/*
 * Copyright (c) 2012-2019 The ANTLR Project. All rights reserved.
 * Use of this file is governed by the BSD 3-clause license that
 * can be found in the LICENSE.txt file in the project root.
 */

module antlr.v4.runtime.tree.AbstractParseTreeVisitor;

import antlr.v4.runtime.tree.ParseTreeVisitor;
import antlr.v4.runtime.tree.ParseTree;
import antlr.v4.runtime.tree.RuleNode;
import antlr.v4.runtime.tree.TerminalNode;
import antlr.v4.runtime.tree.ErrorNode;
import std.variant : Variant;

/**
 * TODO add class description
 */
abstract class AbstractParseTreeVisitor : ParseTreeVisitor
{

    /**
     * @uml
     * {@inheritDoc}
     *
     *  <p>The default implementation calls {@link ParseTree#accept} on the
     *  specified tree.</p>
     */
    public Variant visit(ParseTree tree)
    {
        return tree.accept(this);
    }

    /**
     * @uml
     * {@inheritDoc}
     *
     * <p>The default implementation initializes the aggregate result to
     * {@link #defaultResult defaultResult()}. Before visiting each child, it
     * calls {@link #shouldVisitNextChild shouldVisitNextChild}; if the result
     * is {@code false} no more children are visited and the current aggregate
     * result is returned. After visiting a child, the aggregate result is
     * updated by calling {@link #aggregateResult aggregateResult} with the
     * previous aggregate result and the result of visiting the child.</p>
     *
     * <p>The default implementation is not safe for use in visitors that modify
     * the tree structure. Visitors that modify the tree should override this
     * method to behave properly in respect to the specific algorithm in use.</p>
     * @override
     */
    public override Variant visitChildren(RuleNode node)
    {
        Variant result = defaultResult();
        int n = node.getChildCount();
        for (int i=0; i<n; i++) {
            if (!shouldVisitNextChild(node, result)) {
                break;
            }

            ParseTree c = node.getChild(i);
            Variant childResult = c.accept(this);
            result = aggregateResult(result, childResult);
        }

        return result;
    }

    /**
     * @uml
     * {@inheritDoc}
     *
     * <p>The default implementation returns the result of
     * {@link #defaultResult defaultResult}.</p>
     * @override
     */
    public override Variant visitTerminal(TerminalNode node)
    {
        return defaultResult();
    }

    /**
     * @uml
     * {@inheritDoc}
     *
     * <p>The default implementation returns the result of
     * {@link #defaultResult defaultResult}.</p>
     * @override
     */
    public override Variant visitErrorNode(ErrorNode node)
    {
        return defaultResult();
    }

    /**
     * @uml
     * Gets the default value returned by visitor methods. This value is
     * returned by the default implementations of
     * {@link #visitTerminal visitTerminal}, {@link #visitErrorNode visitErrorNode}.
     * The default implementation of {@link #visitChildren visitChildren}
     * initializes its aggregate result to this value.
     *
     * <p>The base implementation returns {@code null}.</p>
     *
     *  @return The default value returned by visitor methods.
     */
    private Variant defaultResult()
    {
        return Variant(null);
    }

    /**
     * @uml
     * Aggregates the results of visiting multiple children of a node. After
     * either all children are visited or {@link #shouldVisitNextChild} returns
     * {@code false}, the aggregate value is returned as the result of
     * {@link #visitChildren}.
     *
     * <p>The default implementation returns {@code nextResult}, meaning
     * {@link #visitChildren} will return the result of the last child visited
     * (or return the initial value if the node has no children).</p>
     *
     *  @param aggregate The previous aggregate value. In the default
     * implementation, the aggregate value is initialized to
     *  {@link #defaultResult}, which is passed as the {@code aggregate} argument
     * to this method after the first child node is visited.
     *  @param nextResult The result of the immediately preceeding call to visit
     * a child node.
     *
     *  @return The updated aggregate result.
     */
    private Variant aggregateResult(Variant aggregate, Variant nextResult)
    {
        return nextResult;
    }

    /**
     * @uml
     * This method is called after visiting each child in
     * {@link #visitChildren}. This method is first called before the first
     * child is visited; at that point {@code currentResult} will be the initial
     * value (in the default implementation, the initial value is returned by a
     * call to {@link #defaultResult}. This method is not called after the last
     * child is visited.
     *
     * <p>The default implementation always returns {@code true}, indicating that
     * {@code visitChildren} should only return after all children are visited.
     * One reason to override this method is to provide a "short circuit"
     * evaluation option for situations where the result of visiting a single
     * child has the potential to determine the result of the visit operation as
     * a whole.</p>
     *
     *  @param node The {@link RuleNode} whose children are currently being
     * visited.
     *  @param currentResult The current aggregate result of the children visited
     * to the current point.
     *
     *  @return {@code true} to continue visiting children. Otherwise return
     *  {@code false} to stop visiting children and immediately return the
     *  current aggregate result from {@link #visitChildren}.
     */
    private bool shouldVisitNextChild(RuleNode node, Variant currentResult)
    {
        return true;
    }

}