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//               The Next Generation Logic Library                       //
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//  Copyright 2015-20xx Christoph Zengler                                //
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//  Licensed under the Apache License, Version 2.0 (the "License");      //
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//  http://www.apache.org/licenses/LICENSE-2.0                           //
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package org.logicng.functions;

import static org.logicng.formulas.cache.FunctionCacheEntry.DEPTH;

import org.logicng.formulas.Formula;
import org.logicng.formulas.FormulaFunction;

/**
 * A function that computes the depth of a formula. The depth of an atomic formula
 * is defined as 0, all other operators increase the depth by 1.
 * @version 2.3.0
 * @since 2.0
 */
public final class FormulaDepthFunction implements FormulaFunction {

    private static final FormulaDepthFunction INSTANCE = new FormulaDepthFunction();

    /**
     * @deprecated In the next version, the standard constructor will be replaced by a private constructor.
     * In order to instantiate an object of this class, use the {@link #get()} method.
     */
    @Deprecated
    public FormulaDepthFunction() {
        // Intentionally left empty
    }

    /**
     * Returns the singleton instance of this function.
     * @return an instance of this function
     */
    public static FormulaDepthFunction get() {
        return INSTANCE;
    }

    @Override
    public Integer apply(final Formula formula, final boolean cache) {
        final Object cached = formula.functionCacheEntry(DEPTH);
        if (cached != null) {
            return (Integer) cached;
        }
        final int result;
        if (formula.isAtomicFormula()) {
            result = 0;
        } else {
            int maxDepth = 0;
            for (final Formula op : formula) {
                maxDepth = Math.max(maxDepth, apply(op, cache));
            }
            result = maxDepth + 1;
        }
        if (cache) {
            formula.setFunctionCacheEntry(DEPTH, result);
        }
        return result;
    }
}