#!/usr/bin/env python3
"""Regression test for the 256x256 CRTC mode (docs/FINDINGS 23).

Checks tmp/snap256/x68000/0000.png against tmp/frame256.bin:
  1. native snapshot is 256x512 -- 256 dots, and 512 active scanlines of a
     568-line 31.5kHz raster carrying 256 double-scanned graphics rows
  2. double-scan pairing is (1,2),(3,4),... -- MAME halves the ABSOLUTE
     scanline (x68k_v.cpp get_gfx_pixel) and vbegin=41 is odd, so snapshot
     row 0 is a lone half-line and even rows are gfx rows 0..255
  3. the 192 active rows are PIXEL-EXACT against the palette pushed through
     GGGGGRRRRRBBBBBI with I chosen per entry by minimum squared error
  4. the letterbox bars are TRUE black -- needs both a reserved index-0 black
     entry AND I=0 on it, since pal6bit(1) = 4, not 0
"""
import struct, sys
import numpy as np
from PIL import Image

s = np.asarray(Image.open("tmp/snap256/x68000/0000.png").convert("RGB")).astype(int)
d = open("tmp/frame256.bin", "rb").read()
W, H = struct.unpack(">HH", d[4:8])
pal = np.frombuffer(d[8:8+768], np.uint8).reshape(256, 3).astype(int)
idx = np.frombuffer(d[8+768:8+768+W*H], np.uint8).reshape(H, W)

p6 = lambda v: ((v << 2) | (v >> 4)) & 0xFF
f = pal >> 3
render = lambda I: p6((f << 1) | I[:, None])
I = (((render(np.ones(256, int)) - pal) ** 2).sum(1)
     < ((render(np.zeros(256, int)) - pal) ** 2).sum(1)).astype(int)
exp = render(I)[idx]

fail = []
if s.shape[:2] != (512, 256):
    fail.append(f"1. geometry: expected 512x256, got {s.shape[1]}x{s.shape[0]}")
if not all(np.array_equal(s[i], s[i+1]) for i in range(1, s.shape[0]-1, 2)):
    fail.append("2. double-scan pairing (1,2),(3,4),... broken")

g = s[0::2]
yoff = (g.shape[0] - H) // 2
act = g[yoff:yoff+H]
if not np.array_equal(act, exp):
    diff = abs(act - exp)
    fail.append(f"3. active area not pixel-exact: maxdiff {diff.max()}, "
                f"{diff.any(2).sum()} px differ")

bars = np.concatenate([g[:yoff], g[yoff+H:]])
if bars.max() != 0:
    fail.append(f"4. letterbox not true black: max channel {bars.max()}")

for x in fail:
    print("FAIL " + x)
if fail:
    sys.exit(1)

mse = ((act - pal[idx]) ** 2).mean()
print(f"OK  256x512 native, double-scan exact, active {W}x{H} pixel-exact, "
      f"letterbox true black")
print(f"    palette ceiling vs 24-bit palettised source: "
      f"{10*np.log10(255**2/mse):.2f} dB  ({(I==0).sum()}/256 entries use I=0)")