bambuddy/backend/app/services/filament_deficit.py

"""Filament-deficit check used by every queue dispatch path.

The PrintModal warns when an assigned spool can't satisfy a print's per-slot
filament weight (``Pre-print checks now also warn when the spool has
insufficient material`` — #720). That check only runs when the user clicks
"Print" inside PrintModal; ``QueuePage`` Play button, ``start_queue_item``
route, and the VP intake + scheduler auto-dispatch path all skip it (#1496).

This module is the single source of truth for the check. Both the route
handler (``POST /print-queue/{id}/start``) and the dispatch scheduler call
``compute_deficit_for_queue_item`` against live spool state.

Design notes:
* The 3MF parser is the same one used by PrintModal: per-slot ``used_grams``
  comes from ``extract_filament_requirements`` (#1188's filament-overrides
  pipeline) or — when the item points at an unsliced library file — falls
  through to the file's archive copy. Anything that yields no requirements
  is treated as "no deficit" so a malformed or stripped 3MF never blocks.
* Both internal-inventory and Spoolman modes are covered. Internal mode
  resolves via ``SpoolAssignment`` joined to ``Spool`` (``label_weight``
  minus ``weight_used``). Spoolman mode resolves via
  ``SpoolmanSlotAssignment`` then ``SpoolmanClient.get_spool`` for the live
  remaining weight; if Spoolman is unreachable we return no deficit rather
  than wedge the queue on a flaky network call.
* The ``disable_filament_warnings`` user setting is respected at the
  service boundary — callers do not have to know about it.
"""

from __future__ import annotations

import json
import logging
from collections import defaultdict
from dataclasses import dataclass
from pathlib import Path

from sqlalchemy import select
from sqlalchemy.ext.asyncio import AsyncSession
from sqlalchemy.orm import selectinload

from backend.app.api.routes._spoolman_helpers import _map_spoolman_spool
from backend.app.core.config import settings as app_settings
from backend.app.models.print_queue import PrintQueueItem
from backend.app.models.spool_assignment import SpoolAssignment
from backend.app.models.spoolman_slot_assignment import SpoolmanSlotAssignment
from backend.app.services.filament_requirements import extract_filament_requirements

logger = logging.getLogger(__name__)


@dataclass(frozen=True)
class FilamentDeficit:
    """One slot's filament shortfall."""

    slot_id: int
    ams_id: int | None
    tray_id: int | None
    filament_type: str
    required_grams: float
    remaining_grams: float | None  # None = could not determine

    def to_dict(self) -> dict:
        return {
            "slot_id": self.slot_id,
            "ams_id": self.ams_id,
            "tray_id": self.tray_id,
            "filament_type": self.filament_type,
            "required_grams": self.required_grams,
            "remaining_grams": self.remaining_grams,
        }


def _global_to_ams_key(global_tray_id: int) -> tuple[int, int]:
    """Inverse of ``ams_id * 4 + tray_id`` — matches ``usage_tracker``."""
    if global_tray_id >= 254:
        return (255, global_tray_id - 254)
    if global_tray_id >= 128:
        return (global_tray_id, 0)
    return (global_tray_id // 4, global_tray_id % 4)


def _ams_key_to_global(ams_id: int, tray_id: int) -> int:
    """Inverse of ``_global_to_ams_key``.

    Mirrors the frontend ``getGlobalTrayId``: external / VT slots (``ams_id``
    255) land at ``254 + tray_id``, AMS-HT units (``ams_id`` >= 128) use the
    unit id directly, regular AMS slots use ``ams_id * 4 + tray_id``.
    """
    if ams_id >= 255:
        return 254 + tray_id
    if ams_id >= 128:
        return ams_id
    return ams_id * 4 + tray_id


def _resolve_source_3mf(item: PrintQueueItem) -> Path | None:
    """Locate the 3MF file backing this queue item (archive or library).

    ``LibraryFile.file_path`` is stored relative to ``base_dir`` (rows written
    before that convention hold absolute paths, which is why every reader
    guards on ``is_absolute``). Resolving a relative one against the process
    working directory finds nothing, and a source that cannot be found is
    treated as "nothing to verify" — so this check silently passed every
    library-backed item, which is every Slicer Pipeline job and everything
    queued from the Library page (#2779).
    """
    if item.archive is not None and item.archive.file_path:
        return app_settings.base_dir / item.archive.file_path
    if item.library_file is not None and item.library_file.file_path:
        library_path = Path(item.library_file.file_path)
        if library_path.is_absolute():
            return library_path
        # SEC-PATH-OK: file_path is DB-stored and generated by the Library
        # ingest (archive/library/files/<uuid>.<ext>), never request input. The
        # same value already resolves the file for upload in print_queue.py and
        # print_scheduler.py — this check reads what the printer is about to be
        # sent, so it must resolve it identically.
        return app_settings.base_dir / item.library_file.file_path
    return None


async def _spoolman_remaining_grams(spoolman_spool_id: int) -> float | None:
    """Live remaining grams for a Spoolman spool, or None if unavailable."""
    try:
        from backend.app.services.spoolman import (
            SpoolmanClientError,
            SpoolmanNotFoundError,
            get_spoolman_client,
        )
    except ImportError:
        return None
    try:
        client = await get_spoolman_client()
        if client is None:
            return None
        spool = await client.get_spool(spoolman_spool_id)
    except (SpoolmanNotFoundError, SpoolmanClientError):
        return None
    except Exception as e:
        logger.debug("Spoolman fetch failed for spool %s: %s", spoolman_spool_id, e)
        return None

    if not spool:
        return None

    # Spoolman exposes either an absolute remaining_weight, or used_weight +
    # filament.weight. Either is sufficient — prefer remaining_weight when
    # present (the user may have overridden it).
    remaining = spool.get("remaining_weight")
    if isinstance(remaining, (int, float)) and remaining >= 0:
        return float(remaining)

    used = spool.get("used_weight")
    filament = spool.get("filament") or {}
    total = filament.get("weight")
    if isinstance(used, (int, float)) and isinstance(total, (int, float)) and total > 0:
        return max(0.0, float(total) - float(used))

    return None


async def _is_spoolman_mode(db: AsyncSession) -> bool:
    """Check whether the user has opted in to Spoolman inventory mode."""
    try:
        from backend.app.api.routes.settings import get_setting

        spoolman_enabled = await get_setting(db, "spoolman_enabled")
        return bool(spoolman_enabled) and spoolman_enabled.lower() == "true"
    except Exception:
        return False


async def _warnings_disabled(db: AsyncSession) -> bool:
    """Honour the ``disable_filament_warnings`` setting (#720)."""
    try:
        from backend.app.api.routes.settings import get_setting

        disabled = await get_setting(db, "disable_filament_warnings")
        return bool(disabled) and disabled.lower() == "true"
    except Exception:
        return False


def _normalize_color_for_id(raw: str | None) -> str:
    """Canonicalise a hex colour for identity comparison.

    Strips the leading ``#``, uppercases, and drops the alpha channel when
    the hex is 8 chars long (``RRGGBBAA``) so a fully-opaque 8-char hex
    matches a 6-char hex of the same RGB. Empty / None → empty string.

    Anything that is not a string reads as "no colour" rather than raising.
    In Spoolman mode ``raw`` comes straight off the wire as
    ``filament.color_hex``, and this runs on the dispatch path — a record
    holding a number there would otherwise fail a queue start rather than
    merely fail to pool.
    """
    if not isinstance(raw, str):
        raw = None
    s = (raw or "").strip().lstrip("#").upper()
    if len(s) == 8:  # RRGGBBAA → strip alpha
        s = s[:6]
    return s


def _material_identity_internal(spool) -> str:
    """Strict same-material key for backup-peer matching in internal mode.

    Requires a Bambu filament preset ID (``slicer_filament``, e.g. ``GFA00``)
    AND a matching colour. The preset identifies the filament profile (PETG
    HF, PLA Basic, etc.) — same hot-end behaviour — but the firmware's
    switch logic also requires the spool to be the same colour (otherwise
    every PETG HF spool would back every other PETG HF spool regardless of
    colour, which would dye prints mid-run). Spools without a preset
    (user-tagged / non-Bambu) get a per-spool unique key so they NEVER
    pair with anything else; without the Bambu preset the firmware can't
    trust the backup decision.
    """
    preset = (spool.slicer_filament or "").strip() if spool else ""
    if preset:
        color = _normalize_color_for_id(spool.rgba if spool else None)
        return f"preset:{preset}|color:{color}"
    # Unique-per-spool key prevents grouping. Use the spool's primary key so
    # the same spool always resolves to the same key within a request.
    spool_id = getattr(spool, "id", None) if spool else None
    return f"unmatched:{spool_id}"


def _material_identity_spoolman(spool: dict | None) -> str:
    """Strict same-material key for backup-peer matching in Spoolman mode.

    Two spools pair only when they reference the same Spoolman ``filament``
    catalog entry (same ``filament.id``) AND share the same colour. The
    catalog entry pins the profile (PETG HF / PLA Basic / ...); the colour
    pins the variant. Spools without a resolvable filament id get a
    per-spool unique key so they never pair.
    """
    if not isinstance(spool, dict) or not spool:
        return "unmatched:none"
    # Both are free-form JSON off the Spoolman API, and this is the dispatch
    # path: a wrongly-typed member must cost the slot its pool, never the
    # queue its start.
    filament = spool.get("filament") or {}
    if not isinstance(filament, dict):
        filament = {}
    fil_id = filament.get("id")
    if isinstance(fil_id, (int, str)) and str(fil_id).strip():
        # Prefer the per-spool override colour when set (Spoolman lets the user
        # tag a spool with a colour distinct from the filament catalog
        # default); fall back to the filament catalog colour.
        color = _normalize_color_for_id(
            (spool.get("color_hex") if isinstance(spool.get("color_hex"), str) else None) or filament.get("color_hex")
        )
        return f"filament:{fil_id}|color:{color}"
    spool_id = spool.get("id")
    return f"unmatched:{spool_id}"


def _ams_id_from_global(global_tray_id: int) -> int:
    """Inverse of ``_global_to_ams_key`` returning ams_id only."""
    return _global_to_ams_key(global_tray_id)[0]


def _extruder_side_for_ams(
    ams_id: int,
    ams_extruder_map: dict[str, int],
    is_dual_extruder: bool,
) -> int:
    """Resolve the extruder index (0=right, 1=left) for a given AMS unit.

    Single-extruder printers collapse everything to 0. On dual-extruder
    printers (H2D / H2C / X2D), the firmware can't cross extruders even with
    AMS Filament Backup ON, so the pool must be scoped per-side.
    """
    if not is_dual_extruder:
        return 0
    return int(ams_extruder_map.get(str(ams_id), 0))


def _parse_ams_mapping(raw: str | None) -> list[int] | None:
    if not raw:
        return None
    try:
        parsed = json.loads(raw)
    except (json.JSONDecodeError, TypeError):
        return None
    if not isinstance(parsed, list):
        return None
    return [v for v in parsed if isinstance(v, int)]


async def _get_printer_backup_context(
    printer_id: int,
) -> tuple[bool, dict[str, int], bool]:
    """Return ``(backup_on, ams_extruder_map, is_dual_extruder)`` for the printer.

    Read from the live MQTT state via ``printer_manager`` (no DB round-trip).
    Defaults conservatively to ``backup_on=False`` when the state is missing
    or the printer is offline — same fallback as today (per-slot deficit
    accounting), so an offline printer is never treated as backup-capable.
    """
    try:
        from backend.app.services.printer_manager import printer_manager
        from backend.app.utils.printer_models import is_dual_nozzle_model
    except ImportError:
        return False, {}, False

    state = printer_manager.get_status(printer_id)
    if state is None:
        return False, {}, False

    backup_on = state.ams_filament_backup is True
    ams_extruder_map = dict(state.ams_extruder_map or {})
    model = printer_manager.get_model(printer_id)
    is_dual = bool(model and is_dual_nozzle_model(model))
    return backup_on, ams_extruder_map, is_dual


@dataclass(frozen=True)
class SlotSpoolIdentity:
    """How the spool bound to a slot should be *named*, as opposed to matched.

    The printer cannot supply this and never will. A tray record carries no
    brand field at all, and ``tray_sub_brands`` stays empty for anything that
    isn't a Bambu spool, so a client naming a slot from telemetry alone has
    only the type and the colour hex to work with — and turns that hex into
    whichever catalogue colour happens to share it. A Devil Design PLA Basic
    Orange the operator assigned in Bambuddy reads back as "PLA (Sunflower
    Yellow)", because Bambu sell a Sunflower Yellow at the same ``FEC600``.

    Only the assignment knows the answer, which is why it is served alongside
    the pooling key rather than left to the client to resolve: the identity
    rule differs per inventory mode, and the printer card and the print dialog
    disagreeing about what is in a slot is the bug this exists to close.

    Purely descriptive — nothing here takes part in matching, which stays on
    the printer's own telemetry so the dialog and the dispatcher cannot draw
    different conclusions from the same slot.
    """

    brand: str | None
    material: str | None
    subtype: str | None
    color_name: str | None
    rgba: str | None

    def to_dict(self) -> dict:
        return {
            "brand": self.brand,
            "material": self.material,
            "subtype": self.subtype,
            "color_name": self.color_name,
            "rgba": self.rgba,
        }


def _clean(value) -> str | None:
    """Trim a display field, collapsing blanks to None so the client can skip it."""
    text = str(value).strip() if value is not None else ""
    return text or None


def _identity_from_internal(spool) -> SlotSpoolIdentity:
    """Display identity from an internal-inventory ``Spool`` row."""
    return SlotSpoolIdentity(
        brand=_clean(spool.brand),
        material=_clean(spool.material),
        subtype=_clean(spool.subtype),
        color_name=_clean(spool.color_name),
        rgba=_clean(spool.rgba),
    )


def _identity_from_spoolman(spool_dict: dict) -> SlotSpoolIdentity | None:
    """Display identity from a raw Spoolman spool dict, or None if unreadable.

    Goes through ``_map_spoolman_spool`` rather than reading the dict directly:
    brand lives on the nested vendor, subtype is the filament name with its
    material prefix stripped, and ``color_name`` has a three-step read order
    Spoolman itself has no field for. Re-deriving any of that here is how the
    two modes would drift apart.
    """
    # Broad on purpose. This is a name for a dropdown, and the caller is on the
    # dispatch path -- ``compute_deficit_for_queue_item`` runs it before every
    # queue start. ``_map_spoolman_spool`` walks a dozen nested fields off the
    # wire (``filament.vendor.name``, ``extra.tag``, ``filament.color_hex``) and
    # any of them arriving as the wrong type raises AttributeError rather than
    # ValueError, so a narrow catch here would turn one malformed Spoolman
    # record into a failed dispatch. Losing the name costs a fallback to
    # telemetry, which is what every slot did before this existed.
    try:
        mapped = _map_spoolman_spool(spool_dict)
    except Exception as exc:  # noqa: BLE001 - display-only, must never block a dispatch
        logger.debug(
            "Spoolman spool %r has no usable display identity: %s",
            spool_dict.get("id") if isinstance(spool_dict, dict) else spool_dict,
            exc,
        )
        return None
    # Spoolman has no colour-name field, so `_map_spoolman_spool` synthesises
    # one from the subtype when nothing is stored -- which reads fine in an
    # inventory list ("PLA Basic") and badly as a colour ("Devil Design PLA
    # Basic (Basic)"). Drop it and let the client's catalogue lookup name the
    # hex, which is what an unnamed slot got before this existed.
    color_name = None if mapped.get("color_name_is_synthesized") else _clean(mapped.get("color_name"))
    return SlotSpoolIdentity(
        brand=_clean(mapped.get("brand")),
        material=_clean(mapped.get("material")),
        subtype=_clean(mapped.get("subtype")),
        color_name=color_name,
        rgba=_clean(mapped.get("rgba")),
    )


@dataclass(frozen=True)
class SlotMaterial:
    """One inventory-bound AMS slot: what's in it, how much is left, which side."""

    ams_id: int
    tray_id: int
    global_tray_id: int
    # Opaque grouping key. Two slots pool for AMS Filament Backup only when
    # their keys AND extruder sides match. Never parse it — the format is an
    # implementation detail of ``_material_identity_*``.
    material_key: str
    remaining_grams: float
    extruder: int
    # Display-only; see SlotSpoolIdentity. None when the binding resolves to a
    # spool we cannot describe, which callers render from telemetry as before.
    spool: SlotSpoolIdentity | None = None

    def to_dict(self) -> dict:
        return {
            "ams_id": self.ams_id,
            "tray_id": self.tray_id,
            "global_tray_id": self.global_tray_id,
            "material_key": self.material_key,
            "remaining_g": self.remaining_grams,
            "extruder": self.extruder,
            "spool": self.spool.to_dict() if self.spool else None,
        }


async def build_slot_materials(db: AsyncSession, printer_id: int) -> list[SlotMaterial]:
    """Every inventory-bound slot on ``printer_id``, with identity and remaining grams.

    Mode-agnostic: internal inventory resolves via ``SpoolAssignment`` joined to
    ``Spool`` (``label_weight`` minus ``weight_used``), Spoolman mode via
    ``SpoolmanSlotAssignment`` plus a live ``get_spool`` fetch. Slots whose
    remaining weight can't be determined — no spool row, zero label weight,
    Spoolman unreachable — are omitted entirely rather than reported as zero, so
    a missing binding never manufactures a shortfall.

    This is the pool the AMS-Filament-Backup accounting draws on (#1762), and
    the same data the PrintModal pre-flight check consumes through
    ``GET /printers/{id}/inventory-remain``. Both sides share it so the modal's
    warning and the dispatcher's 409 can't disagree about what backs what up.
    """
    _, ams_extruder_map, is_dual = await _get_printer_backup_context(printer_id)
    materials: list[SlotMaterial] = []

    def _append(
        ams_id: int,
        tray_id: int,
        material_key: str,
        remaining: float,
        spool: SlotSpoolIdentity | None = None,
    ) -> None:
        materials.append(
            SlotMaterial(
                ams_id=ams_id,
                tray_id=tray_id,
                global_tray_id=_ams_key_to_global(ams_id, tray_id),
                material_key=material_key,
                remaining_grams=remaining,
                extruder=_extruder_side_for_ams(ams_id, ams_extruder_map, is_dual),
                spool=spool,
            )
        )

    if await _is_spoolman_mode(db):
        sm_all = await db.execute(select(SpoolmanSlotAssignment).where(SpoolmanSlotAssignment.printer_id == printer_id))
        from backend.app.services.spoolman import (
            SpoolmanClientError,
            SpoolmanNotFoundError,
            get_spoolman_client,
        )

        try:
            client = await get_spoolman_client()
        except Exception:
            client = None
        if client is None:
            return []
        for sa in sm_all.scalars().all():
            try:
                spool_dict = await client.get_spool(sa.spoolman_spool_id)
            except (SpoolmanNotFoundError, SpoolmanClientError):
                continue
            except Exception as e:
                logger.debug("Spoolman pool fetch failed for spool %s: %s", sa.spoolman_spool_id, e)
                continue
            if not spool_dict:
                continue
            remaining: float | None = None
            rw = spool_dict.get("remaining_weight")
            if isinstance(rw, (int, float)) and rw >= 0:
                remaining = float(rw)
            else:
                used = spool_dict.get("used_weight")
                total = (spool_dict.get("filament") or {}).get("weight")
                if isinstance(used, (int, float)) and isinstance(total, (int, float)) and total > 0:
                    remaining = max(0.0, float(total) - float(used))
            if remaining is None:
                continue
            _append(
                sa.ams_id,
                sa.tray_id,
                _material_identity_spoolman(spool_dict),
                remaining,
                _identity_from_spoolman(spool_dict),
            )
        return materials

    internal_all = await db.execute(
        select(SpoolAssignment)
        .options(selectinload(SpoolAssignment.spool))
        .where(SpoolAssignment.printer_id == printer_id)
    )
    for assignment in internal_all.scalars().all():
        spool = assignment.spool
        if spool is None:
            continue
        label_weight = float(spool.label_weight or 0)
        weight_used = float(spool.weight_used or 0)
        if label_weight <= 0:
            continue
        _append(
            assignment.ams_id,
            assignment.tray_id,
            _material_identity_internal(spool),
            max(0.0, label_weight - weight_used),
            _identity_from_internal(spool),
        )
    return materials


async def compute_deficit_for_queue_item(
    db: AsyncSession,
    item: PrintQueueItem,
) -> list[FilamentDeficit]:
    """Return per-slot filament shortfalls for ``item``, or [] when it's safe to dispatch.

    Returns an empty list whenever any of the following hold:

    * The ``disable_filament_warnings`` setting is on.
    * The item has no resolved ``printer_id`` (model-based assignment not
      yet picked a printer — the scheduler re-runs the check after it does).
    * No source 3MF is available, or the 3MF carries no per-slot
      requirements (treated as "nothing to verify" rather than an error,
      matching the PrintModal behaviour).
    * No AMS mapping is set yet — the scheduler computes the mapping just
      before dispatch; until it does we cannot map slot → tray.
    * Spoolman mode is on but the Spoolman server is unreachable. We do not
      wedge the queue on a network blip.

    #1762: when the printer reports ``ams_filament_backup=True`` in MQTT
    status, available material is pooled across ALL same-material spools on
    the printer (within the same extruder side for dual-nozzle models, since
    firmware can't cross extruders even with the backup bit set). Per-slot
    shortfalls are then only emitted if the POOL is too small for the
    print's total required of that material — matching how the printer
    actually behaves with Filament Backup ON.
    """
    if await _warnings_disabled(db):
        return []
    if item.printer_id is None:
        return []

    # Refresh the relationships we need without assuming the caller eagerly
    # loaded them — both the route and the scheduler call this from contexts
    # with different loading strategies.
    refreshed = await db.execute(
        select(PrintQueueItem)
        .options(
            selectinload(PrintQueueItem.archive),
            selectinload(PrintQueueItem.library_file),
        )
        .where(PrintQueueItem.id == item.id)
    )
    item = refreshed.scalar_one_or_none() or item

    source_path = _resolve_source_3mf(item)
    if source_path is None:
        # No archive and no library file — nothing was ever attached to check.
        return []
    if not source_path.exists():
        # Dispatch is not blocked: the upload that follows needs the same file
        # and fails within seconds, where wedging the queue here would strand
        # it. But skipping a safety check must leave a trace — a silent skip is
        # what hid #2779 for every library-backed item.
        logger.warning(
            "Filament check skipped for queue item %s: source 3MF not found at %s",
            item.id,
            source_path,
        )
        return []

    requirements = extract_filament_requirements(source_path, item.plate_id)
    if not requirements:
        return []

    mapping = _parse_ams_mapping(item.ams_mapping)
    if not mapping:
        return []

    spoolman_mode = await _is_spoolman_mode(db)
    backup_on, ams_extruder_map, is_dual = await _get_printer_backup_context(item.printer_id)

    # ------------------------------------------------------------------ phase 1
    # Resolve each requirement to (ams_id, tray_id, identity, remaining_grams).
    # Slot identity is the identity of the spool *assigned to that slot*. A
    # ``None`` remaining means "couldn't determine" — treated as "no deficit"
    # below (preserved from pre-#1762 behaviour for non-backup paths too).
    @dataclass
    class _ReqRow:
        slot_id: int
        ams_id: int
        tray_id: int
        global_tray_id: int
        required: float
        identity: str
        remaining: float | None
        filament_type: str
        extruder: int

    resolved: list[_ReqRow] = []

    for req in requirements:
        slot_id = req.get("slot_id")
        used_grams = req.get("used_grams")
        if not isinstance(slot_id, int) or slot_id <= 0:
            continue
        if not isinstance(used_grams, (int, float)) or used_grams <= 0:
            continue
        idx = slot_id - 1
        if idx >= len(mapping):
            continue
        global_tray_id = mapping[idx]
        if global_tray_id is None or global_tray_id < 0:
            continue
        ams_id, tray_id = _global_to_ams_key(global_tray_id)

        identity = "attrs:|||"
        remaining: float | None = None
        if spoolman_mode:
            sm_result = await db.execute(
                select(SpoolmanSlotAssignment).where(
                    SpoolmanSlotAssignment.printer_id == item.printer_id,
                    SpoolmanSlotAssignment.ams_id == ams_id,
                    SpoolmanSlotAssignment.tray_id == tray_id,
                )
            )
            sm_assignment = sm_result.scalar_one_or_none()
            if sm_assignment is None:
                continue
            # Live remaining_weight from Spoolman. The fetch also resolves the
            # filament identity for pooling (material + colour + name).
            from backend.app.services.spoolman import (
                SpoolmanClientError,
                SpoolmanNotFoundError,
                get_spoolman_client,
            )

            try:
                client = await get_spoolman_client()
                spool_dict = await client.get_spool(sm_assignment.spoolman_spool_id) if client else None
            except (SpoolmanNotFoundError, SpoolmanClientError):
                spool_dict = None
            except Exception as e:
                logger.debug("Spoolman fetch failed for spool %s: %s", sm_assignment.spoolman_spool_id, e)
                spool_dict = None
            if spool_dict:
                identity = _material_identity_spoolman(spool_dict)
                rw = spool_dict.get("remaining_weight")
                if isinstance(rw, (int, float)) and rw >= 0:
                    remaining = float(rw)
                else:
                    used = spool_dict.get("used_weight")
                    total = (spool_dict.get("filament") or {}).get("weight")
                    if isinstance(used, (int, float)) and isinstance(total, (int, float)) and total > 0:
                        remaining = max(0.0, float(total) - float(used))
        else:
            internal_result = await db.execute(
                select(SpoolAssignment)
                .options(selectinload(SpoolAssignment.spool))
                .where(
                    SpoolAssignment.printer_id == item.printer_id,
                    SpoolAssignment.ams_id == ams_id,
                    SpoolAssignment.tray_id == tray_id,
                )
            )
            assignment = internal_result.scalar_one_or_none()
            if assignment is None or assignment.spool is None:
                continue
            spool = assignment.spool
            identity = _material_identity_internal(spool)
            label_weight = float(spool.label_weight or 0)
            weight_used = float(spool.weight_used or 0)
            if label_weight <= 0:
                continue
            remaining = max(0.0, label_weight - weight_used)

        if remaining is None:
            # Unable to determine remaining grams — preserve pre-#1762 behaviour
            # (don't block on undetermined data).
            continue

        resolved.append(
            _ReqRow(
                slot_id=slot_id,
                ams_id=ams_id,
                tray_id=tray_id,
                global_tray_id=global_tray_id,
                required=float(used_grams),
                identity=identity,
                remaining=remaining,
                filament_type=str(req.get("type", "")),
                extruder=_extruder_side_for_ams(ams_id, ams_extruder_map, is_dual),
            )
        )

    # ------------------------------------------------------------------ phase 2
    # When backup is OFF, fall back to today's per-slot accounting (one-line
    # equivalence of the original loop), so this path is a strict no-op
    # behaviour-wise vs. the pre-#1762 code.
    if not backup_on:
        return [
            FilamentDeficit(
                slot_id=row.slot_id,
                ams_id=row.ams_id,
                tray_id=row.tray_id,
                filament_type=row.filament_type,
                required_grams=row.required,
                remaining_grams=row.remaining,
            )
            for row in resolved
            if row.remaining is not None and row.remaining < row.required
        ]

    # ------------------------------------------------------------------ phase 3
    # Backup ON: build (identity, extruder)-keyed pool and required-sum maps
    # from EVERY assigned spool on the printer (not just the slots in the
    # print's mapping). Then emit deficits only when the pool for a slot's
    # material is too small for the print's total required of that material.
    pool_by_key: dict[tuple[str, int], float] = defaultdict(float)
    required_by_key: dict[tuple[str, int], float] = defaultdict(float)

    for slot in await build_slot_materials(db, item.printer_id):
        pool_by_key[(slot.material_key, slot.extruder)] += slot.remaining_grams

    for row in resolved:
        required_by_key[(row.identity, row.extruder)] += row.required

    deficits: list[FilamentDeficit] = []
    for row in resolved:
        key = (row.identity, row.extruder)
        # Pool insufficient for the print's TOTAL required of this material on
        # this extruder side → real deficit. The per-slot remaining still gets
        # surfaced so the UI can point at the slot the user assigned.
        if pool_by_key[key] < required_by_key[key]:
            deficits.append(
                FilamentDeficit(
                    slot_id=row.slot_id,
                    ams_id=row.ams_id,
                    tray_id=row.tray_id,
                    filament_type=row.filament_type,
                    required_grams=row.required,
                    remaining_grams=row.remaining,
                )
            )

    return deficits


# Re-export the most useful pieces for callers that just want the data.
__all__ = [
    "FilamentDeficit",
    "compute_deficit_for_queue_item",
]