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import { type RDF } from "../rdf.ts";
import { type IQueryEngine, type QueryContext } from "./types.ts";import {  getResponseTypes,  resolve,  type ResolverType,} from "./query_resolvers.ts";
/** * A query engine that can query a SPARQL endpoint. * * Implements {@link IQueryEngine} interface. * * This engine is used by default if no other engine is configured. * See {@link Options} for more details. * * If you need to query other data sources, or multiple SPARQL endpoints, * you can use [Comunica](https://comunica.dev) instead, extend this engine, * or implement your own. */export class QueryEngine implements IQueryEngine {  protected getSparqlEndpoint(context?: QueryContext): string {    if (!context) {      throw new Error(        "No context supplied to QueryEngine. You need to create a default context or pass one to a resource.",      );    }    if (!Array.isArray(context.sources) || context.sources.length < 1) {      throw new Error(        "Invalid context `sources` attribute defined. You need to provide a URL to a SPARQL endpoint to query.",      );    }    if (context.sources.length > 1) {      throw new Error(        "This query engine supports only one data source, multiple defined in `sources` property in context.",      );    }    const source = context.sources[0];    if (typeof source === "string") {      return source;    }    if ("value" in source && typeof source.value === "string") {      if ("type" in source) {        if (source.type === "sparql") {          return source.value;        }      } else {        return source.value;      }    }    throw new Error(      "Invalid SPARQL source defined - please provide URL to a SPARQL endpoint to query.",    );  }
  protected getFetch(context?: QueryContext): typeof fetch {    return context && context.fetch ? context.fetch : fetch;  }
  protected query(    query: string,    responseType: string,    context?: QueryContext,  ): Promise<Response> {    const endpoint = this.getSparqlEndpoint(context);    const fetchFn = this.getFetch(context);    return fetchFn(endpoint, {      method: "POST",      headers: {        "accept": responseType,        "content-type": "application/x-www-form-urlencoded; charset=UTF-8",      },      body: new URLSearchParams({        query,      }),    });  }
  protected async queryAndResolve<T extends ResolverType>(    type: T,    query: string,    context?: QueryContext,  ) {    const responseType = getResponseTypes(type).join(", ");    const response = await this.query(      query,      responseType,      context,    );
    if (!response.ok) {      await response.body?.cancel();      throw new Error(        `Invalid query response status '${response.status} ${response.statusText}'`,      );    }
    return resolve(type, response);  }
  /**   * Executes a SPARQL SELECT query and returns a stream of bindings.   *   * @param query SPARQL query string   * @param context Engine context   * @returns Stream of bindings   */  public queryBindings(    query: string,    context?: QueryContext,  ): Promise<RDF.ResultStream<RDF.Bindings>> {    return this.queryAndResolve("bindings", query, context);  }
  /**   * Executes a SPARQL ASK query and returns a boolean result.   *   * @param query SPARQL query string   * @param context Engine context   * @returns Boolean result   */  public queryBoolean(    query: string,    context?: QueryContext,  ): Promise<boolean> {    return this.queryAndResolve("boolean", query, context);  }
  /**   * Executes a SPARQL CONSTRUCT query and returns a stream of quads.   *   * @param query SPARQL query string   * @param context Engine context   * @returns Stream of quads   */  queryQuads(    query: string,    context?: QueryContext,  ): Promise<RDF.ResultStream<RDF.Quad>> {    return this.queryAndResolve("quads", query, context);  }
  /**   * Executes a SPARQL UPDATE query and returns nothing.   *   * @param query SPARQL query string   * @param context Engine context   * @returns Nothing   */  async queryVoid(    query: string,    context?: QueryContext,  ): Promise<void> {    await this.query(query, "application/sparql-results+json", context);  }}