"""Derive a custom dashboard palette from one brand hex, then check text contrast.

Usage:  python palette.py "#7a3cff"

Prints the accent tokens for the body{} (light) and body.dark{} blocks of
dashboard.html, which keeps the Tebra Sky base for everything else, then a
contrast table for light and dark. A brand colour that is too light or too dark
is nudged until text on it is readable. Exits 1 if any check still fails.
"""
import sys

WHITE, BLACK = "#ffffff", "#000000"
# Tebra Sky base colours the accents sit on. Keep in step with dashboard.html.
BASE = {"light": {"bg": "#eaf6fb", "ink": "#1c2b33"}, "dark": {"bg": "#182a38", "ink": "#e6eef4"}}
LOG_TEXT = "#f68c82"  # dimmest log line colour; the log box background is --accent-dark


def rgb(h):
    return [int(h[i:i + 2], 16) for i in (1, 3, 5)]


def mix(a, b, t):
    """Blend hex colour a toward b by t (0..1)."""
    return "#" + "".join(f"{round(x + (y - x) * t):02x}" for x, y in zip(rgb(a), rgb(b)))


def contrast(a, b):
    """WCAG 2 contrast ratio between two hex colours."""
    def lum(h):
        c = [v / 255 for v in rgb(h)]
        c = [x / 12.92 if x <= 0.03928 else ((x + 0.055) / 1.055) ** 2.4 for x in c]
        return 0.2126 * c[0] + 0.7152 * c[1] + 0.0722 * c[2]
    hi, lo = sorted((lum(a), lum(b)), reverse=True)
    return (hi + 0.05) / (lo + 0.05)


def checks(t):
    """(mode, what, ratio, needed) for every text pair the accents affect."""
    rows = []
    for mode, m in t.items():
        bg, ink = BASE[mode]["bg"], BASE[mode]["ink"]
        rows += [(mode, "white button text on --accent", contrast(WHITE, m["--accent"]), 3),
                 (mode, "white button text on --accent-2", contrast(WHITE, m["--accent-2"]), 3),
                 (mode, "--accent text on the page", contrast(m["--accent"], bg), 3),
                 (mode, "ink on --accent-soft buttons", contrast(ink, m["--accent-soft"]), 4.5),
                 (mode, "log text on --accent-dark", contrast(LOG_TEXT, m["--accent-dark"]), 4.5)]
    return rows


def nudge(colour, fix):
    """Step colour toward whatever fix() returns (WHITE/BLACK) until it returns None."""
    for _ in range(60):
        toward = fix(colour)
        if toward is None:
            break
        colour = mix(colour, toward, 0.06)
    return colour


def derive(brand):
    brand = brand.strip().lower()
    if len(brand) == 4:  # #abc -> #aabbcc
        brand = "#" + "".join(ch * 2 for ch in brand[1:])
    out = {}
    for mode, lift in [("light", 0.15), ("dark", 0.10)]:
        bg, ink = BASE[mode]["bg"], BASE[mode]["ink"]
        dark_page = mode == "dark"

        def fix_accent(c):
            if contrast(c, bg) < 3:  # accent text must stand out from the page
                return WHITE if dark_page else BLACK
            if contrast(WHITE, mix(c, WHITE, lift)) < 3:  # white text on the button
                return BLACK
            return None

        a = nudge(brand, fix_accent)
        soft = mix(a, BLACK, 0.35) if dark_page else mix(a, WHITE, 0.82)
        out[mode] = {
            "--accent": a,
            "--accent-2": mix(a, WHITE, lift),
            "--accent-dark": nudge(mix(a, BLACK, 0.45 if dark_page else 0.75),
                                   lambda c: BLACK if contrast(LOG_TEXT, c) < 4.5 else None),
            "--accent-soft": nudge(soft, lambda c: None if contrast(ink, c) >= 4.5 else BLACK if dark_page else WHITE),
            "--accent-soft-2": mix(a, BLACK, 0.6) if dark_page else mix(a, WHITE, 0.62),
        }
    return out


if __name__ == "__main__":
    if len(sys.argv) != 2 or not sys.argv[1].startswith("#"):
        sys.exit(__doc__)
    t = derive(sys.argv[1])
    for mode, block in [("light", "body{"), ("dark", "body.dark{")]:
        print(f"/* {mode}: replace these tokens in the {block} block */")
        print(block + " " + "; ".join(f"{k}:{v}" for k, v in t[mode].items()) + "; }")
    print()
    rows = checks(t)
    for mode, what, ratio, need in rows:
        print(f"{mode:<6} {what:<34} {ratio:5.2f}:1  needs {need}:1  {'PASS' if ratio >= need else 'FAIL'}")
    sys.exit(0 if all(r >= n for *_, r, n in rows) else 1)