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__all__ = ["maxCtxFont"]
def maxCtxFont(font):
"""Calculate the usMaxContext value for an entire font."""
maxCtx = 0
for tag in ("GSUB", "GPOS"):
if tag not in font:
continue
table = font[tag].table
if not table.LookupList:
continue
for lookup in table.LookupList.Lookup:
for st in lookup.SubTable:
maxCtx = maxCtxSubtable(maxCtx, tag, lookup.LookupType, st)
return maxCtx
def maxCtxSubtable(maxCtx, tag, lookupType, st):
"""Calculate usMaxContext based on a single lookup table (and an existing
max value).
"""
# single positioning, single / multiple substitution
if (tag == "GPOS" and lookupType == 1) or (
tag == "GSUB" and lookupType in (1, 2, 3)
):
maxCtx = max(maxCtx, 1)
# pair positioning
elif tag == "GPOS" and lookupType == 2:
maxCtx = max(maxCtx, 2)
# ligatures
elif tag == "GSUB" and lookupType == 4:
for ligatures in st.ligatures.values():
for ligature in ligatures:
maxCtx = max(maxCtx, ligature.CompCount)
# context
elif (tag == "GPOS" and lookupType == 7) or (tag == "GSUB" and lookupType == 5):
maxCtx = maxCtxContextualSubtable(maxCtx, st, "Pos" if tag == "GPOS" else "Sub")
# chained context
elif (tag == "GPOS" and lookupType == 8) or (tag == "GSUB" and lookupType == 6):
maxCtx = maxCtxContextualSubtable(
maxCtx, st, "Pos" if tag == "GPOS" else "Sub", "Chain"
)
# extensions
elif (tag == "GPOS" and lookupType == 9) or (tag == "GSUB" and lookupType == 7):
maxCtx = maxCtxSubtable(maxCtx, tag, st.ExtensionLookupType, st.ExtSubTable)
# reverse-chained context
elif tag == "GSUB" and lookupType == 8:
maxCtx = maxCtxContextualRule(maxCtx, st, "Reverse")
return maxCtx
def maxCtxContextualSubtable(maxCtx, st, ruleType, chain=""):
"""Calculate usMaxContext based on a contextual feature subtable."""
if st.Format == 1:
for ruleset in getattr(st, "%s%sRuleSet" % (chain, ruleType)):
if ruleset is None:
continue
for rule in getattr(ruleset, "%s%sRule" % (chain, ruleType)):
if rule is None:
continue
maxCtx = maxCtxContextualRule(maxCtx, rule, chain)
elif st.Format == 2:
for ruleset in getattr(st, "%s%sClassSet" % (chain, ruleType)):
if ruleset is None:
continue
for rule in getattr(ruleset, "%s%sClassRule" % (chain, ruleType)):
if rule is None:
continue
maxCtx = maxCtxContextualRule(maxCtx, rule, chain)
elif st.Format == 3:
maxCtx = maxCtxContextualRule(maxCtx, st, chain)
return maxCtx
def maxCtxContextualRule(maxCtx, st, chain):
"""Calculate usMaxContext based on a contextual feature rule."""
if not chain:
return max(maxCtx, st.GlyphCount)
elif chain == "Reverse":
return max(maxCtx, 1 + st.LookAheadGlyphCount)
return max(maxCtx, st.InputGlyphCount + st.LookAheadGlyphCount)
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