"""DAG validation per Section 6."""

from __future__ import annotations

from abc import ABC, abstractmethod
from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from ect.types import Payload

from ect.validate import DEFAULT_MAX_PRED_LENGTH

DEFAULT_CLOCK_SKEW_TOLERANCE = 30
DEFAULT_MAX_ANCESTOR_LIMIT = 10000


class ECTStore(ABC):
    """Lookup of ECTs by task ID for DAG validation."""

    @abstractmethod
    def get_by_tid(self, tid: str) -> "Payload | None":
        pass

    @abstractmethod
    def contains(self, tid: str, wid: str) -> bool:
        pass


class DAGConfig:
    def __init__(
        self,
        clock_skew_tolerance: int = DEFAULT_CLOCK_SKEW_TOLERANCE,
        max_ancestor_limit: int = DEFAULT_MAX_ANCESTOR_LIMIT,
        max_pred_length: int = 0,
    ):
        self.clock_skew_tolerance = clock_skew_tolerance or DEFAULT_CLOCK_SKEW_TOLERANCE
        self.max_ancestor_limit = max_ancestor_limit or DEFAULT_MAX_ANCESTOR_LIMIT
        self.max_pred_length = max_pred_length or 0


def default_dag_config() -> DAGConfig:
    return DAGConfig()


def _has_cycle(
    target_tid: str,
    pred_ids: list[str],
    store: ECTStore,
    visited: set[str],
    max_depth: int,
) -> bool:
    if len(visited) >= max_depth:
        return True
    for pred_id in pred_ids:
        if pred_id == target_tid:
            return True
        if pred_id in visited:
            continue
        visited.add(pred_id)
        pred = store.get_by_tid(pred_id)
        if pred is not None:
            if _has_cycle(target_tid, pred.pred, store, visited, max_depth):
                return True
    return False


def validate_dag(
    payload: "Payload",
    store: ECTStore,
    cfg: DAGConfig,
) -> None:
    """Section 6.2: uniqueness (by jti), predecessor existence, temporal ordering, acyclicity, predecessor policy."""
    if cfg.max_pred_length > 0 and len(payload.pred) > cfg.max_pred_length:
        raise ValueError("ect: pred exceeds max length")
    if store.contains(payload.jti, payload.wid or ""):
        raise ValueError(f"ect: task ID (jti) already exists: {payload.jti}")

    for pred_id in payload.pred:
        pred = store.get_by_tid(pred_id)
        if pred is None:
            raise ValueError(f"ect: predecessor task not found: {pred_id}")
        if pred.iat >= payload.iat + cfg.clock_skew_tolerance:
            raise ValueError(f"ect: predecessor task not earlier than current: {pred_id}")

    visited: set[str] = set()
    if _has_cycle(payload.jti, payload.pred, store, visited, cfg.max_ancestor_limit):
        raise ValueError("ect: circular dependency or depth limit exceeded")

    # Predecessor policy decision: only when predecessor has policy claims in ext per -01
    for pred_id in payload.pred:
        pred = store.get_by_tid(pred_id)
        if pred and pred.has_policy_claims() and pred.pol_decision() in ("rejected", "pending_human_review"):
            if not payload.compensation_required():
                raise ValueError(
                    "ect: predecessor has non-approved pol_decision; current ECT must be compensation/remediation or have ext.compensation_required true"
                )