#!/usr/bin/env python3
"""Print metric-override fallback @font-face blocks for a web font.

The blocks make a local system font (Arial, Roboto) occupy the same space as the
web font, so switching from the fallback to the web font on load does not shift
layout (CLS).

Usage:
    python3 font-metrics.py <webfont.woff2> [--weight 500] [--name Montserrat] [--weights "400 800"]

Requires Python 3 with fonttools and brotli:  pip install fonttools brotli
"""

import argparse
import sys

from fontTools.ttLib import TTFont

# Advance-width metrics of common fallback fonts, measured over the same
# character set as the web font (a-z A-Z 0-9 space), normalised to unitsPerEm.
# The ascent/descent/lineGap values are not used: only the average width matters
# for size-adjust. Keys are the local() src names.
FALLBACKS = {
    'Arial': {'local': 'Arial', 'suffix': '', 'avg': 0.5742885044642857},
    'Roboto': {'local': 'Roboto', 'suffix': ' Android', 'avg': 0.5608646453373016},
}

CHARS = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 '


def web_metrics(path, weight):
    font = TTFont(path)
    if weight is not None and 'fvar' in font:
        from fontTools.varLib import instancer

        font = instancer.instantiateVariableFont(font, {'wght': weight}, inplace=False)
    head = font['head']
    hhea = font['hhea']
    cmap = font.getBestCmap()
    hmtx = font['hmtx']
    upm = head.unitsPerEm
    widths = [hmtx[cmap[ord(c)]][0] / upm for c in CHARS if ord(c) in cmap]
    return {
        'ascent': hhea.ascent / upm,
        'descent': abs(hhea.descent) / upm,
        'lineGap': hhea.lineGap / upm,
        'avg': sum(widths) / len(widths),
    }


def family_name(path):
    font = TTFont(path, lazy=True)
    for id_ in (16, 1):
        name = font['name'].getDebugName(id_)
        if name:
            return name.split()[0]
    return 'WebFont'


def pct(value):
    text = f'{value:.2f}'.rstrip('0').rstrip('.')
    return f'{text}%'


def main():
    parser = argparse.ArgumentParser(description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter)
    parser.add_argument('webfont', help='path to the web font file (woff2/ttf/otf)')
    parser.add_argument('--weight', type=int, default=500, help='weight to measure variable fonts at (default: 500)')
    parser.add_argument('--name', default=None, help='font family name used in the fallback family names')
    parser.add_argument('--weights', default='400 800', help='font-weight value for the fallback rules (default: "400 800")')
    args = parser.parse_args()

    web = web_metrics(args.webfont, args.weight)
    name = args.name or family_name(args.webfont)

    for key, fb in FALLBACKS.items():
        size_adjust = web['avg'] / fb['avg']
        if not size_adjust:
            print(f'warning: empty width for {key}', file=sys.stderr)
            continue
        print('@font-face {')
        print(f'    ascent-override: {pct(web["ascent"] / size_adjust * 100)};')
        print(f'    descent-override: {pct(web["descent"] / size_adjust * 100)};')
        print(f"    font-family: '{name} Fallback{fb['suffix']}';")
        print(f'    font-weight: {args.weights};')
        print(f'    line-gap-override: {pct(web["lineGap"] / size_adjust * 100)};')
        print(f'    size-adjust: {pct(size_adjust * 100)};')
        print(f"    src: local('{fb['local']}');")
        print('}')
        print()


if __name__ == '__main__':
    main()