#!/usr/bin/env python3
"""Throwaway WCAG 2.x contrast check for the #53 palette candidates.

Same method as the #50 headroom research: relative luminance, (L1+.05)/(L2+.05),
alpha colours flattened over their real backdrop first (source-over, sRGB).
Self-checks against WCAG's published values before printing anything.
"""


def _srgb(c):
    c /= 255
    return c / 12.92 if c <= 0.04045 else ((c + 0.055) / 1.055) ** 2.4


def rgb(h):
    h = h.lstrip("#")
    return tuple(int(h[i : i + 2], 16) for i in (0, 2, 4))


def lum(h):
    r, g, b = rgb(h)
    return 0.2126 * _srgb(r) + 0.7152 * _srgb(g) + 0.0722 * _srgb(b)


def flatten(fg, alpha, bg):
    f, b = rgb(fg), rgb(bg)
    return "#%02x%02x%02x" % tuple(round(f[i] * alpha + b[i] * (1 - alpha)) for i in range(3))


def ratio(a, b):
    la, lb = lum(a), lum(b)
    hi, lo = max(la, lb), min(la, lb)
    return (hi + 0.05) / (lo + 0.05)


def _selfcheck():
    assert round(ratio("#000000", "#ffffff"), 1) == 21.0
    assert round(ratio("#767676", "#ffffff"), 2) == 4.54
    assert round(ratio("#949494", "#ffffff"), 2) == 3.03
    assert ratio("#959595", "#ffffff") < 3.0
    assert flatten("#ffffff", 0.5, "#000000") == "#808080"


_selfcheck()

# candidate surfaces (the invented ones need proving; the rest come from #50)
SURFACES = {
    "bg #150E13": "#150E13",
    "bg-deep #0C0810": "#0C0810",
    "panel #21161C": "#21161C",
    "panel-2 #180F16": "#180F16",
    "panel-3 / Plum Noir #351E28": "#351E28",
    "plum-soft #4A2A38": "#4A2A38",
    "paper / champagne #EFE5D2": "#EFE5D2",
}

INKS = {
    "Bone #E6E0D6": "#E6E0D6",
    "Ash #B9B2A6": "#B9B2A6",
    "Dust #9A9086": "#9A9086",
    "Ledger Ink #D8C28A": "#D8C28A",
    "Tarnished Gold #C2A35A": "#C2A35A",
    "Old Gilt #A8893F": "#A8893F",
    "Gold Lit #E0C878": "#E0C878",
    "Pewter #9EA7B3": "#9EA7B3",
    "Ink-on-light #351E28": "#351E28",
    "Red #8E3438": "#8E3438",
    "Stamp #A84A4E": "#A84A4E",
}

if __name__ == "__main__":
    w = max(len(k) for k in INKS)
    for sname, s in SURFACES.items():
        print(f"\n== {sname}")
        for iname, i in INKS.items():
            r = ratio(i, s)
            tag = "AA" if r >= 4.5 else ("large/non-text" if r >= 3.0 else "FAIL")
            print(f"  {iname:<{w}} {r:6.2f}  {tag}")