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/*
 * Copyright 2013-2024 Netherlands Forensic Institute
 * Copyright 2021-2024 Infix Technologies B.V.
 *
 * 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 static io.parsingdata.metal.expression.value.NotAValue.NOT_A_VALUE;

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

import io.parsingdata.metal.ImmutableObject;
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 SingleValueExpression} implementations of the Fold
 * operation.
 * 
 * Fold has three operands: values (a {@link ValueExpression}),
 * reducer (a {@link BinaryOperator}) and initial (a
 * {@link SingleValueExpression}). First initial is evaluated. If it
 * does not return a valid 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 extends ImmutableObject implements SingleValueExpression {

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

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

    @Override
    public Optional evalSingle(final ParseState parseState, final Encoding encoding) {
        final Optional initialValue = initial != null ? initial.evalSingle(parseState, encoding) : Optional.empty();
        if (initialValue.isPresent() && initialValue.get().equals(NOT_A_VALUE)) {
            return initialValue;
        }
        final ImmutableList unpreparedValues = this.values.eval(parseState, encoding);
        if (unpreparedValues.isEmpty()) {
            return initialValue;
        }
        if (containsNotAValue(unpreparedValues).computeResult()) {
            return Optional.of(NOT_A_VALUE);
        }
        final ImmutableList valueList = initialValue.map(value -> prepareValues(unpreparedValues).add(value))
            .orElseGet(() -> prepareValues(unpreparedValues));
        return Optional.of(fold(parseState, encoding, reducer, valueList.head, valueList.tail).computeResult());
    }

    private Trampoline fold(final ParseState parseState, final Encoding encoding, final BinaryOperator reducer, final Value head, final ImmutableList tail) {
        if (head.equals(NOT_A_VALUE) || tail.isEmpty()) {
            return complete(() -> head);
        }
        return reduce(reducer, head, tail.head).evalSingle(parseState, encoding)
            .map(reducedValue -> intermediate(() -> fold(parseState, encoding, reducer, reducedValue, tail.tail)))
            .orElseThrow(() -> new IllegalArgumentException("Reducer must evaluate to a value."));
    }

    private Trampoline containsNotAValue(final ImmutableList list) {
        if (list.isEmpty()) {
            return complete(() -> false);
        }
        if (list.head.equals(NOT_A_VALUE)) {
            return complete(() -> true);
        }
        return intermediate(() -> containsNotAValue(list.tail));
    }

    protected abstract ImmutableList prepareValues(ImmutableList valueList);

    protected abstract SingleValueExpression 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 immutableHashCode() {
        return Objects.hash(getClass(), values, reducer, initial);
    }

}