"""Step 1 (SPEC L-10): deterministic rules from `config/agent_rules.yaml`, before any LLM call."""

from pathlib import Path

import regex
import yaml
from pydantic import BaseModel, ConfigDict, Field, field_validator


class Rule(BaseModel):
    model_config = ConfigDict(extra="forbid", frozen=True)

    reason: str
    pattern: str

    @field_validator("pattern")
    @classmethod
    def _compiles(cls, value: str) -> str:
        try:
            regex.compile(value)
        except regex.error as e:
            raise ValueError(f"bad pattern: {e}") from e
        return value


class AgentRules(BaseModel):
    model_config = ConfigDict(extra="forbid", frozen=True)

    max_length: int = Field(default=500, gt=0)
    rules: list[Rule]


def load_rules(config_dir: Path) -> AgentRules:
    with (config_dir / "agent_rules.yaml").open(encoding="utf-8") as f:
        return AgentRules.model_validate(yaml.safe_load(f))


TOO_LONG = "слишком длинный запрос"


class Prefilter:
    def __init__(self, rules: AgentRules) -> None:
        self._max = rules.max_length
        self._rules = [(r.reason, regex.compile(r.pattern, regex.IGNORECASE)) for r in rules.rules]

    def check(self, prompt: str) -> str | None:
        """The reason to refuse, or None when the prompt may go on to the guard."""
        if len(prompt) > self._max:
            return TOO_LONG
        for reason, pattern in self._rules:
            if pattern.search(prompt, timeout=0.1):
                return reason
        return None