/**
 * Copyright 2009-2019 the original author or authors.
 * <p>
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * <p>
 * http://www.apache.org/licenses/LICENSE-2.0
 * <p>
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.ibatis.parsing;

/**
 * @author Clinton Begin
 *
 * 功能和流程：
 * 1、传入openToken、closeToken、TokenHandler实现类的实例，进行初始化
 * 2、通过传入openToken、closeToken来对传入内容进行匹配并生成expression
 * 3、将expression传入自定义的TokenHandler处理类中，调用handleToken()方法返回解析出的值
 *
 */
public class GenericTokenParser {
    /**
     * 开始的 Token 字符串
     */
    private final String openToken;
    /**
     * 结束的 Token 字符串
     */
    private final String closeToken;
    /**
     * 处理expression（通过token生成）的handler
     */
    private final TokenHandler handler;

    public GenericTokenParser(String openToken, String closeToken, TokenHandler handler) {
        this.openToken = openToken;
        this.closeToken = closeToken;
        this.handler = handler;
    }

    public String parse(String text) {
        if (text == null || text.isEmpty()) {
            return "";
        }
        // 获取text中openToken的起始位置
        int start = text.indexOf(openToken);
        // 未找到则直接返回
        if (start == -1) {
            return text;
        }
        char[] src = text.toCharArray();
        int offset = 0;
        final StringBuilder builder = new StringBuilder();
        StringBuilder expression = null;
        while (start > -1) {
            if (start > 0 && src[start - 1] == '\\') {
                // this open token is escaped. remove the backslash and continue.
                // 该openToken被转义，删除反斜杠并继续
                builder.append(src, offset, start - offset - 1).append(openToken);
                offset = start + openToken.length();
            } else {
                // found open token. let's search close token.
                // 通过openToken来查找closeToken的位置
                if (expression == null) {
                    expression = new StringBuilder();
                } else {
                    // 初始化StringBuilder的长度，相当于清空其内容，实现复用
                    expression.setLength(0);
                }
                builder.append(src, offset, start - offset);
                offset = start + openToken.length();
                int end = text.indexOf(closeToken, offset);
                while (end > -1) {
                    if (end > offset && src[end - 1] == '\\') {
                        // this close token is escaped. remove the backslash and continue.
                        // 该closeToken被转义，删除反斜杠并继续
                        expression.append(src, offset, end - offset - 1).append(closeToken);
                        offset = end + closeToken.length();
                        end = text.indexOf(closeToken, offset);
                    } else {
                        expression.append(src, offset, end - offset);
                        break;
                    }
                }
                if (end == -1) {
                    // close token was not found.
                    // 如果只有openToken而没有closeToken，那么直接将openToken连同后面的所有字符完全填充入builder中
                    builder.append(src, start, src.length - start);
                    offset = src.length;
                } else {
                    // 将通过openToken和closeToken匹配生成的字符串expression交由TokenHandler进行处理
                    builder.append(handler.handleToken(expression.toString()));
                    offset = end + closeToken.length();
                }
            }
            start = text.indexOf(openToken, offset);
        }
        // 匹配完所有token字符串之后，剩下不需要匹配的，则直接添加入builder中返回（如：#{test} is ok的is ok）
        if (offset < src.length) {
            builder.append(src, offset, src.length - offset);
        }
        return builder.toString();
    }
}