递归下降子程序法实现PL0语法分析器java

PL0语法分析器的递归下降子程序法实现主要包括以下步骤：

1. 根据PL0语法规则定义语法分析函数，每个函数对应一个非终结符号。
2. 读入PL0源代码，将其转换为token序列。
3. 从token序列中读入第一个token，并调用语法分析的入口函数，开始语法分析。
4. 在语法分析过程中，递归调用语法分析函数，根据当前token和语法规则进行判断和处理。
5. 如果当前token与语法规则不匹配，则回溯到上一级语法分析函数，直到找到匹配的语法规则进行处理。
6. 在语法分析过程中，根据语法规则生成语法树，同时进行语义分析和错误检查。
7. 如果语法分析成功，则输出语法树，否则输出错误信息提示。

下面是一个简单的PL0语法分析器的代码实现（基于Java语言）：

import java.util.ArrayList;
import java.util.List;

public class PL0Parser {
    private List<Token> tokens;     // token序列
    private Token currentToken;     // 当前token
    private int index;              // 当前token在序列中的索引

    public PL0Parser(List<Token> tokens) {
        this.tokens = tokens;
        this.currentToken = tokens.get(0);
        this.index = 0;
    }

    public void parse() {
        program();      // 从program开始语法分析
    }

    // program -> block '.'
    private void program() {
        block();
        match(TokenType.PERIOD);   // 匹配'.'
    }

    // block -> [constDeclarations] [varDeclarations] {procedureDeclaration} statement
    private void block() {
        if (currentToken.getType() == TokenType.CONST) {
            constDeclarations();
        }
        if (currentToken.getType() == TokenType.VAR) {
            varDeclarations();
        }
        while (currentToken.getType() == TokenType.PROCEDURE) {
            procedureDeclaration();
        }
        statement();
    }

    // constDeclarations -> const constDeclaration {',' constDeclaration} ';'
    private void constDeclarations() {
        match(TokenType.CONST);
        constDeclaration();
        while (currentToken.getType() == TokenType.COMMA) {
            match(TokenType.COMMA);
            constDeclaration();
        }
        match(TokenType.SEMICOLON);
    }

    // constDeclaration -> ident '=' number
    private void constDeclaration() {
        match(TokenType.IDENT);
        match(TokenType.EQ);
        match(TokenType.NUMBER);
    }

    // varDeclarations -> var ident {',' ident} ';'
    private void varDeclarations() {
        match(TokenType.VAR);
        match(TokenType.IDENT);
        while (currentToken.getType() == TokenType.COMMA) {
            match(TokenType.COMMA);
            match(TokenType.IDENT);
        }
        match(TokenType.SEMICOLON);
    }

    // procedureDeclaration -> 'procedure' ident ';' block ';'
    private void procedureDeclaration() {
        match(TokenType.PROCEDURE);
        match(TokenType.IDENT);
        match(TokenType.SEMICOLON);
        block();
        match(TokenType.SEMICOLON);
    }

    // statement -> [ident ':=' expression | callStatement | compoundStatement |
    //              ifStatement | whileStatement | readStatement | writeStatement] ';'
    private void statement() {
        if (currentToken.getType() == TokenType.IDENT) {
            match(TokenType.IDENT);
            match(TokenType.ASSIGN);
            expression();
        } else if (currentToken.getType() == TokenType.CALL) {
            callStatement();
        } else if (currentToken.getType() == TokenType.BEGIN) {
            compoundStatement();
        } else if (currentToken.getType() == TokenType.IF) {
            ifStatement();
        } else if (currentToken.getType() == TokenType.WHILE) {
            whileStatement();
        } else if (currentToken.getType() == TokenType.READ) {
            readStatement();
        } else if (currentToken.getType() == TokenType.WRITE) {
            writeStatement();
        } else {
            error("Invalid statement");
        }
        match(TokenType.SEMICOLON);
    }

    // callStatement -> 'call' ident
    private void callStatement() {
        match(TokenType.CALL);
        match(TokenType.IDENT);
    }

    // compoundStatement -> 'begin' statement {';' statement} 'end'
    private void compoundStatement() {
        match(TokenType.BEGIN);
        statement();
        while (currentToken.getType() == TokenType.SEMICOLON) {
            match(TokenType.SEMICOLON);
            statement();
        }
        match(TokenType.END);
    }

    // ifStatement -> 'if' condition 'then' statement ['else' statement]
    private void ifStatement() {
        match(TokenType.IF);
        condition();
        match(TokenType.THEN);
        statement();
        if (currentToken.getType() == TokenType.ELSE) {
            match(TokenType.ELSE);
            statement();
        }
    }

    // whileStatement -> 'while' condition 'do' statement
    private void whileStatement() {
        match(TokenType.WHILE);
        condition();
        match(TokenType.DO);
        statement();
    }

    // readStatement -> 'read' ident
    private void readStatement() {
        match(TokenType.READ);
        match(TokenType.IDENT);
    }

    // writeStatement -> 'write' expression
    private void writeStatement() {
        match(TokenType.WRITE);
        expression();
    }

    // condition -> oddExpression | expression relOp expression
    private void condition() {
        if (currentToken.getType() == TokenType.ODD) {
            match(TokenType.ODD);
            expression();
        } else {
            expression();
            relOp();
            expression();
        }
    }

    // expression -> [addOp] term {addOp term}
    private void expression() {
        if (currentToken.getType() == TokenType.PLUS || currentToken.getType() == TokenType.MINUS) {
            addOp();
        }
        term();
        while (currentToken.getType() == TokenType.PLUS || currentToken.getType() == TokenType.MINUS) {
            addOp();
            term();
        }
    }

    // term -> factor {mulOp factor}
    private void term() {
        factor();
        while (currentToken.getType() == TokenType.TIMES || currentToken.getType() == TokenType.SLASH) {
            mulOp();
            factor();
        }
    }

    // factor -> ident | number | '(' expression ')'
    private void factor() {
        if (currentToken.getType() == TokenType.IDENT) {
            match(TokenType.IDENT);
        } else if (currentToken.getType() == TokenType.NUMBER) {
            match(TokenType.NUMBER);
        } else if (currentToken.getType() == TokenType.LPAREN) {
            match(TokenType.LPAREN);
            expression();
            match(TokenType.RPAREN);
        } else {
            error("Invalid factor");
        }
    }

    // relOp -> '=' | '<>' | '<' | '<=' | '>' | '>='
    private void relOp() {
        if (currentToken.getType() == TokenType.EQ || currentToken.getType() == TokenType.NE ||
            currentToken.getType() == TokenType.LT || currentToken.getType() == TokenType.LE ||
            currentToken.getType() == TokenType.GT || currentToken.getType() == TokenType.GE) {
            match(currentToken.getType());
        } else {
            error("Invalid relation operator");
        }
    }

    // addOp -> '+' | '-'
    private void addOp() {
        if (currentToken.getType() == TokenType.PLUS || currentToken.getType() == TokenType.MINUS) {
            match(currentToken.getType());
        } else {
            error("Invalid addition operator");
        }
    }

    // mulOp -> '*' | '/'
    private void mulOp() {
        if (currentToken.getType() == TokenType.TIMES || currentToken.getType() == TokenType.SLASH) {
            match(currentToken.getType());
        } else {
            error("Invalid multiplication operator");
        }
    }

    // 匹配当前token并读入下一个token
    private void match(TokenType type) {
        if (currentToken.getType() == type) {
            if (index < tokens.size() - 1) {
                currentToken = tokens.get(++index);
            }
        } else {
            error("Unexpected token: " + currentToken.getValue());
        }
    }

    // 报告错误信息
    private void error(String message) {
        System.err.println("Syntax error: " + message);
        System.exit(1);
    }
}

在代码中，每个语法分析函数对应一个非终结符号，如program、block、constDeclarations等等。函数的实现过程中，根据当前token和语法规则进行判断和处理，并递归调用其他语法分析函数。如果当前token与语法规则不匹配，则回溯到上一级语法分析函数，直到找到匹配的语法规则进行处理。在语法分析过程中，根据语法规则生成语法树，同时进行语义分析和错误检查。如果语法分析成功，则输出语法树，否则输出错误信息提示。

注：以上代码仅供参考，实际应用中需要根据具体需求进行修改和完善。