from datetime import datetime
from typing import Literal

from pydantic import BaseModel, ConfigDict, Field, field_validator, model_validator


class Message(BaseModel):
    role: Literal["user", "assistant"]
    content: str
    created_at: datetime


class Session(BaseModel):
    model_config = ConfigDict(extra="forbid")

    session_id: str
    system_prompt: str
    created_at: datetime
    updated_at: datetime
    messages: list[Message] = Field(default_factory=list)

    @model_validator(mode="before")
    @classmethod
    def add_timestamp_to_legacy_messages(cls, data: object) -> object:
        if not isinstance(data, dict):
            return data

        fallback = data.get("created_at")
        messages = data.get("messages")
        if fallback is not None and isinstance(messages, list):
            for message in messages:
                if isinstance(message, dict):
                    message.setdefault("created_at", fallback)
        return data


class CreateSessionRequest(BaseModel):
    model_config = ConfigDict(extra="forbid")

    system_prompt: str | None = None

    @field_validator("system_prompt")
    @classmethod
    def validate_system_prompt(cls, value: str | None) -> str | None:
        if value is not None and not value.strip():
            raise ValueError("system_prompt must not be blank")
        return value


class CreateSessionResponse(BaseModel):
    session_id: str
    system_prompt: str
    created_at: datetime


class SendMessageRequest(BaseModel):
    model_config = ConfigDict(extra="forbid")

    content: str

    @field_validator("content")
    @classmethod
    def validate_content(cls, value: str) -> str:
        if not value.strip():
            raise ValueError("content must not be blank")
        return value


class TokenUsage(BaseModel):
    prompt_tokens: int
    completion_tokens: int
    total_tokens: int


class SendMessageResponse(BaseModel):
    session_id: str
    message: Message
    usage: TokenUsage | None


class SessionHistoryResponse(BaseModel):
    session_id: str
    system_prompt: str
    created_at: datetime
    updated_at: datetime
    messages: list[Message]