@jahed/sparql-engine/dist/operators/optional.js

SPARQL query engine for servers and web browsers.

import { Pipeline } from "../engine/pipeline/pipeline.js";
import ExecutionContext from "../engine/context/execution-context.js";
import { PlanBuilder } from "../engine/plan-builder.js";
import { Bindings } from "../rdf/bindings.js";
/**
 * Handles an SPARQL OPTIONAL clause
 * @see {@link https://www.w3.org/TR/sparql11-query/#optionals}
 * @param source - Input {@link PipelineStage}
 * @param patterns - OPTIONAL clause, i.e., a SPARQL group pattern
 * @param builder - Instance of the current {@link PlanBuilder}
 * @param context - Execution context
 * @return A {@link PipelineStage} which evaluate the OPTIONAL operation
 */
export default async function optional(source, patterns, builder, context) {
    const seenBefore = [];
    const engine = Pipeline.getInstance();
    const start = engine.tap(source, (bindings) => {
        seenBefore.push(bindings);
    });
    let leftOp = await builder._buildWhere(start, patterns, context);
    leftOp = engine.tap(leftOp, (bindings) => {
        // remove values that matches a results from seenBefore
        const index = seenBefore.findIndex((b) => {
            return b.isSubset(bindings);
        });
        if (index >= 0) {
            seenBefore.splice(index, 1);
        }
    });
    return engine.merge(leftOp, engine.from(seenBefore));
}
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