"""Write an Acorn DFS single-sided disk image (.ssd) natively.

DFS layout: sectors 0-1 are the catalogue; files follow from sector 2, each
starting on a 256-byte sector boundary.  Addresses are 18-bit (load/exec/length
high bits packed into the per-file flag byte).  Boot option (*OPT 4,n) lives in
sector 1 byte 6 bits 4-5.
"""

from __future__ import annotations

SECTOR = 256
TRACKS = 80
SECTORS = TRACKS * 10                # 800 sectors = 200K, single sided


class DfsError(RuntimeError):
    pass


def _name7(name: str) -> bytes:
    n = "".join(c for c in name.upper() if 32 <= ord(c) < 127)[:7]
    return n.ljust(7).encode("ascii")


def build_ssd(files, title="8BITLENSER", boot_option=0) -> bytes:
    """files: list of (name, load_addr, exec_addr, data). boot_option 0-3
    (*OPT 4,n): 0 none, 1 *LOAD, 2 *RUN, 3 *EXEC of the first matching $.!BOOT."""
    if len(files) > 31:
        raise DfsError("DFS holds at most 31 files")

    cat0 = bytearray(SECTOR)
    cat1 = bytearray(SECTOR)
    t = title.ljust(12)[:12].encode("ascii", "replace")
    cat0[0:8] = t[0:8]
    cat1[0:4] = t[8:12]
    cat1[4] = 0                                   # cycle number
    cat1[5] = len(files) * 8                       # (#files) * 8
    cat1[6] = ((SECTORS >> 8) & 0x03) | ((boot_option & 0x03) << 4)
    cat1[7] = SECTORS & 0xFF

    body = bytearray()
    start_sector = 2
    for i, (name, load, exec_, data) in enumerate(files):
        e = 8 + i * 8
        cat0[e:e + 7] = _name7(name)
        cat0[e + 7] = ord("$")                     # directory '$' (unlocked)
        length = len(data)
        cat1[e + 0] = load & 0xFF
        cat1[e + 1] = (load >> 8) & 0xFF
        cat1[e + 2] = exec_ & 0xFF
        cat1[e + 3] = (exec_ >> 8) & 0xFF
        cat1[e + 4] = length & 0xFF
        cat1[e + 5] = (length >> 8) & 0xFF
        cat1[e + 6] = (((exec_ >> 16) & 0x03) << 6 |
                       ((length >> 16) & 0x03) << 4 |
                       ((load >> 16) & 0x03) << 2 |
                       ((start_sector >> 8) & 0x03))
        cat1[e + 7] = start_sector & 0xFF
        nsec = (length + SECTOR - 1) // SECTOR
        body += bytes(data) + bytes((-length) % SECTOR)
        start_sector += nsec

    img = bytes(cat0) + bytes(cat1) + bytes(body)
    if len(img) > SECTORS * SECTOR:
        raise DfsError("files exceed disk capacity")
    return img + bytes(SECTORS * SECTOR - len(img))


def write_ssd(path: str, files, title="8BITLENSER", boot_option=0) -> str:
    with open(path, "wb") as f:
        f.write(build_ssd(files, title, boot_option))
    return path