java-词法分析器的设计与实现(编译原理)(标识符机内码实现)

╰半夏微凉° 2023-07-25 09:09 72阅读 0赞

# 1.机内码应该的特点: #

一个标识符对应一个机内码  
相同标识符对应一个机内码

# 2.设计思想: #

基于java语言的hashmap结构:  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpdHRsZXdoaXRldmc_size_16_color_FFFFFF_t_70]

# 3.Java hashmap的特点: #

*  存储时：他们会找到相同的bucket位置，发生碰撞，因为HashMap使用链表存储对象（每个Map.Entry都有一个next指针），这个Entry会存储在链表中。
 *  获取时:会用hashCode找到bucket位置，然后调用key.equals()方法找到链表中正确的节点.最终找到要找的值对象.
 *  减少碰撞：使用final修饰的对象、或不可变的对象作为键，使用(Integer、String)
 *  动态扩容: 理论上,只要内存足够,是完全可以放下所有程序里需要的标识符的
 *  产生hash碰撞的元素会放到相同hash值里的链表下
 *  查询效率:O(1),插入效率O(1)

# 4.标识符的hashcode的生成:(属于java的内置方法) #

就是以31为权，每一位为字符的ASCII值进行运算，用自然溢出来等效取模。

哈希计算公式可以计为s\[0\]\*31^(n-1) + s\[1\]\*31^(n-2) + … + s\[n-1\]

# 5.产生机内码流程: #

产生一个标识符->用这个标识符的hashcode去到hashmap里去查是否存在  
\->若存在,则得到这个标识符的机内码  
\->若不存在,则生成一个这个标识符的机内码并放入hashmap  
本文表示形式:var\#id

# 代码: #

public class WordAnalyze {
        private static HashMap<String, Integer> keywords = new HashMap<>();
        static {
            //初始化 关键字对应的机内码
            int i = 0;
            keywords.put("int", ++i);
            keywords.put("float", ++i);
            keywords.put("char", ++i);
            keywords.put("if", ++i);
            keywords.put("else", ++i);
            keywords.put("for", ++i);
            keywords.put("while", ++i);
            keywords.put("return", ++i);
            keywords.put("break", ++i);
            keywords.put("continue", ++i);
            keywords.put("switch", ++i);
            keywords.put("case", ++i);
            keywords.put("default", ++i);
            keywords.put("double", ++i);
            keywords.put("void", ++i);
            keywords.put("struct", ++i);
            keywords.put("static", ++i);
            keywords.put("do", ++i);
            keywords.put("short", ++i);
            keywords.put("+", ++i);
            keywords.put("*", ++i);
            keywords.put("/", ++i);
            keywords.put("%", ++i);
            keywords.put("=", ++i);
            keywords.put(">", ++i);
            keywords.put("<", ++i);
            keywords.put("!", ++i);
            keywords.put("==", ++i);
            keywords.put("!=", ++i);
            keywords.put(">=", ++i);
            keywords.put("<=", ++i);
            keywords.put("++", ++i);
            keywords.put("--", ++i);
            keywords.put("&", ++i);
            keywords.put("&&", ++i);
            keywords.put("||", ++i);
            keywords.put("[", ++i);
            keywords.put("]", ++i);
            keywords.put(",", ++i);
            keywords.put(";", ++i);
            keywords.put("(", ++i);
            keywords.put(")", ++i);
            keywords.put("{", ++i);
            keywords.put("}", ++i);
            keywords.put("\"", ++i);
            //单双引号 自己加,java只有单引号
            keywords.put("", ++i);
            keywords.put(":", ++i);
            keywords.put("#", ++i);
            keywords.put(">>", ++i);
            keywords.put("<<", ++i);
            System.out.println("一共" + i + " 个关键字");
            System.out.println("常数类别标识51, 机内码为二进制表示");
            System.out.println("关键字类别标识52, 机内码暂定 ");
            System.out.println("==========================================");
        }
    
        private FileWriter os;
        private char ch;
        private HashMap<String, String> identifier = new HashMap<>();
        private int identifierCount = 0;
    
        public WordAnalyze() {
    
            try {
                File file = new File("目标文件");
                if (file.exists()){
                    file.createNewFile();
                }
                os = new FileWriter(file, true);
            } catch (IOException e) {
                e.printStackTrace();
            }
    
        }
    
        public static void main(String[] args) throws Exception {
            File file = new File("源文件");//定义一个file对象，用来初始化FileReader
            FileReader reader = new FileReader(file);//定义一个fileReader对象，用来初始化BufferedReader
            int length = (int) file.length();
            //这里定义字符数组的时候需要多定义一个,因为词法分析器会遇到超前读取一个字符的时候，如果是最后一个
            //字符被读取，如果在读取下一个字符就会出现越界的异常
            char buf[] = new char[length + 1];
            reader.read(buf);
            reader.close();
            new WordAnalyze().analyze(buf);
        }
    
        //判断是否是关键字
        boolean isKey(String str) {
            if (keywords.containsKey(str))
                return true;
            return false;
        }
    
        //判断是否是字母
        boolean isLetter(char letter) {
            if ((letter >= 'a' && letter <= 'z') || (letter >= 'A' && letter <= 'Z'))
                return true;
            else
                return false;
        }
    
        //判断是否是数字
        boolean isDigit(char digit) {
            if (digit >= '0' && digit <= '9')
                return true;
            else
                return false;
        }
    
        //词法分析
        void analyze(char[] chars) {
            String arr = "";
            for (int i = 0; i < chars.length; i++) {
                ch = chars[i];
                arr = "";
                if (ch == ' ' || ch == '\t' || ch == '\n' || ch == '\r') {
                } else if (isLetter(ch)) {
                    while (isLetter(ch) || isDigit(ch)) {
                        arr += ch;
                        ch = chars[++i];
                    }
                    //回退一个字符
                    i--;
                    if (isKey(arr)) {
                        //关键字
    //                    System.out.println(arr + "  " + keywords.get(arr));
                        writefile(arr + "  " + keywords.get(arr) + "\n");
                    } else {
                        //标识符
                        if (identifier.containsKey(arr)) {
                            //如果已经存在这个标识符号了,就get出来
    //                        System.out.println(arr + " 52 " + identifier.get(arr));
                            writefile(arr + " 52 " + identifier.get(arr) + "\n");
                        } else {
                            //不存在这个标识符号,就存入并生成一个机内码
                            identifierCount++;
                            identifier.put(arr, "var#" + identifierCount);
    //                        System.out.println(arr + " 52 " + identifier.get(arr));
                            writefile(arr + " 52 " + identifier.get(arr) + "\n");
                        }
    
                    }
                } else if (isDigit(ch) || (ch == '.')) {
                    while (isDigit(ch) || (ch == '.' && isDigit(chars[++i]))) {
                        if (ch == '.') i--;
                        arr = arr + ch;
                        ch = chars[++i];
                        i--;
                    }
                    //属于无符号常数
                    writefile(arr + " 51 " + Integer.toBinaryString(Integer.parseInt(arr))+"\n");
    //                System.out.println(arr + " 51 " + Integer.toBinaryString(Integer.parseInt(arr)));
                } else switch (ch) {
                    //运算符
                    case '+':{
                        ch = chars[++i];
                        if (ch == '+')
    //                        System.out.println("++" + " " + keywords.get("++"));
                            writefile("++" + " " + keywords.get("++") + "\n");
                        else {
    //                        System.out.println("=" + " " + keywords.get("+"));
                            writefile("+" + " " + keywords.get("+") + "\n");
                            i--;
                        }
                    }
                    break;
                    case '-': {
                        ch = chars[++i];
                        if (ch == '-')
    //                        System.out.println("--" + " " + keywords.get("--"));
                            writefile("--" + " " + keywords.get("--") + "\n");
                        else {
    //                        System.out.println("-" + " " + keywords.get("-"));
                            writefile("-" + " " + keywords.get("-") + "\n");
                            i--;
                        }
                    }
                    break;
                    case '=': {
                        ch = chars[++i];
                        if (ch == '=')
    //                        System.out.println("==" + " " + keywords.get("=="));
                            writefile("==" + " " + keywords.get("==") + "\n");
                        else {
    //                        System.out.println("=" + " " + keywords.get("="));
                            writefile("=" + " " + keywords.get("=") + "\n");
                            i--;
                        }
                    }
                    break;
                    case ':': {
    //                    System.out.println(":" + " " + keywords.get(":"));
                        writefile(":" + " " + keywords.get(":") + "\n");
                    }
                    break;
                    case '>': {
                        ch = chars[++i];
                        if (ch == '=')
    //                        System.out.println(">=" + " " + keywords.get(">="));
                            writefile(">=" + " " + keywords.get(">=") + "\n");
                        else if (ch == '>')
    //                        System.out.println(">>" + " " + keywords.get(">>"));
                            writefile(">>" + " " + keywords.get(">>") + "\n");
                        else {
    //                        System.out.println(">" + " " + keywords.get(">"));
                            writefile(">" + " " + keywords.get(">") + "\n");
                            i--;
                        }
                    }
                    break;
                    case '<': {
                        ch = chars[++i];
                        if (ch == '=')
    //                        System.out.println("<=" + " " + keywords.get("<="));
                            writefile("<=" + " " + keywords.get("<=") + "\n");
                        else if (ch == '<')
    //                        System.out.println("<<" + " " + keywords.get("<<"));
                            writefile("<<" + " " + keywords.get("<<") + "\n");
                        else {
    //                        System.out.println("<" + " " + keywords.get("<"));
                            writefile("<" + " " + keywords.get("<") + "\n");
                            i--;
                        }
                    }
                    break;
                    //默认一些
                    default:
                        if (keywords.containsKey(String.valueOf(ch))) {
    //                        System.out.println(ch + " " + keywords.get(String.valueOf(ch)));
                            writefile(ch + " " + keywords.get(String.valueOf(ch)) + "\n");
                        }
                }
            }
            //在分析结束后,关闭流
            try {
                os.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    
        public void writefile(String content) {
            try {
                os.write(content);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

# 代码实现优势: #

单字符Keyword可随意增加,双字符keyword需要修改代码,这样也有了一点点可扩展性;  
要想添加双字符keyword,得在case里面自己加,当然是得看懂代码前提下

有时间其实可以用设计模式之策略模式优化以下,当然了,肯定是没时间了  
对了,这个还可以用有限状态机来写,这才是最正规的做法  
可以搜搜 如何用代码实现有限状态机,这是字符串操作算法里面的一种思路,leetcode碰到就hard级了

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