"""The typed cost-IR of a candidate measure (method-spec §7.1).

Schema invariants are enforced at construction, so a malformed proposal can never
exist as a value (§3 Step 2): ``affected_items`` non-empty with ``quantity >= 0`` and
``unit_cost > 0``, ``claimed_saving_nok > 0`` and never above the affected items' own
total, ``assumptions`` an uncertainty band per cost code (empty = degenerate, no
spread). ALL numbers are finite and band endpoints non-negative (R-2 hardening: a
non-finite number would clear the mandatory validator vacuously — ``p90=inf``
validates everything; note ``json.loads`` accepts the bare ``Infinity`` literal).
Loading the IR projection from a bundle is FAIL-FAST: a missing file raises
(required input — contrast the tolerant inbox, §5).
"""

from __future__ import annotations

import json
from pathlib import Path
from typing import Annotated, Any

from pydantic import BaseModel, Field, model_validator

_VALIDATOR_INPUT_FILENAME = "validator-input.json"

# A band endpoint is a sampled unit cost (§7.1) — finite, never negative.
_BandEndpoint = Annotated[float, Field(ge=0, allow_inf_nan=False)]


class AffectedItem(BaseModel):
    """One affected cost item: ``{code, quantity >= 0, unit_cost > 0}`` (§7.1)."""

    code: str = Field(min_length=1)
    quantity: float = Field(ge=0, allow_inf_nan=False)
    unit_cost: float = Field(gt=0, allow_inf_nan=False)


class SavingsProposal(BaseModel):
    """The candidate measure projected into the typed cost-IR (§7.1)."""

    project_id: str = Field(min_length=1)
    measure: str = Field(min_length=1)
    affected_items: list[AffectedItem] = Field(min_length=1)
    claimed_saving_nok: float = Field(gt=0, allow_inf_nan=False)
    assumptions: dict[str, tuple[_BandEndpoint, _BandEndpoint]] = Field(default_factory=dict)

    @model_validator(mode="after")
    def _claim_within_affected_total(self) -> SavingsProposal:
        # §7.1: a claim above the items' own total is a schema error, not a
        # validator rejection — the value must never exist.
        total = sum(item.quantity * item.unit_cost for item in self.affected_items)
        if self.claimed_saving_nok > total:
            raise ValueError(
                f"claimed_saving_nok ({self.claimed_saving_nok}) exceeds the affected "
                f"items' own total ({total})"
            )
        return self


def load_validator_input(bundle_dir: Path) -> SavingsProposal:
    """Load a bundle's IR projection — FAIL-FAST: a missing file raises (§7.1)."""
    raw: dict[str, Any] = json.loads(
        (bundle_dir / _VALIDATOR_INPUT_FILENAME).read_text(encoding="utf-8")
    )
    # Shared fasit files carry an informative "_note"; extra keys are ignored.
    return SavingsProposal.model_validate(raw)