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RecursiveDescentParser.java
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package parsing;
import java.util.*;
public class RecursiveDescentParser {
enum Lexeme { NUM, ID, IF, ELSE, WHILE, DO, LBRA, RBRA, LPAR, RPAR, PLUS, MINUS, LESS, EQUAL, SEMICOLON, EOF }
static class Lexer {
static final Map<Character, Lexeme> SYMBOLS = new HashMap<>();
static {
SYMBOLS.put('{', Lexeme.LBRA);
SYMBOLS.put('}', Lexeme.RBRA);
SYMBOLS.put('=', Lexeme.EQUAL);
SYMBOLS.put(';', Lexeme.SEMICOLON);
SYMBOLS.put('(', Lexeme.LPAR);
SYMBOLS.put(')', Lexeme.RPAR);
SYMBOLS.put('+', Lexeme.PLUS);
SYMBOLS.put('-', Lexeme.MINUS);
SYMBOLS.put('<', Lexeme.LESS);
}
static final Map<String, Lexeme> WORDS = new HashMap<>();
static {
WORDS.put("if", Lexeme.IF);
WORDS.put("else", Lexeme.ELSE);
WORDS.put("do", Lexeme.DO);
WORDS.put("while", Lexeme.WHILE);
}
final String s;
int pos = 0;
Character ch = ' ';
Integer value;
Lexeme sym;
Lexer(String s) {
this.s = s;
}
void error(String message) {
throw new RuntimeException("Lexer error: " + message);
}
void readCh() {
ch = pos < s.length() ? s.charAt(pos++) : null;
}
void readNextToken() {
value = null;
sym = null;
while (sym == null) {
if (ch == null) {
sym = Lexeme.EOF;
} else if (Character.isSpaceChar(ch)) {
readCh();
} else if (SYMBOLS.containsKey(ch)) {
sym = SYMBOLS.get(ch);
readCh();
} else if (Character.isDigit(ch)) {
int val = 0;
while (ch != null && Character.isDigit(ch)) {
val = val * 10 + ch - '0';
readCh();
}
value = val;
sym = Lexeme.NUM;
} else if (Character.isLetter(ch)) {
String ident = "";
while (ch != null && Character.isLetter(ch)) {
ident += ch;
readCh();
}
if (WORDS.containsKey(ident)) {
sym = Lexer.WORDS.get(ident);
} else if (ident.length() == 1) {
sym = Lexeme.ID;
value = ident.charAt(0) - 'a';
} else {
error("Unknown identifier: " + ident);
}
} else {
error("Unexpected symbol: " + ch);
}
}
}
}
enum Nonterminal { VAR, CONST, ADD, SUB, LT, SET, IF1, IF2, WHILE, DO, EMPTY, SEQ, EXPR, PROG }
static class Node {
Nonterminal kind;
int value;
Node op1;
Node op2;
Node op3;
Node(Nonterminal kind, int value, Node op1, Node op2) {
this.kind = kind;
this.value = value;
this.op1 = op1;
this.op2 = op2;
}
}
static class Parser {
final Lexer lexer;
Parser(Lexer lexer) {
this.lexer = lexer;
}
void error(String message) {
throw new RuntimeException("Parser error: " + message);
}
Node term() {
if (lexer.sym == Lexeme.ID) {
Node node = new Node(Nonterminal.VAR, lexer.value, null, null);
lexer.readNextToken();
return node;
} else if (lexer.sym == Lexeme.NUM) {
Node node = new Node(Nonterminal.CONST, lexer.value, null, null);
lexer.readNextToken();
return node;
} else {
return parenExpr();
}
}
Node summa() {
Node node = term();
while (lexer.sym == Lexeme.PLUS || lexer.sym == Lexeme.MINUS) {
Nonterminal kind = lexer.sym == Lexeme.PLUS ? Nonterminal.ADD : Nonterminal.SUB;
lexer.readNextToken();
node = new Node(kind, 0, node, term());
}
return node;
}
Node test() {
Node node = summa();
if (lexer.sym == Lexeme.LESS) {
lexer.readNextToken();
node = new Node(Nonterminal.LT, 0, node, summa());
}
return node;
}
Node expr() {
if (lexer.sym != Lexeme.ID) {
return test();
}
Node node = test();
if (node.kind == Nonterminal.VAR && lexer.sym == Lexeme.EQUAL) {
lexer.readNextToken();
node = new Node(Nonterminal.SET, 0, node, expr());
}
return node;
}
Node parenExpr() {
if (lexer.sym != Lexeme.LPAR) {
error("'(' expected");
}
lexer.readNextToken();
Node node = expr();
if (lexer.sym != Lexeme.RPAR) {
error("')' expected");
}
lexer.readNextToken();
return node;
}
Node statement() {
Node node;
if (lexer.sym == Lexeme.IF) {
node = new Node(Nonterminal.IF1, 0, null, null);
lexer.readNextToken();
node.op1 = parenExpr();
node.op2 = statement();
if (lexer.sym == Lexeme.ELSE) {
node.kind = Nonterminal.IF2;
lexer.readNextToken();
node.op3 = statement();
}
} else if (lexer.sym == Lexeme.WHILE) {
node = new Node(Nonterminal.WHILE, 0, null, null);
lexer.readNextToken();
node.op1 = parenExpr();
node.op2 = statement();
} else if (lexer.sym == Lexeme.DO) {
node = new Node(Nonterminal.DO, 0, null, null);
lexer.readNextToken();
node.op1 = statement();
if (lexer.sym != Lexeme.WHILE) {
error("'while' expected");
}
lexer.readNextToken();
node.op2 = parenExpr();
if (lexer.sym != Lexeme.SEMICOLON) {
error("';' expected");
}
} else if (lexer.sym == Lexeme.SEMICOLON) {
node = new Node(Nonterminal.EMPTY, 0, null, null);
lexer.readNextToken();
} else if (lexer.sym == Lexeme.LBRA) {
node = new Node(Nonterminal.EMPTY, 0, null, null);
lexer.readNextToken();
while (lexer.sym != Lexeme.RBRA) {
node = new Node(Nonterminal.SEQ, 0, node, statement());
}
lexer.readNextToken();
} else {
node = new Node(Nonterminal.EXPR, 0, expr(), null);
if (lexer.sym != Lexeme.SEMICOLON) {
error("';' expected");
}
lexer.readNextToken();
}
return node;
}
Node parse() {
lexer.readNextToken();
Node node = new Node(Nonterminal.PROG, 0, statement(), null);
if (lexer.sym != Lexeme.EOF) {
error("Invalid statement syntax");
}
return node;
}
}
enum Command { IFETCH, ISTORE, IPUSH, IPOP, IADD, ISUB, ILT, JZ, JNZ, JMP, HALT }
static class Compiler {
List<Object> compile(Node node) {
List<Object> program = new ArrayList<>();
switch (node.kind) {
case VAR:
program.add(Command.IFETCH);
program.add(node.value);
break;
case CONST:
program.add(Command.IPUSH);
program.add(node.value);
break;
case ADD:
compile(node.op1);
compile(node.op2);
program.add(Command.IADD);
break;
case SUB:
compile(node.op1);
compile(node.op2);
program.add(Command.ISUB);
break;
case LT:
compile(node.op1);
compile(node.op2);
program.add(Command.ILT);
break;
case SET:
compile(node.op2);
program.add(Command.ISTORE);
program.add(node.op1.value);
break;
case IF1: {
compile(node.op1);
program.add(Command.JZ);
int addr = program.size();
program.add(0);
compile(node.op2);
program.set(addr, program.size());
break;
}
case IF2: {
compile(node.op1);
program.add(Command.JZ);
int addr1 = program.size();
program.add(0);
compile(node.op2);
program.add(Command.JMP);
int addr2 = program.size();
program.add(0);
program.set(addr1, program.size());
compile(node.op3);
program.set(addr2, program.size());
break;
}
case WHILE: {
int addr1 = program.size();
compile(node.op1);
program.add(Command.JZ);
int addr2 = program.size();
program.add(0);
compile(node.op2);
program.add(Command.JMP);
program.add(addr1);
program.set(addr2, program.size());
break;
}
case DO: {
int addr = program.size();
compile(node.op1);
compile(node.op2);
program.add(Command.JNZ);
program.add(addr);
break;
}
case SEQ:
compile(node.op1);
compile(node.op2);
break;
case EXPR:
compile(node.op1);
program.add(Command.IPOP);
break;
case PROG:
compile(node.op1);
program.add(Command.HALT);
break;
}
return program;
}
}
static class VirtualMachine {
void run(List<Object> program) {
int[] var = new int[26];
Stack<Integer> stack = new Stack<>();
int pc = 0;
m1:
while (true) {
Command op = (Command) program.get(pc);
int arg =
pc < program.size() - 1 && program.get(pc + 1) instanceof Integer ? (int) program.get(pc + 1) : 0;
switch (op) {
case IFETCH:
stack.push(var[arg]);
pc += 2;
break;
case ISTORE:
var[arg] = stack.pop();
pc += 2;
break;
case IPUSH:
stack.push(arg);
pc += 2;
break;
case IPOP:
stack.push(arg);
stack.pop();
pc += 1;
break;
case IADD:
stack.push(stack.pop() + stack.pop());
pc += 1;
break;
case ISUB:
stack.push(-stack.pop() + stack.pop());
pc += 1;
break;
case ILT:
stack.push(stack.pop() > stack.pop() ? 1 : 0);
pc += 1;
break;
case JZ:
if (stack.pop() == 0) {
pc = arg;
} else {
pc += 2;
}
break;
case JNZ:
if (stack.pop() != 0) {
pc = arg;
} else {
pc += 2;
}
break;
case JMP:
pc = arg;
break;
case HALT:
break m1;
}
}
System.out.println("Execution finished.");
for (int i = 0; i < var.length; i++) {
if (var[i] != 0) {
System.out.println((char) ('a' + i) + " = " + var[i]);
}
}
}
}
public static void main(String[] args) {
Lexer lexer = new Lexer("{ a = 10; do { a = a - 2;} while (3 < a); }");
Parser parser = new Parser(lexer);
Node ast = parser.parse();
Compiler compiler = new Compiler();
List<Object> program = compiler.compile(ast);
VirtualMachine vm = new VirtualMachine();
vm.run(program);
}
}