"""Minimal Ogg page reader/writer."""
import struct


def _crc_table():
    t = []
    for i in range(256):
        r = i << 24
        for _ in range(8):
            r = ((r << 1) ^ 0x04C11DB7) if r & 0x80000000 else (r << 1)
        t.append(r & 0xFFFFFFFF)
    return t


_T = _crc_table()


def ogg_crc(b: bytes) -> int:
    c = 0
    for x in b:
        c = ((c << 8) & 0xFFFFFFFF) ^ _T[((c >> 24) & 0xFF) ^ x]
    return c


def read_packets(b: bytes):
    """Return list of (packet_bytes, granule_of_page_if_packet_ends_page_else_None)."""
    pos = 0
    out = []
    cur = b""
    while pos + 27 <= len(b):
        if b[pos:pos + 4] != b"OggS":
            raise ValueError("lost Ogg sync at %d" % pos)
        htype, gran, serial, seq, crc, nseg = struct.unpack_from("<BqIIIB", b, pos + 5)
        segs = b[pos + 27: pos + 27 + nseg]
        p = pos + 27 + nseg
        ends = []
        for s in segs:
            cur += b[p:p + s]
            p += s
            if s < 255:
                ends.append(cur)
                cur = b""
        for i, pk in enumerate(ends):
            out.append((pk, gran if i == len(ends) - 1 else None))
        pos = p
    return out


class OggWriter:
    def __init__(self, serial=0x41434B54):
        self.serial = serial
        self.seq = 0
        self.out = bytearray()

    def page(self, packets, granule, bos=False, eos=False):
        segs = bytearray()
        body = bytearray()
        for pk in packets:
            n = len(pk)
            while n >= 255:
                segs.append(255); n -= 255
            segs.append(n)
            body += pk
        hdr = struct.pack("<4sBBqIIIB", b"OggS", 0, (2 if bos else 0) | (4 if eos else 0),
                          granule, self.serial, self.seq, 0, len(segs)) + bytes(segs)
        page = bytearray(hdr + body)
        struct.pack_into("<I", page, 22, ogg_crc(page))
        self.out += page
        self.seq += 1

    def bytes(self):
        return bytes(self.out)