import tiktoken

from app.database.models import Message, Summary


class ContextBuilder:
    def __init__(self, token_limit: int):
        self.token_limit = token_limit
        try:
            self.encoder = tiktoken.get_encoding("cl100k_base")
        except Exception:
            self.encoder = None

    def count_tokens(self, text: str) -> int:
        return len(self.encoder.encode(text)) if self.encoder else max(1, len(text) // 4)

    def build(self, summary: Summary | None, messages: list[Message]) -> str:
        sections = [f"Current memory summary:\n{summary.text}" if summary else ""]
        recent: list[str] = []
        budget = self.token_limit - self.count_tokens(sections[0])
        for item in reversed(messages):
            line = f"[{item.sent_at.isoformat()}] {item.user_name}: {item.text}"
            cost = self.count_tokens(line)
            if cost > budget:
                break
            recent.append(line)
            budget -= cost
        if recent:
            sections.append("Recent messages:\n" + "\n".join(reversed(recent)))
        return "\n\n".join(part for part in sections if part)