@jahed/sparql-engine/dist/operators/modifiers/select.js

SPARQL query engine for servers and web browsers.

import { Pipeline } from "../../engine/pipeline/pipeline.js";
import { isWildcard, UNBOUND } from "../../utils/rdf.js";
/**
 * Evaluates a SPARQL SELECT operation, i.e., perform a selection over sets of solutions bindings
 * @see {@link https://www.w3.org/TR/2013/REC-sparql11-query-20130321/#select}
 * @param source - Input {@link PipelineStage}
 * @param query - SELECT query
 * @return A {@link PipelineStage} which evaluate the SELECT modifier
 */
export default function select(source, query) {
    if (!("variables" in query)) {
        throw new Error("Not a select query.");
    }
    if (isWildcard(query.variables[0])) {
        return Pipeline.getInstance().map(source, (bindings) => {
            // return bindings.mapValues((k, v) => (rdf.isVariable(k) ? v : null));
            return bindings;
        });
    }
    const variables = query.variables.map((v) => "variable" in v ? v.variable : v);
    return Pipeline.getInstance().map(source, (bindings) => {
        bindings = variables.reduce((obj, v) => {
            if (bindings.has(v.value)) {
                obj.set(v.value, bindings.get(v.value));
            }
            else {
                obj.set(v.value, UNBOUND);
            }
            return obj;
        }, bindings.empty());
        // return bindings.mapValues((k, v) => (rdf.isVariable(k) ? v : null));
        return bindings;
    });
}
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