const requestp = require('request-promise-native');

const {makeCached} = require('./util');

const OVERPASS_ENDPOINT = 'https://lz4.overpass-api.de/api/interpreter';

const fetchLineData = makeCached(async (lineRef) =>
{
    // Retrieve routes, ways and stops from OpenStreetMap
    const rawData = await requestp.post(OVERPASS_ENDPOINT, {form: `\
data=[out:json];

// Find the public transport line bearing the requested reference
relation[network="TaM"][type="route_master"][ref="${lineRef}"];

// Recursively fetch routes, ways and stops inside the line
(._; >>;);

out body qt;
    `});

    const elementsList = JSON.parse(rawData).elements;

    // Extract all routes for the given line
    const rawRoutes = elementsList.filter(elt =>
        elt.type === 'relation'
        && elt.tags.type === 'route'
        && elt.tags.ref === lineRef
    );

    // If no route is found, assume the line does not exist
    if (rawRoutes.length === 0)
    {
        return null;
    }

    // Index retrieved objects by their ID
    const elements = elementsList.reduce((prev, elt) =>
    {
        prev[elt.id] = elt;
        return prev;
    }, {});

    const color = rawRoutes[0].tags.colour || '#000000';

    // Extract stops for each route of the line
    const routes = rawRoutes.map(route => ({
        from: route.tags.from,
        to: route.tags.to,

        // Retrieve each stop’s information (stop order in the relation is
        // assumed to reflect reality)
        stops: route.members
            .filter(({role}) => role === 'stop')
            .map(({ref}) => {
                const elt = elements[ref];
                return {
                    id: ref,
                    lat: elt.lat,
                    lon: elt.lon,
                    name: elt.tags.name,
                };
            }),

        // List of ways making up the route (raw)
        rawWays: route.members
            .filter(({role}) => role === '')
            .map(({ref}) => ref)
    }));

    // Process ways in each route to sort and merge them
    for (let route of routes)
    {
        const {from, to, stops, rawWays} = route;

        // Construct a graph with all connected nodes
        const nodeNeighbors = new Map();

        for (let id of rawWays)
        {
            const {type, nodes} = elements[id];

            if (type === 'way' && nodes.length >= 1)
            {
                let previousNode = nodes[0];

                if (!nodeNeighbors.has(previousNode))
                {
                    nodeNeighbors.set(previousNode, new Set());
                }

                for (let node of nodes.slice(1))
                {
                    if (!nodeNeighbors.has(node))
                    {
                        nodeNeighbors.set(node, new Set([previousNode]));
                    }
                    else
                    {
                        nodeNeighbors.get(node).add(previousNode);
                    }

                    nodeNeighbors.get(previousNode).add(node);
                    previousNode = node;
                }
            }
        }

        // Find way from first stop through the end using DFS
        const numberOfStops = stops.length;
        const ways = [];

        let currentStopIndex = 0;

        while (currentStopIndex + 1 < numberOfStops)
        {
            const currentStop = stops[currentStopIndex];
            const nextStop = stops[currentStopIndex + 1];

            const visited = new Set();
            const stack = [{
                currentNode: currentStop.id,
                way: [currentStop.id],
            }];

            let found = false;

            while (stack.length !== 0)
            {
                const {currentNode, way} = stack.pop();
                visited.add(currentNode);

                if (currentNode === nextStop.id)
                {
                    // Arrived at next stop
                    ways.push(way);
                    found = true;
                    break;
                }

                const neighbors = nodeNeighbors.get(currentNode) || [];

                for (let nextNode of neighbors)
                {
                    if (!visited.has(nextNode))
                    {
                        stack.push({
                            currentNode: nextNode,
                            way: way.concat([nextNode]),
                        });
                    }
                }
            }

            if (!found)
            {
                throw new Error(`No way between stop “${currentStop.name}” \
(${currentStop.id}) and stop “${nextStop.name}” (${nextStop.id}) on line \
${lineRef}’s route from “${from}” to “${to}”`);
            }

            ++currentStopIndex;
        }

        // Only keep geo coordinates for each way node
        route.ways = ways.map(way =>
            way.map(id =>
            {
                const node = elements[id];
                return {lat: node.lat, lon: node.lon};
            })
        );

        delete route.rawWays;
    }

    return {
        ref: lineRef,
        color,
        routes
    };
});

exports.fetchLineData = fetchLineData;