使用C/C++语言编写PL/0编译程序的语法分析代码实现

PL/0是一种简单的编程语言，它的语法分析可以使用递归下降法来实现。下面是一个简单的PL/0语法分析程序的C++代码实现：

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cctype>

using namespace std;

const int MAX_SYMBOL_TABLE_SIZE = 1000;
const int MAX_CODE_LENGTH = 1000;
const int MAX_IDENT_LENGTH = 20;

enum {
    nulsym = 0, identsym, numbersym, plussym, minussym,
    multsym, slashsym, oddsym, eqlsym, neqsym, lessym,
    leqsym, gtrsym, geqsym, lparentsym, rparentsym,
    commasym, semicolonsym, periodsym, becomessym, beginsym,
    endsym, ifsym, thensym, whilesym, dosym, callsym,
    constsym, varsym, procsym, writesym, readsym
};

typedef struct {
    int kind;
    char name[MAX_IDENT_LENGTH + 1];
    int val;
    int level;
    int addr;
} symbol;

typedef struct {
    int op;
    int l;
    int m;
} instruction;

int symbol_table_size = 0;
symbol symbol_table[MAX_SYMBOL_TABLE_SIZE];
instruction code[MAX_CODE_LENGTH];
int code_length = 0;

char input_buffer[1000];
int input_buffer_pos = 0;

int current_token;
char current_ident[MAX_IDENT_LENGTH + 1];
int current_number;

void error(const char* message) {
    cerr << message << endl;
    exit(EXIT_FAILURE);
}

void get_char() {
    if (input_buffer[input_buffer_pos] == 0) {
        fgets(input_buffer, sizeof(input_buffer), stdin);
        input_buffer_pos = 0;
    }
}

void get_token() {
    while (isspace(input_buffer[input_buffer_pos])) {
        input_buffer_pos++;
    }
    if (isalpha(input_buffer[input_buffer_pos])) {
        int i = 0;
        while (isalnum(input_buffer[input_buffer_pos])) {
            current_ident[i++] = input_buffer[input_buffer_pos++];
        }
        current_ident[i] = 0;
        current_token = identsym;
    } else if (isdigit(input_buffer[input_buffer_pos])) {
        current_number = 0;
        while (isdigit(input_buffer[input_buffer_pos])) {
            current_number = current_number * 10 + input_buffer[input_buffer_pos++] - '0';
        }
        current_token = numbersym;
    } else {
        switch (input_buffer[input_buffer_pos]) {
        case '+':
            current_token = plussym;
            break;
        case '-':
            current_token = minussym;
            break;
        case '*':
            current_token = multsym;
            break;
        case '/':
            current_token = slashsym;
            break;
        case '=':
            current_token = eqlsym;
            break;
        case '<':
            if (input_buffer[input_buffer_pos + 1] == '=') {
                input_buffer_pos++;
                current_token = leqsym;
            } else {
                current_token = lessym;
            }
            break;
        case '>':
            if (input_buffer[input_buffer_pos + 1] == '=') {
                input_buffer_pos++;
                current_token = geqsym;
            } else {
                current_token = gtrsym;
            }
            break;
        case '(':
            current_token = lparentsym;
            break;
        case ')':
            current_token = rparentsym;
            break;
        case ',':
            current_token = commasym;
            break;
        case ';':
            current_token = semicolonsym;
            break;
        case '.':
            current_token = periodsym;
            break;
        default:
            error("Invalid character");
        }
        input_buffer_pos++;
    }
}

void enter_symbol(int kind, const char* name, int val, int level, int addr) {
    if (symbol_table_size >= MAX_SYMBOL_TABLE_SIZE) {
        error("Symbol table overflow");
    }
    symbol_table[symbol_table_size].kind = kind;
    strncpy(symbol_table[symbol_table_size].name, name, sizeof(symbol_table[symbol_table_size].name));
    symbol_table[symbol_table_size].val = val;
    symbol_table[symbol_table_size].level = level;
    symbol_table[symbol_table_size].addr = addr;
    symbol_table_size++;
}

int find_symbol(const char* name) {
    for (int i = symbol_table_size - 1; i >= 0; i--) {
        if (strcmp(symbol_table[i].name, name) == 0) {
            return i;
        }
    }
    return -1;
}

void emit(int op, int l, int m) {
    if (code_length >= MAX_CODE_LENGTH) {
        error("Code overflow");
    }
    code[code_length].op = op;
    code[code_length].l = l;
    code[code_length].m = m;
    code_length++;
}

void factor(int level) {
    if (current_token == identsym) {
        int symbol_index = find_symbol(current_ident);
        if (symbol_index < 0) {
            error("Undefined identifier");
        }
        switch (symbol_table[symbol_index].kind) {
        case constsym:
            emit(1, 0, symbol_table[symbol_index].val);
            break;
        case varsym:
            emit(3, level - symbol_table[symbol_index].level, symbol_table[symbol_index].addr);
            break;
        case procsym:
            error("Identifier cannot be a procedure");
        }
        get_token();
    } else if (current_token == numbersym) {
        emit(1, 0, current_number);
        get_token();
    } else if (current_token == lparentsym) {
        get_token();
        expression(level);
        if (current_token != rparentsym) {
            error("Missing right parenthesis");
        }
        get_token();
    } else {
        error("Invalid factor");
    }
}

void term(int level) {
    factor(level);
    while (current_token == multsym || current_token == slashsym) {
        int token = current_token;
        get_token();
        factor(level);
        if (token == multsym) {
            emit(2, 0, 4);
        } else {
            emit(2, 0, 5);
        }
    }
}

void expression(int level) {
    if (current_token == plussym || current_token == minussym) {
        int token = current_token;
        get_token();
        term(level);
        if (token == minussym) {
            emit(2, 0, 1);
        }
    } else {
        term(level);
    }
    while (current_token == plussym || current_token == minussym) {
        int token = current_token;
        get_token();
        term(level);
        if (token == plussym) {
            emit(2, 0, 2);
        } else {
            emit(2, 0, 3);
        }
    }
}

void condition(int level) {
    if (current_token == oddsym) {
        get_token();
        expression(level);
        emit(2, 0, 6);
    } else {
        expression(level);
        if (current_token == eqlsym || current_token == neqsym || current_token == lessym ||
                current_token == leqsym || current_token == gtrsym || current_token == geqsym) {
            int token = current_token;
            get_token();
            expression(level);
            switch (token) {
            case eqlsym:
                emit(2, 0, 8);
                break;
            case neqsym:
                emit(2, 0, 9);
                break;
            case lessym:
                emit(2, 0, 10);
                break;
            case leqsym:
                emit(2, 0, 11);
                break;
            case gtrsym:
                emit(2, 0, 12);
                break;
            case geqsym:
                emit(2, 0, 13);
                break;
            }
        } else {
            error("Invalid condition");
        }
    }
}

void statement(int level) {
    if (current_token == identsym) {
        int symbol_index = find_symbol(current_ident);
        if (symbol_index < 0) {
            error("Undefined identifier");
        }
        if (symbol_table[symbol_index].kind != varsym) {
            error("Identifier cannot be a constant or a procedure");
        }
        get_token();
        if (current_token != becomessym) {
            error("Missing assignment operator");
        }
        get_token();
        expression(level);
        emit(4, level - symbol_table[symbol_index].level, symbol_table[symbol_index].addr);
    } else if (current_token == callsym) {
        get_token();
        if (current_token != identsym) {
            error("Missing procedure identifier");
        }
        int symbol_index = find_symbol(current_ident);
        if (symbol_index < 0) {
            error("Undefined procedure identifier");
        }
        if (symbol_table[symbol_index].kind != procsym) {
            error("Identifier cannot be a constant or a variable");
        }
        emit(5, level - symbol_table[symbol_index].level, symbol_table[symbol_index].addr);
        get_token();
    } else if (current_token == beginsym) {
        get_token();
        statement(level);
        while (current_token == semicolonsym) {
            get_token();
            statement(level);
        }
        if (current_token != endsym) {
            error("Missing end");
        }
        get_token();
    } else if (current_token == ifsym) {
        get_token();
        condition(level);
        int jmp_instr_index = code_length;
        emit(8, 0, 0);
        if (current_token != thensym) {
            error("Missing then");
        }
        get_token();
        statement(level);
        code[jmp_instr_index].m = code_length;
    } else if (current_token == whilesym) {
        int instr_index = code_length;
        get_token();
        condition(level);
        int jmp_instr_index = code_length;
        emit(8, 0, 0);
        if (current_token != dosym) {
            error("Missing do");
        }
        get_token();
        statement(level);
        emit(7, 0, instr_index);
        code[jmp_instr_index].m = code_length;
    } else if (current_token == readsym) {
        get_token();
        if (current_token != identsym) {
            error("Missing identifier");
        }
        int symbol_index = find_symbol(current_ident);
        if (symbol_index < 0) {
            error("Undefined identifier");
        }
        if (symbol_table[symbol_index].kind != varsym) {
            error("Identifier cannot be a constant or a procedure");
        }
        emit(9, 0, 2);
        emit(4, level - symbol_table[symbol_index].level, symbol_table[symbol_index].addr);
        get_token();
    } else if (current_token == writesym) {
        get_token();
        expression(level);
        emit(9, 0, 1);
    }
}

void block(int level) {
    int num_vars = 0;
    while (current_token == constsym) {
        get_token();
        if (current_token != identsym) {
            error("Missing identifier");
        }
        char ident[MAX_IDENT_LENGTH + 1];
        strncpy(ident, current_ident, sizeof(ident));
        get_token();
        if (current_token != eqlsym) {
            error("Missing equal sign");
        }
        get_token();
        if (current_token != numbersym) {
            error("Missing number");
        }
        enter_symbol(constsym, ident, current_number, level, 0);
        num_vars++;
        get_token();
    }
    while (current_token == varsym) {
        get_token();
        if (current_token != identsym) {
            error("Missing identifier");
        }
        enter_symbol(varsym, current_ident, 0, level, num_vars + 3);
        num_vars++;
        get_token();
        while (current_token == commasym) {
            get_token();
            if (current_token != identsym) {
                error("Missing identifier");
            }
            enter_symbol(varsym, current_ident, 0, level, num_vars + 3);
            num_vars++;
            get_token();
        }
        if (current_token != semicolonsym) {
            error("Missing semicolon");
        }
        get_token();
    }
    while (current_token == procsym) {
        get_token();
        if (current_token != identsym) {
            error("Missing procedure identifier");
        }
        enter_symbol(procsym, current_ident, 0, level, code_length);
        get_token();
        if (current_token != semicolonsym) {
            error("Missing semicolon");
        }
        get_token();
        block(level + 1);
        if (current_token != semicolonsym) {
            error("Missing semicolon");
        }
        get_token();
    }
    emit(6, 0, num_vars + 3);
    statement(level);
}

int main() {
    fgets(input_buffer, sizeof(input_buffer), stdin);
    get_char();
    get_token();
    block(0);
    emit(9, 0, 3);
    for (int i = 0; i < code_length; i++) {
        printf("%d %d %d\n", code[i].op, code[i].l, code[i].m);
    }
    return 0;
}

该程序将从标准输入读取PL/0源代码，并将生成的目标代码写入标准输出。