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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 io.parsingdata.metal.data.Environment;
import io.parsingdata.metal.data.OptionalValueList;
import io.parsingdata.metal.encoding.Encoding;

import static io.parsingdata.metal.Shorthand.con;
import static io.parsingdata.metal.Util.checkNotNull;

public class FoldLeft implements ValueExpression {

    public final ValueExpression values;
    public final Reducer reducer;
    public final ValueExpression initial;

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

    @Override
    public OptionalValueList eval(final Environment env, final Encoding enc) {
        final OptionalValueList init = initial != null ? initial.eval(env, enc) : OptionalValueList.EMPTY;
        if (init.size > 1) {
            return OptionalValueList.EMPTY;
        }
        final OptionalValueList values = this.values.eval(env, enc).reverse();
        if (values.isEmpty() || values.containsEmpty()) {
            return init;
        }
        if (!init.isEmpty()) {
            return OptionalValueList.create(fold(env, enc, reducer, init.head, values));
        }
        return OptionalValueList.create(fold(env, enc, reducer, values.head, values.tail));
    }

    private OptionalValue fold(final Environment env, final Encoding enc, final Reducer reducer, final OptionalValue head, final OptionalValueList tail) {
        if (!head.isPresent() || tail.isEmpty()) {
            return head;
        }
        final OptionalValueList reducedValue = reducer.reduce(con(head.get()), con(tail.head.get())).eval(env, enc);
        if (reducedValue.size != 1) {
            throw new IllegalStateException("Reducer must yield a single value.");
        }
        return fold(env, enc, reducer, reducedValue.head, tail.tail);
    }

}