"""Build an iNES (.nes) NROM cartridge: 6502 PPU-init viewer + image data + CHR.

Layout: 16-byte iNES header, 16K PRG-ROM (viewer at $C000, 32-byte palette at
$F000, 1024-byte nametable+attributes at $F020, vectors at $FFFA), then 8K
CHR-ROM (256 background tiles in pattern table 0).
"""
from __future__ import annotations

import os
import shutil
import subprocess
import tempfile

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

PRG_BASE = 0xC000
PRG_SIZE = 0x4000               # 16K
PAL_ADDR = 0xF000              # 32-byte palette
NT_ADDR = 0xF020              # 1024-byte nametable + attribute


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


def _assemble() -> bytes:
    if not have_xa():
        raise RuntimeError("The 'xa' (xa65) assembler was not found (apt install xa65).")
    with tempfile.TemporaryDirectory() as td:
        out = os.path.join(td, "v.bin")
        proc = subprocess.run(["xa", "-o", out, "viewer.s"],
                              capture_output=True, text=True, cwd=VIEWER_DIR)
        if proc.returncode != 0:
            raise RuntimeError(f"xa failed:\n{proc.stdout}{proc.stderr}")
        with open(out, "rb") as f:
            return f.read()


def build_rom(palette: bytes, nametable: bytes, chr_rom: bytes) -> bytes:
    """palette: 32 bytes; nametable: 1024 (960 names + 64 attr); chr_rom: 8192."""
    if len(palette) != 32 or len(nametable) != 1024 or len(chr_rom) != 8192:
        raise ValueError("bad NES data block sizes")
    code = _assemble()                              # 6502 viewer, org $C000
    if len(code) > PAL_ADDR - PRG_BASE:
        raise RuntimeError("viewer code overruns the $F000 data area")
    prg = bytearray(PRG_SIZE)
    prg[0:len(code)] = code                          # start == $C000
    prg[PAL_ADDR - PRG_BASE:PAL_ADDR - PRG_BASE + 32] = palette
    prg[NT_ADDR - PRG_BASE:NT_ADDR - PRG_BASE + 1024] = nametable
    # CPU vectors $FFFA/$FFFC/$FFFE all point at start ($C000)
    for off in (0x3FFA, 0x3FFC, 0x3FFE):
        prg[off] = PRG_BASE & 0xFF
        prg[off + 1] = PRG_BASE >> 8

    header = bytes([0x4E, 0x45, 0x53, 0x1A,        # "NES\x1A"
                    1,                              # 16K PRG units
                    1,                              # 8K CHR units
                    0x00, 0x00] + [0] * 8)          # NROM (mapper 0)
    return header + bytes(prg) + bytes(chr_rom)


def write_nes(rom: bytes, path: str) -> str:
    with open(path, "wb") as f:
        f.write(rom)
    return path