# PartTreeStalactiteCountProjection.java

    package org.codefilarete.stalactite.spring.repository.query.projection;

    import java.util.Set;
    import java.util.function.BiConsumer;
    import java.util.function.Function;
    import java.util.function.Supplier;

    import org.codefilarete.stalactite.engine.ExecutableProjection.ProjectionDataProvider;
    import org.codefilarete.stalactite.engine.runtime.AdvancedEntityPersister;
    import org.codefilarete.stalactite.engine.runtime.projection.ProjectionQueryCriteriaSupport;
    import org.codefilarete.stalactite.engine.runtime.query.EntityQueryCriteriaSupport.EntityQueryPageSupport;
    import org.codefilarete.stalactite.query.Operators;
    import org.codefilarete.stalactite.query.model.operator.Count;
    import org.codefilarete.stalactite.spring.repository.query.derivation.ToCriteriaPartTreeTransformer;
    import org.codefilarete.stalactite.spring.repository.query.derivation.ToCriteriaPartTreeTransformer.Condition;
    import org.codefilarete.stalactite.sql.ddl.structure.Column;
    import org.codefilarete.stalactite.sql.ddl.structure.Table;
    import org.codefilarete.stalactite.sql.result.Accumulator;
    import org.codefilarete.tool.trace.MutableLong;
    import org.springframework.data.repository.query.QueryMethod;
    import org.springframework.data.repository.query.RepositoryQuery;
    import org.springframework.data.repository.query.parser.PartTree;

    /**
     * Creates a repository query dedicated to the "count" case.
     *
     * @param <C> domain entity type
     * @author Guillaume Mary
     */
    public class PartTreeStalactiteCountProjection<C> implements RepositoryQuery {

    	private final Count count;
    	private final QueryMethod method;
    	private final AdvancedEntityPersister<C, ?> entityPersister;
    	private final Accumulator<ProjectionDataProvider, MutableLong, Long> accumulator;
    	private final ToCriteriaPartTreeTransformer<C> criteriaAppender;

    	/**
    	 * @param method the method found by Spring
    	 * @param entityPersister the Stalactite domain persister
    	 * @param partTree result of method parsing
    	 */
    	public PartTreeStalactiteCountProjection(QueryMethod method,
    											 AdvancedEntityPersister<C, ?> entityPersister,
    											 PartTree partTree) {
    		this.method = method;
    		this.entityPersister = entityPersister;
    		Set<Column<Table, ?>> columns = entityPersister.getMapping().getIdMapping().<Table>getIdentifierAssembler().getColumns();
    		count = Operators.count(columns);
    		if (partTree.isDistinct()) {
    			count.distinct();
    		}
    		accumulator = new Accumulator<ProjectionDataProvider, MutableLong, Long>() {
    			@Override
    			public Supplier<MutableLong> supplier() {
    				return MutableLong::new;
    			}

    			@Override
    			public BiConsumer<MutableLong, ProjectionDataProvider> aggregator() {
    				return (modifiableLong, rowDataProvider) -> {
    					modifiableLong.reset(rowDataProvider.getValue(count));
    				};
    			}

    			@Override
    			public Function<MutableLong, Long> finisher() {
    				return MutableLong::getValue;
    			}
    		};
    		criteriaAppender = new ToCriteriaPartTreeTransformer<>(
    				partTree,
    				entityPersister.getClassToPersist());
    	}

    	@Override
    	public Long execute(Object[] parameters) {
    		ProjectionQueryCriteriaSupport<C, ?> executableEntityQuery = entityPersister.newProjectionCriteriaSupport(select -> {
    			select.add(count, "row_count");
    		});
    		// because order-by and limit clauses are compatible with count operator, we pass a page support which is not the one the executable query,
    		// like a local black hole.
    		EntityQueryPageSupport<C> blackHole = new EntityQueryPageSupport<>();
    		Condition condition = criteriaAppender.applyTo(executableEntityQuery.getEntityCriteriaSupport(), blackHole, blackHole);
    		condition.consume(parameters);
    		return executableEntityQuery.wrapIntoExecutable().execute(accumulator);
    	}

    	@Override
    	public QueryMethod getQueryMethod() {
    		return method;
    	}
    }