bambuddy/backend/app/services/opentag3d.py

"""OpenTag3D NDEF encoder for NTAG tags.

Encodes spool data as an OpenTag3D NDEF message ready to write to NTAG
starting at page 4 (after the manufacturer pages).

NDEF structure:
  [CC: E1 10 12 00]              - Capability Container (4 bytes, page 4)
  [TLV: 03 len]                  - NDEF Message TLV (2 bytes)
  [NDEF record header]           - D2 15 payload_len (3 bytes: MB|ME|SR, TNF=MIME, type_len=21)
  [Type: "application/opentag3d"] - 21 bytes
  [Payload: OpenTag3D fields]    - 102 bytes
  [Terminator: FE]               - 1 byte
"""

import struct

from backend.app.models.spool import Spool

OPENTAG3D_MIME_TYPE = b"application/opentag3d"
PAYLOAD_SIZE = 102
TAG_VERSION = 1000  # v1.000


def _build_payload(spool: Spool) -> bytes:
    """Build 102-byte OpenTag3D core payload from spool fields."""
    buf = bytearray(PAYLOAD_SIZE)

    # 0x00: Tag Version (2 bytes, big-endian)
    struct.pack_into(">H", buf, 0x00, TAG_VERSION)

    # 0x02: Base Material (5 bytes, UTF-8, space-padded)
    material = (spool.material or "")[:5].ljust(5)
    buf[0x02:0x07] = material.encode("utf-8")[:5]

    # 0x07: Material Modifiers (5 bytes, UTF-8, space-padded)
    modifiers = (spool.subtype or "")[:5].ljust(5)
    buf[0x07:0x0C] = modifiers.encode("utf-8")[:5]

    # 0x0C: Reserved (15 bytes, zero-fill) — already zero

    # 0x1B: Manufacturer (16 bytes, UTF-8, space-padded)
    brand = (spool.brand or "")[:16].ljust(16)
    buf[0x1B:0x2B] = brand.encode("utf-8")[:16]

    # 0x2B: Color Name (32 bytes, UTF-8, space-padded)
    color_name = (spool.color_name or "")[:32].ljust(32)
    buf[0x2B:0x4B] = color_name.encode("utf-8")[:32]

    # 0x4B: Color 1 RGBA (4 bytes)
    rgba_hex = spool.rgba or "00000000"
    try:
        rgba_bytes = bytes.fromhex(rgba_hex[:8].ljust(8, "0"))
    except ValueError:
        rgba_bytes = b"\x00\x00\x00\x00"
    buf[0x4B:0x4F] = rgba_bytes[:4]

    # 0x4F: Colors 2-4 (12 bytes, zero-fill) — already zero

    # 0x5C: Target Diameter (2 bytes, big-endian) — 1750 = 1.75mm
    struct.pack_into(">H", buf, 0x5C, 1750)

    # 0x5E: Target Weight (2 bytes, big-endian)
    struct.pack_into(">H", buf, 0x5E, spool.label_weight or 0)

    # 0x60: Print Temp (1 byte) — nozzle_temp_min / 5
    buf[0x60] = (spool.nozzle_temp_min or 0) // 5

    # 0x61: Bed Temp (1 byte) — not tracked
    # 0x62: Density (2 bytes) — not tracked
    # 0x64: Transmission Distance (2 bytes) — not tracked
    # All zero — already zero

    return bytes(buf)


def encode_opentag3d(spool: Spool) -> bytes:
    """Encode spool data as OpenTag3D NDEF message (CC + TLV + record + terminator).

    Returns raw bytes ready to write to NTAG starting at page 4.
    """
    payload = _build_payload(spool)
    mime_type = OPENTAG3D_MIME_TYPE

    # NDEF record: MB|ME|SR (0xD0) | TNF=MIME (0x02) => 0xD2
    # Type length = 21
    # Payload length = 102 (fits in SR single byte)
    record_header = bytes([0xD2, len(mime_type), len(payload)])
    ndef_record = record_header + mime_type + payload

    # TLV: type=0x03 (NDEF Message), length
    ndef_len = len(ndef_record)
    if ndef_len < 0xFF:
        tlv = bytes([0x03, ndef_len])
    else:
        tlv = bytes([0x03, 0xFF, (ndef_len >> 8) & 0xFF, ndef_len & 0xFF])

    # Capability Container (page 4)
    cc = bytes([0xE1, 0x10, 0x12, 0x00])

    # Terminator TLV
    terminator = bytes([0xFE])

    return cc + tlv + ndef_record + terminator