io.parsingdata.metal.expression.value.Fold Maven / Gradle / Ivy

Go to download
Show more of this group Show more artifacts with this name
Show all versions of metal-core Show documentation
Core building blocks and API for the Metal library.
There is a newer version: 10.0.0
/*
 * Copyright 2013-2016 Netherlands Forensic Institute
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package io.parsingdata.metal.expression.value;

import static io.parsingdata.metal.Trampoline.complete;
import static io.parsingdata.metal.Trampoline.intermediate;
import static io.parsingdata.metal.Util.checkNotNull;

import java.util.Objects;
import java.util.Optional;
import java.util.function.BinaryOperator;

import io.parsingdata.metal.Trampoline;
import io.parsingdata.metal.Util;
import io.parsingdata.metal.data.ImmutableList;
import io.parsingdata.metal.data.ParseState;
import io.parsingdata.metal.encoding.Encoding;

/**
 * Base class for {@link ValueExpression} implementations of the Fold
 * operation.
 * 
 * Fold has three operands: values (a {@link ValueExpression}),
 * reducer (a {@link BinaryOperator}) and initial (a
 * {@link ValueExpression}). First initial is evaluated. If it
 * does not return a single value, the final result is an empty list. Next,
 * values is evaluated and its result is passed to the abstract
 * {@link #prepareValues(ImmutableList)} method. The returned list is prefixed
 * by the value returned by evaluating initial. On this list, the
 * reducer is applied to the first two values until a single
 * value remains, which is then returned.
 */
public abstract class Fold implements ValueExpression {

    public final ValueExpression values;
    public final BinaryOperator reducer;
    public final ValueExpression initial;

    public Fold(final ValueExpression values, final BinaryOperator reducer, final ValueExpression initial) {
        this.values = checkNotNull(values, "values");
        this.reducer = checkNotNull(reducer, "reducer");
        this.initial = initial;
    }

    @Override
    public ImmutableList> eval(final ParseState parseState, final Encoding encoding) {
        final ImmutableList> initial = this.initial != null ? this.initial.eval(parseState, encoding) : new ImmutableList<>();
        if (initial.size > 1) {
            return new ImmutableList<>();
        }
        final ImmutableList> values = prepareValues(this.values.eval(parseState, encoding));
        if (values.isEmpty() || containsEmpty(values).computeResult()) {
            return initial;
        }
        if (!initial.isEmpty()) {
            return ImmutableList.create(fold(parseState, encoding, reducer, initial.head, values).computeResult());
        }
        return ImmutableList.create(fold(parseState, encoding, reducer, values.head, values.tail).computeResult());
    }

    private Trampoline> fold(final ParseState parseState, final Encoding encoding, final BinaryOperator reducer, final Optional head, final ImmutableList> tail) {
        if (!head.isPresent() || tail.isEmpty()) {
            return complete(() -> head);
        }
        final ImmutableList> reducedValue = reduce(reducer, head.get(), tail.head.get()).eval(parseState, encoding);
        if (reducedValue.size != 1) {
            throw new IllegalArgumentException("Reducer must evaluate to a single value.");
        }
        return intermediate(() -> fold(parseState, encoding, reducer, reducedValue.head, tail.tail));
    }

    private Trampoline containsEmpty(final ImmutableList> list) {
        if (list.isEmpty()) {
            return complete(() -> false);
        }
        return list.head
            .map(t -> intermediate(() -> containsEmpty(list.tail)))
            .orElseGet(() -> complete(() -> true));
    }

    protected abstract ImmutableList> prepareValues(ImmutableList> values);

    protected abstract ValueExpression reduce(BinaryOperator reducer, Value head, Value tail);

    @Override
    public String toString() {
        return getClass().getSimpleName() + "(" + values + "," + reducer + (initial == null ? "" : "," + initial) + ")";
    }

    @Override
    public boolean equals(final Object obj) {
        return Util.notNullAndSameClass(this, obj)
            && Objects.equals(values, ((Fold)obj).values)
            && Objects.equals(reducer, ((Fold)obj).reducer)
            && Objects.equals(initial, ((Fold)obj).initial);
    }

    @Override
    public int hashCode() {
        return Objects.hash(getClass(), values, reducer, initial);
    }

}