import type { VercelRequest, VercelResponse } from '@vercel/node';

const SYSTEM_INSTRUCTION = `
You are an expert Key Account Manager (KAM) and Senior Sales Strategist for "Craze Analytix".
Your specialty is analyzing retail data, advertising performance, and market trends to provide high-level strategic recommendations.

You have access to a detailed report of the currently filtered sales and advertising data.
The data includes Sell Out (€), Units Sold, Product Lines, Customer Markets, Seasonality (Monthly Trends), and Growth/Decline metrics.

Your objective:
1.  **Analyze**: Deeply study the provided data according to the user's specific query.
2.  **Opinion**: Provide expert opinions on the health of the business, product performance, or market position.
3.  **Recommendations**: Give actionable, professional advice to improve sales, ROAS, stock health, or market share.

Guidelines:
- Maintain a professional, consultative, and business-driven tone (KAM style).
- Always justify your recommendations with specific numbers from the data.
- If asked about "Trends" or "Seasonality", look at the Monthly Seasonality section.
- If asked about "Growth" or "Decline", look at the Top/Bottom Movers sections.
- When comparing metrics, calculate variances or ratios if relevant (e.g. Sales vs Ads growth).
- Format your response with clear headers and bullet points for readability.
- Respond in the same language as the user's question (e.g., if they ask in Spanish, answer in Spanish).
`;

export default async function handler(req: VercelRequest, res: VercelResponse) {
    // CORS headers
    res.setHeader('Access-Control-Allow-Origin', '*');
    res.setHeader('Access-Control-Allow-Methods', 'POST, OPTIONS');
    res.setHeader('Access-Control-Allow-Headers', 'Content-Type');

    if (req.method === 'OPTIONS') {
        return res.status(200).end();
    }

    if (req.method !== 'POST') {
        return res.status(405).json({ error: 'Method not allowed' });
    }

    const apiKey = process.env.GEMINI_API_KEY || process.env.API_KEY;

    if (!apiKey) {
        console.error('[ask-gemini] GEMINI_API_KEY is not set in environment variables');
        return res.status(500).json({ error: 'API Key is not configured on the server. Please add GEMINI_API_KEY to Vercel Environment Variables.' });
    }

    try {
        const { question, context } = req.body;

        if (!question || !context) {
            return res.status(400).json({ error: 'Missing question or context in request body' });
        }

        // Call Gemini API directly via REST (no SDK needed server-side)
        const geminiUrl = `https://generativelanguage.googleapis.com/v1beta/models/gemini-2.0-flash:generateContent?key=${apiKey}`;

        const response = await fetch(geminiUrl, {
            method: 'POST',
            headers: { 'Content-Type': 'application/json' },
            body: JSON.stringify({
                system_instruction: {
                    parts: [{ text: SYSTEM_INSTRUCTION }]
                },
                contents: [{
                    parts: [{ text: `Context Data:\n${context}\n\nUser Question: ${question}` }]
                }]
            })
        });

        if (!response.ok) {
            const errorData = await response.text();
            console.error('[ask-gemini] Gemini API error:', response.status, errorData);
            throw new Error(`Gemini API error: ${response.status}`);
        }

        const data = await response.json();
        const text = data?.candidates?.[0]?.content?.parts?.[0]?.text || "I couldn't generate a response.";

        res.status(200).json({ response: text });
    } catch (error: any) {
        console.error('[ask-gemini] Error:', error);
        res.status(500).json({ error: error.message || 'An unexpected error occurred' });
    }
}