"""Assemble the 6502 viewer stubs with `xa` and build self-contained viewer PRGs.

Each viewer is a small ML stub originating at $0801 (behind a BASIC SYS 2061
autostart).  The picture data is appended after the stub, zero-padded so the
bitmap lands exactly at $2000, screen RAM at $3F40, etc.  The whole thing loads
in a single pass -- no second disk access -- so it works identically on real
hardware and in any emulator regardless of device configuration.

`xa -o` emits raw bytes starting at the origin without the 2-byte CBM load
address, which we prepend here.
"""

from __future__ import annotations

import os
import shutil
import subprocess
import tempfile

VIEWER_DIR = os.path.dirname(os.path.abspath(__file__))
LOAD_ADDR = 0x0801
DATA_ADDR = 0x2000               # where appended picture data must land

# mode/viewer key -> source filename
SOURCES = {
    "hires": "hires.s",
    "multicolor": "multicolor.s",
    "fli": "fli.s",
    "fli_ntsc": "fli_ntsc.s",
    "interlace": "interlace.s",
}

_cache: dict[str, bytes] = {}


class AssemblerError(RuntimeError):
    pass


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


def assemble_stub(viewer_key: str) -> bytes:
    """Assemble a viewer stub to raw bytes (origin $0801, no load-address prefix)."""
    if viewer_key in _cache:
        return _cache[viewer_key]
    if not have_xa():
        raise AssemblerError(
            "The 'xa' (xa65) assembler was not found on PATH.\n"
            "Install it with:  sudo apt install xa65   (Debian/Ubuntu)\n"
            "or build from https://www.floodgap.com/retrotech/xa/")

    src = os.path.join(VIEWER_DIR, SOURCES[viewer_key])
    if not os.path.exists(src):
        raise AssemblerError(f"viewer source missing: {src}")

    with tempfile.TemporaryDirectory() as td:
        out = os.path.join(td, "viewer.bin")
        proc = subprocess.run(["xa", "-o", out, src], capture_output=True, text=True)
        if proc.returncode != 0:
            raise AssemblerError(f"xa failed for {src}:\n{proc.stdout}{proc.stderr}")
        with open(out, "rb") as f:
            raw = f.read()
    _cache[viewer_key] = raw
    return raw


def build_viewer_prg(viewer_key: str, data: bytes, data_addr: int = DATA_ADDR) -> bytes:
    """Combine the assembled stub + padding + picture ``data`` into one PRG.

    ``data`` is the block that must reside from ``data_addr`` upward (bitmap,
    screen, colour RAM, background, ...).
    """
    stub = assemble_stub(viewer_key)
    pad_len = (data_addr - LOAD_ADDR) - len(stub)
    if pad_len < 0:
        raise AssemblerError(
            f"viewer stub {viewer_key} is {len(stub)} bytes, exceeds the "
            f"{data_addr - LOAD_ADDR} bytes available before ${data_addr:04x}")
    payload = stub + bytes(pad_len) + bytes(data)
    return bytes([LOAD_ADDR & 0xFF, (LOAD_ADDR >> 8) & 0xFF]) + payload