"""Assemble the VIC-20 viewer with `xa` and lay out the 8K autostart cartridge.

The cartridge occupies $A000-$BFFF (8192 bytes).  The KERNAL recognises the
"A0CBM" signature at $A004 and jumps through the cold vector at $A000, so no 6502
reset vector is needed.  MAME's `vic20 -cart` wants a full 8K image; smaller .a0
files fail with an I/O error.

ROM layout (fixed so the viewer can copy from constant addresses):
    $A000  header + viewer code
    $A800  CHARSRC  character set   (2048 bytes -> RAM $1400)
    $B000  SCRSRC   screen          ( 506 bytes -> RAM $1E00)
    $B200  COLSRC   colour RAM      ( 506 bytes -> RAM $9600)
"""

from __future__ import annotations

import os
import shutil
import subprocess
import tempfile

VIEWER_DIR = os.path.dirname(os.path.abspath(__file__))

CART_BASE = 0xA000
CART_SIZE = 0x2000
CHARSRC = 0xA800
SCRSRC = 0xB000
COLSRC = 0xB200


class AssemblerError(RuntimeError):
    pass


def have_xa() -> bool:
    return shutil.which("xa") is not None


def _assemble(bg: int, border: int, aux: int) -> bytes:
    if not have_xa():
        raise AssemblerError("The 'xa' (xa65) assembler was not found on PATH.\n"
                             "Install it with:  sudo apt install xa65")
    wrapper = (f"* = ${CART_BASE:04X}\n"
               f"#define CHARSRC ${CHARSRC:04X}\n"
               f"#define SCRSRC ${SCRSRC:04X}\n"
               f"#define COLSRC ${COLSRC:04X}\n"
               f"#define BG {bg & 15}\n"
               f"#define BORDER {border & 7}\n"
               f"#define AUX {aux & 15}\n"
               '#include "viewer.s"\n')
    with tempfile.TemporaryDirectory() as td:
        out = os.path.join(td, "v.bin")
        fd, wrap = tempfile.mkstemp(suffix=".s", prefix="_wrap_", dir=VIEWER_DIR)
        try:
            with os.fdopen(fd, "w") as f:
                f.write(wrapper)
            proc = subprocess.run(["xa", "-o", out, os.path.basename(wrap)],
                                  capture_output=True, text=True, cwd=VIEWER_DIR)
            if proc.returncode != 0:
                raise AssemblerError(f"xa failed:\n{proc.stdout}{proc.stderr}")
            with open(out, "rb") as f:
                code = f.read()
        finally:
            os.unlink(wrap)
    return code


def build_cart(data: dict) -> bytes:
    """data carries chardata(2048), screen(506), color(506) and bg/border/aux."""
    code = _assemble(data["bg"], data["border"], data["aux"])
    if len(code) > (CHARSRC - CART_BASE):
        raise AssemblerError(f"viewer code is {len(code)} bytes, overruns CHARSRC")
    chardata = bytes(bytearray(data["chardata"]))
    screen = bytes(bytearray(data["screen"]))
    color = bytes(bytearray(data["color"]))

    rom = bytearray(b"\x00" * CART_SIZE)
    rom[0:len(code)] = code
    rom[CHARSRC - CART_BASE:CHARSRC - CART_BASE + len(chardata)] = chardata
    rom[SCRSRC - CART_BASE:SCRSRC - CART_BASE + len(screen)] = screen
    rom[COLSRC - CART_BASE:COLSRC - CART_BASE + len(color)] = color
    return bytes(rom)