find-ghost-kerning.py

"""Find GDEF variation tables assigned to class * to class 0 kerning, which should
never exist."""

import argparse
import logging
from pathlib import Path

import fontTools.ttLib

logging.basicConfig(level=logging.INFO, format="%(levelname)s: %(message)s")

parser = argparse.ArgumentParser()
parser.add_argument("fonts", nargs="+", type=Path)
args = parser.parse_args()

for p in args.fonts:
    logging.info(p)

    font = fontTools.ttLib.TTFont(p)

    if not "GPOS" in font:
        logging.info(f"No GPOS in {p}, skipping.")
        continue

    class_kerning_tables = [
        t
        for l in font["GPOS"].table.LookupList.Lookup
        for t in l.SubTable
        if t.Format == 2
    ]

    if not class_kerning_tables:
        logging.info(f"No class kerning in {p}, skipping.")
        continue

    for class_kerning_table in class_kerning_tables:
        for class1_index, class1_record in enumerate(class_kerning_table.Class1Record):
            class2_zero = class1_record.Class2Record[0]

            # A XAdvDevice table in class1_record.Class2Record[0] points to a GDEF
            # variation table that should not exist for a class 0.
            if vars(class2_zero.Value1).get("XAdvDevice", None) is not None:
                class_glyphs = ", ".join(
                    sorted(
                        k
                        for k, v in class_kerning_table.ClassDef1.classDefs.items()
                        if v == class1_index
                    )
                )
                logging.warning(
                    f"Ghost kerning against class 0: class {class1_index} ({class_glyphs})"
                )