@jahed/sparql-engine/src/operators/join/index-join.ts

SPARQL query engine for servers and web browsers.

// SPDX-License-Identifier: MIT
import ExecutionContext from "../../engine/context/execution-context.ts";
import type { PipelineStage } from "../../engine/pipeline/pipeline-engine.ts";
import { Pipeline } from "../../engine/pipeline/pipeline.ts";
import { BindingBase, Bindings } from "../../rdf/bindings.ts";
import Graph from "../../rdf/graph.ts";
import type { EngineTriple, EngineTripleValue } from "../../types.ts";
import { isVariable } from "../../utils/rdf.ts";

/**
 * Perform a join between a source of solution bindings (left relation)
 * and a triple pattern (right relation) using the Index Nested Loop Join algorithm.
 * This algorithm is more efficient if the cardinality of the left relation is smaller
 * than the cardinality of the right one.
 * @param source - Left input (a {@link PipelineStage})
 * @param pattern - Triple pattern to join with (right relation)
 * @param graph   - RDF Graph on which the join is performed
 * @param context - Execution context
 * @return A {@link PipelineStage} which evaluate the join
 */
export default function indexJoin(
  source: PipelineStage<Bindings>,
  pattern: EngineTriple,
  graph: Graph,
  context: ExecutionContext
) {
  const engine = Pipeline.getInstance();
  return engine.mergeMap(source, (bindings: Bindings) => {
    const boundedPattern = bindings.bound(pattern);
    return engine.map(
      engine.from(graph.find(boundedPattern, context)),
      (item: EngineTriple) => {
        const obj: Record<string, EngineTripleValue> = {};
        if (isVariable(boundedPattern.subject)) {
          obj[boundedPattern.subject.value] = item.subject;
        }
        if (isVariable(boundedPattern.predicate)) {
          obj[boundedPattern.predicate.value] = item.predicate;
        }
        if (isVariable(boundedPattern.object)) {
          obj[boundedPattern.object.value] = item.object;
        }
        return BindingBase.fromObject(obj).union(bindings);
      }
    );
  });
}