/*******************************************************************************
 *
 * Copyright (c) 2004-2009 Oracle Corporation.
 *
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 * 
 *    Kohsuke Kawaguchi
 *
 *
 *******************************************************************************/ 

package hudson.util;

import java.util.Collection;
import java.util.Map;

/**
 * Resolves variables to its value, while encapsulating how that resolution
 * happens.
 *
 * @author Kohsuke Kawaguchi
 */
public interface VariableResolver<V> {

    /**
     * Receives a variable name and obtains the value associated with the name.
     *
     * <p> This can be implemented simply on top of a {@link Map} (see
     * {@link ByMap}), or this can be used like an expression evaluator.
     *
     * @param name Name of the variable to be resolved. Never null, never empty.
     * The name shouldn't include the syntactic marker of an expression. IOW, it
     * should be "foo" but not "${foo}". A part of the goal of this design is to
     * abstract away the expression marker syntax.
     * @return Object referenced by the name. Null if not found.
     */
    public abstract V resolve(String name);
    /**
     * Empty resolver that always returns null.
     */
    public static final VariableResolver NONE = new VariableResolver() {
        public Object resolve(String name) {
            return null;
        }
    };

    /**
     * {@link VariableResolver} backed by a {@link Map}.
     */
    public static final class ByMap<V> implements VariableResolver<V> {

        private final Map<String, V> data;

        public ByMap(Map<String, V> data) {
            this.data = data;
        }

        public V resolve(String name) {
            return data.get(name);
        }
    }

    /**
     * Union of multiple {@link VariableResolver}.
     */
    public static final class Union<V> implements VariableResolver<V> {

        private final VariableResolver<? extends V>[] resolvers;

        public Union(VariableResolver<? extends V>... resolvers) {
            this.resolvers = resolvers.clone();
        }

        public Union(Collection<? extends VariableResolver<? extends V>> resolvers) {
            this.resolvers = resolvers.toArray(new VariableResolver[resolvers.size()]);
        }

        public V resolve(String name) {
            for (VariableResolver<? extends V> r : resolvers) {
                V v = r.resolve(name);
                if (v != null) {
                    return v;
                }
            }
            return null;
        }
    }
}