/**
 * QuerySourceService — gateway query source router (FED-M3-09).
 *
 * Accepts the federation query-layer `source` selector and routes list-style
 * reads to local storage, one federated peer, or all active outbound peers.
 *
 * `source: "all"` is intentionally tolerant of per-peer failures: local data
 * and successful peer responses are returned, and the envelope is marked
 * `_partial: true`.  Local failures still reject because there is no safe local
 * fallback and the gateway's own storage is expected to be authoritative.
 */

import { Inject, Injectable, Logger } from '@nestjs/common';
import { and, eq, federationPeers, isNotNull, type Db } from '@mosaicstack/db';
import {
  SOURCE_LOCAL,
  tagWithSource,
  type FederationListResponse,
  type SourceTag,
} from '@mosaicstack/types';
import { DB } from '../../database/database.module.js';
import { FederationClientService } from './federation-client.service.js';

export type QuerySource = 'local' | 'all' | `federated:${string}`;

export type QuerySourceErrorCode = 'INVALID_SOURCE' | 'PEER_NOT_FOUND';

export interface QuerySourceErrorOptions {
  code: QuerySourceErrorCode;
  message: string;
  source: string;
}

export class QuerySourceError extends Error {
  readonly code: QuerySourceErrorCode;
  readonly source: string;

  constructor(opts: QuerySourceErrorOptions) {
    super(opts.message);
    this.name = 'QuerySourceError';
    this.code = opts.code;
    this.source = opts.source;
  }
}

export type LocalListExecutor<T extends object> = () => Promise<FederationListResponse<T> | T[]>;

export interface QuerySourceListOptions<T extends object> {
  source: QuerySource;
  resource: string;
  request?: Record<string, unknown>;
  local: LocalListExecutor<T>;
}

export type QuerySourceListResponse<T extends object> = FederationListResponse<T & SourceTag>;

interface OutboundPeer {
  id: string;
  commonName: string;
  endpointUrl: string;
}

interface TaggedList<T extends object> {
  items: Array<T & SourceTag>;
  partial: boolean;
  truncated: boolean;
  nextCursor?: string;
}

@Injectable()
export class QuerySourceService {
  private readonly logger = new Logger(QuerySourceService.name);

  constructor(
    @Inject(DB) private readonly db: Db,
    @Inject(FederationClientService) private readonly federationClient: FederationClientService,
  ) {}

  async list<T extends object>(
    options: QuerySourceListOptions<T>,
  ): Promise<QuerySourceListResponse<T>> {
    const request = options.request ?? {};

    if (options.source === 'local') {
      const local = await this.runLocal(options.local);
      return this.toResponse(this.tagList(local, SOURCE_LOCAL));
    }

    if (options.source === 'all') {
      return this.listAll(options.resource, request, options.local);
    }

    if (options.source.startsWith('federated:')) {
      const host = options.source.slice('federated:'.length).trim();
      if (!host) {
        throw new QuerySourceError({
          code: 'INVALID_SOURCE',
          message: 'Federated source must include a host after federated:',
          source: options.source,
        });
      }

      const peer = await this.findPeerByHost(host, options.source);
      const remote = await this.federationClient.list<T>(peer.id, options.resource, request);
      return this.toResponse(this.tagList(remote, peer.commonName));
    }

    throw new QuerySourceError({
      code: 'INVALID_SOURCE',
      message: `Unsupported query source: ${options.source}`,
      source: options.source,
    });
  }

  private async listAll<T extends object>(
    resource: string,
    request: Record<string, unknown>,
    local: LocalListExecutor<T>,
  ): Promise<QuerySourceListResponse<T>> {
    const peers = await this.listActiveOutboundPeers();

    const localPromise = this.runLocal(local).then((response) =>
      this.tagList(response, SOURCE_LOCAL),
    );
    const remotePromises = peers.map(async (peer: OutboundPeer): Promise<TaggedList<T> | null> => {
      try {
        const response = await this.federationClient.list<T>(peer.id, resource, request);
        return this.tagList(response, peer.commonName);
      } catch (error: unknown) {
        this.logger.warn(
          `Federated query to peer ${peer.commonName} (${peer.id}) failed; returning partial all-source response: ${
            error instanceof Error ? error.message : String(error)
          }`,
        );
        return null;
      }
    });

    const [localResult, ...remoteResults] = await Promise.all([localPromise, ...remotePromises]);
    const successfulRemoteResults = remoteResults.filter(
      (result: TaggedList<T> | null): result is TaggedList<T> => result !== null,
    );
    const allResults = [localResult, ...successfulRemoteResults];
    const peerFailure = successfulRemoteResults.length !== peers.length;

    return this.mergeTaggedLists(allResults, peerFailure);
  }

  private async runLocal<T extends object>(
    local: LocalListExecutor<T>,
  ): Promise<FederationListResponse<T>> {
    const response = await local();
    if (Array.isArray(response)) {
      return { items: response };
    }
    return response;
  }

  private tagList<T extends object>(
    response: FederationListResponse<T>,
    source: string,
  ): TaggedList<T> {
    return {
      items: tagWithSource(response.items, source),
      partial: response._partial === true,
      truncated: response._truncated === true || response.nextCursor !== undefined,
      nextCursor: response.nextCursor,
    };
  }

  private mergeTaggedLists<T extends object>(
    lists: Array<TaggedList<T>>,
    peerFailure: boolean,
  ): QuerySourceListResponse<T> {
    const items = lists.flatMap((list: TaggedList<T>) => list.items);
    const partial =
      peerFailure ||
      lists.some((list: TaggedList<T>) => list.partial || list.nextCursor !== undefined);
    const truncated = lists.some((list: TaggedList<T>) => list.truncated);

    const response: QuerySourceListResponse<T> = { items };
    if (partial) {
      response._partial = true;
    }
    if (truncated) {
      response._truncated = true;
    }
    return response;
  }

  private toResponse<T extends object>(tagged: TaggedList<T>): QuerySourceListResponse<T> {
    const response: QuerySourceListResponse<T> = {
      items: tagged.items,
    };
    if (tagged.nextCursor !== undefined) {
      response.nextCursor = tagged.nextCursor;
    }
    if (tagged.partial) {
      response._partial = true;
    }
    if (tagged.truncated) {
      response._truncated = true;
    }
    return response;
  }

  private async findPeerByHost(sourceHost: string, source: string): Promise<OutboundPeer> {
    const host = normalizeHost(sourceHost);
    const peers = await this.listActiveOutboundPeers();
    const peer = peers.find((candidate: OutboundPeer) => {
      const commonName = normalizeHost(candidate.commonName);
      const endpointHosts = endpointHostKeys(candidate.endpointUrl).map((endpointHost: string) =>
        normalizeHost(endpointHost),
      );
      return commonName === host || endpointHosts.includes(host);
    });

    if (!peer) {
      throw new QuerySourceError({
        code: 'PEER_NOT_FOUND',
        message: `No active outbound federation peer matches source ${source}`,
        source,
      });
    }

    return peer;
  }

  private async listActiveOutboundPeers(): Promise<OutboundPeer[]> {
    const rows = await this.db
      .select({
        id: federationPeers.id,
        commonName: federationPeers.commonName,
        endpointUrl: federationPeers.endpointUrl,
      })
      .from(federationPeers)
      .where(
        and(
          eq(federationPeers.state, 'active'),
          isNotNull(federationPeers.endpointUrl),
          isNotNull(federationPeers.clientKeyPem),
        ),
      )
      .orderBy(federationPeers.commonName);

    return rows.filter((row): row is OutboundPeer => typeof row.endpointUrl === 'string');
  }
}

function normalizeHost(host: string): string {
  return host.trim().toLowerCase();
}

function endpointHostKeys(endpointUrl: string): string[] {
  try {
    const url = new URL(endpointUrl);
    return Array.from(new Set([url.host, url.hostname].filter((host: string) => host.length > 0)));
  } catch {
    return [];
  }
}