import struct

from app.fit.crc import fit_crc


def make_fit(data_records: bytes, *, header_size: int = 14) -> bytes:
    if header_size not in {12, 14}:
        raise ValueError(header_size)
    header = bytearray(header_size)
    header[0] = header_size
    header[1] = 0x20
    struct.pack_into("<H", header, 2, 0x0100)
    struct.pack_into("<I", header, 4, len(data_records))
    header[8:12] = b".FIT"
    if header_size == 14:
        struct.pack_into("<H", header, 12, fit_crc(header[:12]))
    body = header + data_records
    return bytes(body + struct.pack("<H", fit_crc(body)))


def definition(
    local: int,
    global_num: int,
    fields: list[tuple[int, int, int]],
    *,
    endian: str = "<",
    developer_fields: list[tuple[int, int, int]] | None = None,
) -> bytes:
    """Build a FIT definition message.

    ``developer_fields`` is an optional list of (field_num, size, developer_data_index)
    triples. When provided (and non-empty), the record header's "has developer fields"
    bit (0x20) is set and a developer field definition block is appended after the
    native field definitions, per the FIT format.
    """
    architecture = 1 if endian == ">" else 0
    header_byte = 0x40 | local
    if developer_fields:
        header_byte |= 0x20
    payload = bytearray([0, architecture])
    payload.extend(struct.pack(f"{endian}H", global_num))
    payload.append(len(fields))
    for num, size, base_type in fields:
        payload.extend(bytes([num, size, base_type]))
    if developer_fields:
        payload.append(len(developer_fields))
        for num, size, dev_index in developer_fields:
            payload.extend(bytes([num, size, dev_index]))
    return bytes([header_byte]) + bytes(payload)


def data(local: int, payload: bytes) -> bytes:
    return bytes([local]) + payload


def compressed_timestamp_data(local: int, time_offset: int, payload: bytes) -> bytes:
    """Build a compressed-timestamp data record header (bit 0x80 set, local message
    type in bits 5-6, time offset in bits 0-4) followed by the record payload."""
    header = 0x80 | ((local & 0x03) << 5) | (time_offset & 0x1F)
    return bytes([header]) + payload