Check whether two fonts are metrics compatible by testing on random strings

metrics_compatible.py

import random
import string
import uharfbuzz as hb
import sys
from fontTools.ttLib import TTFont

def generate_random_string(fp):
    f = TTFont(fp)
    cmap = f.getBestCmap()
    length = random.randint(3, 100)
    letters = [chr(c) for c in cmap]
    return ''.join(random.choice(letters) for i in range(length))


def string_length(fp, text):
    # Create a font object using a TrueType font file
    blob = hb.Blob.from_file_path(fp)
    face = hb.Face(blob)
    font = hb.Font(face)

    # Set the string to be rendered

    buf = hb.Buffer()
    buf.add_str(text)
    buf.guess_segment_properties()

    hb.shape(font, buf)
    pos = buf.glyph_positions
    return sum(g.x_advance for g in pos)

while True:
    rand_string = generate_random_string(sys.argv[1])
    original_length = string_length(sys.argv[1], rand_string)
    new_length = string_length(sys.argv[2], rand_string)

    if original_length != new_length:
        print(f"{rand_string}, {original_length}, {new_length}")

The above is optimized a bit for execution, and also only takes the intersection of the two fonts’ cmaps minus a disallowed set (plus nbspace which we don’t test for)