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Filename : cursor.js
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/**
 * @fileoverview Define the abstract class about cursors which iterate tokens.
 * @author Toru Nagashima
 */
"use strict";

//------------------------------------------------------------------------------
// Exports
//------------------------------------------------------------------------------

/**
 * The abstract class about cursors which iterate tokens.
 *
 * This class has 2 abstract methods.
 *
 * - `current: Token | Comment | null` ... The current token.
 * - `moveNext(): boolean` ... Moves this cursor to the next token. If the next token didn't exist, it returns `false`.
 *
 * This is similar to ES2015 Iterators.
 * However, Iterators were slow (at 2017-01), so I created this class as similar to C# IEnumerable.
 *
 * There are the following known sub classes.
 *
 * - ForwardTokenCursor .......... The cursor which iterates tokens only.
 * - BackwardTokenCursor ......... The cursor which iterates tokens only in reverse.
 * - ForwardTokenCommentCursor ... The cursor which iterates tokens and comments.
 * - BackwardTokenCommentCursor .. The cursor which iterates tokens and comments in reverse.
 * - DecorativeCursor
 *     - FilterCursor ............ The cursor which ignores the specified tokens.
 *     - SkipCursor .............. The cursor which ignores the first few tokens.
 *     - LimitCursor ............. The cursor which limits the count of tokens.
 *
 */
module.exports = class Cursor {

    /**
     * Initializes this cursor.
     */
    constructor() {
        this.current = null;
    }

    /**
     * Gets the first token.
     * This consumes this cursor.
     * @returns {Token|Comment} The first token or null.
     */
    getOneToken() {
        return this.moveNext() ? this.current : null;
    }

    /**
     * Gets the first tokens.
     * This consumes this cursor.
     * @returns {(Token|Comment)[]} All tokens.
     */
    getAllTokens() {
        const tokens = [];

        while (this.moveNext()) {
            tokens.push(this.current);
        }

        return tokens;
    }

    /**
     * Moves this cursor to the next token.
     * @returns {boolean} `true` if the next token exists.
     * @abstract
     */
    /* istanbul ignore next */
    moveNext() { // eslint-disable-line class-methods-use-this
        throw new Error("Not implemented.");
    }
};