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With inspiration from other libraries
There is a newer version: 2.12.15
/*
 * [The "BSD license"]
 *  Copyright (c) 2011 Terence Parr
 *  All rights reserved.
 *
 *  Redistribution and use in source and binary forms, with or without
 *  modification, are permitted provided that the following conditions
 *  are met:
 *  1. Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *  2. Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *  3. The name of the author may not be used to endorse or promote products
 *     derived from this software without specific prior written permission.
 *
 *  THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 *  IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 *  OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 *  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 *  INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 *  NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 *  DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 *  THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 *  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 *  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
package org.stringtemplate.v4.compiler;

import org.antlr.runtime.Token;

/**
 * Represents the name of a formal argument defined in a template:
 *  *  test(a,b,x=defaultvalue) ::= "<a> <n> <x>"
 *  Each template has a set of these formal arguments or sets
 * {@link CompiledST#hasFormalArgs} to {@code false} (indicating that no
 * arguments were specified such as when we create a template with
 * {@code new ST(...)}).
 *
 * 
 * Note: originally, I tracked cardinality as well as the name of an attribute.
 * I'm leaving the code here as I suspect something may come of it later.
 * Currently, though, cardinality is not used.
 */
public class FormalArgument {
/*
    // the following represent bit positions emulating a cardinality bitset.
    public static final int OPTIONAL = 1;     // a?
    public static final int REQUIRED = 2;     // a
    public static final int ZERO_OR_MORE = 4; // a*
    public static final int ONE_OR_MORE = 8;  // a+
    public static final String[] suffixes = {
        null,
        "?",
        "",
        null,
        "*",
        null,
        null,
        null,
        "+"
    };
    protected int cardinality = REQUIRED;
     */

    public String name;

	public int index; // which argument is it? from 0..n-1

	/** If they specified default value {@code x=y}, store the token here */
	public Token defaultValueToken;
	public Object defaultValue; // x="str", x=true, x=false
	public CompiledST compiledDefaultValue; // x={...}

    public FormalArgument(String name) { this.name = name; }

	public FormalArgument(String name, Token defaultValueToken) {
		this.name = name;
		this.defaultValueToken = defaultValueToken;
	}

    /*
    public static String getCardinalityName(int cardinality) {
        switch (cardinality) {
            case OPTIONAL : return "optional";
            case REQUIRED : return "exactly one";
            case ZERO_OR_MORE : return "zero-or-more";
            case ONE_OR_MORE : return "one-or-more";
            default : return "unknown";
        }
    }
    */

	@Override
    public int hashCode() {
        return name.hashCode() + defaultValueToken.hashCode();
    }

	@Override
    public boolean equals(Object o) {
		if ( o==null || !(o instanceof FormalArgument) ) {
			return false;
		}
		FormalArgument other = (FormalArgument)o;
		if ( !this.name.equals(other.name) ) {
			return false;
		}
		// only check if there is a default value; that's all
		return !((this.defaultValueToken != null && other.defaultValueToken == null) ||
			   (this.defaultValueToken == null && other.defaultValueToken != null));
	}

	@Override
    public String toString() {
		if ( defaultValueToken!=null ) return name+"="+defaultValueToken.getText();
        return name;
    }
}