import os
from typing import Tuple

def build_prompt(prefix: str, suffix: str) -> str:
    """
    改进后的提示词构建函数。
    使用更明确的指令来引导模型生成高质量的续写内容。
    """
    MAX_CONTEXT_LINES = 30
    
    prefix_lines = prefix.split('\n')
    suffix_lines = suffix.split('\n') if suffix else []
    
    recent_prefix = '\n'.join(prefix_lines[-MAX_CONTEXT_LINES:])
    recent_suffix = '\n'.join(suffix_lines[:5])

    prompt = f"""You are an expert writing assistant. Continue the text naturally.

CONTEXT:
⟨CURSOR⟩ marks where to continue.
- Before ⟨CURSOR⟩: existing text
- After ⟨CURSOR⟩: following context (if any)

RULES:
- Match existing style, tone, and terminology
- Maintain logical flow
- Write only the continuation, nothing else

TEXT:
{recent_prefix}⟨CURSOR⟩{recent_suffix}

CONTINUATION:"""
    
    return prompt.strip()