src/main/java/org/apache/sling/scripting/sightly/impl/filter/Filter.java

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 * to you under the Apache License, Version 2.0 (the
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package org.apache.sling.scripting.sightly.impl.filter;

import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.Set;

import org.apache.sling.scripting.sightly.compiler.expression.Expression;

/**
 * A filter is a transformation which performs modifications on expressions. Unlike plugins, filters are always applied on an expression.
 * Whether the filter transformation is actually necessary is decided by the filter. The application order of filters is given by filter
 * priority.
 */
public interface Filter extends Comparable<Filter> {

    /**
     * Transform the given expression
     *
     * @param expression        the original expression
     * @param expressionContext the expression's context
     * @return a transformed expression. If the filter is not applicable
     * to the given expression, then the original expression shall be returned
     */
    Expression apply(Expression expression, ExpressionContext expressionContext);

    /**
     * The priority with which filters are applied. This establishes order between filters. Filters with
     * lower priority are applied first.
     *
     * @return an integer representing the filter's priority
     */
    int priority();

    /**
     * Provides the option names this {@code Filter} will process.
     *
     * @return a set of option names
     */
    default Set<String> getOptions() {
        return Collections.emptySet();
    }

    /**
     * Provides the option names that will trigger a filter's execution.
     *
     * @return the required options from an expression in order to trigger the filter
     */
    default Set<String> getRequiredOptions() {
        return Collections.emptySet();
    }

    /**
     * Provides the applicable contexts for this filter.
     *
     * @return the applicable contexts for this filter
     */
    default Set<ExpressionContext> getApplicableContexts() {
        return new HashSet<>(Arrays.asList(ExpressionContext.values()));
    }

}