import { getSession } from 'next-auth/react';

export default async function handler(req, res) {
    // Authentication check
    const session = await getSession({ req });
    if (!session) {
        return res.status(401).json({ error: 'Unauthorized' });
    }

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

    try {
        const { objects, cables } = req.body;
        
        // Validate input data
        if (!objects || !cables) {
            return res.status(400).json({ error: 'Missing objects or cables data' });
        }

        // Find transformer (source)
        const transformer = objects.find(obj => obj.type === 'transformer');
        if (!transformer) {
            return res.status(400).json({ error: 'No transformer found in the system' });
        }

        // Calculate voltage drops for each path
        const results = calculateVoltageDrops(transformer, objects, cables);

        res.status(200).json({ 
            success: true, 
            data: results,
            timestamp: new Date().toISOString()
        });
    } catch (error) {
        console.error('Voltage drop calculation error:', error);
        res.status(500).json({ 
            error: 'Internal server error',
            details: process.env.NODE_ENV === 'development' ? error.message : undefined
        });
    }
}

function calculateVoltageDrops(transformer, objects, cables) {
    const results = {
        transformer: transformer,
        paths: [],
        summary: {
            totalPower: 0,
            maxVoltageDrop: 0,
            minVoltage: transformer.bottomVoltage
        }
    };

    // Cable resistance values (Ω/km) - simplified values for common cable types
    const cableResistance = {
        'YAKY': {
            16: 1.15, 25: 0.727, 35: 0.524, 50: 0.387, 70: 0.268, 95: 0.193, 120: 0.153, 150: 0.124, 185: 0.099, 240: 0.075
        },
        'NA2XY-J': {
            16: 1.15, 25: 0.727, 35: 0.524, 50: 0.387, 70: 0.268, 95: 0.193, 120: 0.153, 150: 0.124, 185: 0.099, 240: 0.075
        },
        'AL': {
            16: 1.91, 25: 1.20, 35: 0.868, 50: 0.641, 70: 0.443, 95: 0.320, 120: 0.253, 150: 0.206, 185: 0.164, 240: 0.125
        },
        'AsXSn': {
            16: 1.91, 25: 1.20, 35: 0.868, 50: 0.641, 70: 0.443, 95: 0.320, 120: 0.253, 150: 0.206, 185: 0.164, 240: 0.125
        }
    };

    // Find all paths from transformer to end connections
    const paths = findAllPaths(transformer, objects, cables);

    paths.forEach((path, index) => {
        let totalVoltageDrop = 0;
        let pathPower = 0;
        const pathDetails = [];

        // Calculate for each segment in the path
        for (let i = 0; i < path.length - 1; i++) {
            const fromNode = path[i];
            const toNode = path[i + 1];
            
            // Find the cable connecting these nodes
            const cable = cables.find(c => 
                (c.from === fromNode.id && c.to === toNode.id) ||
                (c.from === toNode.id && c.to === fromNode.id)
            );

            if (cable) {
                // Get cable resistance
                const resistance = getCableResistance(cable, cableResistance);
                
                // Calculate current (simplified - assuming balanced load)
                const nodePower = toNode.powerRating || 0;
                const current = nodePower > 0 ? (nodePower * 1000) / (Math.sqrt(3) * transformer.bottomVoltage * 0.9) : 0; // cos φ = 0.9
                
                // Calculate voltage drop for this segment
                const segmentVoltageDrop = Math.sqrt(3) * current * resistance * (cable.length / 1000); // length in km
                
                totalVoltageDrop += segmentVoltageDrop;
                pathPower += nodePower;

                pathDetails.push({
                    from: fromNode.name,
                    to: toNode.name,
                    cable: {
                        label: cable.label,
                        type: cable.cableType,
                        crossSection: cable.crossSection,
                        length: cable.length
                    },
                    current: current.toFixed(2),
                    resistance: resistance.toFixed(4),
                    voltageDrop: segmentVoltageDrop.toFixed(2),
                    power: nodePower
                });
            }
        }

        const finalVoltage = transformer.bottomVoltage - totalVoltageDrop;
        const voltageDropPercentage = (totalVoltageDrop / transformer.bottomVoltage) * 100;

        results.paths.push({
            id: index + 1,
            endNode: path[path.length - 1].name,
            totalVoltageDrop: totalVoltageDrop.toFixed(2),
            finalVoltage: finalVoltage.toFixed(2),
            voltageDropPercentage: voltageDropPercentage.toFixed(2),
            pathPower: pathPower,
            segments: pathDetails,
            isValid: voltageDropPercentage <= 5 // 5% is typical limit
        });

        results.summary.totalPower += pathPower;
        if (totalVoltageDrop > results.summary.maxVoltageDrop) {
            results.summary.maxVoltageDrop = totalVoltageDrop;
            results.summary.minVoltage = finalVoltage;
        }
    });

    return results;
}

function findAllPaths(transformer, objects, cables) {
    const paths = [];
    const visited = new Set();

    function dfs(currentNode, currentPath) {
        visited.add(currentNode.id);
        currentPath.push(currentNode);

        // If this is an end connection, save the path
        if (currentNode.type === 'end') {
            paths.push([...currentPath]);
        } else {
            // Find all connected nodes
            const connectedCables = cables.filter(cable => 
                cable.from === currentNode.id || cable.to === currentNode.id
            );

            connectedCables.forEach(cable => {
                const nextNodeId = cable.from === currentNode.id ? cable.to : cable.from;
                const nextNode = objects.find(obj => obj.id === nextNodeId);
                
                if (nextNode && !visited.has(nextNodeId)) {
                    dfs(nextNode, currentPath);
                }
            });
        }

        currentPath.pop();
        visited.delete(currentNode.id);
    }

    dfs(transformer, []);
    return paths;
}

function getCableResistance(cable, resistanceTable) {
    const cableType = cable.cableType || 'YAKY';
    const crossSection = cable.crossSection || 25;
    
    if (resistanceTable[cableType] && resistanceTable[cableType][crossSection]) {
        return resistanceTable[cableType][crossSection];
    }
    
    // Default resistance if not found
    return 1.0;
}