#!/usr/bin/env python3
"""Guard against invisible-text palettes (BACKLOG item 25).

Asserts, for every themes/*.json, the WCAG contrast ratio of each
hex-on-hex fg/bg pairing the generated swaync CSS uses. Pairings built
from alpha(@text) tints over @base are contrast-by-construction and
need no assert. Extend PAIRINGS as more generated surfaces are fixed to
palette-safe roles (item 27 tracks waybar/rofi, which still use
subtext/surface and would fail here today).

Wired as `checks.theme-contrast` in flake.nix; also runnable directly:
    python3 tools/check-theme-contrast.py themes/
"""
import json
import sys
from pathlib import Path

# (fg role, bg role, minimum ratio, consumer). Text pairings only —
# borders (@accent/@bad on @base in swaync) are decorative and not held
# to a text ratio (miasma's bad-on-base is 2.3, fine for a border).
PAIRINGS = [
    ("text", "base",   4.5, "swaync body/summary/titles on the toast"),
    ("base", "accent", 3.0, "swaync widget-title button:hover"),
]


def channel(c: int) -> float:
    c /= 255
    return c / 12.92 if c <= 0.03928 else ((c + 0.055) / 1.055) ** 2.4


def luminance(hexstr: str) -> float:
    hexstr = hexstr.lstrip("#")
    r, g, b = (int(hexstr[i:i + 2], 16) for i in (0, 2, 4))
    return 0.2126 * channel(r) + 0.7152 * channel(g) + 0.0722 * channel(b)


def ratio(fg: str, bg: str) -> float:
    hi, lo = sorted((luminance(fg), luminance(bg)), reverse=True)
    return (hi + 0.05) / (lo + 0.05)


def main() -> int:
    themes_dir = Path(sys.argv[1] if len(sys.argv) > 1 else "themes")
    failures = []
    themes = sorted(themes_dir.glob("*.json"))
    if not themes:
        print(f"no theme JSONs found under {themes_dir}", file=sys.stderr)
        return 1
    for path in themes:
        colors = json.loads(path.read_text())["colors"]
        for fg, bg, minimum, consumer in PAIRINGS:
            r = ratio(colors[fg], colors[bg])
            if r < minimum:
                failures.append(
                    f"{path.stem}: {fg} on {bg} = {r:.2f} < {minimum}"
                    f" ({consumer}; {colors[fg]} on {colors[bg]})"
                )
    if failures:
        print("theme contrast failures:", file=sys.stderr)
        for f in failures:
            print(f"  {f}", file=sys.stderr)
        return 1
    print(f"{len(themes)} themes x {len(PAIRINGS)} pairings: all pass")
    return 0


if __name__ == "__main__":
    sys.exit(main())