"""Export a ZX Spectrum image as a .SNA snapshot (+ a standard .SCR alongside)."""
from __future__ import annotations

import os

from . import snapshot

_EXTS = (".sna", ".scr", ".z80")


def export_sna(conv, output_path, source_path=None, display="forever",
               seconds=0, video="pal"):
    if not output_path.lower().endswith(_EXTS):
        output_path += ".sna"
    screen = bytes(conv.data)
    border = _dominant_paper(screen)
    sna = snapshot.build_sna(screen, border=border)
    with open(output_path, "wb") as f:
        f.write(sna)
    # also drop the standard raw-screen .scr next to it (interchange format)
    scr_path = os.path.splitext(output_path)[0] + ".scr"
    with open(scr_path, "wb") as f:
        f.write(snapshot.build_scr(screen))
    return output_path


def _dominant_paper(screen: bytes) -> int:
    """Most common paper colour across the 768 attribute cells -> border colour."""
    counts = [0] * 8
    for a in screen[6144:6912]:
        counts[(a >> 3) & 7] += 1
    return max(range(8), key=lambda c: counts[c])