\u202F': 0.2, # narrow nbsp
        '\u205F': 4/18, # math med
        }


## character labels (for UI) ##

self.spacenames = {
        -30: 'nb', # nbsp
        -31: 'em', # em
        -32: 'en', # en
        -33: '1/3', # 1/3
        -34: '1/4', # 1/4
        -35: '1/6', # 1/6
        -36: 'fig', # figure
        -37: 'pn', # punctuation
        -38: '1/5', # thin
        -39: 'h', # hair
        -40: 'nnb', # narrow nbsp
        -41: 'mt', # math med
        }

My old layout engine would simply output the special negative codepoint and
advance like with any other character when it encountered those unicode
whitespace characters. (The negative codepoints are so that Knockout’s
renderer knows how to display them.) How do I do this in Harfbuzz? They are
important for mathematical typesetting.

[image: Inline image 1]
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