"""Builds glyphless.ttf: a TrueType font whose every glyph is empty and half an
em wide, for the invisible text layer of the PDFs made from DjVu books
(patches/pdf-text-and-links.patch embeds it). Text in that layer is drawn in
render mode 3 (invisible), so the glyphs never show; the font only has to be
valid, and the same width for every character so the page can scale each
word to fit its box.

Written for johnmcm.com (2026); public domain. Run with fontTools:
    python make_glyphless.py   (writes glyphless.ttf beside this script)
"""
from pathlib import Path

from fontTools.fontBuilder import FontBuilder
from fontTools.pens.ttGlyphPen import TTGlyphPen

UPM = 1000
ADVANCE = 500


def empty_glyph():
    return TTGlyphPen(None).glyph()


fb = FontBuilder(UPM, isTTF=True)
fb.setupGlyphOrder(['.notdef', 'glyph'])
# The PDF maps every character to glyph 1 itself (CIDToGIDMap), so no cmap
# entries are needed beyond one, which some readers expect to exist.
fb.setupCharacterMap({0x20: 'glyph'})
fb.setupGlyf({'.notdef': empty_glyph(), 'glyph': empty_glyph()})
fb.setupHorizontalMetrics({'.notdef': (ADVANCE, 0), 'glyph': (ADVANCE, 0)})
fb.setupHorizontalHeader(ascent=1000, descent=0)
fb.setupNameTable({'familyName': 'GlyphLessFont', 'styleName': 'Regular'})
fb.setupOS2(sTypoAscender=1000, sTypoDescender=0, usWinAscent=1000, usWinDescent=0)
fb.setupPost()
fb.save(str(Path(__file__).with_name('glyphless.ttf')))