"""Core domain entities, persistence- and framework-agnostic.

Languages are first-class data (`source_lang` / `target_lang`, ISO 639-1) rather
than constants: English is the launch target, but the model must not need a
migration to add Spanish or German later.
"""

from datetime import UTC, datetime
from enum import StrEnum
from typing import Any
from uuid import uuid4

from pydantic import BaseModel, Field


def _new_id() -> str:
    return uuid4().hex


def _now() -> datetime:
    return datetime.now(UTC)


class CEFRLevel(StrEnum):
    A1 = "A1"
    A2 = "A2"
    B1 = "B1"
    B2 = "B2"
    C1 = "C1"
    C2 = "C2"

    @property
    def rank(self) -> int:
        """0-based ordinal position (A1=0 ... C2=5), for ordinal metrics and adjacency."""
        return list(CEFRLevel).index(self)

    def distance(self, other: "CEFRLevel") -> int:
        """Absolute level distance; `<= 1` is the classic 'adjacent accuracy' criterion."""
        return abs(self.rank - other.rank)


class ExerciseType(StrEnum):
    READING_QA = "reading_qa"  # M1 — comprehension questions on a CEFR-graded text
    DICTATION = "dictation"  # M2 — TTS audio, learner types what they hear
    WRITING = "writing"  # M3 — free writing, LLM correction with typed errors
    PRONUNCIATION = "pronunciation"  # M5 — read-aloud with pronunciation scoring


class Learner(BaseModel):
    id: str = Field(default_factory=_new_id)
    display_name: str
    source_lang: str = "fr"
    target_lang: str = "en"
    created_at: datetime = Field(default_factory=_now)


class Exercise(BaseModel):
    """A generated, cacheable exercise.

    `payload` is type-specific content (text, questions, audio reference...).
    Generated exercises are content-addressed upstream (hash of source text +
    prompt version) so LLM cost stays ~0 and evals are reproducible.
    """

    id: str = Field(default_factory=_new_id)
    type: ExerciseType
    target_lang: str
    cefr_level: CEFRLevel
    payload: dict[str, Any] = Field(default_factory=dict)
    created_at: datetime = Field(default_factory=_now)


class Attempt(BaseModel):
    """One learner answer to one exercise, with its scoring/feedback."""

    id: str = Field(default_factory=_new_id)
    learner_id: str
    exercise_id: str
    response: dict[str, Any] = Field(default_factory=dict)
    score: float | None = Field(default=None, ge=0.0, le=1.0)
    feedback: dict[str, Any] = Field(default_factory=dict)
    created_at: datetime = Field(default_factory=_now)


class ReviewItem(BaseModel):
    """An atomic, schedulable knowledge item (vocab word, recurring error, ...).

    `scheduler_state` is an opaque dict owned by the SRS engine (FSRS-ready in
    M4) so the domain model is not coupled to one algorithm's parameters.
    """

    id: str = Field(default_factory=_new_id)
    learner_id: str
    kind: str  # "vocab" | "error" | ... (free-form until M4 hardens it)
    content: dict[str, Any] = Field(default_factory=dict)
    due_at: datetime | None = None
    scheduler_state: dict[str, Any] = Field(default_factory=dict)
    created_at: datetime = Field(default_factory=_now)